AnalyticalMethods

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Department of Chemistry, Faculty of Scienc

Turkey. E-mail: sacmaci@erciyes.edu.tr; Fax

Cite this: DOI: 10.1039/c3ay40727f

Received 2nd May 2013Accepted 3rd June 2013

DOI: 10.1039/c3ay40727f

www.rsc.org/methods

This journal is The Royal Society of

A coprecipitation procedure for the determination ofsome metals in food and environmental samples byflame atomic absorption spectroscopy

Cosxkun Ozdemir, Sxerife Samac* and Sxenol Kartal

A novel, rapid, and simple preconcentration procedure based on the coprecipitation of Cr(III), Cu(II), Fe(III),

Pb(II), Pd(II) and Zn(II) ions with Ni(II)a-benzoin oxime precipitate was developed prior to flame atomic

absorption spectrometric determination. The optimum values of the important analytical parameters

influencing the quantitative coprecipitation of the analytes were as follows: pH 8.5, amount of a-

benzoin oxime: 100 mg, amount of Ni(II) as carrier element: 0.25 mg, time duration: 10 min. The effects

of alkali (400015 000 mg L1) and alkaline earth (10004000 mg L1) metals on the recovery were also

studied. The alkali metal ions (K+ up to 4000 and Na+ up to 15 000 mg L1) and alkaline earth metal

ions (Mg2+ up to 1000 and Ca2+ up to 4000 mg L1) did not show any interference effect on the

recovery of the analytes. Under the optimized conditions, the detection limits (3s, N 10) for theanalytes were in the range of 0.15.3 mg L1. The accuracy of the presented coprecipitation method was

checked by the analysis of certified reference materials. The method was applied to the determination

of the analytes in real samples such as natural water and food samples, and good results were obtained

(relative standard deviation 95%).

1 Introduction

Metals are considered very important and highly toxic in variousenvironmental areas. The input of metals to the environment asa result of anthropogenic activities is difficult to measure due tovery large natural inputs from erosion or rocks, wind-blowndust, volcanic activity and forest res.1 Toxic metals can causedamaging effects even at very low concentrations. They tend toaccumulate in the food chain and in the body, and can be storedin so (e.g., kidney) and hard tissues (e.g., bone).2 They oenexist in a positively charged form and can bind onto negativelycharged organic molecules to form complexes. The importanceof metal pollution control has increased signicantly in recentdecades.3

Air, soil, and water pollution are presently contributing tothe presence of harmful elements in the environment.4

Recently, pollution of water and food samples by an appreciableamount of metals has been considered as the result of humanactivities.5 Excessive accumulation of dietary metals in thehuman body may cause serious health problems, and there isgrowing concern about the effects of dairy food contaminantson human health. Contamination of food products by metals isbecoming an unavoidable problem nowadays.6

Zinc, copper and iron are essential elements that arerequired by a variety of biomolecules to maintain the normal

es, Erciyes University, TR-38039, Kayseri,

: +90 352 4374933; Tel: +90 352 4374937

Chemistry 2013

structure, function, and proliferation of cells. These metals canbe toxic in excessive amounts, especially in certain geneticdisorders.79 In the environment, lead is known to be toxic toplants, animals and microorganisms. Plastics, shing tools,lead crystal glass used in cathode ray tubes, ceramics, solders,pieces of lead ashing and many other minor products arematerials that contain lead. Lead inuences the nervoussystem, slowing down nerval response. Children are exposed tolead right from their birth, as children in the embryonic stagereceive lead from their mothers through blood.10 Cr(VI)-compounds are generally considered to be the most toxic andcarcinogenic. The environmental concerns related to chromiumare focused on applications like tanning, wood preservation,and the use of pigments and dyes for plastics, paint andtextiles.10,11 Some palladium compounds have been reported aspotential health risks to humans, causing asthma, allergy, rhi-noconjunctivitis and other serious health problems.12 The FAO/WHO has declared that for an average adult (60 kg body weight),the provisional tolerable daily intakes of Pb, Cu, Fe and Zn are0.2, 3, 48 and 60 mg, respectively.13 The monitoring and controlof these trace elements in food sources require processing a lotof samples to accurately characterize their abundance and toreach reliable conclusions.14

The determination of trace elements and contaminants incomplex matrices, such as food, oen requires extensive samplepreparation and/or extraction regimes prior to instrumentalanalysis.5 Therefore, it is of prime importance to determine theconcentration of essential and toxic elements in foods. The

Anal. Methods

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Analytical Methods Paper

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determination of trace metals in food samples has routinelybeen carried out by inductively coupled plasma optical emis-sion spectrometry (ICP-OES),15 inductively coupled plasmamass spectrometry (ICP-MS),16 graphite furnace atomicabsorption spectrometry (GFAAS),17 and ame atomic absorp-tion spectrometry (FAAS).1820

The preconcentration of traces of metals is oen requiredprior to instrumental determination to lower the detectionlimits and to improve the precision and accuracy of analyticalresults.21 The coprecipitation method is useful for the pre-concentration of trace metal ions and is one of the most usefulways for the preconcentration/separation of trace elementsfrom the sample matrix.22,23 The coprecipitation method has anadvantage in that many elements other than alkali and alkalineearth elements are preconcentrated quite effectively under thesame experimental conditions.24,25

In this study, a simple and rapid coprecipitation procedureusing a Ni(II)a-benzoin oxime precipitate was studied to deter-mine Cr(III), Cu(II), Fe(III), Pb(II), Pd(II) and Zn(II) ions in water andfood samples by ame atomic absorption spectrometry (FAAS).To the best of our knowledge, there is no study on the copreci-pitation of the analyte elements by using Ni(II)a-benzoin oxime.We found that the Ni(II)a-benzoin oxime system has a goodcollecting ability for this purpose. Also, the Ni(II)a-benzoinoxime precipitate is convenient to handle, because it can beeasily dissolved in about 0.5 mL of conc. nitric acid withoutheating, which is required for the dissolution of the precipitate.

2 Experimental2.1 Instruments

A PerkinElmer model AAnalyst 800 ame atomic absorptionspectrometer (Norwalk, CT, USA) equipped with a deuteriumbackground correction system and an air-acetylene burner wasused for the determination of all the metal ions. The operatingcondition adjustments in the spectrometer were carried outaccording to the standard guidelines of the manufacturer. FormeasuringpHvalues in the aqueousphase, aConsortmodelC533pHmeter combinedwith a glass-electrodewas used and aMLTW-54 model centrifuge was used for centrifugation purposes.

2.2 Reagents and standard solutions

All the reagents used were of the highest available purity and ofat least analytical reagent grade (Merck, Darmstadt, Germany).Deionized ultra pure water was used for the preparation of thesolutions. Standard stock solutions of the analytes, 1000 mgL1, were prepared by dissolving appropriate amounts of theirnitrate salts in 2% (w/v) nitric acid. Working standard solutionswere prepared fresh daily by stepwise dilution of the stocksolutions with deionized water. The calibration curve wasestablished using the standard solutions prepared in 1 mol L1

HNO3 by dilution of the stock solutions. A 1% (w/v) solution ofa-benzoin oxime was prepared by dissolving 1 g of the reagentin 100 mL of methanol. The glassware used was cleaned bysoaking overnight in dilute HNO3 (1 : 5) and then rinsed withdeionized water several times.

Anal. Methods

2.3 Preparation of samples

Ten mL of the certied reference material (SPS-WW2 Batch 108wastewater sample) was taken and diluted to 25 mL withdeionized water. The pH of this sample solution was adjusted to8.5 with ammonia/ammonium chloride buffer solution. Thenthe separation/preconcentration procedure given below wasapplied to these sample solutions.

A 1.0 g aliquot from each of the food and the standardreference material (INCT-TL-1 tea leaves) samples was takenand treated with 15 mL of concentrated nitric acid and thenheated until a clear solution was obtained. The evaporationwas continued almost to dryness, and to the moist residue wasadded 15 mL of nitric acid once more. Again the mixture wasevaporated nearly to dryness and then 3 mL of concentratedH2O2 was added to it. Aer completing the dissolution process,the sample solution was ltered through a cellulose lter paper.The lter paper was washed with 12 mL of 1 mol L1 HNO3.The ltrate was diluted to 25 mL with deionized water. Thesesample solutions were analyzed by using the proposed proce-dure. The nal measurement volume of the sample solutionswas 5 mL. A blank digest was prepared in the same way.

The proposed method was applied to various water samples.First, the pH of the water samples was adjusted to 8.5 and thenthe preconcentration procedure was applied. The analyteconcentrations in the nal solution (5 mL) were determinedby FAAS.

2.4 Proposed procedure

The proposed coprecipitation method was tested with modelsolutions prior to its application to real samples. 25 mL of theaqueous sample solution containing individually 20 mg of Cr(III),Fe(III), Pd(II) and Pb(II), 10 mg of Cu(II) and 2 mg of Zn(II) wasplaced into the centrifuge tube. The pH of the solutions wasadjusted to 8.5 with ammonia/ammonium chloride buffersolution (2 mL). Then 0.25 mL of 1000 mg L1 Ni(II) as a carrierelement and 0.1 mL of 1% (w/v) a-benzoin oxime solution wereadded to this solution. Aer the formation of the precipitate,the solution was centrifuged at 3500 rpm for 10 min. Thesupernatant was removed. The precipitate which remainedadhering to the bottom of the tube was dissolved with 500 mL ofconcentrated HNO3. Then the nal volume was made up to 5mL with deionized water. The analytes in the nal solution weredetermined by FAAS.

3 Results and discussion3.1 Effect of pH

For quantitative coprecipitation efficiency of the analyte ions,the working pH of the media is one of the most importantfactors. The inuence of pH on the recovery of the analytes inthe Ni(II)a-benzoin oxime system was studied in the pH rangeof 110 by using model solutions containing the analyte ions.All pH adjustments were made using the following buffersolutions: KCl/HCl solution for pH 12, H3PO4/NaH2PO4 solu-tion for pH 3, CH3COOH/CH3COONa solution for pH 46, andCH3COONH4 solution for pH 7. An ammonia/ammonium

This journal is The Royal Society of Chemistry 2013

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chloride buffer solution was used between pH 8 and 10. Thiswas prepared from 1 mol L1 solutions of the buffer compo-nents. Two mL of the buffer solution was employed in eachexperimental study. The results are shown in Fig. 1. In the pHrange of 810, all the analyte ions were simultaneously andquantitatively (>95%) recovered. All the further experimentswere performed at pH 8.5.

Fig. 3 Effect of the amount of Ni(II) on the recoveries of the analyte ions (n 3).

3.2 Effect of the amount of a-benzoin oxime

The effect of the amount of a-benzoin oxime reagent on therecovery (%) of the analyte ions was investigated. The results aregiven in Fig. 2. When the experiments were performed withouta-benzoin oxime at pH 8.5, the recovery values were not quan-titative (#45%) for the studied metal ions. The recovery of theanalyte ions increased with increasing amounts of a-benzoinoxime. These results show that the a-benzoin oxime reagent isnecessary for the quantitative recoveries of the studied metalions. For all the subsequent experiments 100 mL of 1% (w/v)a-benzoin oxime solution was used (Fig. 3).

3.3 Amount of nickel as carrier element

The inuence of the amount of Ni(II) as carrier element on therecovery of the analyte ions was also investigated. Withoutnickel, the recovery of the analyte ions was below 56%. Quan-titative recovery values (>95%) for the analyte ions wereobtained for 0.25 mg of Ni(II). All the further experiments wereperformed with 0.25 mL of 1000 mg L1 Ni(II) solution.

Fig. 1 Effect of pH on the recoveries of the analyte ions (n 3).

Fig. 2 Effect of the amount of a-benzoin oxime on the recoveries of the analyteions (n 3).

This journal is The Royal Society of Chemistry 2013

3.4 Effects of time duration

The effects of the duration of the coprecipitation procedure onthe recovery of analyte ions were also studied in the time rangeof 530 min. Quantitative recoveries for all the analytes wereobtained aer 10 min duration. All the further studies wereperformed with 10 min duration time.

3.5 Effect of centrifugation rate and time

One of the most important parameters for coprecipitationprocedures is the centrifugation rate. For this purpose, a seriesof experiments was conducted at different centrifugation ratesvarying from 1000 to 4000 rpm for 10 min. The absorbancesharply increased up to the centrifugation rate of 2500 rpmexcept for Pd, for which a slow increase was observed up to 3000rpm and then it remained almost constant. A plateau wasobserved over the centrifugation rate of 3500 rpm. As theoptimal centrifugation rate, 3500 rpm was selected (Fig. 4).

Another important parameter for coprecipitation proceduresis the centrifugation time. In order to determine the bestcentrifugation time, the experiments were performed in thetime range of 530min under optimized conditions. The resultsshowed that centrifugation time had a signicant inuence onthe signals of the analytes. The best results were obtained with10 min centrifugation time. The analyte signals were nearlyconstant beyond the centrifugation time of 10 min. Theoptimum centrifugation time for this process was chosen as 10min (Fig. 5).

Fig. 4 Effect of the centrifugation rate of on the recoveries of the analyte ions(n 3).

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3.6 Sample volume and preconcentration factor

The effect of the sample volume on the recovery of the analyteions was investigated with sample volumes in the range of251000 mL by using the model solutions. The centrifuge tubesused have a maximum capacity of 50 mL. For sample volumeslarger than 50 mL, we have used the ltration technique.Quantitative metal recoveries were obtained up to 500 mL ofsample volume, except for Cr(III) and Pd(II). The preconcentra-tion factor was calculated as the ratio of the highest samplevolume (500 mL) to the lowest nal measurement volume(5 mL) for Fe(III), Pb(II), Cu(II) and Zn(II). As a result, the pre-concentration factor was calculated as 100 for these metals,while it was only 50 for Cr(III) and Pd(II). If we take intoconsideration iron alone, it had the highest preconcentration

Fig. 5 Effect of the centrifugation time of on the recoveries of the analyte ions(n 3).

Fig. 6 Effect of the sample volume on the recoveries of the analyte ions (n 3).

Table 1 The influence of some ions on the recoveries of analyte ions (n 3)a

Ions Added asConcentration(mg L1)

Recovery (%)

Cr(III)

Na+ NaNO3 15 000 99 1K+ KNO3 4000 100 2Ca2+ Ca(NO3)2$4H2O 4000 100 2Mg2+ Mg(NO3)2$6H2O 1000 99 1SO4

2 Na2SO4 4500 100 2Cl NaCl 12 000 98 2H2PO4

NaH2PO4$2H2O 500 98 1a Average standard deviation.

Anal. Methods

factor of 200, because it still continued to give quantitativerecoveries at a sample volume of 1000 mL (Fig. 6).

3.7 Effect of foreign ions

Alkali and alkaline earth metal ions, and some anions (like Na+,K+, Ca2+, Mg2+, Cl, SO4

2, H2PO4) at mg L1 and higher levels

may have interfering effects on the determination of metal ionsby ame atomic absorption spectrometry. The effects of theseions on the coprecipitation efficiency of Cr(III), Cu(II), Fe(III),Pb(II), Pd(II) and Zn(II) ions were examined by adding knownconcentrations of these ions. Then the coprecipitation proce-dure was applied individually for all the diverse ions mentionedabove. Tolerable concentrations were found to be 15 000 mgmL1 for Na+ ion, 4000 mg mL1 for K+, Ca2+ and SO4

2, 1000 mgmL1 for Mg2+, and 500 mg mL1 for PO4

3. The toleratedamounts of each diverse ion were the concentration valuestested that caused less than 5% absorbance alteration. Theseforeign ions did not interfere under the experimental condi-tions used (Table 1).

3.8 Evaluation of method performance

The quantitation of the analytes was performed by using theexternal calibration curve method. For each analyte, the cali-bration curves were prepared from the standard solutions of theanalyte elements. The correlation coefficients were at least0.998.

The limits of detection (LOD) of the proposed method for theinvestigated elements were studied under the optimum exper-imental conditions by applying the proposed procedure forblank solutions. The detection limits were established byanalyzing 20 blank solutions (3s). The detection limits of Cr(III),Fe(III), Cu(II), Pb(II), Pd(II) and Zn(II) were found to be 1.1, 0.2, 0.4,5.3, 3.3, and 0.1 mg L1, respectively. The relative standarddeviations of the proposed method were

Table 2 The results for the standard reference materials (n 3)

Analyte

INCT-TL-1 tea leaves (mg g1) SPS-WW2 Batch 108 waste water (mg L1)

Certied value Found RE (%)a Certied value Found RE (%)

Cr(III) 1.91 0.22b 1.86 0.05 2.6 1000 5 984 1 1.6Fe(III) 432 429 2 0.7 1000 5 992 2 0.8Cu(II) 20.4 1.5 19.5 0.8 4.4 1000 5 989 1 1.1Pb(II) 1.78 0.24 1.70 0.06 4.5 1000 5 991 1 0.9Pd(II) Zn(II) 34.7 2.7 33.2 1.3 4.3 1000 5 982 6 1.8a RE%: per cent relative error. b Average standard deviation.

Table 3 The determination of analyte ions in red mullet, green tea and tap water samples after the application of the presented procedure (N 3)

Sample Added

Tap water (mg L1) Red mullet (mg g1) Green tea (mg g1)

Found Recovery (%) Found Recovery (%) Found Recovery (%)

Cr 0.05 0.01 0.41 0.06 0.1 0.096 0.008a 96 1 0.145 0.014 97 2 1.0 0.97 0.05 97 1 1.35 0.09 96 2

Fe 0.37 0.02 78 4 178 6 0.5 0.83 0.05 96 1 100 174 6 98 2 269 5 97 2

Cu 0.56 0.06 2.25 0.14 1.0 1.49 0.22 96 2 0.98 0.09 98 1 3.09 0.18 95 22.0 1.9 0.1 95 1 4.1 0.13 96 1

Pb 0.21 0.03 0.2 0.19 0.04 95 1 0.192 0.043 96 1 0.39 0.06 95 1

Pd 1.0 0.96 0.08 96 2 0.97 0.11 97 3 0.98 0.07 98 2

Zn 0.28 0.06 9.7 0.9 18.2 2.1 0.5 0.77 0.11 99 3 10 19 1 96 2 27.7 1.9 98 320 37 2 97 2

a Average standard deviation.

Table 4 Determination of the analytes in some food samples after application of the proposed coprecipitation procedure (n 3, mg g1)

Sample Cr(III) Cu(II) Fe(III) Pb(II) Pd(II) Zn(II)

Black tea 0.45 0.05a 1.72 0.16 152 3 b 14 1Carnation 0.55 0.04 0.23 0.06 137 6 0.31 0.01 5.3 0.5Heath 0.33 0.04 0.19 0.02 179 2 0.39 0.01 3.22 0.65Green tea 0.46 0.08 2.07 0.11 173 5 0.17 0.01 17 2Pomegranate ower 0.34 0.08 2.02 0.06 242 4 14 1Red mullet 0.06 0.01 80 5 8.8 0.6Daisy 4.02 0.08 249 1 21 1Bay leaf 0.46 0.01 0.37 0.06 20 1 3.04 0.07Coffee 23 2 0.23 0.04 0.6 0.1Corn 1.95 0.08 14 1 29 1Rice 2.53 0.08 4.6 0.3 29 3Sesame 0.16 0.02 19 1 38 4Cumin our 0.53 0.06 47 2 0.011 0.003 30 2Linen 0.20 0.06 47 3 0.38 0.02 40 1Bean 0.92 0.16 1.81 0.12 201 18 19.7 0.1a Average standard deviation. b Below detection limit.

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Table 5 Application of the presented procedure for various waters and anode slime samples (n 3)

Sample Cr(III) Cu(II) Fe(III) Pb(II) Pd(II) Zn(II)

Tap water (mg L1) a 5.4 0.3b 10.8 0.5 2.4 0.4Rain water (mg L1) 0.5 0.1 7.0 0.1 2.4 0.1Waste water (mg L1) 4.7 0.2 7.3 0.9 12.0 1.7 350 40Acidic dialysis solution (mg L1) 0.24 0.09 1.2 0.1 33.3 0.6 2.0 0.1 4.3 0.8Sea water (mg L1) 7.0 1.0 7.0 0.8 35.6 1.2 6.6 2.8Anode slime (mg g1) 12.3 0.5 3610 30 2291 62 22 746 427 6.7 0.2 95 4a Below detection limit. b Average standard deviation.

Table 6 Comparison of the results of the proposed method with the other published studies

Preconcentration method Analyte Technique LOD (mg L1) RSD (%) PF Reference

Solvent extraction Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Zn FAAS 0.330.9

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14 Sx. Samac and Sx. Kartal, Clean: Soil, Air, Water, 2011, 39, 577.15 Q. He, X. J. Chang, X. P. Huang and Z. Hu, Microchim. Acta,

2008, 160, 147.16 D. Hammer, M. Nicolas and D. Andrey, At. Spectrosc., 2005,

26, 203.17 R. Manjusha, K. Dash and D. Karunasagar, Food Chem.,

2007, 105, 260.18 L. Liu, L. Ding, M. Qi, X. L. Han and H. Q. Zhang, Spectrosc.

Spectral Anal., 2007, 27, 1436.19 D. Citak, M. Tuzen and M. Soylak, Food Chem. Toxicol., 2009,

47, 2302.20 V. A. Lemos, D. G. da Silva, A. L. de Carvalho, D. D. Santana,

G. D. Novaes and A. S. dos Passos,Microchem. J., 2006, 84, 14.

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21 M. Hiraide, H. Hommi and H. Kawaguchi, Anal. Sci., 1991, 7,169.

22 T. Minami, K. Atsumi and J. Ueda, Anal. Sci., 2003, 19, 313.23 J. Ueda and C. Mizui, Anal. Sci., 1988, 4, 417.24 H. Sawatari, E. Fujimori and H. Haraguchi, Anal. Sci., 1995,

11, 369.25 Sx. Samac and Sx. Kartal, Microchim. Acta, 2010, 170, 75.26 F. Xie, X. Lin, X. Wu and Z. Xie, Talanta, 2008, 74, 836843.27 J. S. Suleiman, B. Hu, H. Peng and C. Huang, Talanta, 2009,

77, 15791583.28 A. Goswami and A. K. Singh, Talanta, 2002, 58, 669678.29 Sx. Samac, Sx. Kartal, M. Samac and C. Soykan, Bull. Korean

Chem. Soc., 2011, 32, 444450.

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http://dx.doi.org/10.1039/c3ay40727fA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopyA coprecipitation procedure for the determination of some metals in food and environmental samples by flame atomic absorption spectroscopy