A versatile vacuum ultraviolet ion source for reduced pressure bipolar chemical ionization mass spectrometry

We present the development of a chemical ionization mass spectrometer ion source specifically designed for in situ measurements of trace gases in the upper troposphere and lower stratosphere. The ion source utilizes a commercially available photoionization krypton lamp, primarily emitting photons in the vacuum ultraviolet (VUV) region at wavelengths of 124 and 117 nm (corresponding to energies of 10 and 10.6 eV, respectively), coupled to a commercially available Vocus proton transfer reaction mass spectrometer. The VUV ion source can produce both negative and positive reagent ions; however, here we primarily focus on generating iodide anions (I−). The instrument's drift tube (also known as ion–molecule reactor) operates at pressures between 2 and 10 mbar, which facilitates ambient sampling at atmospheric pressures as low as 50 mbar. The low operating pressure reduces secondary ion chemistry that can occur in iodide chemical ionization mass spectrometry (CIMS). It also allows the addition of water vapor to the drift tube to exceed typical ambient humidity by more than 1 order of magnitude, significantly reducing ambient humidity dependence of sensitivities. An additional benefit of this ion source and drift tube is a 10- to 100-fold reduction in nitrogen consumed during operation relative to standard I− ion sources, resulting in significantly reduced instrument weight and operational costs. In iodide mode, sensitivities of 76 cps ppt−1 for nitric acid, 35 cps ppt−1 for Br2 and 8.9 cps ppt−1 for Cl2 were achieved. Lastly, we demonstrate that this ion source can generate benzene (C6H6+) and ammonium (NH4+) reagent ions to expand the number of detected atmospheric trace gases.