Proton-transfer-reaction mass spectrometry

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical chemistry technique that uses gas phase hydronium reagent ions which are produced in an ion source.^[1] PTR-MS is used for online monitoring of volatile organic compounds (VOCs) in ambient air and was developed in 1995 by scientists at the Institut für Ionenphysik at the Leopold-Franzens University in Innsbruck, Austria.^[2] A PTR-MS instrument consists of an ion source that is directly connected to a drift tube (in contrast to SIFT-MS no mass filter is interconnected) and an analyzing system (quadrupole mass analyzer or time-of-flight mass spectrometer). Commercially available PTR-MS instruments have a response time of about 100 ms and reach a detection limit in the single digit pptv or even ppqv region. Established fields of application are environmental research, food and flavor science, biological research, medicine, security, cleanroom monitoring, etc.^[1]

Theory

With H₃O⁺ as the reagent ion the proton transfer process is (with $R$ being the trace component)

{\ce {H3O+ + R -> RH+ + H2O}}

(1)

Reaction (1) is only possible if energetically allowed, i.e. if the proton affinity of $R$ is higher than the proton affinity of H₂O (691 kJ/mol^[3]). As most components of ambient air possess a lower proton affinity than H₂O (e.g. N₂, O₂, Ar, CO₂, etc.) the H₃O⁺ ions only react with VOC trace components and the air itself acts as a buffer gas. Moreover, due to the low concentrations of trace components one can assume that the total number of H₃O⁺ ions remains nearly unchanged, which leads to the equation^[4]

{\ce { = 0}}\left(1-e^{-kt}\right)\approx {\ce {0 }}kt

(2)

In equation (2) ${\ce {}}$ is the density of product ions, ${\ce {H3O+0}}$ is the density of reagent ions in absence of reactant molecules in the buffer gas, $k$ is the reaction rate constant and $t$ is the average time the ions need to pass the reaction region. With a PTR-MS instrument the number of product and of reagent ions can be measured, the reaction rate constant can be found in literature for most substances^[5] and the reaction time can be derived from the set instrument parameters. Therefore, the absolute concentration of trace constituents ${\ce {R}}$ 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 dependent on the 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.