Coupling of API Sources to Mass Spectrometers

Coupling an API source (APCI, ESI or APPI) to a mass spectrometer, so as to maximize the analyte signal while disposing of the large amount of vaporized LC mobile phase, is an essential and important component of any API-MS system. The following discussion is based upon two excellent early reviews (Bruins 1991 Niessen 1995) 

Chemical species forming adduct Formula of adduct ion Cause of adduct formation POS and/or NEC m/z shift of adduct ion relative to [A-FH] (POS) or [A-H]-(NEG) 

Water [AiH-FHjO] Aqueous solvent or water as impurity POS/NEG 4-18 

Often the gas flow from the atmospheric pressure side through the nozzle is too large for the vacuum pumps of the mass analyzer. In such cases differential pumping (Section 6.6.2d) can be applied, such that the 

Hawkridge 2004) to provide order-of-magnitude increases in ion transmission especially when used in conjunction with a focusing potential applied to the device. However, this approach does not appear to have heen incorporated into analytical instrumentation as yet, for reasons that are not clear. Other approaches apphed on the atmospheric pressure side have involved electrostatic focusing (Shaffer 1997 Him 2000 Schneider 2002), hut none of these approaches have turned out to he suitably versatile and robust to dehver a reliable advantage in other than specialized instruments. 

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