Analysis of Powdered Coffee and Milk by ICP OES after Sample Treatment with Tetramethylammonium Hydroxide
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A simple method for the solubilization of powdered milk and soluble coffee using tetramethylammonium hydroxide (TMAH) is proposed. While 500 µL of the reagent was used for the solubilization of 350 mg of coffee samples, milk samples required 1000 µL of TMAH. Solubilization at 80 °C takes place within 10, 20 and 30 min for soluble coffee, whole and skim powdered milk, respectively. Subsequently, the volume was topped up to 25 mL with deionized water. Thereafter, the concentrations of Ca, Cu, Fe, K, Mg, Mn, Na, P, Se, Sn and Zn were determined by inductively coupled plasma optical emission spectrometry (ICP OES). When applied to standard reference material and to commercial samples the proposed method showed good results, and it was also compared with two other preparation methods. Limits of detection calculated for Ca, Cu, Fe, K, Mg, Mn, Na, P, Se, Sn and Zn were 2.1, 0.065, 0.11, 103, 0.088, 0.011, 1.5, 2.1, 1.1, 0.66 and 0.11 µg g−1, respectively. The RSD values were less than 10% for the certified reference materials (IAEA A-11 and BCR 63R). Statistical tests showed that the results for the metal concentrations do not vary significantly with the different methods or with the certified reference values, considering a confidence limit of 95%.
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