Abstract
Atomic Absorption Spectrometry is a widely used method for the
determination of single elements. The theory of Atomic Absorption is that excited
molecules will increase their energy levels even more by absorbing wavelengths of
certain frequencies. These absorptions can be measured if a monochromatic light source
is used for illumination.
We are using a “GTA 96” electrothermal atomizer, to measure the amount of lead
in standard solutions and an unknown sample. The atomizer raises the temperature of the
lead to 2400 degrees Celsius, while the atomic absorption spectrometer measures the
absorption of 283.3 nm radiation, emitted from a lead-filament bulb.
A series of standard solutions was measured on the Spectr AA20, and the
concentration of lead in solution vs. the height of the Absorbency peak was found to be
given by the formula A = 0.0123[Lead, ppm]+0.0518, with a linear fit coefficient of
0.9917. Measured against this line the concentration of lead in the unknown sample was
found to be 17.4 ppm.
The Calibration Curve method of determining the concentration of the unknown
is only a good method if the composition of the samples is the same. That includes, the
same chemical composition, pH, etc. A better way to measure the concentration of lead
in sample that displays matrix effect is by standard addition. For example, by preparing a
series of samples with a fixed volume of unknown solution, adding a volume of solution
of a known concentration, and then diluting to a fixed value (“Continuous Variation of
Standard at Constant Total Volume”) Unfortunately we did not have a high enough
concentration of lead in the unknown sample to dilute it and still have above 0.2
absorbency.
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Theory and Objective
Atomic Absorption Spectrometry has for long been “the most widely used
method for the determination of single elements in analytical samples1.
The process of electrothermal atomic absorption couple a furnace with a UV
spectrometer. The process of sample introduction into the machine is automated, done by
a siphon which draws samples successively from a carousel and introduces the sample
into a graphite tube, opened at both ends and ventilated by an inert gas (Argon). The gas
is necessary to prevent the sample and the graphite tube from burning the tube itself is
attached to high-amperage electrodes and is held in water-cooled metal housing. Once a
sample is introduced into the graphite the heating of the sample begins2.
There are several major steps that sample undergoes during atomization: first is
desolvation, when the solvent is evaporated to produce a “solid molecular aerosol” 3
which is then volatilized to produce gaseous molecules at high temperature, these
molecules either remain as excited molecules or dissociate into excited atoms, or ionize
into excited ions which produce emission spectra.
In an electrothermal atomizer, such as the “GTA 96” graphite tube atomizer that
we are using, the temperature program consists of several steps. First, a few microliter of
sample are evaporated at a relatively low temperature, then ashed at a somewhat higher
temperature. After “ashing” the current is rapidly increased causing the temperature to
rise to (in our case) to 2400 degrees Celsius. This is enough to atomize the sample in
several milliseconds, at this time the absorption of atomized particles is measured in the
region above the heated surface4. The absorption is measured by a spectroscope sensitive
to UV radiation which the element of interest absorbs and the source of radiation is
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typically a lamp made out of the same element as the one being measured. The simple
reason for this is that upon excitation the filament of the lamp will produce radiation of
only the particular wavelength desired, the same wavelength that is absorbed by the
element. Typically an element absorbs radiation with a very narrow frequency range
(much less than one nanometer) and it would be impossible to separate such a narrow
frequency range from a source producing a wide spectrum of radiation5.
Atomic absorption is very good for measuring metal concentration because it is a
sensitive method, and metals are generally present in trace quantities. The concentration
of lead in a natural sample of fish for example could be 1 ppm6. Usually, when the
sample is likely to have some matrix effects the method of measuring the concentrations
of sample is standard addition. One way to perform the method of standard addition is to
prepare a series of samples with a fixed volume of unknown solution, add a volume of
solution of a known concentration, and then dilute to a fixed value. This method is called
the “Continuous Variation of Standard at Constant Total Volume7”
Some recently research done by Ross, et. al. has focused on measuring the amount
of lead in calcium preparation made in different ways, including preparation from bonemeal and dolomite, and from calcium carbonate preparations. Lead content, was assayed
using electrothermal atomic absorption, the average doze of a calcium supplement is one
gram and the maximum amount of safe lead intake per day is considered 6 micrograms.
It was found that 4 out of 7 calcium supplements prepared from oyster shells had lead
content, with the average concentration of 1 ppm, or one microgram per gram. No lead
was detected in the calcium acetate or polymer products. Lead was present even in some
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brand name products from major pharmaceutical companies not of natural oyster shell
derivation8.
Our objective is to produce our own standard calibration curve for lead at 0 to 40
ppm and then use it to find the concentration of lead in an unknown sample.
Unfortunately the concentration of lead in our unknown sample was found to be rather
low, so that if it was diluted by standard addition the value for the absorbency which we
would obtain would be less than the minimum 0.2.
1 - Skoog page 206
2 - Skoog page 206
3 - Skoog page 207
4 - Skoog page 210
5 - Skoog page 214
6 - Huang S. J., Jiang S. J. page 1491
7 - Bader page 703
8 - Ross page 1425-6
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Procedure
A stock solution of 1.0 g of Lead per 1.0 ml of 2.0% nitric acid (1000 ppm)
diluted in metal-free filtered deionized water was used to prepare standard solutions of
10, 20, 30 and 40 ppm of Lead in 2.0% nitric acid. In addition to the standard samples
blank samples containing no Lead were used to find the background absorbance of the
solution sans lead. The “unknown” solutions are also in 2% nitric acid.
Optimization on a model “Spectr AA20” atomic absorption spectrometer with a
“Varian” auto sampler and “GTA 96” graphite tube atomizer was performed. The
monitored wavelength was set to 283.3 nm and the slit width is set to 0.5 nm. The lamp
type used to irradiate the Lead samples is a Pb 5 milliamp hollow cathode (with 0.5 nm
slit width and Argon as the inert gas).
The absorbance of each standard sample was measured, starting from the sample
with the blank samples, followed by the samples with the lowest concentrations of lead,
the unknown was also measured. Due to the fact that the average measured value of the
unknown was 0.266, close to the lower limit for the absorbency range on the calibration
curve (0.2 to 0.8) the method of standard addition was not performed. The method that
could have been performed required the continuos variation of standard. The procedure
for standard addition is outlined in Appendix B.
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Experimental Results
The following are the values for the absorbency of the standard lead samples.
Note that some concentrations were sampled more than once, as we seemed to be getting
a relatively large amount of deviation from the expected linear increase in Absorption
with respect to concentration (see Discussion).
Lead Concentration Absorption at 283.3 nm
(ppm)
0
0.017
0
0.023
0
0.071
0
0.056
10
0.182
20
0.333
30
0.448
30
0.457
40
0.524
40
0.51
40
0.523
The average absorbency of the four blank samples is 0.042, the best-fit line
produced by plotting the absorbency vs. Lead concentration has the slope intercept
formula of y = 0.0123x + 0.0518 and a fit coefficient of 0.9917 (see Appendix A).
The absorption of the unknown samples is tabulated below. The unknown
concentration samples were measured against the calibration curve produced by the
standards. The average lead concentration in the three unknown samples is 17.4 ppm
(micrograms/ml).
Corresponding Concentration of unknown using the formula:
[Concentration]=(Absorption-.0518)/0.0123
Absorption
Concentration (ppm)
unknown sample 1
0.255
16.5
unknown sample 2
0.281
18.6
unknown sample 3
0.261
17.0
Average Unknown Concentration =
17.4 ppm
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Discussion
The value obtained for the average concentration of lead in the unknown sample,
seemed somewhat low, even though it was in the range of 0.2 to 0.8 absorbency and it
concentration (17.4 ppm) was in the middle of our range of samples. Standard addition
was not performed because it was feared that if the sample was diluted so that its
absorbency fell below 0.2 the results would be inaccurate.
We did find a good deal of linearity in our samples, even though there was some
deviation between the absorbencies of samples with the same concentration. For
example the blank samples ranged in absorbency from 0.017 to 0.071 with an average
value of 0.041. The fit coefficient of 0.9917 (two nines) for the line produced by
graphing the absorbency versus concentration is probably satisfactory. Obviously it does
not cross the 0 lead concentration at 0 absorbency, this probably means that other
material in the solution contributes to the apparent absorbency reading. Whatever this
material might be its concentration might vary between samples producing deviation. It
is also possible that the preparation of samples could have involved some contamination
in the form lead being introduced into the samples. One obvious way to improve this
experiment would be to increase the number of samples and measure unknowns by
standard addition. Another would be to install a fresh graphite tube, and rinse the siphon
on the automatic sampler more thoroughly in lead-free water.
Conclusion
Our calibration curve closely matched the calibration curve obtained from the
manual of the Spectr AA20, but there was some deviation. Within our results, the
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calibration curve had a linear fit coefficient of 0.9917, and the formula for the calibration
curve (line) is y=0.0123x+0.0518 where x is the concentration of lead in ppm. Using the
calibration curve to compare the concentration of lad in the unknown we obtained the
value of 17.4 ppm (micrograms per milliliter).
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Appendix A - Calibration Curve
Atomic Absorption of Lead
0.6
y = 0.0123x + 0.0518
R = 0.9917
Absorption (283.3 nm)
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
35
40
Concentration ppm
The graph above shows the ppm concentration of lead in a total of 10 standard samples
over 5 concentrations, that were obtained experimentally in lab.
The graph below shows the Absorption (peak height) vs. Lead concentration
Literature value calibration curve from the Spectr AA manual.
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Appendix B – Calculations and Statistical Analysis
Calculations for linear calibration curve
Sum X
Avg. X
Lead Concentration (ppm)
(x-xm)^2
Absorption at 283.3 nm
(y-ym)^2
0
0
0
0
10
20
30
30
40
40
40
364.466281
364.466281
364.466281
364.466281
82.646281
0.826281
119.006281
119.006281
437.186281
437.186281
437.186281
0.017
0.023
0.071
0.056
0.182
0.333
0.448
0.457
0.524
0.51
0.523
0.07225344
0.06906384
0.04613904
0.05280804
0.01077444
0.00222784
0.02630884
0.02930944
0.05673924
0.05026564
0.05626384
5.1316608
5.0171148
4.1007468
4.3871118
0.9436458
0.0429048
1.7694398
1.8676208
4.9805238
4.6877978
4.9596148
210
19.09090909
3090.909091
Sxx
3.144
0.285818182
0.47215364
Syy
37.888182
Sxy
Sum Y:
Avg. Y:
slope = Sxy/Sxx = 37.888/3090.9 = 0.0123
y intercept = ym - slope*xm = 0.2858-(0.0123)19.091 = .0518
fit = Sxy/(SxxSyy)^1/2 =
0.991787211
Calculations for Standard Addition - Continuous Variation of Standard
Theoretical - This procedure was not performed in this lab due to low unknown concentration
A = Absorption
k = proportionality constant
Vx = fixed unit volume of unknown
Vs = fixed unit volume of standard
Vt = Total volume of solution
Cx = stock concentration of unknown
Cs = stock concentration of standard
N= running integers (0, 1, 2...) - these are multipliers for concentration
Several solutions are prepared with a constant volume of unknown solution (Vx) at a concentration of Cx
and NVs units of standard of concentration Cs, each solution is diluted then to a constant volume Vt. The
absorption of each sample would then be:
A = k [(VxCx/Vt)+(NVsCs/Vt)]
to produce a linear plot let fx=Vx/Vt, fs=Vs/Vt
A = k[(fxCx+NfsCs)]
when plotted as Absorption vs. N this equation will produce a line with a slope(m) of kfsCs, and vertical
intercept (m) (at N=0) at kfxCx the unknown concentration will then equal
Cx=bVsCs/mVx
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(x-xm)(y-ym)
References
Bader, Morris A systematic Approach to Standard Addition Methods in
instrumental Analysis. Journal of Chemical education. Volume 57, Number 10, October
1980. Pages 703-706
Huang S. J., Jiang S. J., Determination of lead in fish samples by slurry sampling
electrothermal atomic absorption spectrometry. Analyst. August 2000;125(8): pages
1491-4
Ross E. A., Szabo N. J., Tebbett I. R., Lead content of calcium supplements.:
JAMA 2000 Sep 20;284(11): pages1425-9, 1432-3
Skoog, D. A., Holler, F. J., and Nieman T. A., Principles of Instrumental Analysis
(fifth edition). Saunders College Publishing, New York, 1998.
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