Ion sources hollow cathode discharge

The great maj ority of PTR-MS experiments carried out to date have employed a DC hollow cathode electrical discharge as the means of generating ions. Before considering the details of this ion source as it is applied to PTR-MS, some background information on electrical discharges and the hollow cathode effect will be given. 

It is not immediately obvious that an electrical discharge in water vapour will lead to overwhelming production of H3O +. After all, electron impact ionization of H2O can yield H2O+ or it can produce fragment ions such as H +, H2 +, OH+ and 0+ (H2+ is a very minor product but we include its mention here and below for completeness) [9]. To make H3O+ with a relatively high purity, some secondary chemistry is needed. H2O+ is readily converted into H3O+ by the fast reaction 

However, what about the fragment ions in the discharge It turns out that these too can be converted to H3O+, either through a fast, direct ion-molecule reaction or through formation of H2O+ and subsequent reaction via Reaction 3.6 [11], The chemistry is summarized below and rate coefficients (at 300 K) are taken from the compilation by Ikezoe and co-workers [10]  

The large rate coefficients shown above are all indicative of ion-molecule reactions that occur at essentially a collision-limiting rate. To allow these reactions to fully take place, so that a high yield of H3O+ is attained, a so-called source drift region may be added between the HCD and the drift tube. 

As mentioned earlier, ingress of some air from the analyte gas into the discharge region of a PTR-MS instrument is inevitable. This can generate other types of ions such as N2 + and O2 +. Fortunately, N2 + can be readily removed by a charge exchange reaction with H2O [12], 

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In a later study investigating biomass emissions from controlled laboratory fires it was again concluded that PTR-MS measurements underestimated the concentration of HCN, and in this study by roughly a factor of 5 in comparison to values determined by FTIR spectroscopy [17]. From the collected field data, it was found that the HCHO ratios between PTR-MS and FTIR measurements varied significantly with ambient atmospheric humidity, ranging from 0.2 at the lowest humidity to 0.05 at the highest humidity. It was concluded that the addition of water from the hollow cathode discharge ion source into the drift tube needs to be taken into account in addition to the humidity of the inlet air if HCN and HCHO concentrations are to be accurately determined. 

Figure 3.10 Schematic cross section of an ion source and drift tube from the laboratory of one of the authors. Here the drift tube is constructed from a single block of Teflon and the metal electrodes are located in slots on the outside of the drift tube. A Venturi inlet was employed to introduce the analyte gas into the drift tube. HC refers to the hollow cathode discharge region...

As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

A similar process uses a 30 cm. hollow cathode ion source with its optics masked to 10 cm. Argon is introduced to establish the discharge followed by methane in a 28/100 ratio of methane molecules to argon atoms. The energy level is 100 eV, the acceleration voltage 600 V, and the resulting deposition rate 0.5 to 0.6 im/ hour.t" ]... 

Deng R. C. and Williams P. (1994) Suppression of cluster ion interferences in glow discharge mass spectrometry by sampling high-energy ions from a reversed hollow cathode ion source, Anal Chem 66 1890-1896. 

The atomic vapor is produced by an action similar to the action of a conventional hollow cathode source. Positive ions of the fill-gas strike the cathode, dislodging metal atoms into the optical path. The sensitivity is a function of the discharge current. Walsh used currents of up to 60 mA, but currents of 200-600 mA have been used by others. Higher currents cause some additional heating of the cathode and some additional atomic vapor is produced by the heating process. 

Aside from H3O+, alternative proton donators in PTR-MS have been considered, most notably NH4 [6]. Similarly, NH4+ ions can form when NH3 vapor is used as the discharge gas in the hollow cathode ion source. Proton transfer reaction with NH4+ is less exothermic than with H30 because the PA of NH3 is higher (162 kJ/ mol) than that of H2O (see Table 28.1). Thus, the NH4+ reagent ions might reduce ionic fragmentation and perhaps simplify mass spectral interpretation for those analytes possessing higher PAs than that of NH3. Of course, one should realize that product ions of the M NH4 type hkely appear, which are formed by the three-body ion association reaction [9]. 

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