orchratoxins, aflatoxin, and moniliformin (1). A multi-mycotoxin method for food and feed matrices based on liquid chromatography/electrospray ionization-tandem mass spectrometry (HPLC/ESI-MS/MS) covered the analysis of 186 fungal and bacterial metabolites. The method is based on a single extraction step using an acidified acetonitrile/ water mixture followed by analysis of the diluted crude extract (13). The development of LC/MS methods for mycotoxin determination is impeded to some extent by the chemical diversity of the analytes and compromises that have to be made on the conditions of sample preparation (1). Considering the wide range of polarities of the analytes the seemingly high selective MS/MS detection could lead incorrectly to the perception that matrix interferences could be eliminated effectively and quantitative results may be obtained without any clean-up and with very little chromatographic separation. Unfortunately, co-eluting matrix components influence the ionization efficiency of the analyte positively or negatively, impairing the repeatability and accuracy of the analytical method (1). As a consequence, only a few approaches describe the successful injection of crude extracts, and the majority of publications depict a sample clean-up prior to liquid chromatography with solid-phase extraction (SPE) as the most efficient pro-
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cedure, and in particular the use of Mycosep® columns proved straightforward and efficient (4,5,6,7,8,9).
Stable Isotope Dilution Assay In order to overcome matrix effects and related quantification problems, external matrix calibration for each commodity tested was recommended. This is extremely time-consuming and proved to be very impractical under routine conditions, where one is confronted with a variety of matrices every day. As an alternative approach, the use of [stable] isotope labelled internal standards has been introduced recently (10). These sub-
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