Mass spectrometers vacuum system

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. 

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.)...

In early GC with packed columns several tens of milliliters of carrier gas per minute were eluted. Thus, most of the flow had to be separated before entering the ion source to prevent the mass spectrometer vacuum system from breakdown [4,33,38]. Flow reduction was either effected by a simple split to divide the effluent in the inlet system by a factor of about 1 100 or by means of a more elaborate separator, the jet separator being the best-known of those [57,58]. The advantage of separators over a simple split is the enrichment of the analyte relative to the carrier gas in case of the jet separator this effect is based on a principle related to that of the nozzle-skimmer system in ESI (Chap. 12.2.1). 

A series of highly complex interfaces were described between 1978 and 1980 by Blakley et al. [62-64] in an attempt to develop a system capable of the introduction of up to 1 ml/min aqueous mobile phase into an El mass spectrometer. The systems contained extensive multistage vacuum systems and ingenious heating devices, e.g., laser evaporation or hydrogen-flame heaters, to achieve rapid solvent evaporation. These systems subsequently developed towards the TSP interface (Ch. 4.7). 

In preparing samples for GC/MS, one simple fact must be kept in mind, namely, that everything injected onto the gas chromatographic column will be deposited into the mass spectrometer with the exception of those sample components which remain in the injection port or on the column. For volatile components this is not a concern as they are pumped away by the spectrometer vacuum system without consequence, but semivolatile materials may deposit in the ion source of the spectrometer with resultant loss of sensitivity, increased maintenance, and other unfavorable results. It is not nncommon for normal column bleed to eventually degrade system performance. For particularly valuable samples, such as metabolite extracts, biological samples, or other samples obtained through extensive effort, the contamination threat must be tolerated as the cost of analysis. However, if sample cleanup is possible without significant sample alteration, then a reasonable effort should be made to prevent contamination of the spectrometer. 

Vacuum system. Components associated with lowering the pressure within a mass spectrometer. A vacuum system includes not only the various pumping components but also valves, gauges, and associated electronic or other control devices the chamber in which ions are formed and detected and the vacuum envelope. 

Direct-inlet probe. A shaft or tube having a sample holder at one end that is inserted into the vacuum system of a mass spectrometer through a vacuum lock to place the sample near to, at the entrance of, or within the ion source. The sample is vaporized by heat from the ion source, by heat applied from an external source, or by exposure to ion or atom bombardment. Direct-inlet probe, direct-introduction probe, and direct-insertion probe are synonymous terms. The use of DIP as an abbreviation for these terms is not recommended. 

Moving-belt (ribbon or wire) interface. An interface that continuously applies all, or a part of, the effluent from a liquid chromatograph to a belt (ribbon or wire) that passes through two or more orifices, with differential pumping into the mass spectrometer s vacuum system. Heat is applied to remove the solvent and to evaporate the solute into the ion source. 

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

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