Atmospheric pressure ionisation-mass detection modes

Owing to the anionic character of LAS, an electrospray ionisation (ESI) interface operated in negative ion mode is particularly attractive for the mass spectrometric detection of this surfactant type. Consequently, a great part of the atmospheric pressure ionisation-mass spectrometry (API-MS) work on LAS is devoted to the application of (— )-ESI-MS. 

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). 

Lacorte and Barcelo [36] have described a procedure for the determination of nanogram per litre levels of organophosphorus pesticides in ground waters based on automated online liquid-solid extraction followed by liquid chromatography. The detection used on the liquid chromatograph is an atmospheric pressure chemical ionisation mass spectrometer using negative and positive ion modes of operation. 

Examination of the diethyl ether extract by liquid chromatography (LC) using a Cjg reverse phase gradient elution separation with atmospheric pressure chemical ionisation (APCI) in the positive mode gave the total ion current trace shown in Figure 8.3. The last three peaks showed ions of masses 538, 566 and 594 and are believed to relate to the presence of tellurium dithiocarbamates in the extract (ions +2H from ionised protonic solvent). These species were not detected by GC-MS. 

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