An Atomic Absorption Experiment in Food Chemistry The nutritional importance of trace amounts of certain metals in foodstuffs is well established. Among the metals commonly cited as essential are manganese and iron.' These two metals are often incorporated in feed formulations to assure their proper intake. In our undergraduate analytical service course, atomic absorption is introduced with a determination of iron or manganese in dry eat food. Students go through the entire process of sampling, sample preparation, final measurement, and data reduction with a programmable calculator. The important practical techniques of wet ashing and standard addition are included in the procedure, providing insight into their use in actual practice. Student interest in this experiment has been consistently high, probably due in part to the use of "real" samples. Experimental Dry pet food is thoroughly ground in a mortar and pestle to aid the ashing procedure. Approximately 500 mg samples are weighed to the nearest milligram and transferred to 50 ml Pyrex beakers. Ten milliliters of 1:l nitriclperchloric acid are added, and samples are placed on a hot plate and digested in a fume hood until colorless. After digestion, the liquid is quantitatively transferred t o a 25 ml volumetric flask and diluted to the mark with distilled, deionized water. Two-milliliter portions are taken with a volumetric pipet, to which are added 0.1,0.2, and 0.3 ml of 100 ppm solution of the metal determined. The original solution and three spiked portions are then aspirated into the flame of the instrument, and the absorhances measured and recorded. The absorbance values are then corrected for dilution, and results calculated by a ratio method or hy graphical analysis. Students are encouraged to use a programmable calculator to ohtain the best fitting line thru their data points with the use of a least-squares p r ~ g r a m . ~ A Beekman Model 1301 atomic absorption spectrophotometer with a laminar burner was used for absorbance measurements. The hollow cathode lamp was a Westinghouse multielement lamp which contained both iron and manganese in the cathode element. This lamp makes determination of both iron and manganese very simple, obviating warm up time for additional lamps. Instrumental parameters were in accordance with manufacturer's recommended values." Results and Discussion Good agreement was ohtained with nominal values provided by the manufacturer,' as can he seen in the table. The high values for both iron and manganese permit the use of simple, single-beam instruments in this experiment. Aceording to the manufacturer, zinc is also present a t high concentration levels (70 ppm) in the pet food and could also be easily determined. Concentrations of Iron and Manganese in Dry Cat Food student v a l v e 4
M a n u f a c t u r e r . ~value&
l u d x sl
(!ah; d
Metal
20 determinations.
b 3 determinations.
Several desirable features are apparent from a year's experience with this experiment. First, the sample is readily available, cheap, and well characterized as to metal content by the manufacturer. Secondly, the student is exposed t o a total analytical prohlem, perhaps for the first time. A number of practical considerations such as obtaining a representative sample, overcoming potential matrix effects with standard addition, and ashing a rather complicated sample are all encountered. Finally, since nutritional chemistry is of current interest, the experiment is more meaningful than the analysis of ''artificial" samples.
' Underwood, E. J., "Trace Elements in Human and Animal Nutrition,"
2nd ed., Academic Press, Inc., New York,
1962, pp. 46-50; 194-200.
Wang Laboratories, Ine., "5201600 Series General Program Library," 1972, pp. 3-5. Beckman Instruments, Ine., "Atomic Absorption Accessory," 1965. Miller. L. G.. Director. Pet Foods-Research Laboratories, Carnation Company, Van Nuys, California, private communication Illinois S t a t e University Normal, 61761
Daniel C. Paschal
Volume 53. Number 3, March 1976 / 165
