Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in Milk by Inductively. Coupled Plasma-Optical .... (Millipore Corp., Bedford, MA), specific resistance,. 18 MW/cm.
KIRA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 87, NO. 1, 2004 151 RESIDUES AND TRACE ELEMENTS
Comparison of Partial Digestion Procedures for Determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in Milk by Inductively Coupled Plasma-Optical Emission Spectrometry CARMEN S. KIRA and FRANCA D. MAIO Instituto Adolfo Lutz, Divisno de Bromatologia e Química, Av. Dr. Arnaldo 355, CEP 01246-902, Sno Paulo, SP, Brazil VERA A. MAIHARA Laboratório de Análise por Ativaçno – IPEN/CNEN-SP, CEP 05508-000, São Paulo, SP, Brazil
A fast procedure was developed for determination of Ca, Cr, Cu, Fe, K, Mg, Mn, Na, P, and Zn in milk samples. This procedure consisted of a partial digestion with hydrochloric acid on a hot plate. The results obtained were compared with 3 other digestion procedures (dry ashing and 2 microwave digestions). All the procedures showed similar precision levels, with coefficients of variation 0.05
+
0.691
>0.05
+
4.118
>0.05
+
Cr
0.868
>0.05
+
0.425
>0.05
+
0.291
>0.05
+
3.443
>0.05
+
Cu
0.501
>0.05
+
4.394
0.05
+
1.698
>0.05
+
Fe
2.487
>0.05
+
0.339
>0.05
+
0.625
>0.05
+
2.708
>0.05
+
K
0.653
>0.05
+
2.144
>0.05
+
0.532
>0.05
+
1.258
>0.05
+
Mg
0.519
>0.05
+
1.542
>0.05
+
2.056
>0.05
+
4.124
>0.05
+
Mn
0.252
>0.05
+
0.822
>0.05
+
0.162
>0.05
+
2.912
>0.05
+
Na
0.141
>0.05
+
2.662
>0.05
+
1.025
>0.05
+
0.256
>0.05
+
P
1.621
>0.05
+
0.144
>0.05
+
0.526
>0.05
+
2.397
>0.05
+
Zn
0.043
>0.05
+
0.750
>0.05
+
0.342
>0.05
+
1.371
>0.05
+
a
A: Dry ashing; B: wet digestion on a hot plate; C: wet digestion in a microwave oven for whole milk samples; D: wet digestion in a microwave oven for nonfat milk samples; + = mean concentrations are not considered significantly different at the significance level of 95%; – = mean concentrations are considered significantly different at the significance level of 95%.
156
KIRA ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 87, NO. 1, 2004
Table 7. Limits of detection (LOD) and quantitation (LOQ) in mg/L Element
l (nm)
LOD (mg/L)
LOQ (mg/L)
Ca
422.673
0.11
0.36
Cr
267.716
0.002
0.007
Cu
327.396
0.006
0.02
Fe
259.940
0.007
0.02
K
766.491
0.21
0.70
Mg
280.270
0.03
0.08
Mn
257.610
0.0004
0.001
Na
589.592
0.18
0.61
P
213.618
0.13
0.42
Zn
213.856
0.005
0.02
Table 6 shows good agreement among the different digestion procedures applied for the milk powder samples. The results for whole milk samples digested either by dry ashing or wet digestions on a hot plate or in a microwave oven were not significantly different (p > 0.05), except for copper, because the standard deviation obtained for the sample digested in a microwave oven was smaller than the others. The results obtained by the different sample treatment procedures for nonfat milk samples were not significantly different, indicating that the concentrations for all elements analyzed yielded basically the same results. The limits of detection and quantitation (LOD and LOQ) of the elements were determined according to AOAC procedure (20) and are presented in Table 7. Conclusions Reliable results can be obtained by wet digestions with HCl on a hot plate as well as in a microwave oven. Both digestion procedures have qualities that make them suitable for ICP-OES determination of a wide range of inorganic elements in milk samples, but wet digestion on a hot plate has the advantage of low-cost equipment and little maintenance. In wet digestion on a hot plate, multiple samples can be prepared concurrently, whereas in wet digestion in a microwave oven, as performed in our laboratory, only 2 samples can be prepared simultaneously. In this study, up to 16 samples could be digested at the same time when the hot plate was used and, depending on the size of the hot plate, even more samples may be digested. In sample digestions performed in a microwave oven, throughput is limited by the number of digestion vessels
that can be processed simultaneously, the digestion time due to cooling time, and the small amounts of sample that can be used in this system. The advantage of wet digestion on a hot plate is related to the use of a common laboratory reagent, such as hydrochloric acid. Therefore the procedure proposed, with ICP-OES capabilities, can be successfully used for inorganic element determinations in milk samples. References (1) Williams, S.R. (1997) Fundamentos de Nutriçno e Dietoterapia, Artes Médicas, Porto Alegre, RS, Brazil, pp 144–169 (2) Neville, M.C., Zhang, P., & Allen, J.C. (1995) in Handbook of Milk Composition, R.G. Jensen (Ed.), Academic Press, New York, NY, pp 577–661 (3) Krushevska, A., Barnes, R.M., & Amarasiriwardena, C. (1993) Analyst 118, 1175–1181 (4) Reid, H.J., Greenfield, S., & Edmonds, T.E. (1995) Analyst 120, 1543–1548 (5) Amarasiriwardena, D., Kotrebai, M., Krushevska, A., & Barnes, R.M. (1997) Can. J. Anal. Sci. Spectrosc. 42, 69–78 (6) Nóbrega, J.A., Gélinas, Y., Krushevska, A., & Barnes, R.M. (1997) J. Anal. At. Spectrom. 12, 1243–1246 (7) Borkowska-Burnecka, J., Szmigiel, E., & Zyrnicki, W. (1996) Chem. Anal. (Warsaw) 41, 625–632 (8) Karadjova, I., Girousi, S., Iliadou, E., & Stratis, I. (2000) Mikrochim. Acta 134, 185–191 (9) Sun, D.H., Waters, J.K., & Mawhinney, T.P. (2000) J. AOAC Int. 83, 1218–1224 (10) De La Fuente, M.A., & Juárez, M. (1995) Analyst 120, 107–111 (11) Tahán, J.E., Sánchez, J.M., Granadillo, V.A., Cubillán, H.S., & Romero, R.A. (1995) J. Agric. Food Chem. 43, 910–915 (12) Burguera, M., & Burguera, J.L. (1998) Anal. Chim. Acta 366, 63–80 (13) Jorhem, L. (1995) Mikrochim. Acta 119, 211–218 (14) Amaro-López, M.A., Moreno-Rojas, R., Sánchez-Segarra, P.J., & Zurera-Cosano, G. (1996) Alimentaria 271, 71–78 (15) Novozamsky, I., Lee, V.H., & Houba, V.J.G. (1995) Mikrochim. Acta 119, 183–189 (16) Hoening, M., & Kersabiec, A-M. (1996) Spectrochim. Acta Part B 51, 1297–1307 (17) Subramanian, K. (1996) Spectrochim. Acta Part B 51, 291–319 (18) Bode, P. (1997) J. Radioanal. Nucl. Chem. 215, 87–94 (19) De La Fuente, M.A., Carazo, B., & Juárez, M. (1997) J. Dairy Sci. 80, 806–811 (20) International Accreditation Criteria for Laboratories Performing Food Chemistry Testing (1999) AOAC INTERNATIONAL, Gaithersburg, MD, pp 1–33
