Demonstration of a VUV lamp photoionization source for improved organic speciation in an aerosol mass spectrometer

In recent years, the Aerodyne Aerosol Mass Spectrometer (AMS) has become a widely used tool for determining aerosol size distributions and chemical composition for non-refractory inorganic and organic aerosols. All AMSs to date have used a combination of flash thermal vaporization and 70 eV electron impact (EI) ionization. However, EI causes extensive fragmentation and mass spectra of organic aerosols are difficult to deconvolve because they are composites of the overlapping fragmentation patterns of a multitude of species. In this manuscript we present an approach to gain more information about organic aerosol composition by employing the softer technique of vacuum ultraviolet (VUV) ionization in a Time-of-Flight AMS (ToF-AMS). Our novel design allows for alternation between photoionization (PI) and EI within the same instrument on a timescale of minutes. Thus, the EI-based quantification capability of the AMS is retained while improved spectral interpretation is made possible by combined analysis of the complementary VUV and EI spectra. PI and EI spectra are compared for several compounds and mixtures in multiple dimensions including size distributions and size-segregated mass spectra. In general, VUV spectra contain much less fragmentation than EI spectra and for many compounds the parent ion is the base peak in the VUV spectrum. Results for oleic acid are compared to experiments conducted using tunable VUV radiation from a synchrotron source and were shown to be comparable under similar conditions of photon energy and vaporizer temperature. Future technical modifications for improvements in sensitivity and its potential for ambient measurements will be discussed.