Novel gaseous reference materials for calibrating chemical ionisation mass spectrometers Worton, David R., Moreno, Sergi, Hristova, Yoana R., O’Daly, Kieran and Allen, Nick National Physical Laboratory, United Kingdom Proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow tube mass spectrometry (SIFT- MS) are state-of-the-art approaches that allow for rapid and highly sensitive detection of trace volatile organic compounds (VOCs) in real-time 1 . They are based on soft chemical ionization (CI) of the analyte, which minimizes the fragmentation and the difficulty of analysing the spectra, during a distinct reaction time. Hence, compounds analysed can be quantified using the kinetic rate constant k of the reaction between the analyte and the precursor ion. The applications for both techniques spread across several established disciplines from medicine and food science to cleanroom monitoring and environmental research 2 . One of the crucial advantages of stable CI is that it provides a lasting calibration stability. However, although SIFT-MS can measure numerous VOCs at the pmol/mol (parts-per-trillion) levels without individual calibration, the measurement accuracy is ± 25% 3 . The case of PTR-MS is similar as without calibration, the quantification accuracy of compounds with unknown reaction kinetics is approximately 30%, also presuming lack of considerable fragmentation 4 . Although both approaches utilize a similar principle, there are two distinct differences. The first is how the reagent ions are generated. The second one is the energy differences arising from the effective temperature of the reagent ions causing distinctive fragmentation patterns 2 . Therefore, two independent calibration standards with different compositions are required. Following on from collaborations with the PTR-MS and SIFT-MS communities, NPL created two multicomponent gas mixtures in nitrogen matrix with nominal concentrations of 1 and 2 µmol mol -1 , respectively. These gaseous reference materials will aid in the daily validation of the instruments to establish whether they provide a stable response over the whole mass range 3 . The composition of the gas standards is based on three core factors. Firstly, the molar mass of the compounds covers the largest mass to charge ratio (m/z) range currently available for both quadrupole systems. Additionally, all compounds are adequately volatile and stable for a period of at least 1 year. Lastly, their reaction with the precursor ion results in minimal fragmentation which provides excellent routine repeatability. The development of these gaseous standards will allow for standardization between the different user groups for both the SIFT-MS and PTR-MS communities as well as enhanced accuracy (± 5% rsd) for analytes quantification 3 . For more information, please visit: https://www.npl.co.uk/products-services/gas/standards-for-calibrating-ptr-ms- instruments References 1. Breath: Biogenic Origin and Point-of-Care Analysis Approaches. Volatile Biomarkers , 129-154. 2. Lehnert, A.-S., Behrendt, T., Ruecker, A., Pohnert, G., & Trumbore, S. E. (2019). Performance of SIFT-MS and PTR-MS in the measurement of volatile organic compounds at different humidities. Atmospheric Measurement Techniques . 3. Zhu, J. H., Nones, C., Li, Y., Milligan, D., Prince, B., Poslter, M., & Dearth, M. (2017). Ultra-Trace Real Time VOC Measurements by SIFT-MS for VIAQ. SAE International Journal Engines . 4. Holzinger et al. (2019), Validity and limitations of simple reaction kinetics to calculate concentration of organic compounds from ion counts in PTR-MS, Atmos. Meas. Tech. , 12, 6193-6208
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