In the Laboratory
Solving a Mock Arsenic-Poisoning Case Using Atomic Spectroscopy
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Matthew A. Tarr Department of Chemistry, University of New Orleans, New Orleans, LA 70148;
[email protected] Several new laboratory procedures have been adopted into the University of New Orleans Instrumental Analysis laboratory course (CHEM 4030, junior/senior level course). These new procedures were designed to focus on real-world problems in order to increase the students’ problem-solving capabilities and to present procedures that were both interesting and relevant to the students’ experiences. In this report, a laboratory procedure is described in which students are asked to provide evidence in a criminal investigation of an arsenic poisoning case. Inductively coupled plasma (ICP) atomic emission spectroscopy was used for this procedure, but atomic absorption spectroscopy could also be used. No instrumental laboratories for arsenic determination have been published in this Journal. However, primarily qualitative analysis procedures are available dating from 1959 or earlier (1). Numerous reports of atomic spectroscopy experiments have appeared recently (2–8). None of these reports involved the determination of arsenic, although some did focus on the use of atomic spectroscopy for solving real-world problems (2, 8). Overview In performing this procedure, the students learn a number of skills and fundamentals. Included among these are (i) the ability to assess a problem and devise a sound experimental approach based on available information, (ii) the process of developing a new laboratory protocol, (iii) basic issues of sample preparation, including errors due to matrix effects, (iv) fundamentals of inductively coupled plasma atomic emission spectroscopy or atomic absorbance spectroscopy, (v) preparation of acceptable calibration curves, (vi) analysis of data in the context of a problem, and (vii) use of statistical tools to distinguish differences between measured values. The students receive a mock letter from the FBI. The letter asks for analysis of arsenic-laced soda samples in order to determine the source of the arsenic in the soda. It indicates that sodium arsenate (dibasic) was found in a suspect’s garage. Previous analysis of the sodium arsenate revealed the presence of several contaminants: Ca, Cu, Mg, Mn, Na. The elemental composition of the sodium arsenate sample is provided, along with relevant statistical information from the analysis. These data are provided to avoid use of sodium arsenate solid for safety reasons. The task of the students is to analyze the arseniclaced soda samples to determine if the sodium arsenate sample can be linked to the poisonings. Instead of being provided with a step-by-step procedure for the lab, the students were led in a guided discussion to develop a laboratory procedure. In addition, they were instructed on how to use each piece of equipment or apparatus and were briefed on safety issues relevant to the lab. The procedure was performed in two 3–4hour sessions.
Procedure The laboratory procedure involved determination of the arsenic concentration in each soda sample and determination of at least two additional elements. Through statistical analysis, the ratio of each impurity to arsenic in the soda is compared to the corresponding ratio for the solid material. Based on the statistical comparison, the students make a conclusion regarding whether the solid was the source of the arsenic in the soda samples. In general, the procedure first involved semiquantitative determinations of each element in unadulterated soda and the contaminated soda. On the basis of the background signal of the unadulterated soda and the concentration of each element in the contaminated soda, the students then chose at least two elements (in addition to arsenic) to determine quantitatively. For calibration, they chose the concentration of each element and the matrix for standard solutions. After obtaining suitable calibrations, they performed replicate analyses of the soda samples, deciding for themselves the number of replicates. After completing the measurements, the students performed statistical analyses to determine if the soda and the solid sodium arsenate could be linked. A brief overview of the procedure is outlined below. The ratio of each impurity to As was calculated from the measured concentrations (for example, [Cu]/[As]). This ratio was calculated for each soda sample and for the solid. The errors in these ratios were determined using propagation of errors (9). Finally, the ratio for each impurity to arsenic in the soda samples was compared to the ratio for the solid using a paired t-test (9). From this analysis, the students were able to assess the likelihood that the source of the arsenic in the soda was the solid obtained from the suspect’s home. In addition to the statistical analysis, the students were also encouraged to consider the following: 1. Was the concentration of arsenic in the soda high enough to be lethal? 2. Would the added sodium from the sodium arsenate have had a noticeable effect on the taste of the soda? 3. What precision is necessary in the quantitation of each element to yield a statistically useful comparison of the samples?
The procedure was performed by students working in groups of three. Each group submitted several reports based on their lab work: (i) a reply memo to the FBI stating the results and their reliability based on statistical analysis, (ii) an instrument report detailing the design and function of the equipment used, (iii) a data report detailing all calculations and data manipulations and their relevance to the results, and (iv) a situation report providing background information
JChemEd.chem.wisc.edu • Vol. 78 No. 1 January 2001 • Journal of Chemical Education
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In the Laboratory
about the problem and indicating how the experimental approach related to the required information (e.g., did the precision of the experiments allow meaningful conclusions?). Hazards Arsenic is toxic and carcinogenic. Proper safety precautions should be observed including use of gloves, safety glasses, lab coats, hoods, and ventilation of instrument exhaust. Uncontaminated and arsenic-laced soda samples should be stored in clearly labeled laboratory containers. Original soda containers should not be used at any time. Disposal of waste should be in accordance with all environmental regulations. As in any laboratory procedure, eating and drinking should be prohibited in the lab, and students should wash their hands before leaving. Discussion Incorporating a realistic problem involving common topics (soda, arsenic, poisoning) dramatically improved the students’ interest and enthusiasm. Furthermore, the lab encouraged the students to seek information relevant to the problem from sources other than traditional teaching materials (textbooks, lab manuals, instructors). For example, they utilized such resources as the Merck Index, the ingredients lists on soda bottles, material safety data sheets, medical/health documents, etc. Finally, the procedure gave the students an understanding
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of how experimental details can have a significant impact on the results as well as the meaningfulness of those results with respect to a real problem. This experiment requires an inductively coupled plasma atomic emission spectrometer or an atomic absorbance spectrometer. Alternatively, students could be provided with simulated data and the procedure could be performed as a data analysis and interpretation exercise only. W
Supplemental Material
The “FBI letter”, an outline of the procedure and a safety sheet for students, and detailed notes for the instructor are available as supplemental material in this issue of JCE Online. Literature Cited 1. Boyd, C. C.; Easley, W. K. J. Chem. Educ. 1959, 36, 284. 2. Gilles de Pelichy, L. D.; Adam, C.; Smith, E. T. J. Chem. Educ. 1997, 74, 1192. 3. Stolzberg, R. J. Chem. Educ. 1997, 74, 216. 4. Quigley, M. N.; Vernon, F. A. J. Chem. Educ. 1996, 73, 980. 5. Rocha, F. R. P.; Nobrega, J. A. J. Chem. Educ. 1996, 73, 982. 6. Quigley, M. N.; Reid, W. S. J. Chem. Educ. 1995, 72, 440. 7. Kieber, R. J.; Jones, S. B. J. Chem. Educ. 1994, 71, A218. 8. Quigley, M. N. J. Chem. Educ. 1994, 71, 800. 9. Christian, G. D. Analytical Chemistry, 5th ed.; Wiley: New York, 1994; Chapter 2.
Journal of Chemical Education • Vol. 78 No. 1 January 2001 • JChemEd.chem.wisc.edu
