Towards a universal detector for small molecule application: Direct-EI in LC-MS.

This article describes the operating principles of the direct-electron ionization (EI) interface, which is becoming more popular in many LC-MS applications. Matrix effects and the role of direct-EI as a universal detector for small molecule analysis are also discussed in detail. The advantages and drawbacks of this approach are described and a comparison with atmospheric pressure ionization (API) interfaces is made. The potential of direct-EI is illustrated with a selection of practical applications

Profiling of non-esterified fatty acids in human plasma using liquid chromatography-electron ionization mass spectrometry

This paper focuses on the development of a novel approach to analyze underivatized fatty acids in human plasma. The method is based on liquid–liquid extraction followed by reversed phase liquid chromatography coupled to direct-electron ionization mass spectrometry (LC-Direct-EI-MS). The assay is validated. Calibrations show satisfactory linearity and precision in the investigated range of linearity. Recoveries span from 75% to 104%. The method limits of detection, varying from 0.53 to 5.35 μM, are satisfactory for the quantitation of non-esterified fatty acids (NEFAs) in plasma at physiological levels. The method has been successfully applied to the NEFAs profiling of plasma samples from healthy adult volunteers and subjects affected by diabetes mellitus. Compared with published protocols based on gas chromatography–mass spectrometry and liquid chromatography coupled to electrospray ionization mass spectrometry, this method does not require derivatization and does not show matrix effects, thus simplifying sample preparation procedure and reducing the total time of analysis to approximately 90 min. In addition, Direct-EI-MS allows the acquisition of highquality NIST library-matchable EI spectra, allowing an easy-to-obtain identification of the target NEFAs