Theory
With H3O+ as the primary ion the proton transfer process is (with being the trace component)
- (1).
Reaction (1) is only possible if energetically allowed, i.e. if the proton affinity of is higher than the proton affinity of H2O (691 kJ/mol). As most components of ambient air possess a lower proton affinity than H2O (e.g. N2, O2, Ar, CO2, etc.) the H3O+ ions only reacts with VOC trace components and the air itself acts as a buffer gas. Moreover due to the low number of trace components one can assume that the total number of H3O+ ions remains nearly unchanged, which leads to the equation
- (2).
In equation (2) is the density of product ions, is the density of primary ions in absence of reactant molecules in the buffer gas, is the reaction rate constant and is the average time the ions need to pass the reaction region. With a PTR-MS instrument the number of product and of primary ions can be measured, the reaction rate constant can be found in literature for most substances and the reaction time can be derived from the set instrument parameters. Therefore the absolute concentration of trace constituents can be easily calculated without the need of calibration or gas standards. Furthermore it gets obvious that the overall sensitivity of a PTR-MS instrument is mainly dependent on the primary / reagent ion yield. Fig. 1 gives an overview of several published (in peer-reviewed journals) reagent ion yields during the last decades and the corresponding sensitivities.
Read more about this topic: Proton-transfer-reaction Mass Spectrometry
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