Atomic absorption spectroscopy is an analytical techniqueused for the determination of elements, and at timescompounds, based on the absorption of radiation by unexcitedgaseous free atoms. The determinations of metals in avariety of real-life samples with flame atomic absorption spectroscopy(FAAS) have been reported in this Journal. Thosestudies include the determination of zinc in human hair (1),lead in paint chips and gasoline (2, 3), iron in multivitamintablets (4), iron and manganese in dry pet food (5), calciumin fruit juice and analgesics (6, 7), and calcium in foodstuffs(8). Although the last reference (8) provided results on thedetermination of calcium in foodstuffs, the procedure wasdesigned for a nonscience major course. The quantity of calciumpresent in foodstuffs was determined using the standardcalibration method, which does not compensate fordeterminate errors caused by the matrix effect when analyzingreal-life samples. Also, the experiment used wet-ashingwith concentrated nitric acid, where some samples had to befiltered and the wet-ashing repeated.The experiment described here is designed for sciencemajors who have completed two semesters of general chemistryand are enrolled in the Quantitative Methods of Analysiscourse. This experiment uses the two increment standardaddition method in conjunction with FAAS to determine thequantity of calcium in cereal. It eliminates the need for wetashingwith concentrated nitric acid and minimizes the generationof hazardous waste by dry-ashing the sample in a silicacrucible using a high temperature muffle furnace. This experiment(i) introduces the students to the principles ofatomic absorption spectroscopy, (ii) gives the students handsonexperience in operating and obtaining measurements usinga modern FAAS instrument, (iii) gives experience indry-ashing sample preparation, (iv) gives the opportunity towork with and understand the principle of the two incrementstandard addition approach for the analysis of real-lifesamples, and (v) illustrates the importance of FAAS in theanalysis of samples relevant to their daily lives. This experimentwas successfully used in three different laboratory sectionsof the Quantitative Methods of Analysis course in thefall 2001 term. The data presented in this article were obtainedby 36 students majoring in biochemistry, biology,chemistry, Earth science, or environmental science, the majorityof whom are required to take the course. The experimentis normally completed in six hours of laboratory time.

ProcedureSample PreparationA 5-g sample of the cereal to be analyzed is crushed intoa fine powder using a mortar and pestle. Samples, ∼0.5-g, of the crushed cereal are placed in three weighed glazed silicacrucibles and weighed to the nearest 0.0001 g. The samplesare then placed in a muffle furnace maintained at 600 _C.An empty crucible is also placed in the furnace to serve as a

blank.After two hours, the four crucibles are removed, cooled,and the residue is treated with 6 M HCl. The residue readilydissolves and filtration of the resulting solution is not necessary.The content of each crucible is transferred to 100-mLvolumetric flasks and diluted to volume with distilled water.The two increment standard addition technique is performedby using two 5.00-mL aliquots from each sample. A singleaddition of 20.00 mL of the standard calcium solution isadded to one of the aliquots and both are diluted to volumeusing 50-mL flasks.This treatment generates three sets of unspiked andspiked samples to be used for the FAAS measurements. Ablank solution is also prepared from the empty crucible andmeasured.

Atomic Absorption Measurementand ppm Calculation of Calcium in CerealA Varian SpectrAA-600 instrument with a calcium hollowcathode lamp and nitrous oxide–acetylene flame is used.Because of its higher temperature, the nitrous oxide–acetyleneflame eliminates the need for the addition of a releasingagent, such as lanthanum or strontium ion, to the solutions.A releasing agent is added to eliminate chemical interferencesowing to the formation of refractory compounds when air–acetylene flame is used in the analysis of calcium. Absorbancemeasurements are made using the calcium resonance line atλ 422.7 nm, 0.5-nm slit width, and 5.0-mA lamp current.Additional experimental details are included in the Supplemental

Material.W The calcium level is calculated as,

where Csam and Cstd are the concentrations in g mL_1 of thesample and standard solutions, respectively, and Asam andAsam+std are the corrected absorbance values of the unspikedand spiked solutions, respectively. These values are obtainedby subtracting the absorbance of the blank solution from theirrespective absorbance measurements. Vstd is the number ofmL of the standard solution added and Vsam is the numberof mL of the cereal sample solution. Equations 1 and 2 allowfor the determination of Csam and the ppm Ca in thecereal sample.

HazardsFlame-resistant leather or glass-fiber gloves and jumbo(18 in.) crucible tongs must be used when inserting and removingthe silica crucibles from the muffle furnace. Hydrochloricacid, HCl, is corrosive; preparation of dilute solutionsfrom concentrated HCl must be done in the fume hood.Acid-resistant gloves and laboratory coats are recommendedin addition to mandatory safety goggles. Owing to potentiallfire or explosion hazards when using nitrous oxide as the oxidant,the instrument manufacturer safety practices and recommendationsshould be followed carefully.

Results and DiscussionThe data obtained by the students for the determinationof calcium in two brands of cereal, Kix and Cheerios,from three different laboratory sections in Fall 2001 areshown in Table 1. Each student worked independently andwas responsible for making her or his standard solutions,sample preparation, and measurements using the VarianSpectrAA-600 instrument. Each value represents the averageof three separate determinations. For the Kix results, the RSDof the student triplicate determinations ranged from 0.31 to16.50% with the majority (78%) having values ≤ 5%. Forthe Cheerios results, the RSD for the student triplicate determinationsranged from 0.59 to 11.64% with the majority(67%) having values between 0.59 and 5.25%. Based on theaverages of the students’ results and a value of 1200 mg forthe recommended dietary allowance (RDA) for calcium forthe 19–24 year age group (the pertinent age group for thestudents; ref 9), one 30-g serving of Kix and Cheerios provides17.0% and 13.1% of the RDA, respectively. Typicalabsorbance values for blanks and Kix and Cheerios samplesare shown in Tables 2 and 3. Each sample was measured three

times and the average was used in the calculation in eq 1.Although this experiment is designed for the two incrementstandard addition approach, it may be easily adaptedto the multiple-increment standard addition method. A standardaddition graph for a Cheerios sample, with additionsranging from 0 to 20 mL of the standard calcium solutionto 5.00-mL aliquots of the sample followed by dilution to50.0 mL, is shown in Figure 1. The quantity of calcium inthis sample, Csam, was found to be 4500 ppm. This value,which is comparable to those obtained with the two incrementapproach, was calculated using the following equationin conjunction with eq 2,

where Cstd is the concentration of the standard solutionadded, Vsam is the number of mL of the cereal sample solution,m is the slope, and b is the intercept of the standardaddition graph.

ConclusionThe implementation of this laboratory experiment in theQuantitative Methods of Analysis course was successful. Theanalysis of cereal products was of interest to the students since these are foods they regularly consume. The results for thedetermination of calcium in two cereal brands obtained by36 students from three different laboratory sections showedvery good precision with 72% having RSD ≤ 5%. We planto use this approach in the future for the determination ofother metals such as iron, zinc, and copper in cereal products.

WSupplemental Material

Instructions for the students and notes for the instructorare available in this issue of JCE Online.

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