Mass spectrometry interface with other techniques

It is a feature of current developments in mass spectrometry that this method of analysis can be coupled or interfaced with other techniques. The interfacing may in fact be with further MS equipment (MS/MS), or with various separation techniques such as high-performance liquid chromatography (LC/MS), gas chromatography (GC/MS), electrophoresis, particularly high-resolution capillary electrophoresis (CE/ MS), and biomolecular interaction analysis (BIA/MS) (Krone 1997). 

Frequently industrial hygiene analyses require the identification of unknown sample components. One of the most widely employed methods for this purpose is coupled gas chromatography/ mass spectrometry (GC/MS). With respect to interface with mass spectrometry, HPLC presently suffers a disadvantage in comparison to GC because instrumentation for routine application of HPLC/MS techniques is not available in many analytical chemistry laboratories (3). It is, however, anticipated that HPLC/MS systems will be more readily available in the future ( 5, 6, 1, 8). HPLC will then become an even more powerful analytical tool for use in occupational health chemistry. It is also important to note that conventional HPLC is presently adaptable to effective compound identification procedures other than direct mass spectrometry interface. These include relatively simple procedures for the recovery of sample components from column eluate as well as stop-flow techniques. Following recovery, a separated sample component may be subjected to, for example, direct probe mass spectrometry infra-red (IR), ultraviolet (UV), and visible spectrophotometry and fluorescence spectroscopy. The stopped flow technique may be used to obtain a fluorescence or a UV absorbance spectrum of a particular component as it elutes from the column. Such spectra can frequently be used to determine specific properties of the component for assistance in compound identification (9). 

Identification of the component peaks of a chromatogram, which may be numerous, can be achieved in two ways comparison of retention times (discussed below) trapping the eluted components for further analysis by other analytical techniques such as infrared and mass spectrometry or by direct interfacing of these techniques with a gas chromatograph. This latter approach is discussed on p. 114. 

The identification of GC peaks other than through retention data, which are sometimes ambiguous or inconclusive, can be facilitated by the direct interfacing of GC with infrared spectrometry (p. 378 et seq.) or mass spectrometry (p. 426 etseq.), so-called coupled or hyphenated techniques. The general instrumental arrangement is shown in Figure 4.29(a). 

In addition, the development of even softer (i.e. gentler) ionization techniques (beyond those already discussed) will improve the sensitivity of mass spectrometry as a tool for carotenoid identification and quantitation. Interfacing future ionization techniques with other MS tools may further expand opportunities. For example, a softer ionization technique that preserves carotenoid geometrical isomers in-source might allow isomers to be successfully separated via IMS and eliminate the need for prior separation by HPLC. 

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