Liquid chromatography–tandem mass spectrometry determination of loperamide and its main metabolite desmethylloperamide in biological specimens and application to forensic cases

A liquid chromatographic mass spectrometric (LC/MS/MS) method has been developed for the determination of loperamide in whole blood and other biological specimens. The procedure involves liquid–liquid extraction of loperamide, desmethylloperamide and methadone-D3 (internal standard) with butyl acetate. Confirmation and quantification was done by positive electrospray ionisation with a triple quadrupole mass spectrometer operating in multiple reaction-monitoring (MRM) mode. Two MRM transitions of each compound were established and identification criteria were set up based on the ratio of the responses between the two MRM transitions of each compound. The standard curves were linear over a working range of 0.1–500 μg/kg for all transitions. The limit of quantification was 0.1 μg/kg in whole blood. The repeatability and reproducibility within the laboratory expressed by relative standard deviation were less than 5 and 11%, respectively, and the accuracy was better than 9%. The method was developed to examine a feces sample from a child whose mother was suspected of Münchausen syndrome by proxy and it proved to be suitable for forensic cases being simple, selective and reproducible. The method was also applied for a case investigation involving a overdose of loperamide.