Atomic absorption spectroscopy

ATOMIC ABSORPTION SPECTROSCOPY (AAS)

1 | P a g e

Submitted To Mam Khadijah

Submitted By Anum Shahid – CMS#8168 Aniqa Javed - CMS#13464 Faiza Iftikhar – CMS#8420 Saleha Sayyab – CMS#8571 Kalsoom Saleem – CMS#8107

Submission Date 06-11-2014 Semester 6th Section A

Total Pages 9

Remarks

ASSIGNMENT ATOMIC ABSORPTION

SPECTROSCOPY

v, 2014


2 | P a g e

Table of Contents

Sr # Content Page #

1 Introduction 3

2 Basic Principle of AAS 4

3 Atomic Spectra 4

4 Instrumentation of AAS 5

5 Interpretation of AAS 7

6 Applications of AAS 8

7 References 9


3 | P a g e

1. Introduction

Atomic-absorption spectroscopy quantifies the absorption of ground state atoms in the gaseous state.

The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels.

The analyte concentration is determined from the amount of absorption. Concentration

measurements are usually determined from a working curve after calibrating the instrument with

standards of known concentration. Atomic absorption is a very common technique for detecting

metals and metalloids in environmental samples.

When a solution containing metallic species are introduced into a flame, the vapor of metallic species

will be obtained. Some of the metal atoms may be

raised to higher energy level and emit characteristic

radiation. However, large amount of metal atoms

will remain in non-emitting ground state. These

ground state atoms of particular element are

receptive of light radiation of their own specific

resonance wavelength. Thus, when a light of this

wavelength passed through a flame having atom of

metallic species, part of light will be absorbed and

the absorption will be proportional to the density of

atom in the flame.

Elements determined from this technique as shown in Table 1.

Elements that can be detected by AAS

Aluminium (Al) Copper (Cu) Mercury (Hg) Cadmium (Cd) Calcium (Ca)

Antimony (Sb) Gallium (Ga) Molybdenum (Mo) Cobalt (Co) Chromium (Cr)

Arsenic (As) Hafnium (Hf) Niobium (Nb) Nickel (Ni) Lead (Pb)

Beryllium (Be) Indium (In) Ruthenium (Ru) Manganese (Mn) Lithium (Li)

Barium (Ba) Iron (Fe) Tin (Sn) Magnesium (Mg) Vanadium (V)

Tungsten (W) Vanadium (V) Zinc (Zn) Zirconium (Zr)


4 | P a g e

The atoms in the atomizer get promoted to higher orbitals (excited state) for a short period of time

(nanoseconds) by absorbing a defined quantity of energy (radiation of a given wavelength). This amount

of energy, i.e., wavelength, is specific to a particular electron transition in a particular element. In general,

each wavelength corresponds to only one element, and the width of an absorption line is only of the order

of a few Pico meters (pm), which gives the technique its elemental selectivity. The radiation flux without

a sample and with a sample in the atomizer is measured using a detector, and the ratio between the two

values (the absorbance) is converted to analyte concentration or mass using the Beer-Lambert Law.

2. Principle

The Beer–Lambert law:

Atomic absorption spectroscopy relies on the Beer-Lambert law to determine the concentration of a

particular analyte in a sample. The absorption spectrum and molar absorbance of the desired sample

element are known, a known amount of energy is passed through the atomized sample and by then

measuring the quantity of light, it is possible to determine the concentration of the element being

measured. There is a linear relationship between absorbance and concentration of an absorbing

species.

A= Absorbance l= path lenth of cell (cm) c=molar concentration

= wavelength-dependent molar absorptivity coefficient

Applying Lambert-Beer’s law in atomic absorption spectroscopy is difficult due to variations in the

atomization from the sample matrix and non-uniformity of concentration and path length of analyte

atoms. Concentration measurements are usually determined from a calibration curve generated with

standards of known concentration.

3. Atomic spectra

Atomic spectra feature sharp bands. For example hydrogen spectrum:

n = 1

n = 2

n = 3

energy

DE


5 | P a g e

4. Instrumentation of AAS

Atomic absorption instruments consist of a

a. Radiation Source

b. Monochromator

c. Flame or electrothermal atomizer in which sample is introduced

d. Atomizer

e. Detector

a. Radiation Source

Although radiation in the UV-Vis region is required, we cannot use broadband sources. This is because

even the best monochromators cannot provide a bandwidth that is narrower than the atomic

absorption line. If the bandwidth of the incident radiation is wider than the line width, measurement

will fail as absorption will be only a tiny fraction of a large signal which is difficult to measure and will

result in very low sensitivities (figure a). Therefore, line sources with bandwidths narrower than that

of the absorption lines must be used (figure b). This can be achieved by using a lamp producing the

emission line of the element of interest where analyte atoms can absorb that line. Conditions are

established to get a narrower emission line than the absorption line. This can in fact be achieved by

getting an emission line of interest at the following conditions:

1. Low temperatures: to decrease Doppler broadening

(which is easily achievable since the temperature of the

source is always much less than the temperature in

flames).

2. Lower pressures: this will decrease pressure

broadening and will thus produce a very narrow

emission line. Atomic Line Width Monochromator

Bandwidth (different Scales) this may suggest the need

for a separate lamp for each element, which is

troublesome and inconvenient. However, recent

developments lead to introduction of multi -element

lamps. In this case, the lines from all elements should

not interfere and must be easily resolved by the

monochromator so that, at a specific time, a single line of one element is leaving the exit slit.

Hollow Cathode Lamp (HCL)

This is the most common source in atomic absorption

spectroscopy. It is formed from a tungsten anode and

a cylindrical cathode the interior surface of which is

coated by the metal of interest. The two electrodes

are usually sealed in a glass tube with a quartz window

and filled with argon at low pressure (1-5 torr).

Ionization of the argon is forced by application of


6 | P a g e

about 300 V DC where positively charged Ar+ heads rapidly towards the negatively charged cathode

causing sputtering. A portion of sputtered atoms is excited and thus emits photons as atoms relax to

ground state. The cylindrical shape of the cathode serves to concentrate the beam in a limited region

and enhances redisposition of sputtered atoms at the

hollow surface. High potentials usually result in high

currents, which, in turn, produce more intense radiation.

However, Doppler broadening increases as a result. In

addition, the higher currents will produce high

proportion of unexcited atoms that will absorb some of

the emission beam, which are referred to as self-

absorption (a lower intensity at the center of the line is

observed in this case).

b. Monochromators

This is a very important part in an AA spectrometer. It is used to separate out all of the thousands of

lines. Without a good monochromator, detection limits are severely compromised. A monochromator

is used to select the specific wavelength of light, which is absorbed by the sample, and to exclude

other wavelengths. The selection of the specific light allows the determination of the selected element

in the presence of others.

c. Atomizer

Atomization is separation of particles into individual molecules and breaking molecules into

atoms.This is done by exposing the analyte to high temperatures in a flame or graphite furnace. The

role of the atom cell is to primarily dissolvate a liquid sample and then the solid particles are vaporized

into their free gaseous ground state form. In this form, atoms will be available to absorb radiation

emitted from the light source and thus generate a measurable signal proportional to concentration.

There are two types of atomization: Flame and Graphite furnace atomization.

d. Flame Or Electrothermal Atomizer In Which Sample Is Introduced

There can be significant amounts of emission produced in flames due to presence of flame

constituents (molecular combustible products) and

sometimes impurities in the burner head. This emitted

radiation must be removed for successful sensitive

determinations by AAS, otherwise a negative error will

always be observed. The detector will see the overall signal,

which is the power of the transmitted beam (P) in addition

to the power of the emitted radiation from flame (Pe).

Therefore if we are measuring absorbance, this will result in

a negative error as the detector will measure what it appears

as a high transmittance signal (actually it is P + Pe). In case

of emission measurements, there will always be a positive


7 | P a g e

error since emission from flame is an additive value to the actual sample emission. It is therefore

obvious that we should get rid of this interference from emission in flames.

e. Detector

The light selected by the monochromator is directed onto a detector that is typically a photomultiplier

tube, whose function is to convert the light signal into an electrical signal proportional to the light

intensity. A signal amplifier fulfills the processing of electrical signal. The signal could be displayed for

readout, or further fed into a data station for printout by the requested format.

5. Interpretation of AAS

Atomic theory tells us that the electrons in all atoms are in well -defined orbitals. For example, in

uranium, the electron shells with principal quantum number 1-6 are all filled and the shell with

principal quantum number 7 is partially filled. Numerous orbital are available in each shell that are s,

p, d orbitals, etc. in the filled shell, each orbital accommodates an electron. In the unexcited atoms,

these electrons reside in the orbital with the lowest energy level. However, the upper empty orbital

is available to accommodate an electron. During excitation the electrons with the l owest electron

moves from normal low-energy level to an orbital with a higher energy. This orbital may be in the

same shell or in a higher shell, inasmuch as each orbital is available to accommodate an electron

unless excluded by quantum theory-forbidden transitions.

Example:

In atomic sodium, electron fill the shells with quantum numbers 1 and 2, and one electron is in the

shell with the quantum number 3. When the sodium is in the ground state, this will be in orbital with

the lowest energy, i.e., 3s. if we excite sodium, the 3s electron can move to n orbital with higher

energy. The energy level next to the 3s level is the 3p energy level, hence it is possible for an electron

to go from a 3s to 3p orbital. It is also possible for the 3s electron to go into orbitals of even higher


8 | P a g e

energy, such as 4p, 4d, 5p, 5d, etc. When the valence electrons of sodium is in the 3s orbital, its lowest

energy state, the sodium is said to be in the ground state. When the electron is in any orbital with

higher energy, the sodium is said to be in excited energy state, or excited sodium. When radiation

energy is absorbed, the atom becomes excited. If we use a prism to disperse the radiation falling on

the atoms, the absorption spectrum appears as a series of narrow lines opposed to wide bands. If the

transition is between the ground state and lowest

excited state, then it is said that the absorption line is

the resonance line. Transitions between the ground

state and the upper excited states are possible but are

not often used. The energy levels of sodium are shown

in figure, for sake of clarity, upper state transitions are

omitted. In sodium, the transition between the 3s

orbital and a 3p orbital can be achieved by absorbing

radiations at 589 or 589.5 nm. Similar absorption of

radiation at 33.03 nm will cause sodium to be excited

from the 3s ground state to the 5p excited state orbital.

Transitions between the 3s orbital and orbitals with

principal quantum number 6 requires more energy.

6. Applications

1. It is used for water analysis for the presence of following content (e.g. Ca, Mg, Fe, Si, Al, Ba )

2. Also used in food analysis and soil analysis

3. In clinical analysis it is used for analyzing metals in biological fluids and tissues such as whole

blood, plasma, urine, saliva, brain tissue, liver, muscle tissue, semen

4. In Pharmaceuticals it has applications in some pharmaceutical manufacturing processes, minute

quantities of a catalyst that remain in the final drug product

5. It is used in petroleum industry, metallic impurities in petrol, lubricating oils have been

determined.

6. It is used in alloys, metallurgy and in inorganic analysis.

7. Used in analysis of many ores and minerals.

8. It is used in biochemical analysis such as used in estimation of sodium, potassium, zinc, lead,

cadmium, mercury, calcium, iron and magnesium.

9. Also used in pharmaceutical analysis, for estimation of zinc in insulin preparations, creams and in

calamine, oils, calcium in calcium salts, lead in calcium carbonate and also as impurity in number

of chemical salts have been done.

10. Sodium, calcium, and potassium in saline and ringer solutions are estimated by this method.

11. Analysis of ash for determining the contents of sodium, potassium, calcium, magnesium and iron

is carried out in boiler deposits.

12. Used in cement industry.

13. Used in agriculture, soil, forestry, fertilizer and oceanography etc.

14. Used in assay of intraperitoneal dialysis, activated charcoal, cisplatin.


9 | P a g e

7. References

a. James W. Robinson, Atomic Spectroscopy, Second Edition, page 100-102

b. M.Arora, Aseem Anand, Instrumental Method Of Chemical Analysis, Himalaya Publishin

House, Fifth Edition, 2005, Page 240-242

c. Aurora Biomed Inc., Aurora Instruments. 2014. http://www.aurorabiomed.com/atomic-

absorption-spectroscopy/

d. Hitachi High-Technologies Corporation. 2011-2014.

http://www.hitachi-hitec.com/global/science/aas/aas_basic_3.html

e. Galbraith Laboratories, Inc. 2011-2014. http://www.galbraith.com/spectroscopy.htm

f. M.Arora, Aseem Anand, Instrumental Method Of Chemical Analysis, Himalaya Publishin

House, Fifth Edition, 2005, Page 240-241

Education

Atomic absorption spectroscopy