Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

3 MASS SPECTROMETRY 299Suitable choice of wall material may reduce, but not eliminate, the effect. A molecularbeam technique, involving several orifices, can be used to isolate only thoseparticles which have not made collisions during sampling, although the problem ofthe boundary layer at the first orifice remains. Several studies have been made ofmolecular beam formation in mass spectrometer applications"'- lo 3, and Greeneet ~1.''~-'~ have made an investigation of sampling from 1 atmosphere flames bythe molecular beam technique. They find that a simple picture of beam formation,involving isentropic expansion of gas through the kst orifice into a supersonicmolecular beam, is justified. They also observe mass separation, an effect expectedin molecular beams, and have verified the relationI,+ = k,P,M,between ion intensity (I,+), partial pressure of the species (P,) and molecularweight (MJ.Special procedures must be adopted in the determination of the absolute sensitivityof the mass spectrometer to the unstable species.Some measure of the concentrations of unstable species can be obtained fromthe shape of appearance potential curves, although usually it is not possible todistinguish clearly the contributions from precursors and intermediates. It is alsonecessary to make assumptions about ionization cross-sections. However, in suitablecases some information may be obtained. Foner and Hudsong8, for example,made the reasonable assumption that the ionization cross-sections of 02('Ag) and02(3Cg-) were the same, and were able to place limits on the concentration of02('$) in their oxygen flow of between 10 % and 20 %. Again Jackson andSchiff lo6 were able to obtain approximate concentrations of N atoms in dischargeflowexperiments by this procedure.A typical determination is that of Lossing and Ti~kner''~ for the methyl radical.Methyl radicals were produced by the pyrolysis of mercury dimethyl diluted byhelium, and the mass spectrum showed that only CH3, mercury, ethane and a traceof methane were formed. Sensitivity calibrations were obtained in the usual wayfor the stable substances, and then the "net" peak at mass 15, after subtraction ofthe contributions from mercury dimethyl, ethane and methane, was determined.At high temperatures of pyrolysis, where the methyl radicals were most abundant,the sensitivity for the mass 15 peak of the methyl radical could then be calculatedon the basis of 100 % carbon balance. As discussed earlier, wall reactions may leadto appreciable disappearance of the radical under observation, and such effectsmust be taken into account when oalculating the sensitivity of the apparatus. Correctionsof these kinds were applied to the experiments described above whenIngold and Los~ing''~ discovered that part of the methane observed was producedby reaction in the ionization chamber. The smallest relative concentration of radicalswhich can be determined accurately (Le. where several species give rise to theReferences pp. 336-342