Methane Reagent Gas Plasma

The El mass spectrum of methane has already been discussed (Chap. 6.1). Rising the partial pressure of methane from the standard value in El of about 10 Pa to 10 Pa significantly alters the resulting mass spectrum. [1] The molecular ion, CH/ , m/z 16, almost vanishes and a new species, CHs , is detected at m/z 17 instead. [16] In addition some ions at higher mass occur, the most prominent of which may be assigned as C2H5, m/z 29, [17,18] and CsHs , m/z 41 (Fig. 7.2). The positive ion Cl spectrum of methane can be explained as the result of competing and consecutive bimolecular reactions in the ion source [4,6,10] 

The relative abundances of these product ions change dramatically as the ion source pressure increases from El conditions to 25 Pa. Above 100 Pa, the relative concentrations stabilize at the levels represented by the Cl spectrum of methane reagent gas (Pig. 7.3). [4,19] Portunately, the ion source pressure of some 10 Pa in Cl practice is in the plateau region of Pig. 7.3, thereby ensuring reproducible Cl conditions. The influence of the ion source temperature is more pronounced than in PI because the high collision rate rapidly effects a thermal equilibrium. 

Note Although the temperature of the ionized reagent gas is by far below that of a plasma, the simultaneous presence of free electrons, protons, numerous ions and radicals lead to its description as a reagent gas plasma. 

100 mTorr = 13.33 Pa. Adapted from Ref. [19] by permission. Elsevier Science, 1990. 

The tendency of a (basic) molecule B to accept a proton is quantitatively described by its proton affinity (PA, Chap. 2.12). For such a protonation we have [3]  

To judge the chances of protonation under Cl conditions, one has to compare the PA of the neutral analyte M with that of the complementary base B of the pro-ton-donating reactant ion [BH] (Brpnsted acid). Protonation will occur as long as the process is exothermic, i.e., if PA(b) PAf y The heat of reaction has basically to be distributed among the degrees of freedom of the [Mh-H] analyte ion [12,23]. Accordingly, the minimum internal energy of the [Mh-H] ions is well approximated by  

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The protonated methane ion, CHs", represents a reactive as well as fascinating species in the methane reagent gas plasma. In 1991, its structure had been calculated as shown in Scheme 7.1 [17] and the chemical behavior of the CHs" ion indeed appears to be compatible with a stable structure, involving a three-center two-electron bond associating two hydrogens and the carbon atom. 

Chemical ionization. Chemical ionization spectra result from ion-molecule reaction between the ionic products of a high pressure reagent gas, commonly methane, with a low pressure sample gas. Because of the low abundance of the sample, almost all of the initial ionization by electron impact is of the reagent gas. When methane is ionized at a source pressure of 1 mm Hg, the normal El products CHl and CHs react with neutral CH4 molecules producing a plasma in which CH5 (48% 2) and C2H5 (41% 2) are the principal species available for further ion-molecule reaction ... 

The physical phenomenon utilized for the first time by Field [31a] and Munson [31b] is as old as the universe itself. In the MS source, a gas plasma is produced at a pressure of O.I-l Torr (in electron impact, this pressure is of the order of 10 -10" Torr). If the reagent gas is methane, CH5, CjH, etc., are produced after reaction. These ions have been detected in the gas plasmas surrounding Jupiter and Saturn [32] and those which compose certain stars. 

Chemical ionization (Q) source is very similar to the El source but the beam of electrons is used to create a plasma of ionized reagent gas (e.g., isobutane, methane, ammonia). 

Ionization is achieved in the following way. A reagent gas, frequently methane or other simple hydrocarbon gas, is ionized by electron impact at a high source pressure (about 0.133 kPa). The resulting reactive ion plasma in turn ionizes sample molecules by ion-molecule collision. This can be either by proton transfer from, or hydride extraction by, the reagent gas ions and so give rise to quasimolecular ions from the sample, i.e. (M H)t. 

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