A detailed explanation of the Vocus has been published previously. Briefly, ionization occurs via chemical ionization using hydronium ions (H3O + ) as the reagent ion, which is generated by a low-pressure discharge method. The instrument drift tube was operated at a pressure of 2.1 mbar, reactor temperature of 60 °C, single ion signal of 2.75 mVns, and a reduced field strength (E/N) of 130 Td. The ion source was set at 427 V and 2 mA, and the water for the reagent ion flowed constantly at 20 sccm. We collected data at a 1-second time resolution with an inlet flow of ~50 sccm. Mass spectra were recorded from 0 -497 m/z, and the Big Segmented Quadrupole (BSQ) of the Vocus was set at 320 V, filtering signals below m/z 45.