FASTOF: Fully automated in-situ GC-TOF Observation of atmospheric trace gases
Final Report Abstract
The aim of this project was to establish the technique of time-of-flight mass spectrometry for the analysis of halogenated trace gases in air. Anthropogenic release of substances from this species class to the atmosphere and the ensuing chemical and physical interaction of these substances (and their degradation products) with stratospheric ozone and the radiative budget of the atmosphere demand monitoring and regulation. To keep up with the introduction of new compounds to the atmosphere (e.g. unsaturated Hydro-Fluoro-Carbons, HFCs) and at the same time cover small trends of persistent compounds reliably, sophisticated instrumental analytics are required. Moreover, considering the vast amount of species found in a sample of tropospheric air, the conventional method of quadrupole mass spectrometry seems outdated as it cannot monitor the full mass spectrum and be sensitive enough to quantify species in the ppt to sub-ppt concentration range at the same time. In most applications it is therefore tuned to monitor specific molecule signals; thus making it unresponsive towards other signals at that time. In contrast, the inherent full mass range data acquisition of a time-of-flight mass spectrometer (TOFMS) would make it ideally suited for this task, provided that sensitivity, precision, accuracy and dynamic range are sufficient. A new analytical instrument was set up; comprised of sample stream selection, sample preconcentration and state-of-the-art gas chromatograph (GC) and TOFMS. For possible remote-site monitoring application in the future, the setup is fully automated and field deployable. While GC and TOFMS could be acquired on the market after thorough survey, the sample preconcentration unit could not and was therefore self-built Overall, TOFMS has proven to be a very valuable new tool in the field of atmospheric halocarbon research. The large substance range due to the full mass range spectra and the possibility to identify unknown chromatographic signals with ease thanks to the good mass resolving power are very valuable features of this technique. Despite some new limitations that were indiscernible with the QPMS like the sensitivity towards CO2 and H2O, the advantageous features are backed up by high precision and sensitivity as well as, in case of the Tofwerk instrument, large dynamic range and linear response presuming dedicated data processing. Measurement accuracy was verified by cross-comparison experiments to both in-house and external instruments. Future field deployment for in-situ monitoring of halogenated trace gases in air masses, e.g. ground-based observations at Taunus Observatory, Kleiner Feldberg, Germany have been started in early 2018. As there is no monitoring station equipped for this task in Germany yet, this is a big step forward for the surveillance of halogenated greenhouse gases as well as ozone depleting substances. A combination of site measurement data with inverse modelling would in this case be desirable to deduce emissions. The vast amount of traceable species in each GC-TOFMS measurement call for a higher degree of automation in data processing. IAU_Chrom, a self-written software used to integrate chromatographic signals is well adapted tool for that. TOFMS data offers the possibility to analyse signals that were unknown at the time of measurement in retrospect. Provided that the signal is chromatographically resolved, present in the calibration gas and the detector response is linear, an approximate calibration could be achieved, making the TOFMS data a “digital air archive”. Although this cannot replace “real” air sample archives like e.g. the Cape Grim Archive it is much cheaper in maintenance and requires only the space of a few hard drives. Future applications of GC-TOFMS could also include aircraft operation, where the large substance range would be a key improvement and time resolution could benefit from the high spectra rates possible with the TOFMS.
Publications
- (2016). "A versatile, refrigerant- and cryogen-free cryofocusing-thermodesorption unit for preconcentration of traces gases in air." Atmospheric Measurement Techniques 9(11): 5265-5279
Obersteiner, F., H. Boenisch, T. Keber, S. O'Doherty and A. Engel
(See online at https://doi.org/10.5194/amt-9-5265-2016) - (2016). "An automated gas chromatography time-offlight mass spectrometry instrument for the quantitative analysis of halocarbons in air." Atmos. Meas. Tech. 9(1): 179-194
Obersteiner, F., H. Bönisch and A. Engel
(See online at https://doi.org/10.5194/amt-9-179-2016)
