Mass spectrometer vacuum

Vacuum systems are integral parts of any mass spectrometer, but vacuum technology definitely is a field of its own. [251-255] Thus, the discussion of mass spectrometer vacuum systems will be restricted to the very basics. 

Microprocessor controlled quadrupole mass spectrometer Vacuum, 32,1982,163-168... 

Table 1 lists a number of ionization sources which produce ions at either atmospheric pressure or under vacuum conditions. For atmospheric pressure ionization sources a suitable interface is required which allows a controlled leak of ions into the vacuum region of the mass spectrometer. Vacuum ionization techniques likewise require a controlled leak, or mechanical introduction, of neutral molecules into the vacuum chamber, followed by ionization. 

Fig. 2.1 Mass spectrometer vacuum system. The atomizer and furnace support are shown in the loading position. Movement of a furnace along a rail is produced by means of a guide pin. (Reproduced from [11], with permission.)...

GC/MS with capillary columns has been the gold standard for more than 20 years, but LC/MS has become a complementary method due to the success in interface development with atmospheric pressure ionisation (API) for low molecular weight compounds and the appHcation to biopolymers. For many areas of analytical chemistry, LC/MS has become indispensible due to its advantages over GC/MS for polar and thermolabile analytes. A Hmiting factor for LC/MS has been the incompatibility between the hquid eluting from the LC and the mass spectrometer vacuum. This could be overcome in electrospray ionisation with the use of a nebuliser gas ( ion spray ) or additional heated drying gas ( turbo ion spray ) (70, 71]. Due to its high sensitivity and selectivity, APl-MS has become a standard tool for the stracture elucidation of analytes from complex mixtures. 

Atmospheric pressure ionization (API) techniques encompass a range of techniques in which ionization occurs external to the mass spectrometer vacuum. Ionization can be achieved by a variety of methods, including photoionization, corona discharge at the tip of a needle, or by the use of radioisotopes such as Ni. 

Thus far the only difference between the UV and El detectors is that the observed signal for the former is a measure of transmitted intensity I, while that for the latter is a measure of the absorbed intensity — but it turns out that is of little consequence for the present purpose. Thus, for example, a fluorescence detector records a signal that is a measure of the absorbed intensity of the exciting radiation like the El case, but it is also a concentration dependent detector like the simple UV absorption detector. The difference between the two types arises rather in the relationship between the analyte concentration delivered by the mobile phase (c ) and that within the absorption cell or El source in the former case c = c (see equation [4.2]), but the situation is very different in the El case where the mass spectrometer vacuum pumps continuously remove the analyte from the El source. In fact c ei represents an instantaneous steady state value, a compromise between the flow rate of A into the source and the pumping rate out of the source here instantaneous means simply that the establishment of the steady state value c gi occurs on a timescale appreciably shorter than that of the chromatographic peak. Then at this steady state ... 

Although the majority of MALDI-MS experiments have followed the originators (Tanaka 1988 Karas 1987, 1988) by inserting the matrix-analyte mixture into the high vacuum chamber of the mass spectrometer before irradiation, development of MALDI sources that operate at atmospheric pressure has been described (Laiko 2000 Moyer 2002). Such methods obviously require efficient interfaces between the atmospheric pressure ion source and the mass spectrometer vacuum as for the more usual API sources (Section 5.3.3). Currently this approach does not appear to have been exploited for trace quantitative analyses of small molecules. 

Figure 5.15 (a) Sketch of a gas curtain interface for API-MS coupling (Buckley 1974, 1974a French 1977). The ultra-dry gas (N2) curtain separates the ionization chamber (atmospheric pressure) from the orifice leading to the skimmer cone and thence to the mass spectrometer vacuum, (b) Sketch of an API-MS interface based on a heated glass capillary that connects the atmospheric pressure source to the low vacuum region preceding the sampling cone (Figure 5.17). In both cases an electric field E helps direct the ions into the sampling orifice. Reproduced from Bruins, Mass Spectrom. Revs. 10, 53 (1991), with permission of John Wiley Sons, Ltd.

Figure 5.17 Sketch of a heated pneumatic nebuUzer interface for APCI-MS. The LC effluent is nebulized by a fast gas stream and subjected to rapid heating via a heated make-up gas (typically N2 at 120-200 °C, although the heater temperature can be much higher). The vaporized effluent is then introduced into the APCI plasma activated by the corona discharge, all at atmospheric pressure. The gaseous solvated ions are then introduced into the mass spectrometer vacuum via an API interface that must discriminate in favor of ions vs neutrals and also de-solvate the ions (see text). Reproduced from MDS-Sciex literature (The API Book) with permission.

As a meaningful example, consider a vacuum pump with a pumping speed Sp ,, measured at its inlet (Figure 6.36). Then the pumping speed 8, 5, that can be obtained at a mass spectrometer vacuum housing that is... 

Tandem Mass Spectrometry (MS/MS) Random cleavage of a peptide, similar to that from partial hydrolysis with acid, can also be accomplished with mass spectrometry. An intact protein introduced into a mass spectrometer can be cleaved into smaller fragments by collision with gas molecules deliberately leaked into the mass spectrometer vacuum chamber (a technique called collision-induced dissociation, CID). These peptide fragments can be individually selected for analysis using a technique called tandem mass spectrometry (MS/MS). The mass spectra of these random fragments can be compared with mass spectra databases to determine the protein sequence. 

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