A reliable, accurate and reproducible method to quantify vitamin D3 (Vit. D3) in oily dietary supplements was developed after three Vit. D3 intoxications were diagnosed as reasonably resulting from a dietary administration of a cod liver oil based supplement. Liquid chromatography coupled to mass spectrometry operating in atmospheric pressure chemical ionization conditions (LC-APCI) and by using a deuterium labelled internal standard resulted to be an effective technique to reach the analytical aim. Due to the complexity of the oily matrix, the new analytical approach required a solid phase extraction step prior to analysis. The amount of Vit. D3 declared on the label of the cod liver oil based supplement for each soft capsule is 1.5μg. Consequently, the method was developed to quantify Vit. D3 amounts in the range 1-5μg/mL. To improve reliability of obtained data, both MS and MS/MS acquisition methods were employed. The method was evaluated by measuring the characteristic parameters such as linearity, precision, accuracy and robustness and cross checked against a certified pharmaceutical preparation. The LC-APCI-MS and MS/MS methods were applied in order to assess the Vit. D3 content in the dietary supplements taken by the intoxicated patients, found about three order of magnitude higher than that declared. The Vit. D3 content of other batches of the same commercial product was found as declared.
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