Determination of copper in powdered chocolate samples by slurry-sampling flame atomic-absorption spectrometry

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Abstract
Chocolate is a complex sample with a high content of organic compounds and its analysis generally involves digestion procedures that might include the risk of losses and/or contamination. The determination of copper in chocolate is important because copper compounds are extensively used as fungicides in the farming of cocoa. In this paper, a slurry-sampling flame atomic-absorption spectrometric method is proposed for determination of copper in powdered chocolate samples. Optimization was carried out using univariate methodology involving the variables nature and concentration of the acid solution for slurry preparation, sonication time, and sample mass. The recommended conditions include a sample mass of 0.2 g, 2.0 mol L−1 hydrochloric acid solution, and a sonication time of 15 min. The calibration curve was prepared using aqueous copper standards in 2.0 mol L−1 hydrochloric acid. This method allowed determination of copper in chocolate with a detection limit of 0.4 μg g−1 and precision, expressed as relative standard deviation (RSD), of 2.5% (n=10) for a copper content of approximately 30 μg g−1, using a chocolate mass of 0.2 g. The accuracy was confirmed by analyzing the certified reference materials NIST SRM 1568a rice flour and NIES CRM 10-b rice flour. The proposed method was used for determination of copper in three powdered chocolate samples, the copper content of which varied between 26.6 and 31.5 μg g−1. The results showed no significant differences with those obtained after complete digestion, using a t-test for comparison.

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