Calibration Methodology for PTR-MS
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Proton transfer reaction mass spectrometry (PTR-MS) is a method of measuring the volume mixing ratio (VMR) of trace components in the atmosphere based on the ionisation of molecules through proton donation. This document primarily concerns an innovative new method of gas standard addition for instrument calibration. Instead of a steady dilute flow of gas standard into the instrument, a finite, known amount of gas standard is introduced. This is a considerably faster process. Protocols are developed here to retrieve important parameters with this method of standard addition. The compound specific calibration factor, S (the Sensitivity), is the proportionality between the intensities (in ion counts per second) that make up the output of PTR-MS and VMR’s they represent. XR, the humidity factor, is the relative rate with which compounds receive protons from water clusters in comparison to single water molecules. The retrieval of these two parameters allows for the accurate measurement of compounds contained in the gas standard (i.e. compounds for which the PTR-MS can be calibrated) independent of the humidity in an air sample. Protocols have also been developed to increase the accuracy of the measurement of uncalibrated compounds through the application of kinetic theory. Using the same method of standard addition, the relative efficiency with which the instrument detects different masses is found. This is known as the mass dependent transmission curve. This document also includes a collation of data processing and quality assessment techniques from other PTR-MS projects into a single, clear methodology and the introduction of new quality control and tuning protocols. These methods were developed as a contribution to a larger work concerning the implementation of a standard operating procedure for PTR-MS. The knowledge gained during this project is now being used to create a 5-minute standard addition routine that will allow for the comprehensive calibration of an instrument. Through outreach to the wider PTR-MS community via the ACTRIS (Aerosols, Clouds, and Trace gases Research InfraStructure) network, it is hoped that this routine can become standard practice.