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Chapter 1 Introduction
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1.1 Role of geocycle in the introduction of metals into our environment
Air, water, earth and life are strongly interconnected as shown in Figure 1.1,
which constitute the major segments of the environment- atmosphere, hydrosphere,
geosphere and biosphere. Biogeochemical cycles of matter that involve biological,
chemical and geological processes and phenomena manifests the strong interaction
among living organism and various spheres of the abiotic (nonliving) environment. Water
and air are involved in weathering rocks, producing mineral formations and forming
climate; which have profound effects on the geosphere and interchange matter and energy
with it. Living systems (biosphere) which largely exist on the geosphere in turn have
significant effects on it. However, for better or for worse, the environment in which
human must live have been affected irreversibly by technology and industrial activities.
Metals in the environment may be present in the different states as solid, liquid or gaseous
state or in varying forms as individual elements, organic and inorganic compounds. The
movement of metals between environmental reservoirs may or may not involve changes
of state. Gaseous and particulate metals may be inhaled and solid and liquid (aqueous-
phase) metals may be ingested or absorbed, thereby entering the biosphere [1-3].
Figure 1.1 Interaction of human with the environment
Heavy metals are included within the category of environmental toxins:
“Materials which can harm the natural environment even at low concentration, through
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their inherent toxicity and their tendency to accumulate in the food chain and/or have
particularly low decomposition rates”. The redistribution of many toxic metals into the
environment, caused by the gradual increase in industrial activity, has increased the
possibility of human exposure. Among the various toxic elements, heavy metals like
cadmium, lead, and mercury are especially prevalent in nature due to their high industrial
use. These metals serve no biological function and their presence in tissues reflects
contact of the organism with its environment. They are cumulative poison and are toxic
even at low dose [4,5]. The indication of their importance relative to other potential
hazards is their ranking by the U.S. Agency for Toxic Substances and Disease Registry,
which lists all hazards present in the toxic waste sites according to their prevalence and
severity of their toxicity. The first, second, third and sixth hazards on the list are heavy
metals: lead, mercury, arsenic and cadmium, respectively [6]. The other metal ions that
pose potential dangers to human lives include chromium, copper, zinc, nickel, cobalt, iron
and manganese. Herein, sources and effects of these metal ions are discussed.
Lead
Lead is a highly toxic cumulative poison in humans and animals. Its cumulative
poisoning effects are serious hematological damage, anaemia, kidney malfunctioning,
brain damage etc. In humans, chronic lead poisoning is manifested by abnormalities such
as encephalopathy, nervous irritability, kidney disease, altered heme synthesis and
reproductive functions. Such poisoning is associated with low to intermediate levels of
chronic exposure to lead. The principle risk to children from lead is interference with the
normal development of their brains. A number of studies have found small but significant
neuropsychological impairment in young children due to environmental lead absorbed
either before or after birth. In particular, lead appears to have deleterious effects on
children’s behavior and attentiveness, and possibly also on their IQs.
Environmental contamination of lead is widespread; the main anthropogenic
source of this element is burning of leaded gasoline. Lead is used as a construction
material for equipment used in sulfuric acid manufacture, petrol refining, halogenation,
sulfonation, extraction and condensation. It is used in storage batteries, alloys, solder,
ceramics and plastics. It is also used in the manufacture of pigments, tetraethyl lead and
other lead compounds, in ammunition, and for atomic radiation and x-ray protection.
Lead is used in aircraft manufacture, building construction materials (alloyed with
copper, zinc, magnesium, manganese and silicon), insulated cables and wiring, household
utensils, laboratory equipment, packaging materials, reflectors, paper industry, printing
[7].
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inks, glass industry, water purification and waterproofing in the textile industry. The
primary sources, for low to intermediate levels of chronic exposure to lead, are food,
water and air. About 50% of lead is absorbed with inhalation of dusts, 10–15% absorbed
orally, out of which 90% is distributed to bones [8]. In natural water its typical
concentration lies between 2 and 10 ng mL−1, whereas, the upper limit recommended by
WHO is less than 10 ng mL−1
[9].
Copper
Excess copper interferes with zinc, a mineral needed to make digestive enzymes
[10]. Physical conditions associated with copper imbalance include arthritis, fatigue,
adrenal burnout, insomnia, scoliosis, osteoporosis, heart disease, cancer, migraine
headaches, seizures, fungal and bacterial infections including yeast infection, gum
disease, tooth decay, skin and hair problems and female organ conditions including
uterine fibroids, endometriosis and others. Mental and emotional disorders related to
copper imbalance include depression, mood swings, fears, anxiety, phobias, panic attacks,
violence, autism, schizophrenia, and attention deficit disorder [11]. Copper imbalance in
children is associated with delayed development, attention deficit disorder, anti-social and
hyperactive behavior, autism, learning difficulties and infections such as ear infections.
Copper is primarily used as a metal or an alloy (e.g., brass, bronze, gun metal).
Copper sulfate is used as a fungicide, algaecide and herbicide. Copper particulates are
released into the atmosphere by windblown dust; volcanic eruptions; and anthropogenic
sources, primarily copper smelters and ore processing facilities [12]. Copper particles in
the atmosphere will settle out or be removed by precipitation, but can be resuspended into
the atmosphere in the form of dust. Copper is released into waterways by natural
weathering of soil and rocks, disturbances of soil, or anthropogenic sources (e.g., effluent
from sewage treatment plants) [12]. Another source of copper is drinking water that
remained in copper water pipes, or copper added to your water supply.
Zinc
Zinc is an essential trace element of great importance for humans, plants and
animals. It plays an important role in several biochemical processes and its compounds
have bactericidal activity. Zinc phosphide, which is used as an active ingredient in
rodenticide, reacts with water and acid in the stomach to release phosphine gas which in
turn causes cell toxicity with necrosis of the gastrointestinal tract. Oral zinc increases
faecal excretion of copper and blocks the absorption of ingested minerals. In this case, a
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series of complex zinc-copper relationship and metabolism resulted in the inhibition of
copper absorption and increased faecal loss of copper through saliva, gastric juices and
biliary secretions. Breathing large amounts of zinc (as dust or fumes) can cause a specific
short-term disease called metal fume fever. Inhalation of fumes may result in sweet taste,
throat dryness, cough, weakness, generalized aching, chills, fever, nausea and vomiting.
Zinc chloride fumes have caused injury to mucous membranes and pale gray cyanosis.
Ingestion of soluble salts may cause nausea, vomiting and purging [10,13,14].
Zinc has many commercial uses as coating to prevent rust, in dry cell batteries,
and mixed with other metals to make alloys like brass and bronze. Zinc compounds are
widely used in industry to make paint, rubber, dye, wood preservatives, and ointments
[14]. Also used for galvanizing sheet iron; as ingredient of alloys such as bronze, brass,
Babbitt metal, German silver, and special alloys for die-casting; as a protective coating
for other metals to prevent corrosion, for electrical apparatus, especially dry cell batteries,
household utensils, castings, printing plates, building materials, railroad car linings,
automotive equipment; as reducer (in form of the powder) in the manufacture of indigo
and other vat dyes, for deoxidizing bronze; extracting gold by the cyanide process,
purifying fats for soaps; bleaching bone glue; manufacture of sodium hydrosulfite; as
reagent in analytical chemistry, e.g. in the Marsh and Gutzeit test for arsenic; as a reducer
in the determination of iron. Some zinc is released into the environment by natural
processes, but most comes from activities of people like mining, steel production, coal
burning, and burning of waste.
Cadmium
Cadmium may cause renal injuries and may interfere with the renal regulation of
calcium and phosphate balance. It was seen that exposure to abnormal levels of cadmium
can result in its accumulation in the renal cortex, which causes a series of adverse
subclinical reactions such as hypercalciurium, renal stones and renal tubular dysfunction
besides probable development of carcinogenic activity in organisms [15]. The toxicity of
cadmium may involve its binding to key cellular sulfhydryl groups, its competition with
other metals (zinc and selenium) for inclusion in metalloenzymes, and its competition
with calcium for binding sites on regulatory proteins such as calmodulin. The lack of an
effective elimination pathway is responsible for cadmium’s biologic half-life of 10-30
years. Chronic effects of cadmium exposure are dose-dependent and include anosmia,
yellowing of teeth, emphysema, minor changes in liver function, microcytic hypochromic
anemia unresponsive to iron therapy, renal tubular dysfunction characterized by
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proteinuria and increased excretion of β2-microglobulin and (with prolonged poisoning)
Cadmium is an industrial waste or by-product, which has a great environmental
concern. Cadmium is used in many industrial processes, such as a constituent of easily
fusible alloys, soft solder, electroplating and deoxidizer in nickel plating, engraving
processes, electrodes for vapor lamps, photoelectric cells, and nickel-cadmium storage
batteries [16]. It's wide technological use in fertilizers, mining, pigments, as well as its
delivering from oil and coal burning and residues incineration; bring about an extensive
anthropogenic contamination of soil, air and water.
osteomalacia leading to bone lesions and pseudo fractures [6].
Nickel
Nickel is a moderately toxic element as compared with other transition metals.
However, it is known that inhalation of nickel and its compounds can lead to serious
problems, including respiratory system cancer. Moreover, nickel can cause a skin disorder
known as nickel-eczema. The most common adverse health effect of nickel in humans is
an allergic reaction. People can become sensitive to nickel when things containing it are
in direct contact with the skin, when they eat nickel in food, drink it in water, or breathe
dust containing it. Less frequently, allergic people have asthma attacks following
exposure to nickel. Lung effects, including chronic bronchitis and reduced lung function,
have been observed in workers who breathed large amounts of nickel. Headache,
dizziness, shortness of breath, vomiting, and nausea are the initial symptoms of
overexposure; the delayed effects (10 to 36 h) consist of chest pain, coughing, shortness
of breath, bluish discoloration of the skin, and in severe cases, delirium, convulsions, and
death. [10]. Long-term exposure can cause decreased body weight, heart and liver
damage, and skin irritation. High levels of Ni in the diet may be associated with an
increased risk of thyroid problems, cancer, and heart disease [17,18]. Less frequently,
allergic people have asthma attacks following exposure to nickel. Lung effects, including
chronic bronchitis and reduced lung function, have been observed in workers who
breathed large amounts of nickel.
Major sources of exposure are: tobacco smoke, auto exhaust, fertilizers,
superphosphate, food processing, hydrogenated-fats-oils, industrial waste, stainless steel
cookware, testing of nuclear devices, baking powder, combustion of fuel oil, dental work
and bridges. Humans are exposed to it through breathing of air or smoking of tobacco
containing nickel, eating of food containing nickel, as well as drinking of water
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contaminated with nickel and handling of coins and touching of other metals containing
nickel [19].
Arsenic
Once arsenic is in the body, it binds to hemoglobin, plasma proteins, and
leukocytes and is redistributed to the liver, kidney, lung, spleen, and intestines. Arsenic
produces cellular damage through a variety of mechanisms. Arsenic binds to enzyme
sulfhydryl groups and forms a stable ring, which deactivates the enzyme. The process of
deactivating the enzyme causes widespread endothelial cell damage, vasodilation, and
leakage of plasma. Massive transudation of fluid into the bowel lumen, mucosal vesicle
formation, and tissue sloughing may result in large gastrointestinal fluid losses. Arsenic
binds to dihydrolipoic acid, a pyruvate dehydrogenase cofactor, blocking the conversion
of pyruvate to acetyl coenzyme A and inhibiting gluconeogenesis. Arsenic competes with
phosphates for adenosine triphosphate, forming adenosine diphosphate monoarsine,
causing the loss of high-energy bonds. In some forms, arsenic is caustic, exerting a direct
toxic effect on blood vessels and large organs. Long-term exposure results in nerve
damage and may lead to lung, skin, or liver cancer. Once inhaled, arsine gas combines
with hemoglobin in RBCs, causing severe hemolysis and anemia. Patients develop
hemoglobinuria and hematuria within several hours of exposure. One of the early warning
signs of arsenic poisoning is a "pins and needles" sensation in hands and feet. Long-term
oral exposure to inorganic arsenic can result in skin changes including darkening of the
skin and the appearance of small "corns" or "warts" on the palms, soles, and torso.
Arsenite (should be arsenate) (+5) undergoes biomethylation in the liver to the less toxic
metabolites methylarsenic acid and dimethylarsenic acid; biomethylation can quickly
become saturated, however, and the result is the deposition of increasing doses of
inorganic arsenic in soft tissues [20].
Elevated arsenic (As) levels in the environment are attributable to both natural and
anthropogenic sources, including geothermal discharges, industrial products and wastes,
agricultural pesticides, wood preservatives and mine drainage. It is also used in drugs,
war gases and as a homicidal and suicidal weapon. Other uses of arsenic compounds are
in alloys, manufacturing of arsenic compounds (arsenic oxides) and certain glass. Copper
and lead ores contain small amounts of arsenic. Arsenic is also a major ingredient of
Fowler's solution and continues to be found in some folk remedies [14].
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Mercury
Mercury was recognized as causing neuro-developmental disabilities including
dyslexia, attention deficit hyperactivity disorder, intellectual retardation, and autism [21].
Inhalation of very high concentrations causes acute pulmonary edema and interstitial
pneumonitis, which may be fatal. In non-fatal cases dyspnea and coughing may persist.
Kidney effects may occur at exposure levels lower than those causing central nervous
system effects. Also, mercury vapor may cause “Kawasaki” disease, which seems to be
immunologically mediated and is similar to Pink disease. Mercury intoxication also
causes reproductive effects. Contact dermatitis from mercury amalgam fillings and
mercury sensitivity in dental students has been reported. Repeated or prolonged exposure
to mercury vapor is highly toxic to the central nervous system [20]. Exposures to high
levels of metallic, inorganic, or organic mercury can permanently damage the brain,
kidneys, and developing fetus. Embryo toxicity and teratogenicity of organic mercury
compounds have been reported in many test systems. Exposure to methyl mercury is
worse for young children than for adults, because more of it passes into children’s brains
where it interferes with normal development [22-24].
An organic mercury compound, namely methyl mercury, is produced mainly by
small organisms in water and soil. Metallic mercury is used to produce chlorine gas and
caustic soda and also in thermometers, amalgams (dental fillings), and batteries. Mercury
salts are used in skin-lightening creams and as antiseptic creams and ointments. Mercury
is used in scientific and electrical equipments, in the electrolytic production of chlorine
and sodium hydroxide; and as a catalyst in polyurethane foam production [25]. Inorganic
mercury (metallic mercury and inorganic mercury compounds) enters the air from mining
ore deposits, burning coal and waste, and from manufacturing plants. It enters the water
or soil from natural deposits, disposal of wastes, and the use of mercury-containing
fungicides. Breathing of contaminated air or skin contact during use at workplace (dental,
health services, chemical, and other industries that use mercury) represents occupational
exposures. Inhalation of mercury vapor is the most important route of uptake of elemental
mercury [26].
Chromium
Epidemiological studies have consistently shown that human exposure to Cr (VI)
compounds is associated with a higher incidence of respiratory cancers [27]. Acute toxic
effects occur when one breath very high levels of chromium (VI) in air. It can damage
and irritate nose, lungs, stomach, and intestines. People who are allergic to chromium
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may also have asthma attacks after breathing high levels of either chromium (VI) or (III).
Long term exposures to high or moderate levels of chromium (VI) causes damage to the
nose (bleeding, itching, and sores) and lungs and can increase risk of non-cancer lung
diseases. Ingesting very large amounts of chromium can cause stomach upsets and ulcers,
convulsions, kidney and liver damage, and even death. Skin contact with liquids or solids
containing chromium (VI) may lead to skin ulcers [11,13].
Stainless steel welding, particularly the manual metal arc welding method, may
well be the most common source of occupational exposure to Cr(VI), given the fact that
there are millions of stainless steel welders worldwide. Chromium is used in
manufacturing chrome-steel or chrome-nickel-steel alloys (stainless steel) and other
alloys, bricks in furnaces, and dyes and pigments, for greatly increasing resistance and
durability of metals and chrome plating, leather tanning, and wood preserving. Handling
or breathing sawdust from chromium treated wood, manufacturing, disposal of products
or chemicals containing chromium, or fossil fuel burning release chromium to the air,
soil, and underground water [28].
Manganese
Exposure to atmospheric Mn at high concentration is a risk factor in humans that
can manifest as neuronal degeneration resembling Parkinson's disease (PD). Although the
underlying mechanism of Mn and dopamine (DA) interaction-induced cell death remains
unclear, however, Mn exposure alone to mesencephalic cells for 24h induced minimal
apoptotic cell death [29]. Increased manganese intake impairs the activity of copper
metallo-enzymes. Excess manganese interferes with the absorption of dietary iron. Long-
term exposure to excess levels may result in iron-deficiency anemia. High manganese
levels indicate problems with calcium and/or iron metabolism [11,13]. Symptoms of
toxicity mimic those of Parkinson's disease (tremors, stiff muscles) and excessive
manganese intake can cause hypertension in patients older than 40 [30]. Symptoms of
increased manganese levels include psychiatric illnesses, mental confusion, impaired
memory, loss of appetite, mask-like facial expression and monotonous voice, spastic gait
and neurological problems. Manganese toxicity can cause kidney failure, hallucinations,
as well as diseases of the central nervous system.
Uses of Mn include: (i) iron and steel production; (ii) manufacture of dry cell
batteries; (iii) production of potassium permanganate and other Mn chemicals; (iv)
oxidant in the production of hydroquinone; (v) manufacture of glass; (vi) textile
bleaching; (vii) oxidizing agent for electrode coating in welding rods; (viii) matches and
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fireworks; and (ix) tanning of leather [31]. Organic compounds of Mn are present in the
fuel additive, methylcyclopentadienyl manganese tricarbonyl (MMT), fungicides (e.g.,
maneb and mancozeb), and in contrast agents used in magnetic resonance imaging. The
primary anthropogenic sources of Mn in ambient air include emission of Mn from
industrial sources such as ferroalloy production plants, iron and steel foundries, power
plants, and coke ovens and re-entrainment of soils containing Mn [32]. Well water rich in
manganese can be the cause of excessive manganese intake and can increase bacterial
growth in water. Manganese poisoning has been found among workers in the battery
manufacturing industry.
Cobalt
Cobalt metal particles, when inhaled in association with other agents such as
metallic carbides (hard metals) or diamond dust, may produce an interstitial lung disease
termed "hard metal disease" or "cobalt lung". Acute toxicity of cobalt may be observed as
effects on the lungs, including asthma, pneumonia and wheezing, that have been found in
workers who breathed high levels of cobalt in the air. The International Agency for
Research on Cancer (IARC, USA) has determined that cobalt is a possible carcinogen to
humans. Studies in animals have shown that cobalt causes cancer when placed directly
into the muscle or under the skin [11,13,33].
Vast applications of cobalt in various arrays of products and processes such as its
use in alloys, batteries, catalysts, pigments and coloring, make this element to be
considered as an important metal in various industries [34]. Cobalt enters the environment
from natural sources and from the burning of coal and oil. Workers may be exposed to
cobalt in industries that process it or make products containing cobalt [35].
Iron
Excessive iron leads to tissue damage as a result of formation of free radicals [36].
Long term over consumption of iron may cause hemosiderosis, a condition characterized
by large deposits of the iron storage protein hemosiderin in the liver and other tissues.
Iron overload is most often diagnosed when tissue damage occurs, especially in iron-
storing organs, such as the liver. Infections are likely to develop because bacteria thrive
on iron rich blood. Ironically, some of the signs of iron overload are analogous to those of
iron deficiency: fatigue, headache, irritability, and lowered work performance. Other
common symptoms of iron overload include enlarged liver, skin pigmentation, lethargy,
joint diseases, loss of body hair, amenorrhea, and impotence. Untreated hemochromatosis
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aggravates the risks of diabetes, liver cancer, heart disease and arthritis. In cases of iron
overload the natural storage and transport proteins are overwhelmed and the iron spills
over into other tissues and organs, such as the muscle, spleen and liver [37], proving to be
toxic [38].
Iron is included in the quality control of industrial and commercial products such
as petroleum, alloys, foods, beverages etc [20]. Occasionally, iron pipes may also be a
source of iron in water. Water percolating through soil and rock can dissolve minerals
containing iron and manganese and hold them in solution.
1.2 Significance and characteristics of preconcentration
Determination of toxic metal ions in environmental samples, wastewater, various
natural water bodies and biological fluids is necessary for environment monitoring,
assessment of occupational and environmental exposure to toxic metals and its impact on
the ecosystem.
The low concentrations of the metal ions and the strong interference of matrices
present in association with it, in real samples, poses difficulty in its direct determination
despite the availability of sophisticated instrumental methods, with excellent sensitivity
and multielemental analysis capability [39-46]. A radical way to eliminate matrix effects
is a preliminary separation of macro components by a relative, or absolute,
preconcentration of trace metals.
Preconcentration can be of two types: absolute and relative preconcentration.
Absolute prenconcentration involves the transfer of trace elements from a large mass of
sample into a small mass, e.g. by evaporation or by solvent extraction into a small volume
of an organic phase.
Relative preconcentration involves at least partial separation of the components when
their concentrations differ very much. Its main aim is often to exchange the matrix for a
suitable collector (generally of smaller mass) to prevent its interference in the
determination. In some cases, it is difficult to define a boundary between absolute and
relative preconcentration. Relative preconcentration increases the mass ratio of trace
elements to main components (the solvent is not considered as a major component in this
case).
Depending on the purpose, trace elements can be concentrated selectively or in
groups and either separation of the matrix or separation of the trace components can be
used. Removal of the matrix is reasonable if used in combination with multi element
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determination techniques, e.g. spectrochemical analysis, but only if the matrix is of
simple composition. Matrix removal is used especially in analysis of high-purity metals.
If the matrix contains several elements forming complex compounds (geological and
biological materials), it is better to separate the trace elements. Sometimes, there is no
need to remove the matrix completely; the process is then called "enrichment". However,
it is usually more profitable to change to another matrix which better meets the demands
of the subsequent determination, simplifies a calibration, etc. Several such collectors are
suitable for determinations by different techniques. For instance, carbon powder can be
analyzed by a spectrographic method or by flameless atomic-absorption
spectrophotometry [47].
Some quantitative characteristic features used for description of prenconcentration
are listed in Table 1.1
Table 1.1 Characteristic features of preconcentration Recovery (R) R = QT/Q°T, where QT and Q°T are respectively the
quantities of trace element in the concentrate and in the
sample. It is usually expressed as a percentage.
Concentration coefficient (K)
K = (QT/QM) / (Q°T/Q°M), where Q°M and QM are
respectively the amounts of matrix before and after
preconcentration. If R = 100% then K = Q°M/QM
Separation coefficient (S)
.
S = (QM/QT) / (Q°M/Q°T
Preconcentration factor
) = 1/K
Maximum volume of sample / minimum eluent volume
that gives quantitative recovery of analyte
Preconcentration limit (µg L-1 Minimum concentration up to which preconcentration
(maximum volume) is feasible.
)
Of the analytical techniques for preconcentration and separation, SPE has been
preferred over conventional solvent extraction and coprecipitation. The markedly lower
quantity of reagents required, the fact that solid phase can be repeatedly used, higher
concentration factors, and simplicity in handling and transfer are frequently quoted as
advantages [48-51]. Sorption and ion exchange have been studied for different analytical
applications [52–60] using various support materials. Many different methods are used
for analytical preconcentration [61]. They can be classified according to the nature of the
separations (chemical and physical methods) used and the number and nature of the
phases involved in the separation process.
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Solvent extraction [62-64]
Co-precipitation [65-67]
Evaporation methods [68]
Electrochemical Techniques [69-70]
Crystallization [71]
Flotation [72]
Membrane filtration [73]
Cloud point extraction [74]
Solid-phase extraction (SPE) [75,76]
1.3 Solid-phase extraction (SPE ) as a preconcentration method
It is a separation process that is used to remove solid or semi-solid compounds
from a mixture of impurities based on their physical and chemical properties. Analytical
laboratories use solid phase extraction to concentrate and purify samples for analysis. The
separation ability of solid phase extraction is based on the preferential affinity of desired
or undesired solutes in a liquid (mobile phase) for a solid (stationary phase) through
which the sample is passed. Impurities in the sample are either washed away while the
analyte of interest is retained on the stationary phase, or vice-versa. Analytes that are
retained on the stationary phase can then be eluted from the solid phase extraction
cartridge with the appropriate solvent. The preconcentration methods utilizing solid
sorbents are considered to be superior than the liquid-liquid extraction in terms of
simplicity, rapidity, and the ability to obtain a high enrichment factor. Particularly, solid
phase extraction has been demonstrated in various procedures to be a very effective
preconcentration technique in combination with atomic absorption spectrometry. The
main advantage of this technique is the possibility of using a relatively simple detection
system with flame atomization instead of a flameless technique, which require more
expensive equipment and are usually much more sensitive to interferences from
macrocomponents of various natural matrices [39].
1.3.1 History and development of SPE
The history of using solid phases to isolate drugs from biological samples dates
back to 1923 when permutite was used [77] and then subsequently silicic acid [78] for
extracting adrenaline. In the 1950s, alumina columns were used [79] for extracting
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adrenaline and noradrenaline from blood samples. Then successful extraction of catechol
bases, adrenaline and histamine from crude extracts of glands, using Amberlite IRC-50
[80] was performed. Later a method was developed to extract catechol amines from
tissues using cation exchange resin Dowex 50 [81]. By the mid 1960s more complex
samples were being tried. A method using cation exchange paper chromatography [82],
which was qualitative in nature, was developed to detect a number of narcotics,
tranquilizers, amphetamines and barbiturates from urine samples but the method was
more. In the 1970s, the stress was to develop techniques which were more sensitive.
Amberlite XAD-2 resin columns were used to quantify narcotic analgesics from urine and
could determine as low as 0.6 mg mL of urine [83]. It is noted that, in most of the above
cases, the principle of ion exchange was used to separate drugs from biological samples.
Subsequently, adsorption phenomenon was tried on charcoal to concentrate a number of
drugs (barbiturate, glutathimide, ethchlorvynol, amphetamine, phenothiazine, quinine,
morphine, cocaine and its metabolites) from urine and achieved an average detection limit
of 1 mg mL-1
Solid-phase extraction is now emerging as a very important sample preparation
technique. It is preferred over other traditional procedures, such as liquid-liquid extraction
(LLE), mainly because it is more efficient and much less time-consuming.
of urine [84]. By the late 1970s, HPLC technology had made rapid progress
and one of the major developments was the use of silica and bonded silica as the
stationary phase. Waters Associates and Analytichem International were among the first
to develop the concept of using sorbents, similar to those used in HPLC, packed in
miniature columns, to isolate drugs and chemicals from other interfering impurities of test
samples. Specifically, small disposable cartridges containing silica, bonded silica and
other phases were developed for commercial use and these were called SPE columns. Cl8
SepPak@ columns (Waters Associates) were evaluated for the extraction of tricyclic
antidepressants from biological samples [85].
1.3.2 Basic principles
The principle of SPE is similar to that of liquid-liquid extraction (LLE), involving
a partitioning of solutes between two phases. However, instead of two immiscible liquid
phases, as in LLE, SPE involves partitioning between a liquid (sample matrix) and a solid
(sorbent) phase. This sample treatment technique enables the concentration and
purification of analytes from solution by sorption on a solid sorbent.
The basic approach involves passing the liquid sample through a column, a
cartridge, a tube or a disk containing an adsorbent that retains the analytes. After the
14
entire sample has been passed through the sorbent, retained analytes are subsequently
recovered upon elution with an appropriate solvent. The first experimental applications of
SPE started fifty years ago [86,87].
1.3.3 Technique
An SPE method always consists of three to four successive steps, as illustrated in
Figure 1.2.
Figure 1.2 SPE operation steps.
STEP 1: The solid sorbent should be conditioned using an appropriate solvent, followed
by the same solvent as the sample solvent.
Significance:
This step is crucial, as it enables the wetting of the packing material and the
solvation of the functional groups.
In addition, it removes possible impurities initially contained in the sorbent or the
packaging.
Also, this step removes the air present in the column and fills the void volume with
solvent.
The nature of the conditioning solvent depends on the nature of the solid sorbent.
Care must be taken not to allow the solid sorbent to dry between the conditioning
and the sample treatment steps, otherwise the analytes will not be efficiently
retained and poor recoveries will be obtained.
If the sorbent dries for more than several minutes, it must be reconditioned.
STEP 2: The second step is the percolation of the sample through the solid sorbent.
Depending on the system used, volumes can range from 1 mL to 1000 mL. The
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sample may be applied to the column by gravity, pumping, aspirated by vacuum
or by an automated system.
Significance:
The sample flow-rate through the sorbent should be low enough to enable efficient
retention of the analytes, and high enough to avoid excessive duration.
During this step, the analytes are concentrated on the sorbent.
Even though matrix components may also be retained by the solid sorbent, some of
them pass through, thus enabling some purification (matrix separation) of the
sample.
STEP 3: The third step (which is optional) may be the washing of the solid sorbent with
an appropriate solvent, having low elution strength, to eliminate matrix
components that have been retained by the solid sorbent, without displacing the
analytes.
Significance:
A drying step may also be advisable, especially for aqueous matrices, to remove
traces of water from the solid sorbent.
This will eliminate the presence of water in the final extract, which, in some cases,
may hinder the subsequent concentration of the extract and or the analysis.
STEP 4: The final step consists in the elution of the analytes of interest by an appropriate
solvent, without removing retained matrix components.
Significance:
The solvent volume should be adjusted so that quantitative recovery of the analyte is
achieved with subsequent low dilution.
In addition, the flow-rate should be correctly adjusted to ensure efficient elution.
It is often recommended that the solvent volume be fractionated into two aliquots,
and before the elution to let the solvent soak the solid sorbent.
1.3.4 Mechanism of retention
Adsorption of trace elements on the solid sorbent is required for preconcentration
(Figure 1.3). The mechanism of retention depends on the nature of the sorbent, and may
include simple adsorption, chelation or ion-exchange. Active functional groups of the
16
ligand moiety are responsible for the selective chelation with the metal ions whereas
macro porous polymeric support offers large surface area.
Adsorption
Trace elements are usually adsorbed on solid phases through van der Waals forces
or hydrophobic interaction. Hydrophobic interaction occurs when the solid sorbent is
highly non-polar (reversed phase). The most common sorbent of this type is octadecyl-
bonded silica (C18-silica). More recently, reversed polymeric phases have appeared,
especially the styrene-divinylbenzene copolymer that provides additional pi-pi interaction
when p-electrons are present in the analyte [88]. Elution is usually performed with
organic solvents, such as methanol or acetonitrile. Such interactions are usually preferred
with online systems, as they are not too strong and thus they can be rapidly disrupted.
However, because most trace element species are ionic, they will not be retained by such
sorbents.
Chelation
Several functional group atoms are capable of chelating trace elements. The
atoms most frequently used are nitrogen (e.g. N present in amines, azo groups, amides
and nitriles), oxygen (e.g. O present in carboxylic, hydroxyl, phenolic, ether, carbonyl,
phosphoryl groups) and sulfur (e.g. S present in thiols, thiocarbamates and thioethers).
The nature of the functional group will give an idea of the selectivity of the ligand
towards trace elements. In practice, inorganic cations may be divided into 3 groups:–
1. Group I-‘hard’ cations: these preferentially react via electrostatic interactions (due to
a gain in entropy caused by changes in orientation of hydration water molecules); this
group includes alkaline and alkaline-earth metals (Ca2+, Mg2+, Na2+
2. Group II-‘borderline’ cations: these have an intermediate character; this group
contains Fe
) that form rather
weak outer-sphere complexes with only hard oxygen ligands.
2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Mn2+
3. Group III-‘soft’ cations: these tend to form covalent bonds. Hence, Cd
. They possess affinity for both hard
and soft ligands. 2+and Hg2+
Chelating agents may be directly added to the sample for chelating trace elements,
the chelates being further retained on an appropriate sorbent. An alternative is to
possess strong affinity for intermediate (N) and soft (S) ligands. For soft metals, the
following order of donor atom affinity is observed: O<N<S.
17
introduce the functional chelating group into the sorbent. For that purpose, three different
means are available:
1. The synthesis of new sorbents containing such groups (new sorbents);
2. The chemical bonding of such groups on existing sorbents (functionalized sorbents);
and
3. The physical binding of the groups on the sorbent by impregnating the solid matrix
with a solution containing the chelating ligand (impregnated, coated or loaded
sorbents).
The latter remains the most simple to be used in practice. Its main drawback is the
possible flush of the chelating agent out of the solid sorbent during sample percolation or
elution that reduces the lifetime of the impregnated sorbent. Different ligands
immobilized on a variety of solid matrices have been successfully used for the
preconcentration, separation and determination of trace metal ions. Chelating agents with
a hydrophobic group are retained on hydrophobic sorbents (such as C18-silica). Similarly,
ion-exchange resins are treated with chelating agents containing an ion exchange group,
such as a sulfonic acid derivative of dithizone (i.e. diphenylthiocarbazone) (DzS), 5-sulfo-
8-quinolinol, 5-sulfosalicylic acid, thiosalicylic acid, chromotropic acid, or
carboxyphenylporphyrin (TCPP) [89-92].
Binding of metal ions to the chelate functionality is dependent on several factors:
nature, charge and size of the metal ion;
nature of the donor atoms present in the ligand;
buffering conditions which favor certain metal extraction and binding to active
donor or groups; and
nature of the solid support (e.g. degree of cross-linkage for a polymer).
In some cases, the behavior of immobilized chelating sorbents towards metal
preconcentration may be predicted using the known values of the formation constants of
the metals with the investigated chelating agent [93]. However, the presence of the solid
sorbent may also have an effect and lead to the formation of a complex with a different
stoichiometry than the one observed in a homogeneous reaction [94, 95]. In fact, several
characteristics of the sorbent should be taken into account, namely the number of active
groups available in the resin phase [93-96], the length of the spacer arm between the resin
and the bound ligand [97], and the pore dimensions of the resin [98].
18
Ion-pairing
When a non-polar sorbent is to be used, an ion-pair reagent (IP) can be added to
the sorbent [99]. Such reagents contain a nonpolar portion (such as a long aliphatic
hydrocarbonated chain) and a polar portion (such as an acid or a base). Typical ion-pair
reagents are quaternary ammonium salts and sodium dodecylsulfate (SDS) [100,101]. The
non-polar portion interacts with the reversed-phase non-polar sorbent, while the polar
portion forms an ion-pair with the ionic species present in the matrix (that could be either
free metallic species in solution or complexes).
Ion exchange
Ion-exchange sorbents usually contain cationic or anionic functional groups that
can exchange the associated counter-ion. Strong and weak sites refer to the fact that
strong sites are always present as ion-exchange sites at any pH, while weak sites are only
ion-exchange sites at pH values greater or less than the pKa. Strong sites are sulfonic acid
groups (cation-exchange) and quaternary amines (anion-exchange), while weak sites
consist of carboxylic acid groups (cation-exchange) or primary, secondary and tertiary
amines (anion-exchange). These groups can be chemically bound to silica gel or polymers
(usually a styrene-divinylbenzene copolymer), the latter allowing a wider pH range. An
ion-exchanger may be characterized by its capacity, resulting from the effective number
of functional active groups per unit of mass of the material. The theoretical value depends
upon the nature of the material and the form of the resin. However, in the column
operation mode, the operational capacity is usually lower than the theoretical one, as it
depends on several experimental factors, such as flow-rate, temperature, particle size and
concentration of the feed solution. As a matter of fact, retention on ion-exchangers
depends on the distribution ratio of the ion on the resin, the stability constants of the
complexes in solution, the exchange kinetics and the presence of other competing ions.
Even though ion-exchangers recover hydrated ions, charged complexes and ions
complexed by labile ligands, they are of limited use in practice for preconcentration of
trace elements due to their lack of selectivity and their retention of major ions [102]. Yet,
for some particular applications they may be a valuable tool. Hence, iron speciation is
possible through selective retention of the negative Fe(III)-ferron complex on an anion-
exchanger [103]. Selenium speciation is also feasible by selectively eluting Se(IV) and
Se(VI) retained on a anion-exchanger [104].
19
Figure 1.3 Interactions occurring at the surface of the solid sorbent
F-functional group; TE-trace element; MS-matrix solvent; MI-matrix ions; ES-elution
solvent.
1.3.5 Mechanism of elution of trace elements from the sorbent
Similar kinds of interactions (as mentioned in the previous section) usually occur
during the elution step. The type of solvent must be correctly chosen to ensure stronger
affinity of the trace element to the solvent, to ensure disruption of its interaction with the
sorbent (Figure 1.3). Thus, if retention on the sorbent is due to chelation, the solvent
could contain a chelating reagent that rapidly forms a stronger complex with the trace
metal. Elution may also be achieved using an acid that will disrupt the chelate and
displace the free trace element. Similarly, if retention is due to ion exchange, its pH
dependence enables the use of eluents with different pH to be used, such as acids. Of
prime importance is to selectively elute only the target species. So, if they are more
strongly retained on the sorbent than the interfering compounds, a washing step with a
solvent of moderate elution strength is highly advisable before elution of the target
species with the appropriate solvent.
1.3.6 Selection of Solid Sorbents
The nature and properties of the sorbent are of prime importance for effective
retention of metallic species. In practice, the main requirements for a solid sorbent are:
20
The possibility to extract a large number of trace elements over a wide pH range
(along with selectivity towards major ions);
Kinetically faster quantitative sorption and elution;
A high capacity;
Regenerability; and
Accessibility
Solid sorbents may be reversed-phase sorbents or normal-phase sorbents.
Reversed-phase sorbents usually refer to the packing materials that are more
hydrophobic than the sample, which are frequently used with aqueous samples. When
hydrophobic supports are used, retention of ionic metal species will require the formation
of hydrophobic complexes. This can be achieved through addition of the proper reagent to
the sample or through immobilization of the reagent on the hydrophobic solid sorbent.
Normal-phase sorbents refer to materials more polar than the sample and they are
used when the sample is an organic solvent containing the target compounds.
Nascent polymeric resins as sorbent
Nascent styrene–DVB resins such as Amberlite XAD-1180 [105,106], XAD-4
[107], and XAD-16 [108-112] are used directly for enrichment of inorganic species as
their halide or thiocyanate complex. The type and quantity of sorbent, hydrophobicity,
ionizability of the analytes, sample volume and pH interactively determine the
breakthrough volume. Using a styrene–divinylbenzene sorbent, the primary interaction
mechanism is via Vander Waals forces; therefore, the more hydrophobic the compound
the larger the breakthrough volume will be and the larger the sample size from which
quantitative recovery can be expected. This observation can be generalized to other
sorbents by stating that regardless of the primary interaction mechanism between the
analyte and the sorbent, it holds true that the stronger the interaction, the larger the
breakthrough volume will be.
Modification of nascent polymeric resins
. Macroporous hydrophobic resins of the Amberlite XAD series
[polystyrenedivinylbenzene (PS-DVB) resins] are good supports for developing chelating
matrices. Amberlite XAD resins, as the copolymer backbone for the immobilization of
chelating ligands, have some physical superiority, such as porosity, uniform pore size
distribution, high surface area and chemical stability towards acids, bases, and oxidizing
agents, as compared to other resins. In addition to the hydrophobic interaction that also
21
occurs with C18-silica, such sorbents also allow π-π interactions. Due to the hydrophobic
character of PS-DVB, retention of trace elements on such sorbents requires the addition
of a ligand to the sample. The use of surface-modified PS–DVB copolymers with
different polar substituent overcomes the following disadvantages suffered by standard
silica-based material used for SPE:
lack of pH stability under acidic or basic conditions,
low breakthrough for polar analytes,
they are not wettable by water alone and always need a conditioning step with a
wetting solvent, such as methanol.
However, in practice, the resins prepared by impregnation of the ligand are difficult
to reuse, due to partial leaching of the ligand (thus resulting in poor repeatability). To
overcome this problem, the resin may be chemically functionalized. Chemical
modification of PS–DVB copolymers have been carried out by immobilizing varying
substituents through different bridging groups.
Amberlite XAD-2
Surface modification: Amberlite XAD-2 resin modified by surface adsorption with oxime
[113,114] 1-(2-thiazolylazo)-2-naphthol [113,115], pyrocatechol violet [113], 4-(2-
pyridylazo) resorcinol [114], eriochrome blue black R [114], ammonium pyrollidine
dithiocarbamate (APDC) [116], tropolone [117], 1-(2-pyridylazo)- 2-naphthol (PAN)
[118], 2-(2-thiazolylazo)-p-cresol [119], calmagite [120,121], and organophosphinic acid
[122] were used as solid phase extractant sorbents in off-line or online column
preconcentration modes.
Chemical modification: Singh et al. chemically immobilized AmberliteXAD-2 with
alizarin Red S [123], tiron [124], catechol [125], thiosalicylic acid [126], o-aminophenol
[127], chromotropic acid [128], catechol violet [129], salicylic acid [130], and pyrogallol
[125] via azo (-N=N-) spacer. A similar synthetic scheme was employed by Jain et al.
[131, 132] to chemically immobilize o-vanilline thiosemicarbazone on to Amberlite
XAD-2 resin. Dogutan et al. [133] synthesized palmitoyol-8-hydroxyquinoline
functionalized Amberlite XAD-2 by a modified procedure described by Suebert et al.
[134] through chloromethylation. In 1992, Trojanowicz group [135] chemically
immobilized Eriochrome blue black R onto Amberlite XAD-2. Yuan and Shuttler [136]
immobilized quinoline-8-ol onto Amberlite XAD-2 and controlled pore glass, and
22
reported higher enrichment factors with the former as it gave higher flow rates during
quantitation of aluminum by FIA-ETAAS.
Amberlite XAD-4
Surface modification: Surface adsorption of Amberlite XAD-4 resin beads with oxine
[137-140], APDC [137, 138], 2[2-(5-chloropyridyl)azo]- 5-dimethyl amino phenol (5-
ClDMPAP) [139,140], butane–2,3-dionebis(N-pyridinoacetylhydrazone)[141], 2-(5-
bromo-2-pyridylazo)-5-diethyl aminophenol (5-Br PADAP) [142-144], 5-phenyl azo-8-
quinolinol [145], 1-nitroso-2-napthol [146], and N-benzoylphenylhydroxylamine [147]
were used as solid-phase extractants for the trace determination of inorganics using a
variety of detection techniques which include spectral and X-ray techniques.
Chemical modification: Azotization was used for immobilzation of o-aminobenzoic acid
onto Amberlite XAD-4 resin by Cekic et al. [148]. Jain et al. [149] employed similar
synthetic scheme for functionalizing Amberlite XAD-4 with o-vanilline-semicarbazone.
Amberlite XAD-4 was functionalized with N-hydroxy ethyl ethylene diamine via
acylation by Hirata et al. [150] as per the synthesis procedure described by Dev and Rao
[151]. Acid chloride was grafted onto Amberlite XAD-4 [152], Yakin and Apak [153]
immobilized maleic acid by electrophilic substitution of the Amberlite XAD-4 resin with
maleic anhydride by a Friedel-Crafts reaction. Quinoline-8-ol functionalized Amberlite
XAD-4 resin was synthesized by Gladis and Rao [154] through acetylation.
Amberlite XAD-16
Surface modification: Traces of inorganics were enriched on Amberlite XAD-16 resin
beads after surface adsorption with a variety of chelates, namely PAN [155], NaDDTC
[156,157], 4-(2-thiazoylazo) resorcinol [158,159], N,N-dibutyl-N-benzoylthiourea
(DBBT) [160], and di-(2-ethylhexyl phosphoric acid (D2EHPA) [161].
Chemical modification: Azotization was employed to functionalize (bis-2,3,4-trihydroxy
benzyl)ethylene diamine(BTBED) [162], 2-{[1-(3,4-Dihydroxyphenyl)methylidene]
amino}-benzoic acid (DMABA) [163], 4-{[(2-Hydroxyphenyl)imino]methyl}-1,2-
benzenediol (HIMB) [164] and Nitrosonaphthol [165] on to Amberlite XAD-16. D.
Prabhakaran et. al. immobilized 1,3-dimethyl-3-aminopropan-1-ol onto Amberlite XAD-
16 via simple condensation mechanism [166].
23
Apart from this a number of different solid sorbents have been investigated for the
preconcentration of trace metals from an aqueous solution. They include:
Silica gel (Inorganic based sorbents) [167-172]
C-bonded silica gel (Inorganic based sorbents) [173-178]
Other inorganic oxides (Inorganic based sorbents)[179-182]
Divinylbenzene-vinylpyrrolidone copolymers (polymeric organic sorbents)[183-
184]
Polyacrylate polymers (polymeric organic sorbents)[185-187]
Polyurethane polymers (polymeric organic sorbents)[188-192]
Polyethylene polymers (polymeric organic sorbents)[193]
Polytetrafluoroethylene polymers (polymeric organic sorbents)[194-196]
Polyamide polymers (polymeric organic sorbents)[197]
Iminodiacetate-type chelating resins (polymeric organic sorbents)[198-201]
Propylenediaminetetraacetate-type chelating resins (polymeric organic
sorbents)[202]
Polyacrylonitrile based resins (polymeric organic sorbents)[203-206]
Ring-opening metathesis polymerization based polymers (polymeric organic
sorbents)[207,208]
Carbon sorbents (non-polymeric organic sorbents)[209-215]
Cellulose (non-polymeric organic sorbents)[216-218]
Naphthalene based sorbents (non-polymeric organic sorbents)[219-226]
Molecularly Imprinted Polymers (MIP) [227,228]
1.3.7 Advantages of SPE
Reduced detection limit
This step is used if the detection limit of the analytical technique is higher than
the concentration of trace elements in the sample. Concentration often involves separation
of the matrix or the bulk of it, and sometimes a number of interfering minor constituents
as well. In a prepared concentrate, the relative concentration of trace elements is usually
higher than in the initial sample. Moreover, the possibility of increasing the amount of
sample analyzed means that the absolute amounts of elements to be determined can also
be increased. As a result, it is possible to reduce the detection limit of trace elements
(sometimes very significantly; by a factor of 100 or 1000). This is the main but not the
only reason for the widespread use of preconcentration [229].
24
Preparation of a representative sample
Preconcentration is almost essential if trace elements are non-homogeneously
distributed in the material. In this case, a representative sample must be quite large; it is
difficult to analyze it directly, especially if the method of determination needs a small
sample as, for instance, spark-source mass-spectrometry or spectrochemical analysis. In
many other cases, preconcentration with preliminary dissolution and production of a
small volume of concentrate facilitates the preparation of a representative sample.
Samples can be homogenized during other operations also. SPE enjoys superiority over
solvent extraction as it is free from difficult phase separation, which is caused by the
mutual solubility between water and organic solvent layers [230].
Facilitates calibration
Concentration facilitates calibration, especially if there is a lack of standard
reference materials. It makes it possible to obtain concentrates with identical matrices in
analysis of quite different materials, for example, concentrates on carbon powder in
spectrochemical analysis. Reference samples are prepared as concentrates of the same
type. There is then no necessity to have standard reference materials for all substances
analyzed. Preconcentration with exhaustive removal of the matrix is desirable in the
analysis of toxic, radioactive or, if the matrix can be recovered, very expensive materials.
Moreover, it is convenient to add elements as internal standards, if necessary, during
decomposition of the sample and concentration. Sometimes, preconcentration allows an
increase in the number of trace elements which can be determined by a selected technique
or makes it possible for the determination technique to be used at all. These advantages of
preconcentration make it an important part of trace analysis. In spite of the progress in
sensitive instrumental methods of direct analysis, the significance of concentration does
not diminish. On the contrary, its possibilities increase, particularly because of new
combinations with methods of determination [39,231].
Other attractive features
This technique is attractive as it reduces consumption of and exposure to solvents,
their disposal costs and extraction time [232]. It also allows the achievement of high
recoveries, along with possible elevated enrichment factors. However, different results
between synthetic and real samples may be observed [233]; recoveries should be
estimated in both cases as far as possible. Its application for preconcentration of trace
metals from different samples is also very convenient due to sorption of target species on
25
the solid surface in a more stable chemical form than in solution. Use of carcinogenic
organic solvents is avoided and thus the technique is ecofriendly to nature. Finally, SPE
affords a broader range of applications due to the large choice of solid sorbents.
High preconcentration factor
The use of SPE enables the simultaneous preconcentration of trace elements,
removal of interferences, and reduces the usage of organic solvents that are often toxic
and may cause contamination. Upon elution of the retained compounds by a volume
smaller than the sample volume, concentration of the extract can be easily achieved.
Hence, concentration factors of up to 1000 may be attained [229].
Preservation and storage of the species
SPE allows on-site pre-treatment, followed by simple storage and transportation
of the pre-treated samples with stability of the retained metallic species for several days
[234,235]. This point is crucial for the determination of trace elements, as the transport of
the sample to the laboratory and its storage until analysis may induce problems, especially
changes in the speciation. In addition, the space occupied by the solid sorbents is minimal
and avoids storage of bulky containers and the manpower required to handle them.
Selective extraction
SPE offers the opportunity of selectively extracting and preconcentrating only the
trace elements of interest, thereby avoiding the presence of major ions. This is crucial in
some cases, such as with spectrophotometric detection, since the determination of heavy
metals in surface waters may necessitate the removal of non-toxic metals, such as Fe or
Zn, when they occur at high concentrations [236]. It may also be possible to selectively
retain some particular species of a metal, thereby enabling speciation. For example, salen
I modified C18-silica is quite selective towards Cu(II) [237], while chemical binding of
formylsalicylic acid on amino-silica gel affords selectivity towards Fe(III) [238]. This
high selectivity may also be used to remove substances present in the sample that may
hinder metal determination, such as lipid substances in the case of biological samples
[239]. The chelating resin method is an economical method since it uses only a small
amount of ligand and extraction solvent and this also increases the sensitivity of the
system.
26
Automation and possible on-line coupling to analysis techniques
SPE can be easily automated, and several commercially available systems have
been recently reviewed [239]. In addition, SPE can be coupled online to analysis
techniques. On-line procedures avoid sample manipulation between preconcentration and
analysis steps, so that analyte losses and risk of contamination are minimized, allowing
higher reproducibility [240]. In addition, all the sample volume is further analyzed, which
enables smaller sample volume to be used. However, in the case of complex samples, off-
line SPE should be preferred due to its greater flexibility, and the opportunity to analyze
the same extract using various techniques.
1.3.8 Challenges
Some of the challenges of preconcentration are as follows:
Preconcentration increases the analysis time and complicates the analysis for large
volume. However, this can be minimized with online systems using flow injection
(FI) techniques because of their potential for automation, minimization of reagent
and/or sample consumption, and reduced risk of contamination. It ffers all
essential prerequisites for trace analysis [241].
It may also lead to losses or contamination of trace elements to be determined.
Special working procedures, reagents of high purity, specially equipped laboratories
and special materials for equipments are necessary.
1.3.9 Common SPE applications
The common SPE application includes:
Metal ions in various complex real samples,
Pharmaceutical compounds and metabolites in biological fluids,
Drugs of abuse in biological fluids,
Environmental pollutants in drinking and waste water,
Pesticides and antibiotics in food/agricultural matrices,
Desalting of proteins and peptides,
Fractionation of lipids, and
Water and fat soluble vitamins
1.4 Types of analytical techniques coupled with preconcentration method
27
Some frequently used analytical techniques in combinations with preconcentration
may be described as follows:
1.4.1 Adsorptive stripping Voltammetry
In trace analysis, mainly of heavy metal ions, Anodic Stripping Voltammetry
(ASV) is popular because of the low limit of determination – ranging to sub ppb
concentrations, its accuracy and precision, as well as the low cost of instrumentation for
this analytical method. ASV is based on previous electrolytical accumulation of the
compound to be determined on the working electrode, followed by voltammetric
dissolution (oxidation) of the reduced substance formed. In addition, some anions or
organic compounds can be accumulated on a mercury electrode to form an insoluble
compound with the mercury ions obtained by dissolution of the mercury electrode at
positive potentials. In this type of cathodic stripping voltammetry (CSV), the reduction
process of the mercury compound on the electrode surface is studied. The most important
step, leading to a substantial increase in the sensitivity in both
types of methods is electrolytic accumulation of the species on the working electrode.
The high sensitivity of adsorptive stripping method is obviously their greatest
advantage. On the other hand, a serious drawback is interference from other surface-
active substances that may be present in the solution. In this case, competitive adsorption
usually occurs and leads to a decrease in the measured current or, at very high surface-
active substance (s.a.s.) concentrations, to significant suppression of the signal.
Interfering effects depend on the nature of both the analyzed and interfering substances
and on their concentration ratio in the determination.
Evidently, the interfering effect of s.a.s. can be minimized by employing short
accumulation times; however, this approach is not suitable in the determination of trace
amounts of analyte. It is then necessary to employ suitable separation of interfering
compounds, e.g. the application of LC or gel chromatography and extraction procedures
[242-244].
1.4.2 X –Ray fluorescence analysis
For the analysis of liquid samples by X-ray fluorescence (XRF) spectrometry, the
liquid in a specially designed liquid sample holder is irradiated directly with X-rays.
However, for direct analysis of liquid samples, the abundance of effective amounts of
analytes in the irradiated volume is insufficient for the determination of trace metals in
natural water. Therefore, for the quantitation of trace metals in aqueous samples by XRF,
28
it is necessary to preconcentrate the analytes through accepted sample pretreatments
[245,246].
In this case, group concentratration, and more rarely individual concentration, is
used and the concentrate should preferably be a solid which can be analyzed directly.
Otherwise, several operations must be successively performed. Thus for example, extracts
are often decomposed and the trace elements sorbed on cellulose powder or silica gel
carrying functional groups. It is more convenient to do the extraction with low-melting
reagents at increased temperature. In this case, the solidified concentrate may at once be
pressed into a tablet and analyzed.
1.4.3 Spectrophotometric determination
Combinations of concentration, especially extractive, with spectrophotometry in
the visible and ultraviolet regions are widely known. Individual concentration steps or
successive separations of several elements are usually used. The matrix is very rarely
separated in this case. Most frequently, the reagent used for concentration also gives a
colored complex with the element to be determined. However, two reagents may also be
used: first, the most selective reagent is used for the separation and then, for the
determination, a reagent which may not be selective but is the most suitable form for the
photometry is employed. The second reagent may also be added after back-extraction or
mineralization of the extract [248-250].
1.4.4 Neutron-activation analysis
There are two distinct ways of trace concentration in this method: before
irradiation and after it. Separation of the matrix before irradiation is necessary if it is
strongly activated and the radioisotopes formed have long half-lives. In the concentration,
trace components which are strongly activated (but are not to be determined) may also be
separated. However, concentration before irradiation nullifies one of the main advantages
of activation analysis-that a blank correction is unnecessary. This advantage is still valid,
however, in the case of concentration after irradiation. In analytical practice, both variants
are used but the second is used more frequently [251-253].
1.4.5 Electrothermal Atomic Absorption Spectrometry
Nowadays, electrothermal atomic absorption spectrometry (ETAAS) is most
powerful and popular analytical tools for the determination of low concentration of metal
ions present in environmental samples and biological materials due to their high
29
selectivity and sensitivity for analyte determination. Nevertheless, they are potentially
prone to spectroscopic and/or non-spectroscopic interferences. Various schemes have
been suggested to alleviate the interfering effects and facilitate reliable analysis such as
protocols ranging from instrument modifications (e.g. background correction) to
experimental designs (e.g. standard addition or internal standardization). However,
instead of implementing such approaches, there is a much simpler and effective solution
to the problem, namely to subject the sample to appropriate pretreatments before it is
presented to the detector. Preconcentration addresses these serious problems for the
determination of metal ions. Concentration of the desired trace elements can extend the
detection limits, remove interfering constituents, and improve the precision and accuracy
of the analytical results [254-256].
1.4.6 Inductively coupled plasma optical emission spectrometry (ICP-OES) Inductively coupled plasma optical emission spectrometry is an analytical
technique often employed to determine metal ions in various types of samples [257]. But,
their sensitivity and selectivity are usually insufficient for direct determination of these
contaminants at a very low concentration level in complex matrix environmental samples.
Moreover, several types of spectral interference have been reported in the determination
of metal ions by ICP-OES. Thus, preconcentration and separation procedures have been
devised to allow trace amounts of metal ions to be determined in complex matrices using
ICP-OES [258,259].
1.4.7 Inductively coupled plasma mass spectrometry (ICP-MS)
Although inductively coupled plasma-mass spectrometry (ICP-MS) capable of the
rapid simultaneous analysis of multiple elements over a large range of concentrations
from a single aliquot of sample, the high salinity concentrations of open ocean samples
can cause substantial salt precipitation and build-up, unpredictable suppression or
enhancement effects as well as mask the analyte signal through the formation of isobaric
and polyatomic interferences due to the presence of high salt content in the samples. In
particular, the matrix elements in the sample can combine with carbon in the atmosphere
and/or argon in the plasma and result in the formation of polyatomic species which may
interfere with the determination of numerous analytes including transition metals and rare
earth elements. In addition, when the sample contains a very high concentration of
dissolved salts, e.g. seawater; clogging of the sample introduction system or of the
injector tube of the torch may occur. Therefore, direct sample injection is impractical,
30
requiring sample pre-treatment to remove the high salinity matrix by either dilution of the
sample or using analyte extraction/separation techniques. Despite the sensitivity of ICP-
MS, open ocean trace metal concentrations are often at or below the detection limit,
prohibiting the further dilution of samples. It is therefore advisable to concentrate and
separate the trace metals from the seawater matrix prior to ICP-MS analysis [260,261].
1.4.8 Inductively coupled plasma atomic emission spectrometry (ICP-AES)
Inductively coupled plasma atomic emission spectrometry (ICP-AES) is widely
recognized as a multi-element technique for the determination of elemental species,
though direct determinations in environmental and biological samples at trace level is
difficult, because the aspiration of solutions with high salt concentrations in the plasma
can cause problems such as blockage of the nebulizer, considerable background emission,
and transport and chemical interferences with a consequent drop in sensitivity and
precision. This limitation can be overcome by using enrichment methods in that metals
ions of interest from solutions are selectively separated and preconcentrated into smaller
volumes to achieve better detection by ICP-AES [262,263].
1.4.9 Atomic absorption spectrometry
Flame atomic absorption is a very common technique for detecting metals and
metalloids in environmental samples [264]. It is a well established technique for the
quantification of nearly 70 elements in a variety of sample types with sensitivity at the
ppm level or less. Aqueous samples can be determined with no sample preparation; solid
samples must be dissolved or digested. This sample preparation for solids can be time
consuming. Although atomic absorption spectroscopy dates to the nineteenth century, the
modern form was largely developed during the 1955s by a team of Australian chemists.
They were led by Alan Walsh and worked at the CSIRO (Commonwealth Science and
Industry Research Organization) Division of Chemical Physics in Melbourne, Australia.
The time since 1955 can be divided into seven years period. The first was an induction
period (1955-1961) when atomic absorption received attention from only a very few
people. This was followed by a growth period (1962-1969) when most of what we see
today was developed, and then by a period of relative stability (1969-1976) when atomic
absorption contributed greatly to other fields. We are now in a period of great change,
which started in about 1876, due to the impact of computer technology on individual
laboratory instruments [265]. Flame atomic absorption spectrometry is among the most
widely used methods for the determination of the heavy metals at trace levels. This
31
technique presents desirable characteristics such as operational facilities, good selectivity
and low cost. However, in the presence of very high excess of diverse ions compared with
the level of analyte, some limitations, mainly those related to the sensitivity are observed.
In trace analysis, therefore, a preconcentration and/or separation of trace elements from
the matrix are frequently necessary to improve the detection limit and selectivity of their
determination [39,266]. Particularly, solid phase extraction has been demonstrated in
various procedures to be a very effective preconcentration technique in combination with
atomic absorption spectrometry. The main advantage of this technique is the possibility of
using a relatively simple detection system with flame atomization instead of a flameless
technique, which require more expensive equipment and are usually much more sensitive
to interferences from macro components of various natural matrices [39].
The technique (Figure 1.4) makes use of absorption spectrometry to assess the
concentration of an analyte in a sample. It relies therefore heavily on Beer-Lambert law.
In short, the electrons of the atoms in the atomizer can be promoted to higher orbitals for
a short amount of time by absorbing a set quantity of energy (i.e. light of a given
wavelength). This amount of energy (or wavelength) is specific to a particular electron
transition in a particular element, and in general, each wavelength corresponds to only
one element. This gives the technique its elemental selectivity.
Principles
As the quantity of energy (the power) put into the flame is known, and the
quantity remaining at the other side (at the detector) can be measured, it is possible, from
Beer-Lambert law, to calculate how many of these transitions took place, and thus get a
signal that is proportional to the concentration of the element being measured.
In order to analyze a sample for its atomic constituents, it has to be atomized. The
sample should then be illuminated by light. The light transmitted is finally measured by a
detector. In order to reduce the effect of emission from the atomizer (e.g. the black body
radiation) or the environment, a spectrometer is normally used between the atomizer and
the detector.
The technique typically makes use of a flame to atomize the sample [267], but
other atomizers such as a graphite furnace [268] or plasmas, primarily inductively
coupled plasma, are also used [269].
Types of Atomizer
32
Figure 1.4 Atomic absorption spectrometer block diagram
When a flame is used it is laterally long (usually 10 cm) and not deep. The height
of the flame above the burner head can be controlled by adjusting the flow of the fuel
mixture. A beam of light passes through this flame at its longest axis (the lateral axis) and
hits a detector.
A liquid sample is normally turned into an atomic gas in three steps:
Analysis of liquids
1. Desolvation (Drying) – the liquid solvent is evaporated, and the dry sample remains.
2. Vaporization (Ashing) – the solid sample vaporises to a gas.
3. Atomization – the compounds making up the sample are broken into free atoms.
The radiation source chosen has a spectral width narrower than that of the atomic
transitions.
Radiation Sources
Hollow cathode lamps are the most common radiation source in atomic absorption
spectroscopy. Inside the lamp, filled with argon or neon gas, is a cylindrical metal
cathode containing the metal for excitation, and an anode. When a high voltage is applied
across the anode and cathode, gas particles are ionized. As voltage is increased, gaseous
ions acquire enough energy to eject metal atoms from the cathode. Some of these atoms
Hollow cathode lamps
33
are in excited states and emit light with the frequency characteristic to the metal [270].
Many modern hollow cathode lamps are selective for several metals.
Atomic absorption spectroscopy can also be performed by lasers, primarily diode
lasers because of their good properties for laser absorption spectrometry [271]. The
technique is then either referred to as diode laser atomic absorption spectrometry
(DLAAS or DLAS) [272], or since wavelength modulation most often is employed,
wavelength modulation absorption spectrometry.
Diode lasers
Interferences
Various factors which may interfere with the determination of metal ions are as follows:
Chemical interferences may also be eliminated by separating the metal from the
interfering material. Although complexing agents are employed primarily to
increase the sensitivity of the analysis, they may also be used to eliminate or reduce
interferences.
The presence of high dissolved solids in the sample may result in interference from
non-atomic absorbance such as light scattering. If background correction is not
available, a non-absorbing wavelength should be checked. Preferably, samples
containing high solids should be extracted.
Ionization interferences occur when the flame temperature is sufficiently high to
generate the removal of an electron from a neutral atom, giving a positively charged
ion. This type of interference can generally be controlled by the addition, to both
standard and sample solutions, of a large excess (1,000 mg/L) of an easily ionized
element such as K, Na, Li or Cs.
Spectral interference can occur when an absorbing wavelength of an element
present in the sample but not being determined falls within the width of the
absorption line of the element of interest. The results of the determination will then
be erroneously high, due to the contribution of the interfering element to the atomic
absorption signal.
Interference can also occur when resonant energy from another element in a
multielement lamp, or from a metal impurity in the lamp cathode, falls within the
band-pass of the slit setting when that other metal is present in the sample. This type
of interference may sometimes be reduced by narrowing the slit width.
34
The interference, known as background absorption, arises from the presence in the
flame of gaseous molecules, molecular fragments and some time smoke. In addition
background effects can be caused by light scatter.
Procedures for reduction of interferences
Ensure if possible that standard and sample solutions are of similar bulk
composition to eliminate matrix effects.
Alteration of flame composition or of flame temperature can be used to reduce the
likelihood of stable compound formation within the flame.
Selection of an alternative resonance line will overcome spectral interferences from
other atom or molecules and from molecular fragments.
Separation, for example by solvent extraction or an ion exchange process, may
occasionally be necessary to remove an interfering element.
The narrow bandwidth of hollow cathode lamps makes spectral overlap rare. That
is, it is unlikely that an absorption line from one element will overlap with another.
Molecular emission is much broader, so it is more likely that some molecular absorption
band will overlap with an atomic line. This can result in artificially high absorption and
an improperly high calculation for the concentration in the solution. Three methods are
typically used to correct this, namely
Background Correction methods
Zeeman correction- A magnetic field is used to split the atomic line into two sidebands.
These sidebands are close enough to the original wavelength to still overlap with
molecular bands, but are far enough not to overlap with the atomic bands. The absorption
in the presence and absence of a magnetic field can be compared, the difference being the
atomic absorption of interest.
Smith-Hieftje correction- The hollow cathode lamp is pulsed with high current, causing a
larger atom population and self-absorption during the pulses. This self-absorption causes
a broadening of the line and a reduction of the line intensity at the original wavelength
[273].
Deuterium lamp correction- In this case, a separate source (a deuterium lamp) with broad
emission is used to measure the background emission. The use of a separate lamp makes
this method the least accurate, but its relative simplicity (and the fact that it is the oldest
of the three) makes it the most commonly used method.
35
1.5 Statistical Treatment of Data
Analytical chemistry besides providing the methods and tools needed for insight
into our material world [274], seeks to improve the reliability of existing techniques to
meet the demands for better chemical measurements which arise constantly in our society
[275]. Statistical analysis is necessary to understand the significance of collected data and
to set limitations on each step of the analysis. The design of experiments is determined
from proper understanding of what the data will represent. It is impossible to perform
chemical analysis that is totally free from errors, or uncertainties. Every measurement is
influenced by many uncertainties, which combine to produce a scatter of results. It is
seldom easy to estimate the reliability of experimental data. However, the probable
magnitude of the error in a measurement can often be evaluated statistically. Limits
within which the true value of a measured quantity lies at a given level of probability can
then be defined.
Chemical analyses are affected by at least two types of errors namely systematic
and random based on their source. Systematic errors have definitive value, assignable
cause and unidirectional of nature. Systematic errors can be reduced to a negligible level
if an analyst pays careful attention to the details of the analytical procedure including
methods, periodic calibration of the instruments and self discipline. Random errors are the
accumulated effect of the individual indeterminate uncertainties. It is caused by the
uncontrollable variables that are an inevitable part of physical and chemical measurement.
It causes the data to scatter more or less symmetrically around the mean in a random
manner which are assumed to be distributed according to the normal error law (Gaussian
curve). They are revealed by small differences in replicate measurements of a single
quantity and affect the precision of the results. They are more difficult for an analyst to
eliminate, but they can be minimized by increasing the number of replicate
measurements. The random error in the result of an analysis can be evaluated by the
method of statistics [276].
The most common applications of statistics to analytical chemistry include:
To establish confidence limits for the mean of a set of replicate data.
To determine the number of replicate measurements required to decrease the
confidence limit for a mean to a given probability level.
To determine at a given probability whether an experimental mean is different from
the accepted value for the quantity being measured (t test or test for bias in an
analytical method).
36
To determine at a given probability level whether two experimental means (different
methods) are different.
To determine at a given probability level whether the precision of two sets of
measurements differs (F test).
To decide whether a questionable data is probably the result of a gross error and
should be discarded in calculating a mean (Q test).
To define an estimate detection limit of a method.
The commonly used terms in the statistical analysis
Mean- The mean (x ) is obtained by dividing the sum of the replicate measurements
(Σxi
) by the number of observations (N) performed in a set. The mean is considered to be
the best estimate of the true value, which can only be obtained if an infinite number of
measurements are performed.
Accuracy- The term accuracy as used in analytical chemical literature is a measure of the
degree to which a mean (x ) obtained from a series of experimental measurements,
agrees with the value, which is accepted as the true or correct value for the quantity. It is
expressed by the error; either absolute or relative error. There are two types of errors
according to their nature and source- systematic errors and random errors. Systematic
error causes the mean of a set of data to differ (unidirectional) from the accepted value.
They have definitive value, assignable cause and affect the accuracy of results. Random
error causes data to be scattered more or less symmetrically around a mean value. It
mainly affects the precision of measurement.
Precision- Precision describes the agreement among the replicate measurements and is
generally expressed as standard deviation, coefficient of variation (relative standard
deviation) and variance.
Standard deviation- Standard deviation(s) measures how closely the data are clustered
about the mean. When measurements are repeated, the scatter of the results will be around
the expected value of the results, if no bias exists.
In analytical chemistry, this scatter is more often than not of such a nature that it can be
described as a normal distribution. The mean gives the centre of the distribution. Standard
deviation measures the width of the distribution. Therefore, an experimental technique
( )2
1−−Σ
=N
xxis
Nix
x∑
=
37
that produces a small standard deviation is more reliable (precise) than one that produces
a large standard deviation provided that they are equally accurate. The square of the
standard deviation, s2
Coefficient of variation- Precision is more often expressed as the coefficient of variation
which is the standard deviation divided by the average and multiplied by 100. Since the
average and standard deviation have same dimension, the coefficient of variation is
dimensionless; it is only a relative measure of precision. Therefore, it is sometimes also
known as relative standard deviation (RSD).
, is known as variance.
Confidence limit and confidence interval- Calculation of s (standard deviation) for a set of
data provides an indication of the precision inherent in a particular method of analysis.
But unless there is a large number of data, it does not by itself give any information about
how close the experimentally determined mean (x ) might be to the true mean value µ
(population mean). Statistical theory, though, allows us to estimate the limits around an
experimentally determined mean(x ) within which the population mean or true value (µ
) expected to lie, within a given degree of probability. The likelihood that the true value
lies within the range is called confidence level or probability usually expressed as a
recent.
Confidence limit for µN
tsx ±=
where t is the student’s t value which depends on the desired confidence level and this
equation holds when σ, population standard deviation is not known.
Correlation coefficient- The correlation coefficient is a measure of the linear relationship
between two variables. In order to establish the linear relationship between variables xi
and yi
RSD sx
= ×100
the Pearson’s correlation coefficient r is used. The r may have values between +1
and –1. When the r is +1, there is a perfect correlation, i.e. an increase in one variable is
associated with an increase in the other. When the r is –1, there is perfect negative
correlation, i.e., one variable increases where as the other decreases. When r is zero, one
variable has no linear relationship to the other. The correlation coefficient r is calculated
by using the above equation.
( )[ ] ( )[ ]r
n x y x y
n x x n y yi i i i
i i i i
=−
− −
Σ Σ
Σ Σ Σ Σ2 2 2 2
38
Regression equation- When two variables exhibit a linear relationship, we may be
interested in quantifying the relationship so that a value of one variable may be estimated
from the other. A common example is the construction of calibration curve, which relates
the concentration of analyte to absorbance or any measurable property. The least square
curve fitting technique is the most commonly accepted mathematical procedure known
for this purpose. The equation derived by this technique produces a line whose position is
such that the sum of the squares of the vertical distances of each data point to the line is a
minimum if the line is to be used to predict y from x values, or the sum of the squares of
the horizontal distances is a minimum if x is to be predicted from y. If y = bx + a
represents the equation for a straight line, where y is dependent variable and x is
independent variable, then slope ‘b’ and intercept ‘a’ are derived from the following
equations [276-278].
Sensitivity- It is measure of the ability of an instrument or method to discriminate between
small differences in analyte concentration. There are two factors which limit the
sensitivity:
the slope of the calibration curve.
reproducibility or precision of the measuring device.
Of two methods that have equal precision, the one that has the steeper calibration
curve will be the more sensitive. A corollary to this statement is that if two methods have
calibration curves with equal slopes, the one that exhibits the better precision will be the
more sensitive. The quantitative definition of sensitivity that is accepted by the
International Union of Pure and Applied Chemists (IUPAC) is calibration sensitivity,
which is the slope of the calibration curve at the concentration of interest. Most
calibration curves that are used in analytical chemistry are linear and may be represented
by the equation: S = mc + Sbl, where S is the measured signal, c is the concentration of
the analyte, Sbl is the instrumental signal of the blank, and m is the slope of the straight
line. The quantity Sbl
( )b
n x y x yn x x
i i i i
i
=−
−
Σ Σ Σ
Σ Σ2 2
should be the intercept of the straight line. With such curves, the
calibration sensitivity is independent of the concentration c and is equal to m. The
calibration sensitivity as a figure of merit suffers from its failure to take into account the
precision of individual measurements.
a y bx= −
39
To include precision in a meaningful mathematical statement of sensitivity,
analytical sensitivity is proposed (γ): γ = m/ss; where ss = standard deviation of
measurement and m is the slope of the calibration
curve [270,279].
Detection limits- It is defined as the minimum concentration or mass of analyte that can
be detected at a known confidence level. This limit depends upon the ratio of the
magnitude of the analytical signal to the size of the statistical fluctuations in the blank
signal. The limit of detection (CL) according to the definition of International Union of
Pure and Applied Chemists can be expressed as [280]: CL = x + k.sbl ; where x = mean of
blank signal, sbl
The estimation of the detection limit is best understood by considering a
calibration graph. Using the linear regression method, it is possible to obtain the intercept
and slope of the best-fit line. The value of the intercept can be used as x; and errors in the
slope and the intercept of the regression line is acceptable as a measure of s
= standard deviation of the blank measures and k = numerical constant. A
value of k=3 was strongly recommended by IUPAC. The Analytical Methods Committee
of the Royal Society of Chemistry sought to clarify the IUPAC definition [281].
bl. Hence, the
detection limit (CL) may also be expressed as: CL = 3sbl
/b; where b is the slope of the
calibration line [270,277,282,283].
1.6 Present work and its scope
SPE has found its most effective use as a sample clean-up and concentration
method prior to further analysis. Solid phase extraction, in combination with atomic
absorption spectrometry, has been developed as the technique for preconcentration prior
to determination of trace metal ions in varying complex matrices, which includes both
biological and environmental samples.
Amberlite XAD-4 and 16 resins have been successfully used as the polymeric
supports in previous works (Table 1.2 and 1.3). Hence, we have used the same because of
its good surface area (725 m2 g-1 and 800 m2 g-1) and high porosity. The functional groups
used satisfactorily for this work are salicylic acid, o-hydroxybenzamide, salicylanilide
and p-aminobenzene sulfonic acid on account of their smaller size and good number of
chelating sites. This functionalities would render the modified resin more hydrophilic so
as to facilitate faster equilibrium (of solute between solid and aqueous phases) whereby
enhancing the extraction ability.
40
The modification of polystyrene-divinyl benzene adsorbent resin has been
accomplished through azotization reaction, by conventional reaction techniques, by
incorporating neutral organic hydrophilic group through –N=N- spacer. It is important to
note that these functional groups are neutral, that is they bear no positive or negative
charge but, however, they possess the characteristic features of a chelating agent. This is
important in order to allow for effective contaminant removal of the inorganics since
either anionic or cationic charged resins may often pick up undesirable materials that are
present such as other matrices. However, neutral functionalized or modified resins such as
those described here in act differently than charged resins and take up the inorganics by
adsorbtion/chelation rather than ion exchange.
Chelating resin and the aqueous solution containing the heavy metals along with
other matrices may be contacted using both batch and column methods which would
result in intimate contact between the resin and the solution. The columns used for
extractions were packed with 100 to 500 mg of resin. Varying volume of aqueous
solutions of a sample, containing metals in the concentration of the order of parts per
million, is passed through the column at the optimum flow rate whereby the solute gets
retained onto the resin.
Investigations of the optimum experimental conditions sorption and elution have
been carried out by considering the following influential parameters.
Synthesis and Characterization
The introduction of the organic groups onto the polymeric material (XAD-series
resin) can make the stable chelating compounds for the uptake of trace metal ions. The
aim of chemical modification with different organic reagent, which has mainly N and O
donor atoms, is to make the material have excellent coordination properties with trace
metal ions and to obtain a novel sorbent with high loading of metal ions. The FT-IR
analysis is a very useful technique in identifying the immobilization process by
comparing the precursor and modified resin. The thermogravimetric analysis (TGA)
curve of the chelating resin shows mass loss in two steps. In first step, it may be due to
the loss of sorbed water of the resin and second step, due to the loss of functional moieties
of the chelating resin. It was also use to performed, to study the water-regaining capacity
of the resin matrix in order to evaluate its hydrophilic character. The CHN analysis was
performed during each stage of chemical modification to study the extent of ligand
functionalization to the polymeric matrix.
41
The modified resin soaked in acidic (HCl/HNO3, 1-5 mol L-1) and basic medium
(NaOH, 1-4 mol L-1) for 6 h, filtered and then washed with triply distilled water until free
from acid or alkali. The resin shows no loss in sorption capacity. Hence, the resin
exhibited high chemical stability. The water regain capacity and hydrogen ion capacity
have also been studied. This value reflects the high hydrophilicity of the resin which is
satisfactory for column operation. In case of AXAD-16-SALD, the overall hydrogen ion
capacity amounts to 6.08 mmol g-1 of resin, which may be contributed both by the
hydroxylic and amide groups present within the molecule. Theoretically, if 2.65 mmol of
the reagent constituted per gram of the resin, the hydrogen ion capacity, due to the
hydroxylic group should have been 2.65 mmol g-1
. The durability and reusability nature
of the chelating resin was tested with metal ions solutions by batch equilibration method.
Thereafter, the sorption and desorption of metal ions were repeated on the same resin.
The capacity of the resin found to be constant up to the several cycles (35-55) showing
the multiple use of chelating resin without any loss in its physical and chemical
properties.
Sorption studies
Effect of pH of the sample
Sample pH is of prime importance for efficient retention of the trace elements on
the sorbent. Its influence strongly depends on the nature of the sorbent used. Careful
optimization of this parameter is thus crucial to ensure quantitative retention of the trace
elements and in some cases selective retention. In particular with ion exchangers, correct
adjustment of sample pH is required to ensure preconcentration. Thus, in the case of
cationic-exchangers, low pH usually results in poor extraction due to competition
between protons and cationic species for retention on the sorbent. When retention of trace
elements is based on chelation (either in the sample or on the solid sorbent), the sample
pH is also a very important factor as most chelating ligands are conjugated bases of weak
acid groups and accordingly, they have a very strong affinity for hydrogen ions. The pH
will determine the values of the conditional stability constants of the metal complexes. By
contrast, pH may have no influence with some non-ionizable organic ligands [284]. For
inorganic oxides, pH is also of prime importance. In particular, on amphoteric oxides
such as TiO2 or Al2O3
, cations are adsorbed at elevated pH due to the deprotonation of
functional groups, whereas anion retention requires acidic conditions for the protonation
of functional groups.
42
Adsorption isotherm
The isotherm parameters of Langmuir and Frendlich for the sorption of metal ions
have been studied. The results showed that the regression coefficients R2
obtained from
Langmuir model were very close to 1, suggesting the Langmuir model could well
interpret the studied adsorption procedure. From the comparison of correlation
coefficients, it can be concluded that the data were fitted better by Langmuir equation
than by Freundlich equation. Langmuir equation is applicable to homogeneous
adsorption, where the adsorption of each adsorbate molecule onto the surface had equal
adsorption activation energy. The fact shows that the adsorption of the hybrid sorbent is
attributed to monolayer adsorption [285].
Kinetics of sorption
In order to determine the uptake rate of metal ions on the synthesized resin and get
access to the equilibrium time, studies on sorption kinetics were carried out. The
sorbents characterized by good kinetic properties determined by the macroporous
structure of the support, large surface area and total accessibility of the functional groups
(without steric hindrance). The kinetic studies also showed that the temperature affected
the rate constants significantly, that is, saturation was reached at a faster rate at higher
temperature. This temperature effect may be a manifestation of the fact that the resin
swells more completely at higher temperature, which allows metal ions to diffuse more
easily into the interior of the resin, and that the sorption was an endothermic process and
hence high temperature facilitates higher sorption. Moreover, both pseudo-first-order
equation and pseudo second- order equation were used to express the sorption process of
the chelating resin. The results showed that regression coefficients values (R2
) of the
pseudo-second-order model (>0.99) were better than those of the pseudo-first-order
model for the sorbent, suggesting the pseudo-second-order model was more suitable to
describe the sorption kinetics of chelating resin [285,286].
Sample flow-rate
The sample flow-rate has been optimized to ensure quantitative retention along
with minimization of the time required for sample processing. This parameter may have a
direct effect on the breakthrough volume, and elevated flow-rates may reduce the
breakthrough volume [284]. As a rule, cartridges and columns require lower maximum
flow-rates than disks ranging typically from 0.5 to 5 mL min-1. This value may be
43
increased by a factor of 10 using disks. High flow rate has been found for the sorption of
metal ions, such high flow rates support the superiority of present chelating resins.
Elution studies
Nature of the eluent
The nature of the eluent is of prime importance and should optimally meet three
criteria: efficiency, selectivity and compatibility, as discussed below. In addition, it may
be desirable to recover the analytes in a small volume of eluent to ensure a significant
enrichment factor. The eluent may be an organic solvent (when reversed-phase sorbents
are used), an acid (usually with ion-exchangers), or a complexing agent. Firstly, the
eluting agent has been carefully chosen to ensure efficient recovery of the retained target
species and quantitative recovery as far as possible [287]. A further characteristic of the
eluent arises with the possibility of introducing selectivity. Using an eluent with a low or
moderate eluting power, the less retained analytes can be recovered without eluting the
strongly retained compounds. Thus, if the elements of interest are those that remain on the
sorbent another elution step with a more eluent will ensure their quantitative recovery. In
that way interfering analytes were removed during the first eluting step (also called
washing step). On the opposite, if the compounds of interest are the less retained on the
sorbent their elution with a low or moderate eluting agent ensures their selective recovery,
as the interferent compounds will remain on the sorbent due to stronger interactions with
the solid support. In some cases, this selectivity may favour speciation. For example, 1
mol L-1 HCOOH removed only Se4+ from an anion-exchange resin, leaving Se6+ retained
on the sorbent, which was further eluted using 2 mol L-1 HCl [104]. Finally, the eluent
should be compatible with the analysis technique. In particular, when using both flame
and electrothermal AAS, HNO3 has been preferred to other acids (namely H2SO4
, HCl),
as nitrate ion is a more acceptable matrix [288].
Effect of pH of eluent
As retention of trace elements on solid sorbents is usually pH-dependent, careful
choice of the eluent pH may enhance selectivity in the SPE procedure. As an example,
once retained on Eriochrome black-T (ERT)-functionalized- silica gel, Mg2+ could be
eluted first at a pH approximately 4, while increasing the pH to 5–6 was required for
eluting Zn2+
[237].
44
Elution mode
Most of the time, for practical reasons, sample loading and elution steps are
performed in a similar manner. However, to avoid irreversible adsorption and ensure
quantitative recoveries, elution in the back flush mode is recommended in some cases.
This means that the eluent is pumped through the sorbent in the opposite direction to that
of the sample during the preconcentration step. This is especially crucial when carbon-
based sorbents have to be used due to possible irreversible sorption of the analytes.
Flow-rate of eluent
The flow-rate of the elution was found to be high enough to avoid excessive
duration, and low enough to ensure quantitative recovery of the target species. Typical
flow-rates are in the range of 0.5 to 5 ml min-1 for cartridges/ columns and of 1 to 20 ml
min-1
for disks [289]. As a rule, the higher the flow-rate, the larger the eluent volume
required for complete elution [290,291].
Volume of Eluent
The elution volume may be determined either experimentally or estimated
theoretically [292]. The elution volume can usually be reduced by increasing the
concentration of the eluent (e.g. acid). However, in this case, problems with subsequent
analysis may be encountered (e.g. FAAS). Alternatively, the use of micro-sized disks may
allow reduced solvent volume [293]. The elution step should enable sufficient time and
elution volume to permit the metallic species to diffuse out of the solid sorbent pores. As
a rule, 2 elution cycles are usually recommended as compared to a single step (e.g. two 5
ml elution should be preferred to a single 10 ml elution). Soaking time is also critical and
2 to 5 minute soak is most of the time allowed before each elution.
Breakthrough profile
Sample volume to be percolated
An important parameter to control in SPE is the breakthrough volume, which is
the maximum sample volume that has been percolated through a given mass of sorbent
after which analytes start to elute from the sorbent resulting in non-quantitative recoveries
(Figure 1.5). The breakthrough volume is strongly correlated to the chromatographic
retention of the analyte on the same sorbent and depends on the nature of both the sorbent
and the trace element, as well as on the mass of sorbent considered and the analyte
concentration in the sample [294]. In addition, it depends on the sorbent containers, as
45
disks usually offer higher breakthrough volumes than cartridges. This volume may be
determined experimentally or estimated using several methods [292]. For that purpose the
nature of the sample has to be taken into account, as the possible presence of ligands may
dramatically reduce the breakthrough volume [295].
Figure 1.5 Typical representation of the breakthrough curve (i.e. concentration of
the analyte at the outlet of the SPE system vs. sample volume percolated through the system).
VB is the breakthrough volume, VR the chromatographic elution volume, VC the sample
volume corresponding to the retention of the maximum amount of analyte and C0
Preconcentration is a process in which the ratio of the quantity of a desired trace
element to that of original matrix is increased but it does not necessarily mean increase in
the concentration of the analyte. In order to demonstrate the preconcentration factor,
preconcentration limit column procedure has been applied. The closeness of the dynamic
capacity to the total sorption capacity reflects the applicability of the column technique
for preconcentration.
is the
initial analyte concentration in the sample.
Interference of sample matrix
The presence of ligands in the sample matrix may affect trace element retention
when stable complexes are formed in the sample with these ligands, as trace elements are
less available for further retention. Thus, if metals are present in the sample as strong
complexes, they may not dissociate resulting in no retention of the free metal on the
sorbent. As an example, reduction in the retention of Cu2+ on Amberlite CG50 occurs in
the presence of ligands such as glycine [296]. In the case of real samples, the presence of
natural organic matter is of great concern as it may form complex with trace elements as
observed for Cu2+ [296,297]. The most important class of complexing agents that occurs
46
naturally are the humic substances [298]. Their binding of metals through chelation is one
of the most important environmental qualities. Yet, in some cases the presence of ligands
may be a valuable tool for adding selectivity to the SPE step. This requires that the added
ligands be correctly chosen to complex only the elements that are not of interest so that
they are not retained on the sorbent [235]. The presence of ions other than the target ones
in the sample may also cause problems during the SPE step. In particular, due to their
usually high levels (e.g. Ca2+
), they may hinder the preconcentration step by overloading
the sorbent or cause interferences during spectrophotometric analysis. Therefore, their
influence has been studied before validating a SPE method. Sometimes the addition of a
proper masking agent (such as EDTA, thiourea or ethanolamine for example) may
prevent the formation of interferences due to ions present in the sample [287]. Finally, the
ionic strength of the sample is another parameter to control for an efficient SPE, as it may
influence the retention of trace elements, and thus the value of the breakthrough volume
for a given sorbent [299,300]. The chelating resin was found to tolerate high contents of
various naturally occurring alkali and alkaline earth metals, anions and complexing agents
in the determination of these metal ions.
Analytical method validation
Great care has been taken to ensure that accurate results are obtained in the
analysis. Every measurement has some imprecision associated with it, which results in
random distribution of results. The experiment can be designed to narrow the range of
this (confidence limit is set around the mean at some degree of probability), but it cannot
be eliminated. Precision of the method is expressed either standard deviation or relative
standard deviation. Systematic errors of instrumental and personal type are minimized by
periodic calibration of the instruments and volumetric glassware, and self-discipline.
However, systematic method errors are difficult to detect and introduces bias in the
method. Bias of the analytical method has been estimated by the following procedures:
Analyzing Standard Reference Materials (SRM) of biological, environmental and alloy
type having complex sample matrix obtained from National Institute of Environmental
studies (NIST), Iron and Steel institute of Japan (JSS) and National bureau of Standards
Using a Second Independent and Reliable Analytical Method in parallel with the method
being evaluated when standard samples are not available. The independent method should
(NBS). In order to demonstrate whether the difference of the observed mean of the
replicate analysis of SRM and its certified value is due to merely the random error of the
measurement or bias in the method, statistical t test is applied.
47
differ as much as possible from the one under study. This minimizes the possibility that
some common factor in the sample has the same effect on both methods. Then a statistical
t and f test must be used to determine whether any difference is a result of random errors
in the two methods or due to bias in the method under study.
Performing Recovery Experiment which requires the spiking of the sample with known
amounts of the analyte. The amount determined (recovered) by the method is expressed in
terms of percentage recovery which shows accuracy of the developed method. By varying
the sample size, the presence of a constant error can be detected.
Applications
In order to explore the potential applicability of the chelating resin the developed
methods were successfully applied for the determination of metal ions in natural and
sewage water, mint leaves, mango pulp, fish, urine, multivitamins tablets, infant milk
substitute and hydrogenated oil (ghee).
1.7 Merits of the present work
The main advantage of the present procedure is the simple and fast preparation of
the chelating resin and no requirement of organic solvents in the metal elution step.
The excellent ability for the exclusion of alkali and alkaline earth elements makes it
desirable for use in the separation and preconcentration of trace elements because
their presence often interfere in the subsequent FAAS determination.
The use of a column preconcentration technique allows for the assessment of low
trace metal concentrations, even by less sensitive determination methods such as
FAAS.
Preconcentration by this material from river water samples do not require any prior
digestion of the samples. It can be successfully applied for the analysis of both
environmental and biological samples as indicated by the high precision (low
relative standard deviation).
The designing and characterization of these four Amberlite XAD-4/16 based
chelating resins and the investigation of their metal sorption behavior with their
subsequent applications for analysis of various real samples containing varying complex
matrices define the scope of the present thesis.
48
Table 1.2 Chelating agents used for modification of Amberlite XAD-4 resin
Reagent Mode of
preparation Techniques coupled Metals Ref.
N-(dithiocarboxy)- sarcosine Chelate
complex Spectrophotometry Co 301 2+
8-Hydroxyquinoline (Oxine) Impregnation Spectrophotometry UO2 302 2+
o-Aminobenzoic acid Chemically
modified FAAS Pb2+,Cd2+,Ni2+, Co2+,Zn 303 2+
2,3-Dihydroxy-naphthalene Chemically
modified ICP-AES Cu2+, Ni2+, Co2+, Cd 304 2+
Ammoniumpyrrolidine-
dithiocarbamate Impregnation ICP-AES
Cd2+,Cu2+,Mn2+,Ni2+,Pb2+,
Zn305 2+
2-Acetylmercapto-
phenyldiazoaminoazobenzene
Chelate
complex FAAS Cd2+,Co2+,Cu2+,Ni2+, Zn 306 2+
2,6-dihydroxyphenyl-
diazoaminoazobenzene
Chelate
complex FAAS Cd2+, Co2+, Cu2+, Zn 307 2+
Diethyldithiocarbamates Chelate
complex FAAS
Cu2+, Fe2+, Pb2+, Ni2+,
Cd2+, Bi308 2+
1-Hydrazinophthalazine Chelate
complex AAS
Cd2+, Co2+,Cu2+, Ni2+,
Pb2+, Fe2+, Cr3+Zn309 2+
1-(2-pyridylazo)-2-naphthol Impregnation FI-FAAS Cu 234 2+
1-(2-pyridylazo)-2-naphthol Chelate
complex AAS
Cd2+, Cu2+, Ni2+, Pb2+,
Cr3+, Mn310 2+
Salen Chemically
modified AAS Cu2+, Pb2+, Bi 311 2+
Succinic acid Chemically
modified Spectrophotometry U 312 6+
Schiff bases Chemically
modified FI–FAAS
Cd2+, Co2+, Cu2+, Ni2+,
Pb313 2+
O,O-Diethyldi-thiophosphate Chelate
complex FI-FAAS Cd 314 2+
m-Phenylendi-amine Chemically
modified ICP-AES
Rh
3+ 315
2,3-Diamino-naphtalene Chemically
modified Se 316 6+
Chemically
modified 2-Aminothiophenol FAAS Cd2+, Ni 317 2+
49
Table 1.3 Chelating agents used for modification of Amberlite XAD-16 resin
Reagent Mode of
preparation
Techniques coupled Metals Ref.
1,6-bis(2-carboxy
aldehydephenoxy)butane
Chemically
modified FAAS Cu2+, Cd 318 2+
2,6-dichlorophenyl-3,3-
bis(indolyl)methane Impregnation FAAS Cu2+,Zn2+, Mn 319 2+
Gallic acid Chemically
modified FAAS Cr3+,Mn2+,Fe3+,Co2+,
Ni2+, Cu320
2+ 4-{[(2-hydroxyphenyl)imino]
methyl}-1,2-benzenediol Chemically
modified FAAS
Zn2+, Mn2+, Ni2+, Pb2+,
Cd2+, Cu2+, Fe3+, Co164 2+
Acetylacetone Chemically
modified AAS Cr 3+, Cr 321 6+
3,4-dihydroxy benzoyl methyl
phosphonic acid
Chemically
modified FAAS U6+ ,Th4+ 322
Phthalic acid Chemically
modified AAS Pb 323 2+
N,N-dibutyl-N_-
benzoylthiourea Impregnation Spectrophotometry U6+ 160
(bis-2,3,4-trihydroxybenzyl)
ethylene diamine
Chemically
modified Spectrophotometry U6+, Th4+, Pb2+, Cd 162 2+
Thiocyanate Impregnation FAAS Ag 324 2+
1,5-diphenylcarbazone Impregnation FAAS Cr6+ ,Cr 325 3+
Nitrosonaphthol Chemically
modified AAS Ni2+, Cu 165 2+
50
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