Sterol analysis in cancer cells using atmospheric pressure ionization techniques

The analysis of sterol biosynthesis poses analytical challenges such as vast concentration differences between precursor molecules and cholesterol as well as the substantial interferences between the sterols and the matrix, resulting in strong ion suppression. LC-MS and 2D-LC-MS may address some of these challenges, but for sensitive and robust detection, ionization plays a key role. Electrospray ionization (ESI) stands out for its capability to generate ions from polar and nonpolar compounds, rendering it suitable for a broad spectrum of analytes, including sterols. Atmospheric pressure chemical ionization (APCI) demonstrates efficacy for compounds exhibiting moderate to low polarity, thereby facilitating efficient ionization of sterols. Tube plasma ionization (TPI) is a relatively new technique that promises sensitive ionization across a wide polarity range, indicating its potential utility for sterol analysis. Here, a new tube plasma ion source for LC-MS was developed and compared with established ion sources (ESI and APCI) for sterol analysis. The TPI source showed comparable LOQs to APCI and clearly outperformed ESI in terms of sensitivity. Both TPI and APCI provided stable signals during extended measurement series, while ESI suffered from pronounced ion suppression. Application to human plasma, HepG2 cells, and liver tissue demonstrated that TPI provided results in close agreement with APCI, highlighting its robustness for sterol quantification in complex biological matrices.