Electrospray ionisation quadrupole analyser

Undoubtedly, mass spectrometric detection has a substantial role to play in condensed-phase chromatographic analyses of toxic impurities. As in GC/MS, it can be highly sensitive, although this is probably more analyte-specific than in GC/MS. Selectivity can be gained by SIM on single quadrupoles or, if necessary, SRM on MS/MS instruments. What must be considered is the appropriate ionisation mode to be used in LC/MS. Most modern instruments use atmospheric pressure ionisation sources, including electrospray ionisation (ESI), atmospheric pressure chemical ionisation (APCI) and more recently atmospheric pressure photoionisation (APPI). 

Two techniques that have become preffered for ionisation of proteins/peptides is electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). Although different combinations of ionisation techniques and mass analyser exist, MALDI usually is coupled with a time-of-flight (TOF) (Figure 7) tube as a mass analyser while ESI is tradionally combined with quadrupole mass analysers. Instruments capable of MS/MS have the ability to select ions of particular m/z ratio from a mixture, to fragment selected ions and to record the precise masses of the resulting fragment ions. If this process is applied to the analysis of peptide ions, in principle the amino acid sequence of the peptide can be deduced. 

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. 

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