Validity and limitations of simple reaction kinetics to calculate concentrations of organic compounds from ion counts in PTR-MS

<p>In September 2017, we conducted a proton-transfer-reaction mass-spectrometry (PTR-MS) intercomparison campaign at the CESAR observatory, a rural site in the central Netherlands near the village of Cabauw. Nine research groups deployed a total of 11 instruments covering a wide range of instrument types and performance. We applied a new calibration method based on fast injection of a gas standard through a sample loop. This approach allows calibrations on timescales of seconds, and within a few minutes an automated sequence can be run allowing one to retrieve diagnostic parameters that indicate the performance status. We developed a method to retrieve the mass-dependent transmission from the fast calibrations, which is an essential characteristic of PTR-MS instruments, limiting the potential to calculate concentrations based on counting statistics and simple reaction kinetics in the reactor/drift tube. Our measurements show that PTR-MS instruments follow the simple reaction kinetics if operated in the standard range for pressures and temperature of the reaction chamber (i.e. 1–4&thinsp;mbar, 30–120<span class="inline-formula">^∘</span>, respectively), as well as a reduced field strength <span class="inline-formula"><i>E</i>∕<i>N</i></span> in the range of 100–160&thinsp;Td. If artefacts can be ruled out, it becomes possible to quantify the signals of uncalibrated organics with accuracies better than <span class="inline-formula">±30</span>&thinsp;%. The simple reaction kinetics approach produces less accurate results at <span class="inline-formula"><i>E</i>∕<i>N</i></span><span id="page6194"/> levels below 100&thinsp;Td, because significant fractions of primary ions form water hydronium clusters. Deprotonation through reactive collisions of protonated organics with water molecules needs to be considered when the collision energy is a substantial fraction of the exoergicity of the proton transfer reaction and/or if protonated organics undergo many collisions with water molecules.</p>