Ion sources gas discharge

The Duoplasmatron (Eig. 3.18). In the Duoplasmatron, gas-discharge ion sources are used for bombardment with oxygen or argon. In dynamic SIMS, especially, the use of O2 ions is common because of the chemical enhancement effect. With a duoplasmatron ion beam currents of several microamps can be generated. The diameter of the beam can be focused down to 0.5 pm. 

A more sophisticated, though less common version of PTR-MS is selected ion flow tube mass spectrometry (SIFT-MS). In this technique, a mixture of reagent ions is generated in a gas discharge ion source and then a Q mass filter is used to select one reagent ion, which is then injected into an inert carrier gas (usually He), for... 

Figure 11-39. General schematic of a gas-discharge ion source (1) cathode (2) anode (3) discharge chamber (4) electromagnetic coil (5) forming electrode ...

In a later study,an ion mobility spectrometer equipped with a corona discharge ion source was used with pyridine as an alternate reagent gas to enhance selectivity and sensitivity for the determination of ammoniacal nitrogen in river and tap water samples. The limit of detection was about 9.2 x 10 pg/mL, and the linear dynamic range was obtained from 0.03 to 2.00 pg/mL. The RSD was about 11%, and analytical results on actual environmental samples compared favorably with the Nessler method. 

Glow discharge ion source uses a gas discharge between two electrodes, so the sample must be conductive and formed into the cathode. 

Beyer, C., Feldmann, I., Gilmour, D., Hoffmann, V, Jakubowski, N. (2002) Development and analytical characterization of a Grimm-type glow discharge ion source operated with high gas flow rates and coupled to a mass spectrometer with high mass resolution. Spectrochimica Acta Part B Atomic Spectroscopy, 57,1521-1533. 

Figure 8 A SIFT apparatus configured for trace gas analysis, with the stable discharge ion-source for H3O+, NO+ and 0+ ions, and a single air/breath sample inlet port. 

As with O2 +, once NO+ is formed it is unable to undergo charge-exchange reactions with other constituents in the source gas mixture because of the low ionization energy of NO. Consequently, NO+ is another well-established contaminant ion from discharge ion sources. 

The various stages of this process depend critically on the type of gas, its pressure, and the configuration of the electrodes. (Their distance apart and their shapes control the size and shape of the applied electric field.) By controlling the various parameters, the discharge can be made to operate as a corona, a plasma, or an arc at atmospheric pressure. All three discharges can be used as ion sources in mass spectrometry. 

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