A versatile fast polarity switching CIMS platform for studies of atmospheric composition

Chemical ionization mass spectrometry (CIMS) is a powerful analytical technique for the detection of trace gases in the atmosphere. It is uniquely suited for measuring temporal variations of trace volatile organic compounds (VOCs) as well as atmospheric oxidation products including extremely low volatility organic compounds that play a key role in secondary organic aerosol formation.

By choosing an appropriate reagent ion, CIMS enables highly sensitive, online detection of a broad range of chemical species. However, the combination of multiple reagent ions within a single measurement can be technically challenging, especially when both positive and negative ion chemistries are required. Here, we present a versatile fast polarity switching CI-TOFMS platform featuring millisecond transition times between positive and negative ions. This capability enables quasi-simultaneous measurements of diverse molecular families, making the system uniquely suited for comprehensive studies of atmospheric composition. In particular, we present how our novel VUV driven PTR reactor featuring traditional positive ion chemistries such as H₃O⁺ and O₂⁺ can be combined with negative ion chemistries, for example iodide or bromide adducts on a single high resolution instrument platform. We present the characterization of the new system in terms of sensitivities, dynamic range, time response, humidity dependence, as well as reagent ion switching timescales. We also present the first results from test ambient measurements performed in Thun, Switzerland.