Formation of the N-methylpyridinium ether derivative of propofol to improve sensitivity, specificity and reproducibility of its detection in blood by liquid chromatography–mass spectrometry

In spite of comparatively high therapeutic concentrations in blood, the quantitation of propofol may cause analytical challenges due to its pharmacokinetic (e.g. short half life, high distribution volume) and physicochemical (significant volatility) particularities. Moreover, a considerable and concentration dependent protein binding and a substance association to erythrocytes result in an irreproducible distribution between whole blood and serum. A possible redistribution in stored samples needs to be compensated during sample preparation or included into the result interpretation. The new analytical approach is based on a formation of N-methylpyridinium derivatives of propofol and corresponding internal standards and permits a significant (∼300 fold) increase of detection limits in LC–MS/MS. Derivatization is achieved by a direct conversion of the acetonitrile supernatant of a protein precipitation with 2-fluoro-1-methyl-pyridinium-p-toluene-sulfonate using triethylamine as catalyst. The derivative exhibits a high solvent stability and provides – in contrast to the unchanged parent compound – a sufficient number of diagnostic qualifier fragments to fulfil common identification criteria. By using 2-tert-butyl-6-methylphenol as internal standard (instead of the commonly applied thymol), a better compensation of matrix effects could be achieved. Owing to its high robustness, appropriate quantitation limits (LLOQ ∼ 13 ng/mL), minimum sample amount and preparation effort, the assay could efficiently be applied for quantitative propofol analyses in pharmacokinetic studies with high sampling rates.