LC/MS is a hyphenated technique, combining the separation power of HPLC, with the detection power of mass spectrometry. Even with a very sophisticated MS instrument, HPLC is still useful to remove the interferences from the sample that would impact the ionisation. Closely related to LC/MS are some other techniques, like flow injection/MS, CE or CEC/MS, capillary LC or nano LC/MS In all cases, there is the need for an interface that will eliminate the solvent and generate gas phase ions, then transferred to the optics of the mass spectrometer. Most instruments now atmospheric pressure ionisation (API) technique where solvent elimination and ionisation steps are combined in the source and take place at atmospheric pressure. When electron impact ionisation (EI) is the choice, the solvent elimination and ionisation steps are separate. The interface is a particle beam type, which separates the sample from the solvent, and allows the introduction of the sample in the form of dry particles into the high vacuum region. Electron impact is of interest for molecules which do not ionise with API technique, or when an electron impact spectrum is necessary, since it provides spectral information independent of the sample introduction technique (GC or LC, or direct introduction) and instrument supplier.