Liquid-phase microextraction combined with liquid chromatography-mass spectrometry. Extraction from small volumes of biological samples

Liquid-phase microextraction (LPME) is a sample preparation technique based on disposable polypropylene hollow fibres, which results in efficient sample clean-up and high preconcentration. The present paper describes the combination of LPME with LC-MS utilising electrospray ionisation for high sensitivity. Nine antidepressant drugs were extracted from 50 or 500 muL of plasma or whole blood samples, through a thin layer of dodecyl acetate immobilised in the pores of the hollow fibre, and into 15 muL of 200 mM formic acid as acceptor solution inside the hollow fibre. Analyte recoveries in the range 12-68% and 9-52% were obtained from 50 muL of plasma and whole blood respectively. The acceptor solution (15 muL) was diluted with 60 muL of 5 mM ammonium formate pH = 2.7 prior to injection into the LC-MS system. The system was qualitatively investigated for matrix effects utilising a post-column infusion system. Whole blood from 5 different persons was cleaned-up by LPME and injected onto the analytical column while a solution of the 9 model compounds was continuously infused post-column. No signs of ion suppression were seen for any of the model compounds. Limits of quantification (S/N = 10) were in the low ng/mL range for 6 of the 9 model compounds utilising a whole blood sample volume of only 50 muL. The repeatability of the extractions was investigated utilising paroxetine as internal standard. Acceptable RSDs (%) were obtained (< 20%) for 5 of the antidepressants. By increasing the sample volume from 50 to 500 mu L of whole blood RSDs below 20% (3-16%) were observed for all 8 antidepressants.