ESI - Source and Interface

Electrospray ionisation is achieved at atmospheric pressure, the mass analyser, however, operates under high vacuum. A special interface is therefore necessary to transfer the ions from the ionisation chamber into the mass spectrometer. A schematic of such an interface is shown in Fig. 4.14. Usually a zone of intermediate pressure separates the ionisation chamber and the mass analyser. The liquid sample together with a curtain or nebulising gas is introduced into the heated ionisation chamber. An electrospray is generated by applying a potential difference between the needle and the opposite interface plate. A small proportion of the desolvated analyte ions exit the ionisation chamber through a submillimeter orifice and enter the zone of intermediate pressure. The analyte ions then pass via another small orifice into the mass analyser. This is usually a quadrupole which is operated under high vacuum. 

A characteristic feature of ESI is that the sample can be pumped into the mass analyser continuously. MALDI, on the other hand, is a pulsed method which requires a dry sample. Thus, ESI-MS can be coupled directly to liquid separation methods such as RP-HPLC (section 2.3.1) and CE (section 3.3). As the sample emerges from the separation column it is directly pumped into the electrospray chamber. As outlined earlier, MALDI-TOE is capable of separating 

Depending on the amount of sample available, different rates are used. A low flow rate allows for long measurement times to optimise instrument parameters. With the pneumatically assisted electrosprays as shown in Fig. 4.13, 

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