Atmospheric CFC-11 and CCl₄: A free calibration standard for PTR-MS

Proton-transfer-reaction mass spectrometry (PTR-MS) is an analytical technique used to monitor volatile organic compounds in real-time. For quantitative analysis, compounds of interest are typically calibrated using gas standards, but PTR-MS is quantitative to uncalibrated compounds if the mass-dependent transmission is well defined. However, long-term measurements are challenging due to the drift in transmission over time. Performing frequent calibrations helps, but the methods are time-consuming and tedious, often leading to instruments being under-calibrated. Here we show the use of long-lived and globally monitored compounds in the atmosphere as a tool to constrain the transmission between calibrations. The major ion of trichlorofluromethane (CFC-11) and carbon tetrachloride (CCl₄) is found at the mass-to-charge ratio (m/z) 116.9, which we propose using to retrieve the transmission of a PTR-MS. We determined the pseudo-reaction rate constants of CFC-11 and CCl₄ to be 0.82 × 10⁻⁹ ± 0.05 × 10⁻⁹ cm³ s⁻¹ molecule⁻¹, and 1.65 × 10⁻⁹ ± 0.08 × 10⁻⁹ cm³ s⁻¹ molecule⁻¹, respectively. The method introduced here can improve data quality and accuracy, especially for long-term atmospheric measurements.