A quantitative analysis of ion-molecule reactions occurring within the extraction orifice of a mass-spectrometric sampling probe

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Abstract
Ion-molecule reactions occurring within a sampling probe orifice are studied with the help of a hollow cathode discharge. The change in intensity of a particular ion due to reactions in and behind the orifice will be negligible only if the radius and the length of the opening are kept small. A relation is given for the probability of orifice reactions as a function of orifice dimensions, reaction cross section and pressure. The effect was demonstrated and the relation was studied for the reaction H2O++H2O to H3O++OH in a pure water vapour discharge and the reactions N2 ++H2O to N2H++OH and N2H++H2O to H3O++N2 in a nitrogen-water vapour mixture.