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Methods of Ion Generation Chem. Rev. 2001, 101, 361−375 361 Methods of Ion Generation Marvin L. Vestal PE Biosystems, Framingham, Massachusetts 01701 Received May 24, 2000 Contents I. Introduction 361 II. Atomic Ions 362 A. Thermal Ionization 362 B. Spark Source 362 C. Plasma Sources 362 D. Glow Discharge 362 E. Inductively Coupled Plasma (ICP) 363 III. Molecular Ions from Volatile Samples. 364 A. Electron Ionization (EI) 364 B. Chemical Ionization (CI) 365 C. Photoionization (PI) 367 D. Field Ionization (FI) 367 IV. Molecular Ions from Nonvolatile Samples 367 Marvin L. Vestal received his B.S. and M.S. degrees, 1958 and 1960, A. Spray Techniques 367 respectively, in Engineering Sciences from Purdue Univesity, Layfayette, IN. In 1975 he received his Ph.D. degree in Chemical Physics from the B. Electrospray 367 University of Utah, Salt Lake City. From 1958 to 1960 he was a Scientist C. Desorption from Surfaces 369 at Johnston Laboratories, Inc., in Layfayette, IN. From 1960 to 1967 he D. High-Energy Particle Impact 369 became Senior Scientist at Johnston Laboratories, Inc., in Baltimore, MD. E. Low-Energy Particle Impact 370 From 1960 to 1962 he was a Graduate Student in the Department of Physics at John Hopkins University. From 1967 to 1970 he was Vice F. Low-Energy Impact with Liquid Surfaces 371 President at Scientific Research Instruments, Corp. in Baltimore, MD. From G. Flow FAB 371 1970 to 1975 he was a Graduate Student and Research Instructor at the H. Laser Ionization−MALDI 371 University of Utah, Salt Lake City. From 1976 to 1981 he became I. IR MALDI 373 Associate Professor of Chemistry at the University of Houston. From 1981 to 1987 he was Professor of Chemistry at the University of Houston. J. Delayed Extraction 373 From 1983 to 1993 he was President of Vestec Corp. in Houston, TX. K. New Mass Analyzers 373 From 1993 to present, he has been Vice President, Biospectrometry, at V. Present Status and Future Prospects 374 PerSeptive Biosystems, Framingham, MA. VI. References 374 Mass spectrometry traces its roots to the work of J. J. Thomson1 beginning nearly a century ago. I. Introduction During the first half of the century, the technique was developed and practiced by physicists and the Mass spectrometry separates ions according to applications were mainly limited to precise determi- their mass-to-charge ratio. Except for the quadrupole nation of the masses of the stable nuclides and ion trap, all conventional mass analyzers require ions natural abundance of isotopes.2 During and im- in the gas phase in a sufficiently good vacuum that mediately following World War II, analytical applica- collisions with background gas are insignificant. With tions and fundamental physical measurements on few exceptions, such as flames and plasmas, samples molecules began to emerge. The early work required of interest are not charged and are present in intense beams of atomic ions. The fact that most of condensed phases or as gases at atmospheric pres- the elements of interest were naturally available as sure or above. The challenge in selecting an ioniza- components of molecules or crystalline solids was a tion method for mass spectrometry is to choose a nuisance to be overcome by the choice of suitably technique that preserves the properties of the sample energetic means of producing ions such as gaseous that are of interest while at the same time converting discharges, arcs, and sparks. During the last half of it to ions which can be mass analyzed. This review the century, there has been a relatively continuous will concentrate on ionization methods of most prac- shift toward applications involving molecules of tical utility at present but will also include a brief greater and greater complexity, and within the past summary of some that are of historical or fundamen- decade biological applications have become, by far, tal importance. the most important and fastest growing segment. 10.1021/cr990104w CCC: $36.00 © 2001 American Chemical Society Published on Web 01/25/2001 362 Chemical Reviews, 2001, Vol. 101, No. 2 Vestal II. Atomic Ions developed plasma ionization sources but is still used in many laboratories, particularly for determination A. Thermal Ionization of trace impurities in metals. This is one of the earliest techniques in mass C. Plasma Sources spectrometry and was originally used for producing beams of positive and negative atomic ions for precise The dominant techniques presently in use for measurements of masses of the nuclides and relative elemental analysis by mass spectrometry fall into the intensities of the isotopes. This ionization source general category of plasma sources. An excellent produces intense, stable beams that can be main- review with an extensive bibliography has been tained for long periods to allow precise measurements published by Hieftje.10 These may be separated into but is limited to elements of relatively low ionization those operating at reduced pressure, ca. 1 Torr, potential. An excellent review of the theory of ther- producing a glow discharge (GD), and those operating mal ionization and early experimental results has at atmospheric pressure using inductively coupled been given by Ivanov.3 plasma (ICP) generators. Glow discharge ionization In thermal ionization, one or more metal filaments has a long history in mass spectrometry, beginning are heated, directly or indirectly, by passage of an with its early use in isotopic studies and as a means electrical current. The sample may be either in the of ionizing volatile organic compounds. These ap- gas phase or deposited directly on the surface. A plications generally employ other techniques at widely used practical source is the triple-filament present, but glow discharges are still used extensively design of Inghram and Chupka.4 In this arrangement in the analysis of trace elements in semiconductors, 11 the sample is deposited on the outer two filaments metals, and especially high-purity metals. that are heated to produce a neutral vapor of the sample and the inner, hotter filament causes the D. Glow Discharge ionization. The ratio of ions to neutrals at the surface A schematic diagram of a glow discharge ion source of the hotter filament is given by the well-known for a magnetic mass spectrometer is shown in Figure Langmuir-Saha equation. As discussed in detail by 12 3 1. The cathode is formed as a metal pin, ca. 2 mm Ivanov, the effective work function is strongly de- in diameter, mounted on a removable probe. The pendent on the chemical and physical properties of stainless steel anode contains the ion exit aperture, the surface and may not be simply related to those typically 0.5 mm in diameter. The anode is separated for the clean, crystalline metal surface. This mode accounts satisfactorily for ionization of alkali metal and other easily ionized atoms on heated metal surfaces. This source is still widely used for accurate determinations of isotope ratios in geochemistry applications5 and in nuclear technology.6 B. Spark Source The thermal ionization source is simple and well- behaved but is highly selective. Elements with low ionization potentials are efficiently ionized, but others such as most transition metals are difficult or impos- sible to ionize thermally, and the ionization efficiency may vary by many orders of magnitude. The vacuum spark was originally developed by Dempster7 to extend mass spectrometry to analysis of metals. In this source an electrical spark is produced between a rod electrode of the material to be analyzed and the wall of an aperture in a tantalum disk. Both Tesla-coil and pulsed-RF sources have been used for excitation of the spark. Ions passing through the aperture in the tantalum electrode are accelerated to ca. 15 kV and analyzed in a double-focusing mass spectrometer.8 The ions from the source have a very wide energy spread (ca. 1000 eV), so double focusing is required to obtain useful resolution. Another problem with this source is that the spark is subject to random fluctuations in ion intensity and produces considerable RF noise which may interfere with Figure 1. Schematic diagram of the glow discharge source electrical detection. Despite these limitations, the for an MS9 magnetic mass spectrometer: A, beam focusing plates; B, ion exit (anode) plate; C, machined ceramic spark source provides a very sensitive method for the insulator; D, source block; E, sample pin (cathode); F, analysis of trace impurities in solids that give ap- insulated cathode “cap”; G, stainless steel probe shaft; H, proximately equal sensitivity for all elements.9 It has quartz support plates. (Reprinted with permission from ref been replaced for some applications by more recently 12. Copyright 1989.) Methods of Ion Generation Chemical Reviews, 2001, Vol. 101, No. 2 363 from the source block by a machined ceramic insula- tor. The anode to cathode gap is about 5 mm. The cathode is insulated from the source block by a shaped cap of either ceramic or vespel through which the sample pin protrudes. The discharge gas, usually high-purity argon, is supplied through a needle valve and a length of fused silica capillary to maintain a pressure of about 1 Torr in the discharge. A floating power supply provides a voltage of about -1000 V to the cathode, and a series resistor limits the discharge current to 1-2 mA. The sample of interest is either the cathode material itself, in the case of metals, or a layer of material depositied on the surface of the cathode. In this configuration the glow discharge is limited to analysis of conductive solids, but a RF- excited version has also been developed and applied to nonconductive solids.13 The processes occurring within a glow discharge Figure 2. ICP ion source and sampling interface: A, torch have been studied extensively14,15 For a given dis- and load coil; B, induction region of ICP; C, solution aerolsol charge voltage and gas pressure, the voltage drop being injected in axial channel; D, initial radiation zone; E, normal analytical zone; F, nickel cone with sampling adjacent to the cathode (the cathode fall) occurs orifice in tip; G, skimmer cone; H, boundary layer of ICP mostly within the thin dark space near the cathode.
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