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METALS ANALYSIS TRAINING
14.1 ELEMENTAL ANALYSIS 14.2 EXERCISES 14.2.1 Lead and Cadmium in Ceramics 14.2.2 Cadmium and Lead in Food 14.2.3 Total Mercury in Fish 14.2.4 Arsenic and Selenium in Food 14.2.5 References
People are exposed every day to a tremendous number of substances in our environment. These substances include major and trace elements that may or may not be essential for sustaining life. Major elements reported to be essential are C, H, O, N, S, Ca, P, K, Na, Cl, and Mg. Trace elements reported to be essential are Fe, Zn, Cu, Mn, I, F, Sn, Si, V, and As.
Other elements are not known to be essential but are constantly found in living tissues. These include Al, Sb, Cd, Hg, Ge, Rb, Ag, Pb, Au, Bi, Ti, and Zr. The levels of these elements in the body appear to parallel environmental exposure. Of these elements that have no known nutritional value, some have been found to be toxic at concentrations well below those of other nonessential elements.
Lead, cadmium, and mercury are examples of elements that are toxic when
present at relatively low levels. A survey of imported earthenware in 1970 denied over 400 entries offered for import from 19 different countries because high levels of Pb and/or Cd could be leached from them under conditions of normal use.
Food contact surfaces are covered under the Food Additives provision of the
FD&C Act. Under these provisions, no harmful substance is permitted to migrate from the surface onto a food or beverage. In the absence of specific regulations or tolerances, the FDA has established an interim action guideline of 7 ppm for Pb and 0.5 ppm for Cd.
Application of sewage sludge to croplands prompted a survey to establish the naturally occurring levels of Pb, Cd, and other elements in selected raw agricultural crops throughout the country in noncontaminated production areas. These background
data also permit monitoring any contribution of these elements to the product during preparation for consumption.
The problem of mercury in seafood became an important regulatory matter
in 1970. A survey of levels in tuna and swordfish led to the decline and virtual elimination of the swordfish industry. The interim action guideline of 0.5 ppm was established for total Hg in fish.
Arsenic and selenium occur naturally in foods, and are present in groundwaters and in soils. Arsenical sprays are used in agriculture as insecticides and as defoliants. Selenium is present at elevated levels in tuna and swordfish. The presence of Se enables fish to tolerate higher levels of Hg. As and Se are interesting because they are essential elements that become toxic when ingested at high levels.
The experiments in this section are intended to expose the analyst to some of the more frequently used methods and types of instrumentation in elemental analysis. For more detailed information on the determination of additional elements, standards, sample preparation, digestions, instrumentation, or data treatment consult your laboratory copy of the "Elemental Analysis Manual" (EAM).
14.2.1 Lead and Cadmium in Ceramics
A. Introduction
This exercise is designed to develop proficiency in the operation of the atomic absorption (AA) spectrophotometer for the determination of trace quantities heavy metals (13).
B. Assignment
Using the guidelines in the manufacturer's manual for your instrument, evaluate the effects of the following parameters on the measured absorbance for lead and cadmium:
Slit width
Nebulizer adjustment
Fuel/air ratio
Burner height
The fuel/air ratio and burner height are not entirely independent of each other. Investigate several burner heights at each of several fuel/air ratios. Using conditions that give the best ratio of absorbancetobaseline noise, generate standard calibration curves for lead and cadmium.
Obtain a sample of ceramicware and examine it for lead and cadmium using the atomic absorption method ("Elemental Analysis Manual" Method 13).
C. Questions
1. Using a very simple energylevel diagram, show the difference between
absorption and emission for atoms of Pb and Cd.
2. To absorb light at the 228.8 nm wavelength, cadmium must exist as free atoms. What are some other chemical forms in which cadmium might exist in the flame when a typical real sample is analyzed?
3. What is the chief advantage of a doublebeam AA spectrophotometer relative to a singlebeam model?
4. When in doubt as to the presence of a chemical interference, should the analyst use a standard calibration curve or the melhod of standard additions? Why?
5. Why doesn't the standard additions method correct for a background absorption problem?
6. Does a good background correction system compensate for chemical interferences in the flame?
14.2.2 Cadmium and Lead in Food
This experiment acquaints the trainee with dry ashing techniques and the use
of differential pulse anodic stripping voltammetry (DPASV) in a tracelevel multielement determination (38).
B. Assignment
Before starting the exercise, your trainer will discuss the dry ashing technique, including operation of the muffle furnace, and will also discuss voilammetry and demonstrate operation of the voltammeter. A National Institute of Standards and Technology (NIST) standard reference material will be provided. Use the official method of analysis (5).
C. Questions
1. Define the following: direct current (DC) polarography, differential pulse polarography, anodic strippinc voltammetry (ASV), and square wave ASV.
2. Which parameters affect the current in an ASV experiment? How do these parameters affect the current?
3. What is the principal advantage of pulse techniques over DC techniques?
4. What is the principal advantage of square wave ASV over differential pulse ASV?
5. What is the purpose of nitrogen purge of the electrochemical cell?
6. Why is the method of standard additions used for quantification in this experiment as opposed to a calibration curve (external standardization)?
7. How would one compensate for a systematic lead contamination in a sample analysis?
8. When using a mercury working electrode in an acetate buffer/electrolyte, there is a potential region that can be used analytically. However, scanning the potential too far in either the positive or negative direction results in a large increase in the faradaic current. What is being oxidized when the
potential exceeds the positive limit of the analytically useful potential region? What is being reduced when the negative limit is exceeded?
A. Introduction
This experiment acquaints the trainee with acid digestions and with the flame
less atomic absorption method for cold vapor determination of mercury in fish (913).
B. Assignment
Before you start the experiment, your trainer will discuss acid digestion, including precautions that must be taken when working with oxidizing solutions. Your trainer will also discuss the principles involved in a flameless atomic absorption
determination and the operation of the atomic absorption instrumentation. Samples containing known amounts of mercury will be provided. Use the official method of analysis.
D. Questions
1. This determination requires destruction of the organic matrix. Why is wet oxidation chosen over dry oxidation for mercury?
2. What steps are taken to obtain an analytical sample for analysis that is representative of the sample as collected?
3. What is the role of SnCI2 in this determination?
4. Why should your working standard be prepared fresh daily?
5. After generation of a calibration curve, a recovery is run and is found to be low. Can it be assumed that mercury loss was caused by volatilization? How could you check this?
6. Why is a flame not required in this atomic absorption experiment?
14.2.4 Arsenic and Selenium in Food
This optional section is provided for laboratories equipped with hydride generation
apparatus.
A. Introduction
This exercise provides the analyst with experience in performing perchloric acid digestions and introduces a hydride generation technique for atomic absorption
(14-18).
B. Assignment
Your trainer will describe digestion and hydride generation techniques and provide you with a sample. Analyze the sample for arsenic and selenium using the method in LIB 1900 (16).
C. Questions
1. The entrained air/hydrogen flame is much more transparent to 200 nm radiation than is the more commonly used air/acetylene flame. Why is this significant in the analysis of As and Se by hydride generation/atomic absorption spectrometry?
2. A normal AA nebulizer is about 5% efficient, uhich means that only 5% of the generated aerosol reaches the flame. By what factor would sensitivity be enhanced using a continuousflow hydride generation system for arsenic and selenium analysis, assuming complete chemical conversion to the volatile hydride species?
3. Why is the iodide ion added for arsenic analysis but not for selenium?
4. The hydride generation reaction requires acidic conditions. Why is the sodium borohydride reagent prepared in base?
5. Some hydride generation techniques utilize coldtrapping of the metal hydride vapor followed by rapid desorption of the analyte into the flame. Describe two advantages that are realized by this approach.
6. Why is it dangerous to perform a perchloric acid digestion without also usinl~ nitric acid?
7. What must the analyst do if a sample begins to char during perchloric acid digestion?
8. How should perchloric acid spills be handled?
(1) Skoog. D.A., West. D.M. "Principles of Instrumental Analvsis," 4th ed.;
Saunders College Publishing: Philadelphia, 1985; Chapter 9.
(2) Willard, H.H., Merritt. L.L., Dean. J.J., Settle, FA. "Instrumental Methods of Analysis," 6th ed.; Wadsworth Publishing Co.: Belmont, CA, 1981; Chapter 5.
(3) FDA "Elemental Analysis Manual"; Method 13 and references therein.
(4) "Official Methods of Analysis," 15th ed., 1990; AOAC: Arlington, VA; 973.32.
(5) "Official Methods of Analysis," 15th ed., 1990: AOAC: Arlington, VA; 986.15.
(6) FDA "Elemental Analysis Manual"; Methods 4 and 4A.
(7) Gorsuch. T.T. "The Destruction of Organic Matter"; Pergamon Press: New York,
1970: Chapters 5 and 8.
(8) Willard, H.H., Merritt, L.L., Dean, J.A., Settle, FA. "Instrumental Methods of Analysis," 6th ed.; Wadsworth Publishing Co.: Belmont, CA, 1981; Chapter 24.
(9) Kissinger, P.T., Heineman, W.R. "Laboratory Techniques in Electroanalytical Chemistry"; Marcel Decker: New York, 1981; Chapters 5 and 19.
(10) "Official Methods of Analysis," 15th ed., 1990; AOAC: Arlington, VA; 977.15, 971.14.
(11) FDA "Elemental Analysis Manual"; Methods 10, 10A, 10B, 10C, 11, 11A, 11B.
(12) Gorsuch, T.T. "The Destruction of Organic Matter"; Pergamon Press: New York, 1970; Chapters 4 and 8.
(13) Montague, K., Montague, P. "Mercury"; Sierra Club: New York. 1971.
(14) Boss, C.B., Hieftje G.M. "Theoretical Study of the Spatial Distribution of Atoms Surrounding an Individual Solute Particle Vaporizing in an Analytical Flame," Anal. Chem. 1979. 51, 895901.
(15) FDA "Elemental Analysis Manual"; Methods 1 and 2.
(16) Laboratory Information Bulletin (LIB) 1900 and 1764 series.
(17) Smith. G.F. "The Wet Chemical Oxidation of Organic Compositions
Employing Perchloric Acid"; G. Frederick Smith Chemical Co.:
Columbus, OH. 1965 (available on request from G.F. Smith Co.).
(18) Schilt. A.A. "Perchloric Acid and Perchlorates": G. Frederick Smith Chemical Co.: Columbus, OH (available on request from G.F Smith Co.).
