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Slurry sampling and high-resolution continuum source flame atomic absorption spectrometry using secondary lines for the determination of Ca and Mg in dairy products
Microchemical Journal 98 (2011) 231–233
Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Slurry sampling and high-resolution continuum source flame atomic absorption spectrometry using secondary lines for the determination of Ca and Mg in dairy products Geovani C. Brandao, Geraldo D. Matos, Sergio L.C. Ferreira ⁎ Universidade Federal da Bahia, Instituto de Química, Grupo de Pesquisa em Química e Quimiometria, Campus Ondina, Salvador 40170-290, Bahia, Brazil
a r t i c l e
i n f o
Article history: Received 31 January 2011 Accepted 6 February 2011 Available online 15 February 2011 Keywords: Slurry sampling High-resolution continuum source flame atomic absorption spectrometry Secondary lines Calcium Magnesium Dairy products
a b s t r a c t The present paper proposes a simple and fast analytical procedure for the sequential multi-element determination of Ca and Mg in dairy products employing sampling slurry and high resolution-continuum source flame atomic absorption spectrometry (HR-CS FAAS). Considering the high concentration of these species in these matrices, the analytical measurements were carried out at the secondary lines of 239.856 and 202.852 for Ca and Mg, respectively. The experimental conditions established for the preparation of the slurries during the optimization step were: 2.0 mol L− 1 hydrochloric acid, sonication time of 20 min and sample mass of 1.0 g for a slurry volume of 25 mL. Experiments demonstrated that the analytical curves can be established using the external calibration technique employing aqueous standards. The method allows the determination of Ca and Mg with limits of quantification of 0.038 and 0.016 mg g− 1, respectively. The precision was evaluated under reproducibility and repeatability conditions and expressed as relative standard deviation. The results varied from 2.7 to 2.9% (all tests with n = 10) and using a yogurt sample containing Ca and Mg concentrations of 1.40 and 0.13 mg g− 1, respectively. The accuracy was confirmed by the analysis of a certified reference material of non-fat milk powder furnished by the National Institute of Standard and Technology. The proposed method was applied for the determination of Ca and Mg in yogurt, cow milk and milk powder samples. The samples were also analyzed after complete acid digestion and Ca and Mg determination by HR-CS FAAS. No statistical difference was observed between the results obtained by both of the procedures performed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dairy foods are widely consumed in the world because they are important sources of several essential nutrients in the human diet. This way, the determination of chemical elements in these matrices is often required in order to establish quality control of these products [1–5]. Generally, these analyses can be easily performed employing atomic absorption spectrometry (AAS), that calcium and magnesium are macronutrients and, for this reason the determination of these elements using the main lines requires high dilution of the samples. Another difficulty is the fat content that sometimes complicates the sample preparation step [6]. The high-resolution continuum source atomic absorption spectrometry (HR-CS AAS) uses as continuum radiation source a highintensity xenon short-arc lamp, which provides a better signal-to-
⁎ Corresponding author. Fax: +55 71 32355166. E-mail address: [email protected] (S.L.C. Ferreira). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.02.005
noise ratio than hollow cathode lamps used in line source-FAAS [7]. This feature allows the use of less sensitive secondary lines to reduce sensitivity during the determination of major elements and avoid a high dilution of the samples [8]. This strategy has been used for the determination of some elements in sugar-cane leaves [9] and also cobalt and vanadium in undiluted crude oil [10]. Procedures involving simplification of the sample preparation step for analyses of complex matrices have been developed [11]. The solid and slurry sampling techniques employing spectrometric methods such as: high-resolution continuum source graphite furnace atomic absorption spectrometry (HR CS GF AAS) [12,13] flame atomic absorption spectrometry (FAAS) [14,15], inductively coupled plasma optical emission spectrometry (ICP OES) [16,17], hydride generation atomic absorption spectrometry (HG AAS) [18,19], electrothermal atomic absorption spectrometry (ET AAS) [20] and others have been proposed for analyses of several samples [21]. This paper proposes a fast analytical procedure that employs slurry sampling for the sequential multi-element determination of calcium and magnesium in dairy products by HR-CS FAAS. Secondary lines were used in order to avoid a high dilution of the samples.
232
G.C. Brandao et al. / Microchemical Journal 98 (2011) 231–233
2. Experimental 2.1. Instrumentation An Analytik Jena Model ContrAA 300 High Resolution-Continuum Source Flame Atomic Absorption Spectrometer (GLE, Berlin, Germany) equipped with a xenon short-arc lamp XBO 301 with a nominal power of 300 W operating in a hot-spot mode as a continuum radiation source was used for the analysis. A nitrous oxide-acetylene flame was used for the atomization of Ca and Mg. All measurements of absorbance were carried out in triplicates for blanks, analytical solutions and samples using the secondary lines of Ca (239.586 nm) and Mg (202.582 nm). The equipment was operated under optimum conditions. An Ultrasonic Benchtop Cleaner VWR Model 75 D (Cortland, New York, USA) was used for slurry preparation. A Tecnal Model TE-040/25 Aluminum Heating Block (Piracicaba, São Paulo, Brazil) was used for mineralization samples. 2.2. Reagent and samples All reagents were of analytical grade unless otherwise stated. Ultrapure water was obtained from an EASY pure RF (Barnstedt, Dubuque, IA, USA). Nitric and hydrochloric acids were of Suprapur quality (Merck, Darmstadt, Germany). Laboratory glassware was kept overnight in 10% v/v nitric acid solution. Before use the glassware was rinsed with deionized water and dried in a dust-free environment. Multi-element analytical solutions containing calcium (10.0– 100.0 mg L− 1) and magnesium (1.0–10.0 mg L− 1) were prepared daily by appropriate dilution from 1000 μg mL− 1 calcium and magnesium stock solutions (Merck) in 0.5 mol L− 1 hydrochloric acid solution. The certified reference material used for confirmation of the accuracy was the SRM 1549 non-fat milk powder (National Institute of Standard and Technology, NIST, Gaithersburg, MD, USA). The yogurt, cow milk and milk powder samples investigated in this study were locally available brands, collected in supermarkets from Salvador City, Brazil. 2.3. Slurry preparation A 1.0 g mass of yogurt sample (0.1 g of milk powder or 1.0 mL of milk) was directly weighted in 25 mL volumetric flask and diluted with a 0.5 mol L− 1 hydrochloric acid solution. Then, the slurries were placed in an ultrasonic bath for 20 min and afterwards the slurries were aspirated directly through the nebulizer for the determination of calcium and magnesium by HR-CS FAAS. All samples were analyzed in triplicate. The blanks were prepared in the same way as the slurries. Standard calibration technique was used for quantification and the analytical curves were performed using aqueous standards. 2.4. Complete digestion of the real samples A mass of 1.0 g of yogurt sample (or 0.1 g of milk powder or 1.0 mL of cow milk) was directly weighted in digestion tube and volumes of 5.0 mL of concentrated nitric acid and 4 mL of 30% (v/v) hydrogen peroxide were added. The mixture was heated and evaporated to almost dryness in a heating block. After, the residual solution was transferred to 25 mL volumetric flask and diluted with 1% (v/v) acid nitric solution.
employing univariate methodology. In this step, all the experiments were carried out using slurry volumes of 25 mL and a yogurt sample containing calcium and magnesium with concentrations of 1.40 and 0.13 mg g− 1, respectively. In order to determine the hydrochloric acid concentration used as liquid phase, some slurries were prepared at the experimental condition with sample mass of 1.0 g and sonication time of 20 min and acid concentration varied in the range of 0.1 to 2.5 mol L− 1. The results demonstrated that there is no difference in the analytical signals for both analytes in the studied concentration range. However, it was observed that slurries prepared with acid concentrations less than 2.0 mol L− 1 caused occasional blockage in the nebulization system of the spectrometer. Thus, hydrochloric acid at a 2.0 mol L− 1 concentration was established as the liquid phase for slurry preparation. The sonication time for the preparation of the slurries was studied in the range of 5 to 40 min. The results demonstrated that slurries prepared with sonication times high than 10 min provide better results for both the analytes. Thus, a sonication time of 20 min was established for the preparation of the slurries. The sample mass was studied in the range of 0.25 to 2.0 g. For magnesium, the analytical signal was linearly proportional in all studied range (r = 0.999). However, the results obtained for calcium showed that the slurries could be prepared without problems using a mass sample up to 1.75 g (r = 0.999). This way, a sample mass of 1.0 g was established for the preparation of the slurries. The experiments were carried out using secondary lines for both elements. 3.2. Analytical validation The calibration technique used in this method was established involving comparison of the slopes of the curves obtained by external calibration technique (with aqueous standards) and of the slopes of curves found by application of the analyte addition technique in a yogurt sample. The results showed good similarity among the slopes of the curves, all with correlation coefficients N 0.999, for both the analytes, as can be seen in Table 1. It demonstrates that calcium and magnesium in dairy product samples can be determined using the external calibration technique. The limits of detection (LOD) and quantification (LOQ), calculated with three and ten times the standard deviation of the blank divided by the slope of the analytical curve [22], were found to be 0.012 mg g− 1 and 0.038 mg g− 1 for calcium, respectively. For magnesium, it was found 0.005 mg g− 1 and 0.016 mg g− 1, respectively. The precision, expressed as relative standard deviation (RSD) was under repeatability conditions 2.8 (for calcium) and 2.7% (for magnesium) and 2.9 and 2.7% under reproducibility conditions, for calcium and magnesium, respectively. These results were found for a yogurt sample containing calcium and magnesium with concentrations of 1.40 and 0.13 mg g− 1, respectively and n = 10 for all the experiments. The accuracy of the method was confirmed by determination of calcium and magnesium in the certified reference material (CRM) Table 1 Equations of the analytical curves. System
Linear regression equations for calcium
Aqueous media
Abs = (0.00222 ± 0.00016)CCa + (0.00136 ± 0.00492) r = 0.9983 ± 0.0068 Abs = (0.00207 ± 0.00014)CCa + (0.15079 ± 0.00437) r = 0.9984 ± 0.0060
Yogurt
3. Results and discussion System
Linear regression equations for magnesium
3.1. Optimization of the experimental conditions
Aqueous media
The optimization of the experimental factors: concentration of hydrochloric acid, sonication time and sample mass was performed
Yogurt
Abs = (0.01363 ± 0.00079)CMg + (0.00104 ± 0.00238) r = 0.9990 ± 0.0033 Abs = (0.01328 ± 0.00078)CMg + (0.08967 ± 0.00236) r = 0.9989 ± 0.0033
G.C. Brandao et al. / Microchemical Journal 98 (2011) 231–233
References
Table 2 Determination of calcium and magnesium in dairy foods. Digestion
Slurry
Sample
Slurry
Yogurt
Calcium, mg g− 1
Digestion
Whole 1 Skimmed Whole 2
1.40 ± 0.08 1.34 ± 0.04 1.63 ± 0.12
Cow milk
Calcium, mg mL− 1
Magnesium, mg mL− 1
Whole Skimmed Skim
1.23 ± 0.01 1.18 ± 0.06 1.22 ± 0.04
0.12 ± 0.01 0.13 ± 0.01 0.12 ± 0.01
Milk powder
Calcium, mg g− 1
Whole 1 Whole 2 Whole 3
9.37 ± 0.70 8.92 ± 0.41 9.11 ± 0.50
Magnesium, mg g− 1 1.43 ± 0.07 1.27 ± 0.06 1.61 ± 0.02
1.15 ± 0.08 1.19 ± 0.07 1.18 ± 0.03
0.13 ± 0.01 0.14 ± 0.01 0.17 ± 0.01
0.14 ± 0.01 0.12 ± 0.01 0.15 ± 0.02
0.11 ± 0.01 0.11 ± 0.01 0.12 ± 0.01
Magnesium, mg g− 1 9.11 ± 0.57 8.84 ± 0.69 8.98 ± 0.43
0.86 ± 0.03 0.83 ± 0.04 0.86 ± 0.02
233
0.85 ± 0.06 0.83 ± 0.06 0.85 ± 0.05
NIST SRM 1549 of non-fat milk powder, which has certified values of 1.30 ± 0.05 and 0.120 ± 0.003 mg g− 1 for calcium and magnesium, respectively. Using the proposed method calcium and magnesium concentrations of 1.21 ± 0.05 and 0.119 ± 0.004 mg g− 1 were found. 3.3. Application The proposed method was applied for the determination of calcium and magnesium in milk, milk dessert and yogurt samples bought in supermarkets from Salvador City, Brazil. The samples were also analyzed after complete digestion using nitric and hydrogen peroxide and the elements were determined by HR-CS FAAS using secondary lines. All results expressed with interval confidence (at the 95%) are shown in Table 2. For all samples it is observed that calcium content is about ten higher than magnesium content. The results showed also that the calcium and magnesium concentrations for the milk powder samples are about eight higher than the concentrations obtained for the milk and yogurt samples. All these results are in agreement with data reported in the literature. Applying paired t-test at the 95% confidence level no statistical difference was observed among the values obtained by slurry method and those after complete digestion. 4. Conclusion The proposed method presented analytical features adequate for the sequential multi-element determination of calcium and magnesium in dairy product samples. The use of the secondary lines allowed the determination of these macronutrients avoiding a high dilution of the samples. Furthermore, the results showed that the method developed is feasible, fast and simple being suitable for routine analyses. Acknowledgements The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnalógico (CNPq), Fundação de Amparo a Pesquisa à Pesquisa do Estado da Bahia (FAPESB) and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for providing grants and fellowships and for financial support.
[1] I.R. do Nascimento, R.M. de Jesus, W.N.L. dos Santos, A.S. Souza, W.D. Fragoso, P.S. dos Reis, Determination of the mineral composition of fresh bovine milk from the milk-producing areas located in the State of Sergipe in Brazil and evaluation employing exploratory analysis, Microchem. J. 96 (2010) 37. [2] G.V. Pashkova, X-ray fluorescence determination of element contents in milk and dairy products, Food Anal. Methods 2 (2009) 303. [3] C. Frazzoli, B. Bocca, Validation, uncertainty estimation and application of a sector field ICP MS-based method for As, Cd and Pb in cow's milk and infant formulas, Microchim. Acta 162 (2008) 43. [4] C.M. Silva, E.S. Alcoforado, R.S. Amaral, J.A. Santos, R.S.C. Menezes, J.D. Bezerra, D.S. Santos, L.E. Lima, C.A.S. Filho, E.V. Honorato, F.C.A. Ribeiro, J.N. Tabosa, Stable lead in milk and derivates, Food Anal. Methods 3 (2010) 85. [5] F.R. de Amorim, M.B. Franco, C.C. Nascentes, J.B.B. da Silva, Direct determination of molybdenum in milk and infant food samples using slurry sampling and graphite furnace atomic absorption spectrometry, Food Anal. Methods 4 (2011) 41. [6] M.G.A. Korn, E.S.B. Morte, D.C.M.B. dos Santos, J.T. Castro, J.T.P. Barbosa, A.P. Teixeira, A.P. Fernandes, B. Welz, W.P.C. dos Santos, E.B.G.N. dos Santos, M. Korn, Sample preparation for the determination of metals in food samples using spectroanalytical methods—a review, Appl. Spectrosc. Rev. 43 (2008) 67. [7] B. Welz, S. Mores, E. Carasek, M.G.R. Vale, M. Okruss, H. Becker-Ross, Highresolution continuum source atomic and molecular absorption spectrometry—a review, Appl. Spectrosc. Rev. 45 (2010) 327. [8] B. Welz, H. Becker-Ross, S. Florek, U. Heitmann, High-resolution Continuum Source AAS, Wiley-VCH, Weinheim, 2005. [9] S.R. de Oliveira, J.L. Raposo, J.A.G. Neto, Fast sequential multi-element determination of Ca, Mg, K, Cu, Fe, Mn and Zn for foliar diagnosis using high-resolution continuum source flame atomic absorption spectrometry: feasibility of secondary lines, side pixel registration and least-squares background correction, Spectrochim. Acta B 64 (2009) 593. [10] I.M. Dittert, J.S.A. Silva, R.G.O. Araujo, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of cobalt and vanadium in undiluted crude oil using high-resolution continuum source graphite furnace atomic absorption spectrometry, J. Anal. At. Spectrom. 25 (2010) 590. [11] S.L.C. Ferreira, M. Miro, E.G.P. da Silva, G.D. Matos, P.S. dos Reis, G.C. Brandao, W.N. L. dos Santos, A.T. Duarte, M.G.R. Vale, R.G.O. Araujo, Slurry sampling—an analytical strategy for the determination of metals and metalloids by spectroanalytical techniques, Appl. Spectrosc. Rev. 45 (2010) 44. [12] M. Resano, J. Briceno, M.A. Belarra, Direct determination of phosphorus in biological samples using a solid sampling-high resolution-continuum source electrothermal spectrometer: comparison of atomic and molecular absorption spectrometry, J. Anal. At. Spectrom. 24 (2009) 1343. [13] F. Vignola, D.L.G. Borges, A.J. Curtius, B. Welz, H. Becker-Ross, Simultaneous determination of Cd and Fe in sewage sludge by high-resolution continuum source electrothermal atomic absorption spectrometry with slurry sampling, Microchem. J. 95 (2010) 333. [14] E.G.P. da Silva, V. Hatje, W.N.L. dos Santos, L.M. Costa, A.R.N. Nogueira, S.L.C. Ferreira, Fast method for the determination of copper, manganese and iron in seafood samples, J. Food Compos. Anal. 21 (2008) 259. [15] G.C. Brandao, R.M. Jesus, E.G.P. da Silva, S.L.C. Ferreira, Use of slurry sampling for the direct determination of zinc in yogurt by high resolution-continuum source flame atomic absorption spectrometry, Talanta 81 (2010) 1357. [16] E.J. dos Santos, A.B. Herrmann, F. Vieira, C.S. Sato, Q.B. Correa, T.A. Maranhao, L. Tormen, A.J. Curtius, Determination of Hg and Pb in compact fluorescent lamp by slurry sampling inductively coupled plasma optical emission spectrometry, Microchem. J. 96 (2010) 27. [17] B. Kollander, M. Andersson, J. Pettersson, Fast multi-element screening of nondigested biological materials by slurry introduction to ICP-AES, Talanta 80 (2010) 2068. [18] S.M. Macedo, D.C. dos Santos, R.M. de Jesus, G.O. da Rocha, S.L.C. Ferreira, J.B. de Andrade, Development of an analytical approach for determination of total arsenic and arsenic (III) in airborne particulate matter by slurry sampling and HGFAAS, Microchem. J. 96 (2010) 46–49. [19] M.A.D. Saleh, A. Mayra, R.D.F. Neves, R. de Cassio, F.A. Silva, P.M. de Moraes, V.R. Loureiro, P.D. Roldan, P. dos Santos, P.D. Padilha, P. de Magalhaes, GFAAS determination of zinc in fish feed and feces using slurry sampling, Food Anal. Methods 2 (2000) 162. 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Contents lists available at ScienceDirect
Microchemical Journal j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m i c r o c
Slurry sampling and high-resolution continuum source flame atomic absorption spectrometry using secondary lines for the determination of Ca and Mg in dairy products Geovani C. Brandao, Geraldo D. Matos, Sergio L.C. Ferreira ⁎ Universidade Federal da Bahia, Instituto de Química, Grupo de Pesquisa em Química e Quimiometria, Campus Ondina, Salvador 40170-290, Bahia, Brazil
a r t i c l e
i n f o
Article history: Received 31 January 2011 Accepted 6 February 2011 Available online 15 February 2011 Keywords: Slurry sampling High-resolution continuum source flame atomic absorption spectrometry Secondary lines Calcium Magnesium Dairy products
a b s t r a c t The present paper proposes a simple and fast analytical procedure for the sequential multi-element determination of Ca and Mg in dairy products employing sampling slurry and high resolution-continuum source flame atomic absorption spectrometry (HR-CS FAAS). Considering the high concentration of these species in these matrices, the analytical measurements were carried out at the secondary lines of 239.856 and 202.852 for Ca and Mg, respectively. The experimental conditions established for the preparation of the slurries during the optimization step were: 2.0 mol L− 1 hydrochloric acid, sonication time of 20 min and sample mass of 1.0 g for a slurry volume of 25 mL. Experiments demonstrated that the analytical curves can be established using the external calibration technique employing aqueous standards. The method allows the determination of Ca and Mg with limits of quantification of 0.038 and 0.016 mg g− 1, respectively. The precision was evaluated under reproducibility and repeatability conditions and expressed as relative standard deviation. The results varied from 2.7 to 2.9% (all tests with n = 10) and using a yogurt sample containing Ca and Mg concentrations of 1.40 and 0.13 mg g− 1, respectively. The accuracy was confirmed by the analysis of a certified reference material of non-fat milk powder furnished by the National Institute of Standard and Technology. The proposed method was applied for the determination of Ca and Mg in yogurt, cow milk and milk powder samples. The samples were also analyzed after complete acid digestion and Ca and Mg determination by HR-CS FAAS. No statistical difference was observed between the results obtained by both of the procedures performed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Dairy foods are widely consumed in the world because they are important sources of several essential nutrients in the human diet. This way, the determination of chemical elements in these matrices is often required in order to establish quality control of these products [1–5]. Generally, these analyses can be easily performed employing atomic absorption spectrometry (AAS), that calcium and magnesium are macronutrients and, for this reason the determination of these elements using the main lines requires high dilution of the samples. Another difficulty is the fat content that sometimes complicates the sample preparation step [6]. The high-resolution continuum source atomic absorption spectrometry (HR-CS AAS) uses as continuum radiation source a highintensity xenon short-arc lamp, which provides a better signal-to-
⁎ Corresponding author. Fax: +55 71 32355166. E-mail address: [email protected] (S.L.C. Ferreira). 0026-265X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2011.02.005
noise ratio than hollow cathode lamps used in line source-FAAS [7]. This feature allows the use of less sensitive secondary lines to reduce sensitivity during the determination of major elements and avoid a high dilution of the samples [8]. This strategy has been used for the determination of some elements in sugar-cane leaves [9] and also cobalt and vanadium in undiluted crude oil [10]. Procedures involving simplification of the sample preparation step for analyses of complex matrices have been developed [11]. The solid and slurry sampling techniques employing spectrometric methods such as: high-resolution continuum source graphite furnace atomic absorption spectrometry (HR CS GF AAS) [12,13] flame atomic absorption spectrometry (FAAS) [14,15], inductively coupled plasma optical emission spectrometry (ICP OES) [16,17], hydride generation atomic absorption spectrometry (HG AAS) [18,19], electrothermal atomic absorption spectrometry (ET AAS) [20] and others have been proposed for analyses of several samples [21]. This paper proposes a fast analytical procedure that employs slurry sampling for the sequential multi-element determination of calcium and magnesium in dairy products by HR-CS FAAS. Secondary lines were used in order to avoid a high dilution of the samples.
232
G.C. Brandao et al. / Microchemical Journal 98 (2011) 231–233
2. Experimental 2.1. Instrumentation An Analytik Jena Model ContrAA 300 High Resolution-Continuum Source Flame Atomic Absorption Spectrometer (GLE, Berlin, Germany) equipped with a xenon short-arc lamp XBO 301 with a nominal power of 300 W operating in a hot-spot mode as a continuum radiation source was used for the analysis. A nitrous oxide-acetylene flame was used for the atomization of Ca and Mg. All measurements of absorbance were carried out in triplicates for blanks, analytical solutions and samples using the secondary lines of Ca (239.586 nm) and Mg (202.582 nm). The equipment was operated under optimum conditions. An Ultrasonic Benchtop Cleaner VWR Model 75 D (Cortland, New York, USA) was used for slurry preparation. A Tecnal Model TE-040/25 Aluminum Heating Block (Piracicaba, São Paulo, Brazil) was used for mineralization samples. 2.2. Reagent and samples All reagents were of analytical grade unless otherwise stated. Ultrapure water was obtained from an EASY pure RF (Barnstedt, Dubuque, IA, USA). Nitric and hydrochloric acids were of Suprapur quality (Merck, Darmstadt, Germany). Laboratory glassware was kept overnight in 10% v/v nitric acid solution. Before use the glassware was rinsed with deionized water and dried in a dust-free environment. Multi-element analytical solutions containing calcium (10.0– 100.0 mg L− 1) and magnesium (1.0–10.0 mg L− 1) were prepared daily by appropriate dilution from 1000 μg mL− 1 calcium and magnesium stock solutions (Merck) in 0.5 mol L− 1 hydrochloric acid solution. The certified reference material used for confirmation of the accuracy was the SRM 1549 non-fat milk powder (National Institute of Standard and Technology, NIST, Gaithersburg, MD, USA). The yogurt, cow milk and milk powder samples investigated in this study were locally available brands, collected in supermarkets from Salvador City, Brazil. 2.3. Slurry preparation A 1.0 g mass of yogurt sample (0.1 g of milk powder or 1.0 mL of milk) was directly weighted in 25 mL volumetric flask and diluted with a 0.5 mol L− 1 hydrochloric acid solution. Then, the slurries were placed in an ultrasonic bath for 20 min and afterwards the slurries were aspirated directly through the nebulizer for the determination of calcium and magnesium by HR-CS FAAS. All samples were analyzed in triplicate. The blanks were prepared in the same way as the slurries. Standard calibration technique was used for quantification and the analytical curves were performed using aqueous standards. 2.4. Complete digestion of the real samples A mass of 1.0 g of yogurt sample (or 0.1 g of milk powder or 1.0 mL of cow milk) was directly weighted in digestion tube and volumes of 5.0 mL of concentrated nitric acid and 4 mL of 30% (v/v) hydrogen peroxide were added. The mixture was heated and evaporated to almost dryness in a heating block. After, the residual solution was transferred to 25 mL volumetric flask and diluted with 1% (v/v) acid nitric solution.
employing univariate methodology. In this step, all the experiments were carried out using slurry volumes of 25 mL and a yogurt sample containing calcium and magnesium with concentrations of 1.40 and 0.13 mg g− 1, respectively. In order to determine the hydrochloric acid concentration used as liquid phase, some slurries were prepared at the experimental condition with sample mass of 1.0 g and sonication time of 20 min and acid concentration varied in the range of 0.1 to 2.5 mol L− 1. The results demonstrated that there is no difference in the analytical signals for both analytes in the studied concentration range. However, it was observed that slurries prepared with acid concentrations less than 2.0 mol L− 1 caused occasional blockage in the nebulization system of the spectrometer. Thus, hydrochloric acid at a 2.0 mol L− 1 concentration was established as the liquid phase for slurry preparation. The sonication time for the preparation of the slurries was studied in the range of 5 to 40 min. The results demonstrated that slurries prepared with sonication times high than 10 min provide better results for both the analytes. Thus, a sonication time of 20 min was established for the preparation of the slurries. The sample mass was studied in the range of 0.25 to 2.0 g. For magnesium, the analytical signal was linearly proportional in all studied range (r = 0.999). However, the results obtained for calcium showed that the slurries could be prepared without problems using a mass sample up to 1.75 g (r = 0.999). This way, a sample mass of 1.0 g was established for the preparation of the slurries. The experiments were carried out using secondary lines for both elements. 3.2. Analytical validation The calibration technique used in this method was established involving comparison of the slopes of the curves obtained by external calibration technique (with aqueous standards) and of the slopes of curves found by application of the analyte addition technique in a yogurt sample. The results showed good similarity among the slopes of the curves, all with correlation coefficients N 0.999, for both the analytes, as can be seen in Table 1. It demonstrates that calcium and magnesium in dairy product samples can be determined using the external calibration technique. The limits of detection (LOD) and quantification (LOQ), calculated with three and ten times the standard deviation of the blank divided by the slope of the analytical curve [22], were found to be 0.012 mg g− 1 and 0.038 mg g− 1 for calcium, respectively. For magnesium, it was found 0.005 mg g− 1 and 0.016 mg g− 1, respectively. The precision, expressed as relative standard deviation (RSD) was under repeatability conditions 2.8 (for calcium) and 2.7% (for magnesium) and 2.9 and 2.7% under reproducibility conditions, for calcium and magnesium, respectively. These results were found for a yogurt sample containing calcium and magnesium with concentrations of 1.40 and 0.13 mg g− 1, respectively and n = 10 for all the experiments. The accuracy of the method was confirmed by determination of calcium and magnesium in the certified reference material (CRM) Table 1 Equations of the analytical curves. System
Linear regression equations for calcium
Aqueous media
Abs = (0.00222 ± 0.00016)CCa + (0.00136 ± 0.00492) r = 0.9983 ± 0.0068 Abs = (0.00207 ± 0.00014)CCa + (0.15079 ± 0.00437) r = 0.9984 ± 0.0060
Yogurt
3. Results and discussion System
Linear regression equations for magnesium
3.1. Optimization of the experimental conditions
Aqueous media
The optimization of the experimental factors: concentration of hydrochloric acid, sonication time and sample mass was performed
Yogurt
Abs = (0.01363 ± 0.00079)CMg + (0.00104 ± 0.00238) r = 0.9990 ± 0.0033 Abs = (0.01328 ± 0.00078)CMg + (0.08967 ± 0.00236) r = 0.9989 ± 0.0033
G.C. Brandao et al. / Microchemical Journal 98 (2011) 231–233
References
Table 2 Determination of calcium and magnesium in dairy foods. Digestion
Slurry
Sample
Slurry
Yogurt
Calcium, mg g− 1
Digestion
Whole 1 Skimmed Whole 2
1.40 ± 0.08 1.34 ± 0.04 1.63 ± 0.12
Cow milk
Calcium, mg mL− 1
Magnesium, mg mL− 1
Whole Skimmed Skim
1.23 ± 0.01 1.18 ± 0.06 1.22 ± 0.04
0.12 ± 0.01 0.13 ± 0.01 0.12 ± 0.01
Milk powder
Calcium, mg g− 1
Whole 1 Whole 2 Whole 3
9.37 ± 0.70 8.92 ± 0.41 9.11 ± 0.50
Magnesium, mg g− 1 1.43 ± 0.07 1.27 ± 0.06 1.61 ± 0.02
1.15 ± 0.08 1.19 ± 0.07 1.18 ± 0.03
0.13 ± 0.01 0.14 ± 0.01 0.17 ± 0.01
0.14 ± 0.01 0.12 ± 0.01 0.15 ± 0.02
0.11 ± 0.01 0.11 ± 0.01 0.12 ± 0.01
Magnesium, mg g− 1 9.11 ± 0.57 8.84 ± 0.69 8.98 ± 0.43
0.86 ± 0.03 0.83 ± 0.04 0.86 ± 0.02
233
0.85 ± 0.06 0.83 ± 0.06 0.85 ± 0.05
NIST SRM 1549 of non-fat milk powder, which has certified values of 1.30 ± 0.05 and 0.120 ± 0.003 mg g− 1 for calcium and magnesium, respectively. Using the proposed method calcium and magnesium concentrations of 1.21 ± 0.05 and 0.119 ± 0.004 mg g− 1 were found. 3.3. Application The proposed method was applied for the determination of calcium and magnesium in milk, milk dessert and yogurt samples bought in supermarkets from Salvador City, Brazil. The samples were also analyzed after complete digestion using nitric and hydrogen peroxide and the elements were determined by HR-CS FAAS using secondary lines. All results expressed with interval confidence (at the 95%) are shown in Table 2. For all samples it is observed that calcium content is about ten higher than magnesium content. The results showed also that the calcium and magnesium concentrations for the milk powder samples are about eight higher than the concentrations obtained for the milk and yogurt samples. All these results are in agreement with data reported in the literature. Applying paired t-test at the 95% confidence level no statistical difference was observed among the values obtained by slurry method and those after complete digestion. 4. Conclusion The proposed method presented analytical features adequate for the sequential multi-element determination of calcium and magnesium in dairy product samples. The use of the secondary lines allowed the determination of these macronutrients avoiding a high dilution of the samples. Furthermore, the results showed that the method developed is feasible, fast and simple being suitable for routine analyses. Acknowledgements The authors are grateful to Conselho Nacional de Desenvolvimento Científico e Tecnalógico (CNPq), Fundação de Amparo a Pesquisa à Pesquisa do Estado da Bahia (FAPESB) and to Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for providing grants and fellowships and for financial support.
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