Liquid Chromatography-Mass Spectrometry Techniques

Three major difficulties have been generally met in directly combining LC with MS. The first concerns the ionization of nonvolatile and/or thermolabile analytes. The second is related to the mobile-phase incompatibility as result of the frequent use of nonvolatile mobile-phase buffers and additives in LC. The third is due to the apparent flow rate incompatibility as expressed in the need to introduce a mobile phase eluting from the column at a flow rate of 1 ml/min into the high vacuum of the MS. 

At present, the most powerful and promising interfaces for drug residue analysis are die particle-beam (PB) interface that provides online EI mass spectra, the thermospray (TSP) interface diat works well with substances of medium polarity, and more recently the atmospheric pressure ionization (API) interfaces that have opened up important application areas of LC to LC-MS for ionizable compounds. Among die API interfaces, ESP and ISP appear to be the most versatile since diey are suitable for substances ranging from polar to ionic and from low to high molecular mass. ISP, in particular, is compatible with the flow rates used with conventional LC columns (70). In addition, both ESP and ISP appear to be valuable in terms of analyte detectability. These interfaces can further be supplemented by preanalyzer collision-induced dissociation (CID) or tandem MS as realized with the use of triple quadrupole systems. Complementary to ESP and ISP interfaces with respect to the analyte polarity is APCI with a heated nebulizer interface. This is a powerful interface for both structural confirmation and quantitative analysis. 

The particle-beam interface is an analyte-enrichment interface in which the column effluent is pneumatically nebulized into a near atmospheric-pressure desolvation chamber connected to a momentum separator, where the high-mass analytes are preferentially directed to the MS ion source while the low-mass solvent molecules are efficiently pumped away (71, 72). With this interface, mobile phase flow rates within the range O.l-l.O ml/min can be applied (73). Since the mobile phase solvent is removed prior to introduction of the analyte molecules into the ion source, both EI and CI techniques can be used with this interface. 

PB-MS appears to have high potential as an identification method for residues of some antibiotics in foods, since it generates library-searchable EI spectra and CI solvent-independent spectra. Limitations of the PB-MS interface. 

LC-PB-MS has been investigated as a potential confirmatory method for the determination of malachite green in incurred catfish tissue (81) and of cephapirin, furosemide, and methylene blue in milk, kidney, and muscle tissue, respectively (82). Results showed that the mobile-phase composition, nebulization-de-solvation, and source temperature all play an important role in the sensitivity of the method. The sensitivity increases with decreasing heat capacity of the mobile phase in the order methanol acetonitrile isopropanol water and with decreasing flow rate. A comparison of the PB with the thermospray interface showed that less structural information was provided by the latter, whereas the sensitivity was generally lower with the thermospray interface. 

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Yergey, A.L., Edmonds, C.G., Lewis, I.A.S., and Vestal, M.L., Liquid Chromatography/Mass Spectrometry Techniques and Applications, Plenum Press, New York, 1990. 

Vestel, "Liquid Chromatography/Mass Spectrometry. Techniques and Applications", Plenum Press, New York, NY, 1989. 

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

P Arpino. Combined liquid-chromatography mass spectrometry. Techniques and mechanisms of thermospray. Mass Spec Rev 9 631, 1990. 

Mass spectrometry has been suggested for the identification of phenylthio-hydantoins produced during the classical and automated sequencing process and a combined gas-liquid chromatography-mass spectrometry technique for amino-acids from oligopeptide hydrolysates has been discussed. "... 

Recent Advances in Liquid Chromatography-Mass Spectrometry Techniques... 

Yergey AL, Edmonds CG, Lewis IAS and Vestal ML (1990) Liquid Chromatography/Mass Spectrometry, Techniques and Applications, pp 31-85. New York Plenum Press. 

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