A new Tunable VUV Tabletop Light Source for Selective, High Sensitivity, Near-Threshold Ionization of Organics in the Range 7.3 to 10.2eV

Over the past two decades aerosol mass spectrometry has proven to be an invaluable tool in atmospheric research. However, one enduring challenge has been in developing instruments specifically for studying organic aerosols. The large number of different organic compounds that can be present in an aerosol, and the relative structural complexity of these molecules, makes it difficult to construct instruments that are both broadly applicable and sufficiently sensitive. In addition, since many organic molecules are relatively fragile, aerosol mass spectra can be complicated by large numbers of fragment ions making the parent molecule difficult to identify. In the following we describe a laser-based vacuum ultraviolet (VUV) light source which is coupled to an ion trap mass spectrometer and an aerosol inlet. The VUV light source allows for near-threshold, single photon ionization of many organic molecules and the ion trap allows for subsequent MS/MS analysis of a given compound. Near-threshold VUV laser photoionization is extremely useful for ionization of organic molecules because it produces ions with very little internal energy, minimizing fragmentation, and reducing the complexity of the mass spectra obtained. Furthermore, it is broadly applicable to many classes of organic compounds. The VUV source delivers high intensity pulses (109 -1012 photons/pulse, 5 ns FWMH) of light with a continuously tunable wavelength from 122 nm (10.2 eV) to 170nm (7.3 eV). These pulses are highly monochromatic (>107 discrimination of input laser pulses) and have a very low bandwidth (<1 cm- 1). Since this source is designed to be used in a single aerosol mass spectrometer, the setup allows for tight (<1 mm2) and precise focusing (<50 μm resolution) of the VUV light, as well as the ability to generate pulses on demand. When coupled with an ion trap mass spectrometer, the light source has proven to be very efficient for detection of a wide array of environmentally relevant organic molecules in the gas phase, with little to no fragmentation in most cases. Photoionization efficiency curves were collected by scanning the VUV light source and recording the ion signal as a function of photon energy. This allowed for fast determination of first ionization energies, which is a useful piece of information when trying to identify a particular compound. This also allowed for determination of the onset of fragmentation and the degree of fragmentation as a function of photon energy. The continuous scanning of the VUV source can also be used to separate the components of organic mixtures by ionization energy.