An Introduction to Quantitative Aspects of PTR-MS

The measurement of ion signals by the mass spectrometer in PTR-MS provides a means for determining the absolute concentration of a specific constituent of a gas mixture, as will be shown below. With H3O+ as the proton source and assuming reaction with only a single organic gas, designated M, the proton transfer reaction is as shown in Reaction 1.5. This reaction is a second-order elementary reaction and will satisfy the rate equation 

Assuming that [M] [H3O + ], which is reasonable since M is a neutral gas (even if present at trace levels), then [M] is constant (the reaction is pseudo first order) and Equation 1.6 can be integrated to yield 

The reaction time t is the time it takes for the reagent ion, in this case H3O+, to travel from the point where it is first mixed with the analyte to the end of the drift tube (beyond which reaction essentially stops). So far the kinetic analysis is directly analogous to that already provided for the FA technique in Section 1.3.3. The concentration of H3O+ can be related to that of MH + by 

Rearranging Equation 1.8 so that [H30+]r is the argument and substituting into Equation 1.7 gives 

Two final assumptions yield a particularly simple expression. First, we assume that conditions are chosen such that only a small proportion of H3O+ is consumed by the reaction, that is, [H30+]o [H3O+], or equivalently [MH+] [H3O+]. A second 

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