Online analysis is when mobile phase is pumped, at low flow rates, through a small diameter emitter into a mass spectrometer. High voltage is applied to the mobile phase to generate an even spray into the mass spectrometer. The source of the mobile phase, most commonly a syringe pump or a liquid chromatograph (LC), delivers sample that has passed through chromatographic media.

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Offline analysis is when a discrete sample is loaded by syringe or pipette into an emitter that is then mounted into the mass spectrometer's source. To generate the sample flow offline techniques rely on the electrostatic attraction of the liquid inside the emitter, which has been charged by application of high voltage, towards the counter-electrode.