A false positive case due to matrix interference in the analysis of ronidazole residues in muscle tissue using LC–MS/MS

In contrast with the information of the inspection body concerning the use of ronidazole, several non compliant muscle samples were detected using a liquid chromatography–tandem mass spectrometry (LC–MS/MS) method in accordance with confirmation criteria of Decision 2002/657/EC. This led to the suspicion that non compliance could be due to false positive results. In this context, a liquid chromatography–high resolution mass spectrometry (LC–HRMS) method was developed and sample extracts were re-analyzed, resolving the co eluting isobaric interfering peak, which also has an interfering product ion with the transition product (m/z 201 > 140).