In the Laboratory
W
Atomic Absorption Spectroscopy of Calcium in Foodstuffs in Non-Science-Major Courses Keith S. Kostecka Institute For Science Education and Science Communication, Columbia College–Chicago, Chicago, IL 60605-1996;
[email protected] Inspired by Markow’s “analysis of lead in paint chips” experiment (1), Sherren’s use of real-life, familiar samples in quantitative analysis (2), and student interest in osteoporosis, a decision was made to augment our student AAS experience of analyzing lead in water to include the measurement of calcium in items “eaten” by students. Background Calcium, the fifth most abundant element in the human body (after C, H, N, and O), comprises up to 2.0% of our body’s mass and is largely contained in our bones and tooth enamel (3). It is found in milk and dairy products, sardines, broccoli and other dark green vegetables, and legumes, and is added to vitamins and other dietary supplements (4 ). In addition to being in our bones and teeth, calcium plays a key role in maintaining cell membrane integrity and in cell-to-cell adhesion; it also regulates such cell activities as release of neurotransmitters, chemotaxis, muscle contraction, exocytosis/endocytosis, blood clotting, chemiluminescence, and fertilization (5). Recent evidence also suggests a major role for calcium in battling major illnesses such as high blood pressure, colon cancer, and diabetes (6 ). At least 42 articles dealing with calcium have been published in this Journal. Reported analyses of Ca using AAS have included determination of the phosphate interference in the flame analysis of calcium (7), calcium level in fruit juice (8), metal concentration in pure water by graphite furnace (9), calcium percentage in an analgesic tablet (10), calcium in serum samples through use of fuel-lean flame (11), analysis of calcium in brine (12), and optimization of signal/noise ratio using simplex optimization (13). Experimental Procedure In this experiment, non-science-major students are given a brief introduction to the terms ppm (parts per million) and ppb (parts per billion). They then measure out 1 mL of any food color and prepare a 1-ppm solution of this food color in water. How this solution is prepared (and how much there is of it) and what materials are needed in its preparation are left to the discretion of each group. Little instructor input, other than confirming the final 1-ppm solution, is offered during this cooperative learning exercise. After students complete this introductory activity, a demonstration on how to conduct a serial dilution is presented. Students are then given information on how to properly handle standard solutions and what the linear AAS range of Ca is (0–5 ppm), and are told that the solution to begin their work with contains 1000 mg/L of calcium (Aldrich Chemical Company catalog number 30,590-1; density of 1.010). Each group then prepares a set of standard solutions for the AAS analysis of Ca.1
From the “food” item they brought in, students take representative samples in triplicate, massing out between 1 and 10 grams each time. They decide what form (chunks, small pieces, powder, etc.) their samples will be in when put into labeled 150-mL beakers and subjected to digestion with concentrated nitric acid. At the completion of this digestion, the resulting solution is allowed to cool, transferred to a 100mL volumetric flask, and brought to the 100-mL mark with deionized water.2 Hazards CAUTION: Extreme care must be taken to avoid spilling the concentrated nitric acid [7697-37-2]. Use of acid-resistant gloves, aprons, and safety goggles is absolutely necessary. Any spilled acid must be immediately neutralized with either NaHCO3 [144-55-8] or Na2CO3 [497-19-8]. Acid addition and digestion on a hot-plate must be done in fume hood; toxic nitrogen oxide gas is evolved. AAS Analysis and Mass % of Calcium The AAS instrument used in our calcium analyses was a Perkin-Elmer 3100 using nitrous oxide–acetylene for its flame. Analysis was done at 422.7 nm with a slit width of 0.2 nm. Data have been collected from laboratory experiments in Columbia College courses “Modern Methods in Science: Discovering Molecular Secrets” (14), “From Ozone to Oil Spills: Chemistry, the Environment and You”, and “The Extraordinary Chemistry of Ordinary Things”. The calcium level obtained (in mg/L) for each sample was determined through use of the formula mg concn. of calcium in × 0.1 L L mass % calcium = × 100 mass of digested sample in mg and the mass percentages determined are listed in Table 1.3 Data Analysis and Comparison With these values in hand, students must compare their data to documented levels of calcium in different foodstuffs. Questions to the instructor on how and where to find these levels were answered with the response “find the most up-todate information available and compare it to the calcium content of your sample”. Students readily understood that this meant searching the Internet for information dealing with calcium and food. Student searches led to the most effective and active site Calcium for Life (15). Foods analyzed by Columbia College students and information for these foods are given in Table 2.
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In the Laboratory Table 1. Calcium Mass Percentages in Selected Foods Food Type
Samplea
Bread products
Bagel, plain White bead
Mass Percent Trial 1
Trial 2
Trial 3
0.011 0.038
0.013 0.044
0.013 0.046
Cereal
Fruit-Loops
0.018
0.019
0.015
Cheese
American:.F97 .S98 .F98 .S99
0.68 0.76 0.81 0.73
0.72 0.71 0.70 0.71
0.71 0.77 0.72 0.81
Brie
0.16
0.15
0.21
Camembert
0.33
0.36
0.34
Cheddar
0.66
0.71
0.63
Swiss: S98 Sw iss: F98
1.02 1.14
0.98 1.03
0.99 1.05
Drinks
Milk, whole Orange juiceb
0.11 0.06
0.12 0.07
0.17 0.06
Snacks c
Cheetos Doritos Fritos Lay’s classic potato chips
0.04 0.11 0.11 0.12
0.05 0.12 0.13 0.12
0.05 0.14 0.11 0.10
Seafood
Shrimp
0.04
0.07
0.07
Yogurt
Peach, fortified
0.15
0.18
0.14
a ”F97”, “S98”, etc. indicate the semester in which the data were obtained. b VeryFine 100%, from concentrate. c All data in the snack category were collected in the S98 term.
Table 2. Calcium Content per Serving of Some Foods Calcium Content
Food Type
Sample
Serving Size
mg
%
Cheese
Brie Camembert Cheddar Processed (thin slice) Swiss
50 g 50 g 50 g 42 g 50 g
92 193 350 256 480
0.18 0.39 0.70 0.61 0.96
Milk, whole a Orange juice b
250 mL 1 fl. oz
315 18.7
0.12 0.06
41
0.06
259
0.15
Drinks Seafood
Shrimp: cooked, canned
70 g
Yogurt
Fruit flavor
175 g
aFor
milk, density ranges from 1.028 to 1.035 g/mL (16a). Here, we will take the density to be 1.03 because we used cold, whole milk. bAs noted by Stoker, 1 fl. oz = 29.57 mL (16b) and the density of 100% VeryFine orange juice, as determined by the instructor, is 1.04 g/mL.
Overall, it is good to see that Columbia College student results compare favorably for the common items (Tables 1 and 2). This is especially encouraging because our students are non-science majors and precise analytical technique is not something that I expected from them. Reasons that may have accounted for this are (i) possibly, my background as an environmental analytical chemist and student observation and emulation of me, (ii) more than likely, the fact that students were conducting analyses on items of their choice and tying together wet chemistry with understandable use of instrumental techniques, and (iii) the use of the Internet in a cooperative, active learning exercise (also noted as important by Steehler [17 ]). These factors together make this experiment one that is very highly regarded by students. NOTE: It is critical for students to realize that the Internet does not hold all the answers, but can instead provide only information—information that may be contradictory from site to site and in some cases scientifically incorrect.
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Conclusions and Extensions This activity gives non-science majors at Columbia College–Chicago the freedom to analyze foodstuffs of their own choice in an experimental module that provides them with (i) an introduction to ppm and experience in (ii) preparation of calcium standards for later AAS calibration, (iii) nitric acid sample digestion, (iv) AAS analysis for calcium in foodstuffs, (v) determination of the mass % of calcium, and (vi) use of the Internet as a tool for comparison. Extensions of this activity have occurred, and are still the object of further study, in student projects dealing with the analysis of calcium in samples of hair from 20 people (this analysis also includes the determination of Zn, Cd, Fe, and Pb levels); Centrum and Centrum Silver vitamins, supplements such as Tums, and the analysis of chicken and turkey bones. Acknowledgments I wish to thank the National Science Foundation for its assistance through instrumentation grant USE-9052085 (AAS system). I also wish to thank the students from the spring 1997 course The Extraordinary Chemistry of Ordinary Things for suggesting the idea of analyzing foodstuffs for calcium. W
Supplemental Material
Student laboratory handouts and instructor notes are available as supplemental material in this issue of JCE Online. Notes 1. Because our analysis uses the higher-temperature nitrous oxide/acetylene flame, formation of nonvolatile calcium pyrophosphate in the flame is not a concern. Use of this flame makes the difficult preparation (and use) of the lanthanum reagent not necessary. 2. If a heterogeneous system results upon cooling, the solid material can be filtered, dissolved in another portion of concentrated nitric acid, and added to the extract in the 100-mL volumetric flask. This will ensure that all calcium in the analyzed item is present for AAS. 3. Three separate trials were conducted for each food and during each AAS analysis five replicates were run per sample. No data analysis was performed in this study other than determining the mean ppm level.
Literature Cited 1. Markow, P. G. J. Chem. Educ. 1996, 73, 178–179. 2. Sherren, A. T. J. Chem. Educ. 1991, 68, 598–599. 3. Snyder, C. H. The Extraordinary Chemistry of Ordinary Things, 3rd ed.; Wiley: New York, 1998; p 494. 4. Ibid., p 497. 5. Ochiai, E.-I. J. Chem. Educ. 1991, 68, 10–12. 6. Couzin, J. Calcium’s powerful, mysterious ways. U.S. News & World Report, May 3, 1999, p 64. 7. Jackman, D. C. J. Chem. Educ. 1985, 62, 161–162. 8. Strohl, A. N. J. Chem. Educ. 1985, 62, 343. 9. Pohl, B.; Dickels, K. The Determination of Sodium, Calcium and
Journal of Chemical Education • Vol. 77 No. 10 October 2000 • JChemEd.chem.wisc.edu
In the Laboratory Silicon in Pure Water by Graphite Furnace AA; AA Instruments at Work AA-96; Varian Associates: Palo Alto, CA, April 1990; pp 1–4. 10. Quigley, M. N. J. Chem. Educ. 1994, 71, 800. 11. Sommer, M. J.; Rutman, M. G.; Wask-Rotter, E.; Wagoner, H.; Fritsche, E. T. Determination of Calcium in Serum Samples by AAS Using a Fuel Lean Flame; AA Instruments at Work AA-117; Varian Associates: Palo Alto, CA, March 1995; pp 1–5. 12. Moffet, J. H. Determination of Magnesium, Calcium and Potassium in Brines by Flame AAS Using the SIPS-10 Accessory for Automated Calibration and Online Sample Dilution; AA
13. 14. 15.
16.
17.
Instruments at Work AA-123; Varian Associates: Palo Alto, CA, July 1995; pp 1–5. Stolzberg, R. J. J. Chem. Educ. 1999, 76, 834–838. Kostecka, K. S. J. Coll. Sci. Teach. 1995, 24 (5), 322–326. Calcium for Life; B.C. Dairy Foundation: British Columbia, Canada; http://mindlink.bc.ca/Bcdf/calcium.htm (accessed Jun 2000). (a) Stoker, H. S. Introduction to Chemical Principles, 5th ed.; Prentice Hall: Upper Saddle River, NJ, 1996; p 69. (b) Ibid., p 58. Steehler, J. K. J. Chem. Educ. 1998, 75, 274–275.
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