analytical exp 7 cigarette icp-aes
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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.
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