Formation of the N-methylpyridinium derivative to improve the detection of buprenorphine by liquid chromatography-mass spectrometry

The legally defensible identification of the narcotic, analgesic buprenorphine, in biological specimen requires considerable sensitivity due to its low therapeutic dosages and corresponding target concentrations. Application of liquid chromatography-electrospray ionisation-mass spectrometry, which became the default method for buprenorphine detection, is impeded by the disadvantageous fragmentation of the stable precursor ion producing unspecific product ions of comparatively low abundance. A chemical modification to form the N-methylpyridinium ether derivative of buprenorphine is presented to improve the selectivity and sensitivity of its detection by liquid chromatography-mass spectrometry (LC-MS). The reaction of buprenorphine with 2-fluoro-1-methyl-pyridinium-p-toluene-sulfonate and triethylamine as catalyst was accomplished in acetonitrile at an ambient temperature yielding a chemically stable derivative. Fragmentation of the permanently charged precursor ion (m/z = 559) leads to the formation of diagnostic and abundant fragments (e.g. m/z = 443 and 450) representing all parts of the molecule. The application of the technique to the identification of buprenorphine in hair samples demonstrates a high specificity, availability of sufficient qualifier ions and a significant (approximately 8-fold) improvement of detection limits with respect to comparable experiments based on underivatised buprenorphine.