Over 100 anthropogenic halogenated compounds are known to be present in the atmosphere^[1], manufactured mostly as refrigerants, or foam blowing agents. They are potent greenhouse gases and many are ozone-depleting substances.

To search for emerging halogenated gases relevant for climate or air quality, suspect approaches are still mostly used, i.e. to screen air samples based on prior knowledge on substances newly produced by industry^[2]. A radically different approach is non-target screening: to search in a sample for all present substances, suspected and unknown, with very little prior knowledge about the sample. Such approaches have been introduced already more than a decade ago in the field of e.g. water analysis, but are still very scarce in the field of indoor and ambient trace gas measurements, despite the urgent need for a better understanding of the composition of the air.

In atmospheric measurements, so far the instrument of choice for target screening of many compounds in the same sample is preconcentration, gas chromatography separation, and electron impact (EI) ionisation followed by mass spectrometry (MS) detection. Recently, high-resolution MS (HRMS) using the time-of flight (TOF) detection has been introduced for air measurements.

Here, we present a new and innovative method to identify unknown substances detected in Dübendorf ambient air using GC-EI-HR-TOF-MS. We developed an algorithm combining the identification of atom assemblage for the detected fragments and the reconstruction of a pseudo-fragmentation tree, linking fragments belonging to the same substance. This makes possible to identify substances for which no mass spectrum is registered in databases. We will present method performance characteristics for various types of chemical substances, in particular halogenated substances.

[1] Prinn R., Earth Syst. Sci. Data, 2018, 10, 985-1018.
[2] Vollmer M.K., Environ. Sci. Technol. 2015, 49(5), 2703-2708.