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
