Xenobiotics recovery from plasma using solid phase extraction with C-18 sorbent - The reasons of literature discrepancies

Solid phase extraction (SPE) is one of the most popular methods of preparing plasma samples before determining the xenobiotics they contain. The present paper shows that the recovery degree of xenobiotics from plasma samples using SPE with C18 sorbent strongly depends on their storage time and temperature. While xenobiotics can be isolated and recovered in 100 % from fresh plasma samples under optimal conditions of the SPE procedure, their SPE recovery degree from stored plasma is lower. It diminishes with the time increase and temperature reduction of plasma storage. Moreover, a part of xenobiotic in stored plasma samples is not sorbed on SPE-C18 column at all and leaves it together with the waste during loading the column with the examined sample. According to the NMR data, the main reason of the presence of xenobiotics in the waste of the SPE column during its loading are structural-chemical changes occurring in plasma during its storage, leading to the formation of some complex(es) of hydrophilic character with xenobiotic. Another reason for lower SPE recovery degree of xenobiotic from stored plasma is the formation of plasma sediment, which binds/occludes xenobiotic. The presented results expand the current knowledge on why the SPE recovery degree of xenobiotics from plasma reported in the literature may differ. They are important for proper calibration of the chromatographic system in the analysis of xenobiotics in plasma and for achieving high accuracy of the analytical procedure involving SPE in xenobiotic estimation.