Atmospheric Pressure Photoionisation APPI

This is a relatively recent addition to the ionisation techniques used with hyphenated liquid chromatography. In this type of detector, a vaporiser converts the eluent (from the LC) into the gas phase, much like with APCI. The difference with this technique is that instead of the production of electrons from a corona, here we have a discharge lamp producing photons (known as vacuum ultraviolet photons) in a narrow range of ionisation energies. 

The analyser is the part of the mass spectrometer that filters the fragments and ions produced in the ion source before they are detected and amplified and the output viewed. Again, a number of different types of analysers are 

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Fast atom bombardment (FAB) Plasma desorption (PD) Liquid secondary-ion mass spectrometry (LSIMS) Thermospray (TSP)/plasmaspray (PSP) Electrohydrodynamic ionisation (EHI) Multiphoton ionisation (MPI) Atmospheric pressure chemical ionisation (APCI) Electrospray ionisation (ESI) Ion spray (ISP) Matrix-assisted laser desorption/ionisation (MALDI) Atmospheric pressure photoionisation (APPI) Triple quadrupole (QQQ) Four sector (EBEB) Hybrid (EBQQ) Hybrid (EB-ToF, Q-ToF) Tandem ToF-ToF Photomultiplier... 

ESI operating in the negative ion (NI) mode has been the interface most widely used for the analysis of anionic perfluorinated surfactants. In addition, ESI has also been optimised for the determination of neutral compounds such as the sulphonamides PFOSA, Et-PFOSA and t-Bu-PFOS. The use of atmospheric pressure photoionisation (APPI) has been explored in few works [78-80]. Takino et al. [78] found as the main advantage of this technology, the absence of matrix effects, but the limits of detection were considerably higher than those obtained by LC-ESI-MS/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). 

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