Application of gas chromatography-triple quadrupole mass spectrometry to the determination of sterol components in biological samples in consideration of the ionization mode

The hyphenation of gas chromatography (GC) and triple quadrupole mass spectrometry is a promising approach to increase sensitivity and selectivity as compared to single quad mass spectrometry. We present in this paper the application of GC-triple quadrupole mass spectrometry for determination of sterol components in biological samples. Due to the fact that sterols are quite small molecules an appropriate ionization mode has to be found for advantageous exploitation of the triple quad function. Electron ionization (EI), positive and negative chemical ionization (PCI, NCI) have been tested regarding sensitivity improvement in oxysterol and bile acid analysis in plasma samples. Target analytes were 24-, 25- and 27-hydroxycholesterol, 7β-hydroxycholesterol, 7-ketocholesterol, 3β,5α,6β-cholestanetriol, cholic acid, chenodeoxycholic acid, deoxycholic acid and lithocholic acid. In contrast to bile acids, oxysterols could be analyzed with the highest degree of sensitivity by application of PCI in multiple reaction monitoring mode whereas 7β-hydroxycholesterol and 7-ketocholesterol showed even better results with NCI.