Abstract. 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 mbar, 30–120∘,
respectively), as well as a reduced field strength E∕N in the range of 100–160 Td. If
artefacts can be ruled out, it becomes possible to quantify the signals of
uncalibrated organics with accuracies better than ±30 %. The
simple reaction kinetics approach produces less accurate results at E∕N levels
below 100 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.
Validity and limitations of simple reaction kinetics to calculate concentrations of organic compounds from ion counts in PTR-MS
