Reagent Inlet Systems

Practical Aspects of Trapped Ion Mass Spectrometry, Volume V 

As mentioned previously, gas-phase H/D exchange reactions are a form of ion/mole-cule reaction in which an analyte ion reacts with a neutral deuterated molecule within the confines of an ion-trap or ion-drift tube. The product of the reaction is a covalent exchange of hydrogen for deuterium in what amounts to a double replacement reaction that is expected to proceed through a stable ion-molecule complex. Like other organic reactions, the extent of gas-phase H/D exchange reactions depends on the number density of molecules of each reactant species present and the time during which they are allowed to react. 

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Example The vacuum system of non-benchtop mass spectrometers consists of one to three rotary vane pumps and two or three turbo pumps. Rotary vane pumps are used for the inlet system(s) and as backing pumps for the turbo pumps. One turbo pump is mounted to the ion source housing, another one or two are operated at the analyzer. Thereby, a differentially pumped system is provided where local changes in pressure, e.g., from reagent gas in Cl or collision gas in CID, do not have a noteworthy effect on the whole vacuum chamber. 

Bombick et al. [3] presented a simple, low cost method for producing thermal potassium metal ions for use as Cl reagents. All studies were performed on a commercial gas chromatography-mass spectrometiy (GC-MS) system. Thermionic emitters of a mixture of silica gel and potassium salts were mounted on a fabricated probe assembly and inserted into the Cl volume of the ion source through the direct insertion probe inlet. Since adduct ions (also referred to as cationized molecular ions or pseudomolecular ion ) of the type (M + K)+ have been observed, molecular weight information is easily obtained. The method is adaptable to any mass spectrometer with a Cl source and direct inlet probe (DIP). In addition, the technique is compatible with chromatographic inlet systems, i.e., GC-MS modes, which will provide additional dimensions of mass spectral information. 

Figure 5. Schematic of flow system for on-line mass spectrometric studies of F atom reactions. A, flow of F atoms in He carrier from discharge B, silica flow tube C, first pinhole (0.7 mm) separating flow tube from differentially-pumped chamber D, second pinhole (0.7 mm) separating differentially-pumped chamber from quadrupole mass spectrometer Q(E.A.I. 150/A) E, access to discharge tube F, furnace G, 50 mm gate valve H, connections to total pressure manometer IG, ion gage I, connection to cold traps and large pump (60 ls ) M, multiplier Pi, Pi, pumping lines for quadrupole chamber atm for differentially-pumped chamber R, reagent inlets.

The common features of all ion sources are that they incorporate techniques for producing ions and giving them kinetic energy (acceleration used for introduction into the spectrometer). The electron-impact source is the most widely used (Fig. 18). In this device, molecules in the gas phase, obtained from the inlet system, are subjected to a stream of accelerated electrons (usually at a potential of 70 V), and the resulting collisions between the particles cause molecular ionization and fragmentation. These species are then injected into the spectrometer by the accelerating slits. Another source is based on chemical ionization. Here, the analyte molecule is ionized by an ion-molecule reaction, not directly by electrons as mentioned above. A reagent gas is ionized by electrons, for example, methane. 

Figure 3.34 — Manifolds for implementation of a sensor containing a packed non-regenerable reagent and a regenerable fluorophore. (A) Flow-through sensor system 1 eluent vessel 2 pump 3 injection valve 4 TCPO reactor 5 CL cell 6 light-tight box with PMT 7 amplifier 8 recorder. (B) Design of the packed two-layer sensor 1 inlet capillary 2 inlet cap with frit 3 quartz tube 4 TCPO layer 5 frit 6 luminophore layer 7 outlet cap with frit 8 outlet capillary. (C) Manifold for implementation of the previous cell in biochemical applications (Reproduced from [240] and [241] with permission of the American Chemical Society and Elsevier Science Publishers, respectively).

The advent of atmospheric pressure chemical ionization (APCI) is a relatively recent development, in which the same processes occur as in CI, outlined previously, but at atmospheric pressure. By a very similar mechanism to CI, the reagent gas (water) becomes protonated and can act as an acid towards the analyte, leading to the addition of a proton. Once again the species formed in positive ion mode is [M + H]. In the case of negative ion mode, the reagent gas acts as a base towards the analyte, and deprotonation occurs leading to the formation of [M—H], Once ions have been formed, they are funnelled towards the analyser inlet of the MS instrument by the use of electric potentials. APCI is also employed in LC-MS systems (see Section 5.6). 

Two primary methods exist for the introduction and removal of reagents from reactors a flow-through system employing inlet and outlet valves or a liquid-handling robot with stand-alone reactors. Although a valving system always has a closed architecture, a robotic system may have an open or closed architecture. 

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