Flexible Microtube Plasma for the Consecutive-Ionization of Cholesterol in Nano-Electrospray Mass Spectrometry.

Electrospray ionization mass spectrometry (ESI-MS) is an established method for the identification of biomarkers. By nano-ESI (nESI), the polar molecular fraction of complex biological samples can be successfully ionized. In contrast, the less-polar free cholesterol, which serves as an important biomarker for several human diseases, is barely accessible by nESI. Although, complex scan functions of modern high-resolution MS devices are able to increase the signal-to-noise ratio, they are limited by the ionization efficiency of the nESI. One possible method to increase the ionization efficiency is the derivatization with acetyl chloride, however interferences with cholesteryl esters must be considered, so chromatographic separation or complex scan functions may be required. A novel approach to increase the yield of cholesterol ions of the nESI could be the application of a second consecutive-ionization process. This publication presents the flexible microtube plasma (FμTP) as a consecutive-ionization source, which allows the determination of cholesterol in nESI-MS analysis. Focusing on the analytical performance, the nESI-FμTP approach increases the cholesterol signal yield in a complex liver extract by a factor of 49. The repeatability and long-term stability could be successfully evaluated. A linear dynamic range of 1.7 orders of magnitude, a minimum detectability of 5.46 mg/L, and a high accuracy (deviation, -8.1%) demonstrates the nESI-FμTP-MS as an excellent approach for a derivatization-free determination of cholesterol.