CFC-11 and CCl4 in the Atmosphere: A free calibration standard for Proton-Transfer-Reaction Mass-Spectrometry (PTR-MS)

The Proton-transfer-reaction mass spectrometer (PTR-MS) has become an important tool for real-time monitoring of volatile organic compounds (VOCs) in the atmosphere. PTR-MS is semi-quantitative in untargeted analysis if the transmission of the analytes is well constrained. The transmission at different m/z is determined by using commercial calibration standards.

We present an additional method to constrain the instrument transmission using well-monitored ozone-depleting substances in the atmosphere. Ozone-depleting substances such as chlorofluorcarbons (CFCs)  are banned and monitored through the Montreal Protocol, but their concentration in the atmosphere is relatively stable, and due to their long lifetime they are well mixed. Two of these banned substances can be detected using PTR-MS, namely Trifchloroluoromethane (CFC-11) and carbon tetrachloride (CCl₄). Their major ion CCl₃⁺ is measured at m/z 116.906. A clear signal at this mass is observed in all atmospheric measurements. Since the concentrations of CFC-11 and CCl₄ in the atmosphere are measured frequently by global monitoring networks, they can be used to determine the transmission at m/z 116.906. We experimentally determined the pseudo-reaction rate constants of CFC-11 and CCl₄. Utilizing this, the transmission of m/z 116.906 can be determined using any atmospheric measurements in the past or present. This innovative approach can be a useful tool for quality control of PTR-MS data and checking instrument performance during measurements.