Brigham Young University BYU ScholarsArchive Theses and Dissertations 2012-03-06 Optical and Mass Spectrometric Studies of a Helium Dielectric- Barrier Atmospheric-Pressure Plasma Jet Used as an Ambient Desorption Ionization Source Matthew Spencer Heywood Brigham Young University - Provo Follow this and additional works at: https://scholarsarchive.byu.edu/etd Part of the Biochemistry Commons, and the Chemistry Commons BYU ScholarsArchive Citation Heywood, Matthew Spencer, "Optical and Mass Spectrometric Studies of a Helium Dielectric-Barrier Atmospheric-Pressure Plasma Jet Used as an Ambient Desorption Ionization Source" (2012). Theses and Dissertations. 2980. https://scholarsarchive.byu.edu/etd/2980 This Dissertation is brought to you for free and open access by BYU ScholarsArchive. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Optical and Mass Spectrometric Studies of a Helium Dielectric-Barrier Atmospheric-Pressure Plasma Jet Used as an Ambient Desorption Ionization Source Matthew S. Heywood A dissertation submitted to the faculty of Brigham Young University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Paul B. Farnsworth Jaron C. Hansen Milton L. Lee Paul B. Savage Adam T. Woolley Department of Chemistry and Biochemistry Brigham Young University April 2012 Copyright © 2012 Matthew S. Heywood All Rights Reserved ABSTRACT Optical and Mass Spectrometric Studies of a Helium Dielectric-Barrier Atmospheric-Pressure Plasma Jet Used as an Ambient Desorption Ionization Source Matthew S. Heywood Department of Chemistry and Biochemistry, BYU Doctor of Philosophy Recently there has been a surge in the field of mass spectrometry centered around the concept of rapid analysis of target analytes with minimal or no sample preparation. The target analyte undergoes desorption from its surface of origin and is subsequently ionized under ambient conditions. The technique is termed ambient desorption/ionization mass spectrometry (ADI-MS). Since the introduction of ADI-MS in 2004, there has been an explosion of research based around the development of novel ambient desorption/ionization (ADI) sources with the capability of desorbing and ionizing a variety of target analytes from various sampling surfaces. One type of ADI source uses the properties of an electrical discharge, typically a helium gas plasma, for desorption and ionization. For electrical-discharge-based sources, ionization is the result of an atmospheric pressure chemical ionization (APCI) process. The initiation of the APCI process it generally attributed to the Penning ionization of atmospheric nitrogen (N2) by highly energetic helium metastable species (Hem). In this work, I describe the direct imaging of the densities of helium metastable atoms in atmospheric pressure plasma jet (APPJ) of a helium- based dielectric-barrier discharge (DBD) using collisionally-assisted laser-induced fluorescence. * Axial Hem distributions are compared to the emission of excited helium (He ) and nitrogen ion +* (N2 ) species in the plasma. A correlation is found between Hem densities and the performance of the ionization source in ADI-MS. Fluorescence images also show that Hem densities increase substantially when a glass slide is placed 10 mm from the discharge capillary in a geometry typical for desorption/ionization experiments. Advantage is taken of the time-varying nature of the plasma to produce axial profiles of temporally and spectrally resolved fluorescence images of Hem atoms and ground state nitrogen ions in the plasma jet. The axial distribution and similarities in the temporal behavior of the helium metastable and ground state nitrogen ion species give strong evidence that nitrogen ion species are created via Penning ionization by * +* * helium metastable atoms. Although axial distributions of He ,N2 , and N2 emission support the fluorescence data, temporally-resolved emission measurements show that emission from key plasma species is almost entirely the result of excitation by a temporal energy wave. The effect that hydrogen (H2) has on the helium metastable atom densities is also presented. The addition of hydrogen to the discharge gas severely quenches the metastable state, leaving it virtually undetectable. The addition of 0.9% H2 to the helium in the source provides an order of magnitude increase in ADI-MS signal for target analytes despite the quenching of the Hem population. Keywords: Helium Dielectric-Barrier Discharge, Atmospheric Pressure Plasma Jet, Ambient Desorption/Ionization Mass Spectrometry, Helium Metastable Imaging ACKNOWLEDGEMENTS This dissertation represents the dedication, support, and sacrifice of many people. To my advisor, Dr. Paul Farnsworth, I am grateful for the opportunity to work in his research group. His example of patience and scientific excellence will always be a model to follow. I thank my many friends and colleagues from Dr. Farnsworth’s group for their encouragement and support. I am thankful to my family for their support, especially that of my parents, whose prayers in my behalf have been a force for good in my life. I am most grateful to my wife, who has been a constant source of strength and encouragement. Thank you, Marin, for your endless sacrifice and support. I dedicate this dissertation to you and to our beautiful children, Emma, Spencer, and Lindsay. Table of Contents Table of Contents ......................................................................................................................... iv List of Tables ............................................................................................................................... vii List of Figures ............................................................................................................................. viii 1 Introduction ........................................................................................................................... 1 1.1 Ambient Desorption Ionization Mass Spectrometry ....................................................... 1 1.1.1 Background ................................................................................................................. 1 1.1.2 Accomplishments ........................................................................................................ 2 1.2 ADI Sources .................................................................................................................... 3 1.2.1 Solvent-Based Sources................................................................................................ 4 1.2.2 Electrical Discharge-Based Sources ........................................................................... 7 1.3 Fundamental Studies of a Helium-Based Dielectric-Barrier Discharge Atmospheric- Pressure Plasma Jet Used as an Ambient Desorption/Ionization Source .................................. 20 1.3.1 Fundamental Processes of Plasma-Based ADI Sources ........................................... 21 1.4 Future Studies with ADI-MS ........................................................................................ 27 1.5 References ..................................................................................................................... 28 2 Imaging of Hem Distributions in a Helium Dielectric-Barrier Discharge Ionization Ambient Desorption Ionization Source by Collisionally-Assisted Laser-Induced Fluorescence ......................................................................................................................... 39 2.1 Hem Imaging via Collisionally-Assisted Laser-Induced Fluorescence ........................ 39 2.1.1 Collisionally-Assisted LIF Imaging .......................................................................... 40 2.2 Experimental Conditions .............................................................................................. 42 2.2.1 The Dielectric-Barrier Discharge Atmospheric Pressure Plasma Jet ....................... 42 2.2.2 ADI-MS .................................................................................................................... 45 2.2.3 Laser Excitation for Fluorescence Imaging .............................................................. 47 2.2.4 Collection Optics and iCCD ..................................................................................... 48 2.2.5 Spectral Imaging of Plasma Jet Emission ................................................................. 50 2.3 Imaging Results ............................................................................................................ 51 2.3.1 Images ....................................................................................................................... 51 2.3.2 Hem Density Dependence on Power, Frequency, and Flow Rate ............................. 54 +* 2.3.3 Correlation of Hem Fluorescence, He Emission, and N2 Emission ........................ 56 2.3.4 Radially-Resolved Fluorescence Images .................................................................. 59 iv 2.4 Conclusions ................................................................................................................... 61 2.5 References ..................................................................................................................... 62 3 Temporally and Spectrally Resolved Fluorescence and Emission Imaging of Key Reaction Species