Direct-EI may offer a clear advantage over ESI in several applications, but it may excel in the following cases:
*Large number of compounds of different polarities and chemical properties: EI can offer a shortcut, do-it-all solution when hard-to-detect substances are included or and when a combination of positive and negative ion detection runs are required for complete coverage of analyte detection.
*Characterization of unknowns: library matching offer an invaluable tool for compound identification.
*Detection  of non chromophoric compounds that also give poor or no signal with API: for these compounds additional HPLC detectors such as evaporative light scattering detector (ELSD), refractive index (RI) or corona discharge aerosol detector (CAD) are also available but each of them has limitations which restrain obtaining a universal detection with reasonable sensitivity. EI-MS would offer a suitable solution for this type of compounds, in terms of sensitivity and universal response. GC is anyway feasible only for compounds with high to medium volatility and therefore cannot be adopted for a full characterization of mixtures of complex nature. The possibility of hyphenating EI to HPLC separation represents an ideal solution.
*Quantitative analyses in presence of matrix effects: EI-MS offers a superior performance compared to ESI or APCI when intruding interferences from complex matrices pass cleanup procedure and cause signal suppression or enhancement.