LC/MS involves the combination of a liquid chromatography separation column with a mass spectrometry detector. There are several types of ion generation and sample introduction systems in routine use.
In electrospray (ES), ions generated at atmospheric pressure are drawn into the MS using several atmospheric pressure inlets typically comprising one to three stages of pressure reduction. This allows the analysis of macromolecules in the vapor phase such as proteins and DNA as well as highly polar or labile small molecules. Electrospray yields very sensitive and quantitative results but knowledge of solution chemistry and chromatographic skills are required for optimal results.
Atmospheric Pressure Chemical Ionization (APCI) involves an aerosol generator, a desolvation process and an efficient atmospheric pressure ionization process. Ionization generally involves a protonation process involving water and thus APCI is very sensitive for compounds with proton affinities greater than water. Like in ES, ions generated at atmospheric pressure are drawn into the MS using several atmospheric pressure inlets typically comprising one to three stages of pressure reduction. APCI is complementary to electrospray and is an alternative ionization technique with atmospheric pressure systems. It is very sensitive, rugged and allows operation at conventional flow rates. This technique is most useful in the analysis of Phase I metabolites in drug discovery and clinical trials.
Particle Beam (PB)is a transport/enrichment interface for LC/MS designed to accommodate conventional flow rates of reversed phase HPLC into conventional mass analyzers. PB transports a high percentage of the low volatility sample into the MS while removing most of the solvent (enrichment) within the PB interface. The PBinterface is most useful because of its compatibility with conventional electron impact and chemical ionization processes, thus allowing the use of well established methods of tuning and calibration as well as comparison with commercially available EI spectral data bases (bases containing hundreds of thousands of compounds). In general, particle beam is limited to compounds less than 1000amu molecular weight.
PB is sensitive for many compounds at the low nanogram level in the scanning mode and in the picogram range with selected ion monitoring. This technique has particular use for the identification of unknown compounds in formulation, product, and process integrity applications.
Thermospray (TS)involves the spraying of heated LC effluent into a ca. 10 torr evacuated region and generating ions. These ions are then sampled into the mass analyzer through an aperture from this region. Three ionization modes exist for thermospray: filament off (ions generated by spraying), filament-on (ions generated from electron beam directed into region), and discharge (ions generated by high voltage discharge). This technique is both sensitive, versatile, and accommodates high flow rates. Thermospray can determine analytes with basic functional groups making it useful for determining many pharmaceutical chemicals.