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. 2011 Feb;32(1):5–31.

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Figure 1. — Principal components of a tandem mass spectrometer. (A) The sample is ionised in the source, passes into the 1^st mass filter (Q1), then into the collision cell (Q2), followed by the 2^nd mass filter (Q3), and finally the detector. (B) and (C) depict schematically the two principal types of ionisation-sources that are in use in current clinical LC-MS/MS instruments, electrospray ionisation (ESI, B) and atmospheric pressure chemical ionisation (APCI, C). In ESI, the solvent-analyte flow from the LC passes into the source through a positively charged, very narrow capillary, and gets nebulised as microscopic, positively charged solvent-analyte droplets. These droplets fly towards the negatively-charged faceplate, with solvent evaporating on the way, until they disintegrate in a Coulomb explosion, when the repulsive charge of their ionised components exceeds their surface tension. The individual ionised analyte molecules then pass through the faceplate entry hole into the mass spectrometer. In APCI, the solvent-analyte stream from the LC is vaporised by heated nebuliser gas and the polar components of the solvent(s) vapour are ionised by a high-current discharge of a Corona needle. The solvent molecules subsequently transfer their charge to ionisable analyte molecules, which pass through the faceplate entry hole into the mass spectrometer.

Figure 1.