EXPERIMENT 7: MEASUREMENT OF TRACE METALS IN WATER, TOBACCO AND

CIGARETTE

ASH BY INDUCTIVELY-COUPLED PLASMA ATOMIC EMISSION

SPECTROSCOPY (ICP-AES)

OBJECTIVES

From the experiment, we are able:

1. to analyze trace metals present in cigarette tobacco, the cigarette filter, and

the ash obtained when the cigarette is burned.

2. to learn the uses of inductively-coupled plasma atomic emission spectroscopy

(ICP-AES).

INTRODUCTION

Inductively-coupled plasma atomic emission spectroscopy (ICP-AES) and

Inductively coupled plasma mass spectrometry (ICP-MS) are techniques well suited

for the determination of trace elements in water samples, as the low levels of most

elements in these samples eliminates the use of other, less sensitive, analytical

techniques. It is a type of emission spectroscopy that uses the inductively coupled

plasma to produce excited atoms and ions that emit electromagnetic radiation at

wavelengths characteristic of a particular element. The intensity of this emission is

indicative of the concentration of the element within the sample.

The ICP-AES is composed of two parts, the ICP and the optical spectrometer.

Components of ICP typically include sample introduction system (nebulizer), ICP

torch, high frequency generator, transfer optics and spectrometer, also computer

interface. ICP-AES is often used for analysis of trace elements in soil, and for that

reason it is often used in forensics to ascertain the origin of soil samples found at

crime scenes or on victims by taking one sample from a control and determining the

metal composition and taking the sample obtained from evidence and determine

that metal composition allows a comparison to be made.

Advantages of using the ICP is including its ability to identify and quantify all

elements; since many wavelengths of varied sensitivity are available from ultratrace

levels to major components; detection limits are generally low for most elements

with a typical range of 1 - 100 g / L. Probably the largest advantage of employing

ICP when performing quantitative analysis is the fact that multi-elemental analysis

can be accomplished, and quite rapidly. A complete multi-elemental analysis can be

undertaken in a period as short as 30 seconds, consuming only 0.5 ml of sample

solution. Although in theory, all elements except Argon can be determined using

and ICP, certain unstable elements require special facilities for handling the

radioactive fume of the plasma. Also, ICP has difficulty handling halogens--special

optics for the transmission of the very short wavelengths become necessary.

Determination of any one element, the ICP is suitable for all type of concentrations.

The most important advantages of ICP-MS include multi-element capability,

high sensitivity, and the possibility to obtain isotopic information on the elements

determined. Disadvantages inherent to the ICP-MS system include the isobaric

interferences produced by polyatomic species arising from the plasma gas and the

atmosphere. The isotopes of argon, oxygen, nitrogen, and hydrogen can combine

with themselves or with other elements to produce isobaric interferences.

CHEMICAL

Camel cigarette

15.9M HNO3

19M NaOH

40-50mL of ice

Deionized water

APPARATUS

ICP-AES

Syringe

Mortar and pestle

Stir rod

Beaker

pH paper

Fritted glass and acrodic

METHODOLOGY

Tobacco preparation:

The camel cigarette was weighed with its outer wrap. To continue with the

preparation of tobacco, the outer wrap was removed and tobacco was grind with

mortar and pestle. After enough grinding, tobacco was dissolved in 15 mL of

concentrated 15.9M HNO3. While dissolving, tobacco mixture was mash-up with

stirring rod until the tobacco was properly dissolved. Tobacco mixture was then

placed in 40-50mL of ice to give a low temperature condition for reaction with

dropwise addition of 19M NaOH. Addition of NaOH was continued until the pH of

solution was reaching pH 2. Solution was then diluted to 20 times by diluting 5mL of

mixture to 100mL. Before taking the sample for analysis using the ICP-AES, further

filtration was done on the sample solution.

These steps were continued on preparation of solution with the pH of 11.

Ash preparation:

To prepare ash, cigarette was lighted and the ash was collected in a beaker. The

weight of ash was taken and dissolved in 5 mL of HNO3. NaOH was added to the

mixture until the pH of the mixture was reaching 2-3. This step was done by

monitoring the change in pH by using pH paper. The sample was then prepared for

ICP-AES analysis by filtering with fritted glass and acrodic.

Blank solution:

Blank solution for the tobacco preparation and ash preparation experiments was

prepared by adding the same chemicals without dissolving tobacco and ash. The

step for preparing tobacco and ash was repeated to the blank solution without

adding tobacco and ash.

RESULTS

A) Standards

B) Samples

Camel cigarette tobacco at pH 2

Elements Concentration

(ppm) [Jeremy]

Concentration

(ppm) [Fenny]

Average X 20 (20

= dilution factor)

As -0.00039 0.001173 0.01173

Sb -0.00075 0.000621 0.00621

Zn 0.01690 0.008584 0.25484

Co -0.00015 -0.00014 ND

Se 0.000173 0.002687 0.02860

Fe 0.026912 0.044853 0.71765

Cr 0.000234 0.000255 0.00489

Cu 0.003316 0.003594 0.06910

Camel cigarette tobacco at pH 11

Elements Concentration

(ppm) [Ain]

Concentration

(ppm) [Razi]

Average X 20 (20

= dilution factor)

As -0.00076 0.000105 0.00105

Sb 0.000101 -2.71E-05 0.00101

Zn 0.003733 0.002329 0.06062

Co -0.00015 -0.00021 ND

Se 0.001810 0.001108 0.02918

Fe 0.008805 0.010682 0.19487

Cr 0.000445 0.000264 0.00709

Cu 0.000793 0.000692 0.01485

Camel cigarette tobacco ash

Elements Concentration

(ppm) [Khaliq]

Concentration

(ppm) [Faezah]

Average X 20 (20

= dilution factor)

As 0.000174 -0.00097 0.00174

Sb -0.00043 0.000204 0.00101

Zn 0.028860 0.02824 0.00204

Co -7.18E-05 -2.24E-05 ND

Se -5.62E-05 -0.00023 ND

Fe 0.134739 0.136729 2.71468

Cr 0.000402 0.000448 0.00850

Cu 0.010131 0.010469 0.20600

Water samples

Elements Concentration

(ppm)

[Water cooler]

Concentration

(ppm)

[Distilled water]

Concentration

(ppm)

[Tap water]

Concentration

(ppm)

[Drinking

water]

As -3.79E-05 -0.00063 -0.00015 0.000171

Sb 0.000250 -0.00035 7.87E-05 -0.00042

Zn 0.000149 0.000411 0.004675 0.001503

Co 1.82E-05 -7.84E-05 -0.00015 -0.00015

Se 0.001281 -0.00034 0.001344 -0.00015

Fe -0.00052 -0.00066 -0.00044 -0.00050

Cr 0.000154 0.000129 6.95E-05 9.30E-05

Cu 0.000207 4.63E-05 0.000126 -8.5E-05

DISCUSSION

Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) is one of

several techniques available in analytical atomic spectroscopy. Its high specificity,

multi-element capability and good detection limits result in the variety of

applications. All kinds of dissolved samples can be analyzed, varying from solutions

containing high salt concentrations to diluted acids. ICP-AES utilizes plasma as the

atomization and excitation source. A plasma source is used to dissociate the sample

into its constituent atoms or ions, exciting them to a higher energy level. They

return to their ground state by emitting photons of a characteristic wavelength

depending on the element present. This light is recorded by an optical spectrometer

and when calibrated against standards, the technique provides a quantitative

analysis of the original sample. Plasma is an electrically neutral, highly ionized gas

that consists of ions, electrons, and atoms. The energy that maintains analytical

plasma is derived from an electric or magnetic field. Most analytical plasmas

operate with pure argon or helium, which makes combustion impossible. Plasmas

are characterized by their temperature, as well as their electron and ion densities,

where analytical plasmas typically range in temperature from 600 to 8,000 K.

There are six steps involved in the sample detection of ICP-AES; sample

preparation, nebulization, desolvation/volatilization, atomization,

excitation/emission and separation/detection. Figure 1 summarizes the steps

involved in determining the elemental content of an aqueous phase sample by ICP-

AES.

For sample preparation, some sample requires special preparation step,

including treatment with acids, heating or microwave digestion. The next step is

nebulization, where the sample which is in liquid is converted to aerosol. Followed

by desolvation/volatilization, where water is driven off and the remaining solid and

liquid portions are converted to gases. When all the samples are in gaseous state, it

is atomized by atomizer, where gas phase bonds are broken leaving only atoms in

here. At this stage, plasma temperature and inert chemical environment are

important. The atoms present then are gaining energy from collisions, thus excited

from the low energy level to higher energy level, emitting the light of a

characteristic wavelength. Lastly, grating dispersers light is quantitatively

measured by a detector. In ICP-AES, air or nitrogen is not use as the carrier gas in

an ICP-AES, but Argon. Argon is used here because of 0.9% of the earth’s

atmosphere, so it is readily available. N2 emits several molecular bands in the

ultraviolet and visible, so overlaps with analytical lines are possible which might

interfere with the results.

Figure 1: steps involved in the analysis of aqueous samples by ICP-AES

The typical limits of detection obtained with ICP-AES are for over 70

elements. The units are in parts per billion (ng mL -1 or µg L-1). Inert gases and some

prominent nonmetals (C, N, O, and H) are not analyzed by ICP-AES, but most of the

nonmetals for example P, S, and halogens have strong emission lines that are in the

vacuum ultraviolet. As more instruments come equipped with UV/Vis capabilities,

analysis of nonmetals by ICP-AES will expand.

In this experiment, we used ICP-AES to measure trace metals in cigarette

tobacco at pH 2 and pH 11, cigarette ash and four types of water sample which are

water cooler, distilled water, tap water and drinking water. For the standard

solution, the calibration curve for each trace metal is linear. For cigarette tobacco at

pH 2, the highest element that is contained in it is iron (Fe) which the value is

0.71765. For cigarette tobacco at pH 11, iron (Fe) also the highest element

determined, which the value is 0.19487. From the data, we could see that most of

the elements in acidic pH are in higher concentration than the alkaline. It could be

the reason of in acidic condition; these trace metal elements are easily dissolved.

Thus the concentrations of these metal ions are higher. Most of heavy metals are

likely preferred to dissociate in acidic solution, thus which is the reason why in pH 2,

the concentration for each trace elements are higher. Same with both cigarette

tobacco at pH 2 and 11, element that contains the most in cigarette ash is iron (Fe)

and its value is 2.71468. Among three of these samples, the highest amount of Fe

found is cigarette ash. The amount of element in the cigarette depends on cigarette

brand because of different brand contains different amount of element in it. So, for

Camel brand, the results show that iron (Fe) has the highest amount in it.

Furthermore, in cigarette, the amount of heavy metals presence are vary, as we

know that cigarette is one of dangerous kind of thing to human as it contains a lot

of carcinogenic and toxic substances.

For water sample, there is not much to compare between the elements

contained in each sample. For water cooler, an element that contains the most is

Selenium (Se) which the value is 0.001281 and for distilled water, tap water and

drinking water, they have the same element that contains the most in them which is

Zinc (Zn). Their values are 0.000411, 0.004675 and 0.001503. In water sample, the

trace elements could be presence from the waste from industrial factory, or from

water pollutants. But the concentration is not much higher compared to the

tobacco, and especially the cigarette ash as the tobacco which being burned in the

presence of oxygen will have some chemical reaction that will forming other

substances, making the concentration become higher.

CONCLUSION

The most abundance trace metal element in cigarette ash and tobacco sample is Fe

ion, with 0.71765 for pH 2 tobacco, 0.19487 for the pH 11 tobacco and 2.71468 for

cigarette ash. While for water samples, each of the race metal is present in a small

concentration.

QUESTIONS

1. Why can you measure many elements simultaneously with ICP-AES but

typically only one at time with AA?

ICP-AES is able to measure many elements simultaneously since all of the elements

are emitting at once in the plasma compared to AA. With AA, we have to use a lamp

which was specific for the particular element of interest. In addition, the sensitivity

of ICP-AES is generally greater than AA.

2. ICP-AES is generally more sensitive than AA, i.e. has lower detection limits for

most elements. Explain why this is might be expected, based on how the two

technique work. Think about what signals you are actually measuring to

obtain the concentrations.

Each run of a set of samples must be accompanied by a set of known standards.

Only by drawing a calibration curve plotting the emitted intensities from these

standards versus their known concentrations can the emitted intensities from the

unknown samples be converted to a meaningful concentration value. ICP-AES is

used to determine levels in the parts per million and higher. It is easier to detect a

small signal in the absence of background (emission), than a small change in signal

in the presence of a large background signal (absorption). This is why emission

techniques are more sensitive than absorption techniques.

3. Compare your results for the acid and basic tobacco samples. Is there any

difference? Why might this be the case?

There is a bit different between the acid and basic tobacco samples, where the

concentration of each trace metal is higher in acidic condition compared to the

basic. It is because most of the heavy metal is easily dissolves and dissociates in

acidic solution rather than the basic. The metal then dissociates to become ion and

easily detected by ICP-AES. Most of the heavy metal like Fe, As, Sb, Co, Cu and Cr is

reactive in acidic solution. Thus, it results to the higher concentration in acidic

solution than the basic solution.

4. You will see that some elements such as Arsenic have quite high background

signal with Millipore water alone, while the other elements have much smaller

signals.

a) Comment on why this might be the case

The applicability of ICP emission spectrometry to various elements is different. The

detection limit for the best line for each of the element is indicated by the colour

and degree of shading. The area of shading indicates the number of line for each

element that yields a detection limit within a factor of 3 of the best line. The more

such lines that are available, the greater the chance that a usable line can be found

that is free from interference when the matrix yield a line-rich spectrum. Arsenic, in

comparison to the other element have detection limit of 100-300 (ng/mL) and 3-6

detection lines. Arsenic yields a detection limit within a factor of 3 and it found free

from interference. These causes the Arsenic have quite high background signal

compare to other elements.

b) The detection limits typically cited for As, Se and Sb are about an order of

magnitude worse than for the other elements and much worse for Cr. Why

would this be the case? (hint: look at the “blank” counts and at the

wavelengths used).

For all three As, Se and Sb their number of lines for detection limit are in the 3-6

range. However, for Cr, its detection limit is in the range of 7-10. Other elements,

such as Fe have 11-16 number of lines, thus, it detects better than As, Sb and Se.

As for Cr, although the numbers of lines are 7-10, but, its detection limit is below

than 10.

Eleme

nt

Wavelengt

h

Sb 206.836

Se 196.026

As 188.979

Cr 267.716

REFERENCES

Tyler, G. 1992. AA or ICP - which do you choose? Chemistry in Australia.

Vol. 59 (4). 150-152 pp.

Olesik, J. 1991. Elemental Analysis Using ICP-AES and ICP-MS. Anal. Chem. Vol. 63

(1).

12A-21A pp.

Manning, T. J. and Grow, W. R. 1997. Inductively Coupled Plasma-Atomic Emission

Spectrometry. The Chemical Educator. Vol. 2 (1). 1-19 pp.

Holler, F. J., Skoog, D. A., Crouch, S. R. 2007. Principles of Instrumental Analysis. 6th

Edition.

Thomson Higher Education: Belmont, Canada. 267-269 pp.