Okanagan An atmospheric pressure photo-ionization source based on a window-less atmospheric pressure spark discharge Faezeh Dousty 1 , Hendrik Kersten 2 , Trent Hammer 1 , Rob O'Brien 1 , and Thorsten Benter 2 . 1) Department of Chemistry, I.K. Barber School of Arts & Science, UBC Okanagan, Kelowna, CANADA. V1V1V7. 2) .) Department of Physical and Theoretical Chemistry, University of Wuppertal, Gauss Str. 20, 42119 Wuppertal, Germany 1.0 Introduction Atmospheric Pressure Photo-Ionization (APPI) is an established commercial ionization source that typically uses a krypton discharge lamp as a source of energetic photons 1,2 . We have developed an innovative VUV lamp based on an electrical discharge between two metal capillaries. The design of this source allows for discharge gasses to be rapidly changed to vary the nature of the VUV light emitted. The operating conditions can also be varied to switch from a primary PhotoIonization mode to a duel PhotoIonization and Chemical ionization mode. 4.0 Discussion The commercial Photomate™ Photoionization source can only emit photons and when only simple analyte vapours are present, M + , is the dominate ion observed 4 as demonstrated in Spectra 1. The reaction mechanism producing this ion is described by Equation 1 below. Equation 1: M + hν → M + + e - ; hν is the photon energy: 10.0 eV and 10.6 eV for Kr Lamp The home built Type I source generates a very similar spectra, although there are other ions present. This is a function both the higher energy setting on the cone voltage (48V compared to 34V) that leads to CID reactions in the cone region as well as photon induced reactions. This higher voltage setting is needed because there is no repeller in this version of the discharge source. With the Type I source there is no impact on the observed spectra when discharge gas flows are varied or pumping on the output discharge capillary is turned off. The Type II source is mounted directly on a capillary that directs all flow to the inlet cone. In this case no cone voltage is applied. Changing the flow dynamics with in the discharge region does impact the observed spectra. In fact, it is possible to switch from a set of conditions that predominately produce PhotoIonization generated ions to a second state that produces a mix of PhotoIonization (PI) generated ions and Chemical Ionization (CI) generated ions. The CI process are generated from a cascade of reactions originating from the following 2 initiation reactions 5 . Equation 2: Ar + + N 2 → N 2 + + Ar Charge exchange with carrier gas Equation 3: Ar * + N 2 → N 2 + + e - + Ar Penning Ionization with carrier gas 3.0 Results Spectra were collected under a series of different conditions. In the first set, headspace samples were drawn from septa capped vials and then directed past the VUV source using a concentric K type nebulizer. The following spectra are from Naphthalene headspace. Spectra 1 was collected using the commercial Photomate™ Photoionization source. Spectra 2 was collected using our home made Type I source operated in “PhotoIonization mode”. Spectra 3-6 were collected using the Type II source. In the “PhotoIonization mode” (PI) of the Type II source pumping on the “discharge” electrode prevents energetic reagents from mixing with analyte stream. In the “Chemical Ionization” (CI) and (PI) mode energetic reagents are mixed with analyte stream. 2.0 Experimental Mass spectra were collected on a Waters Quattro Premier LC/MSMS system. This system is equipped with a Photomate™ Photoionization source and this was used to generate “traditional” APPI Mass Spectra as “bench mark” spectra. Home built discharge lamps were constructed using 1/32” (0.79 mm) OD 316 stainless steel tubing with 0.010“ (0.25 mm) ID tubes as electrodes. These were aligned in aluminum oxide tubing 0.062” OD (1.6mm) with 0.031” ID (0.8mm) which was notched to expose the discharge region. The discharge occurs in a < 1 mm gap between electrodes, one of which supplies “discharge” gas and a second that can be actively pumped to remove discharge gas reagents. UHP argon was used as the discharge gas and was supplied at various flows. The discharge was sustained using a home built power supply that provided DC voltage of 1.5 kV pulsed at 1.5 kHz delivering 15 mA of current 3 . Two different types of sources were constructed, Type 1 was mounted on a silanized glass injection liner and was installed as shown in Figure 3 below. Gas samples were introduced down the injection liner in a stream of Nitrogen using a “K Type” concentric nebulizer. The Type 2 source was mounted in a similar orientation to the traditional Photomate™ Photoionization source as demonstrated in Figure 4 below. 5.0 Conclusion We have produced an APPI source based on a novel VUV lamp design. This source can be operated in a mode that introduces reactive species from the discharge region into the region containing analyte. In this mode the source is operating in duel PI and CI modes. The placement of the source onto a glass capillary will allow us to determine reaction mechanisms occurring in APPI and other discharge based ionization sources. 7.0 References 1) Robb, D.B., Covey, T.R., Bruins, A.P. Atmospheric pressure photoionization: An ionization method for liquid chromatography–mass spectrometry. Anal. Chem. 2000; 72: 3653. 2) Syage, J.A., Evans, M.D. and, Hanold, K.A. Photoionization Mass Spectrometry. Am. Lab. 2000. 32. 24-29. 3) Hendrik Kersten, Development of an Atmospheric Pressure Ionization source for in situ monitoring of degradation products of atmospherically relevant volatile organic compounds. Ph.D dissertation, 2011, Bergische Universität Wuppertal, Germany 4) Syage, J.A., Mechanism of [M+H]+ Formation in Photoionization Mass Spectrometry. J. Am. Soc. Mass Spectrom. 2004, 15, 1521-1533. 5) Gross, M.L. & Caprioli, R.M. editors The Encyclopedia of Mass Spectrometry – Volume 6 – Ionization Methods., 2007, Elsevier, ISBN 978-0-08-0438016 6.0 Acknowledgments This work was made possible by instrumentation grants from the Western Economic Diversification program and the British Columbia Knowledge Development Fund. The MITACS accelerate program provided essential operation funds. Contact Information: Rob.OBrien@ubc.ca Reprint Download: http://www.orcac.ca/2011ASMS.pdf Figure 2: Discharge lamp mounted on Peek rod, showing 0.5 mm discharge gap. A “Type II” source. Figure 4: Type II discharge lamp in operation. Figure 1: Image of discharge lamp mounted on silanized injection liner with 1.5 mm ID capillary. A “Type I” source. Figure 3: Image of Type I discharge lamp source mounted in front of MS inlet cone. 11.6 eV and 11.8 eV for Ar Lamp Spectra 1: Vapor phase spectra of Naphthalene (m/z 128) collected using Photomate™ Photoionization source. Spectra 2: Vapor phase spectra of Naphthalene (m/z 128) collected using Type I discharge lamp Spectra 3: Vapor phase spectra of Naphthalene (m/z 128) collected using Type II source in PI mode. Spectra 4: Vapor phase spectra of Naphthalene (m/z 128) collected using Type II source in PI and CI mode. Spectra 5: Vapor phase spectra of Biphenyl (m/z 154) collected using Type II source in PI mode. Spectra 6: Vapor phase spectra of Biphenyl (m/z 154) collected using Type II source in PI and CI mode.