Ionization surface

If the molecules could be detected with 100% efficiency, the fluxes quoted above would lead to impressive detected signal levels. The first generation of reactive scattering experiments concentrated on reactions of alkali atoms, since surface ionization on a hot-wire detector is extremely efficient. Such detectors have been superseded by the universal mass spectrometer detector. For electron-bombardment ionization, the rate of fonnation of the molecular ions can be written as... 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

Surface ionization. Takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization occurs only when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule have an appropriate relationship. 

Cesium ions are also sometimes used to enhance the secondary-ion yield of negative elemental ions and that of some polymer fragments [3.6]. They are produced by surface ionization with an extraction technique similar to that of FI sources. 

Electron-tunneling Model. Several models based on quantum mechanics have been introduced. One describes how an electron of the conducting band tunnels to the leaving atom, or vice versa. The probability of tunneling depends on the ionization potential of the sputtered element, the velocity of the atom (time available for the tunneling process) and on the work function of the metal (adiabatic surface ionization, Schroeer model [3.46]). 

Surface Ionization Sources. In this system, a low ionization potential atom (e.g. caesium) is adsorbed on a high work function metal (e.g. iridium). The temperature is raised so that the rate of desorption exceeds the rate of arrival of the atoms at the surface, and the Cs is then desorped as ions with very small energy spread (< 1 eY). The spot size - current characteristics of these sources lie between liquid metal and plasma discharge sources. 

Davis, J.A. and Leckie, J.O., Surface ionization and complexation at the oxide/water interface, II surface properties of amorphous iron oxyhydroxide and adsorption of metal ions, J. Colloid Interface Sci. 67, 90-107, 1978. 

With the surface ionization source it is generally assumed that the reactant ion internal state distribution is characterized by the source temperature and that the majority of the reactant ions are in their ground electronic state. This contrasts with the uncertainty in reactant state distributions when transition metal ions are generated by electron impact fragmentation of volatile organometallic precursors (10) or by laser evaporation and ionization of solid metal targets (11). Many examples... 

Figure 1. Schematic view of ion beam apparatus and surface ionization source.

M. G. Inghram and W. A. Chupka. Surface Ionization Source Using Multiple Filaments. Rev. Sci. Instrum., 24(1953) 518-520. 

The terms surface ionization mass spectrometry (SIMS) and ion sputtering are often used when accelerated atoms such as Xe or ions such as Ar+ strike a surface causing ionization of the material on the surface. The surface can be solid or liquid in the form of a solution or a suspension in the solvent. In this section, the terms fast atom bombardment (FAB) and fast ion bombardment (FIB) will be used. 

The surface ionization (SI) method refers to ions thermally evolving from the surface of a hot filament. The vapor of a volatile inorganic compound may be passed over a heated filament where it decomposes and ionizes to form bare metal... 

Point of zero charge PZC (-) Surface ionization constants ApK (-) Dielectric constant of the oxide e (-) Hydroxyl site density Ns (1/nm2)... 

Davis, J. A., and J. O. Leckie (1978a), "Surface Ionization and Complexation at the Oxide/Water Interface. II. Surface Properties of Amorphous Iron Oxyhydroxide and Adsorption of Metal Ions," J. Colloid Interface Sci. 67, 90-107. 

Dodson MH (1963) A theoretical study of the use of internal standards for precise isotopic analysis by the surface ionization technique Part I General first-order algebraic solutions. J Sci Instrum 40 289-295 Douglas DJ (1989) Some current perspectives on ICP-MS. Canad J Spectrosc 34 38-49 Douglas DJ, French JB (1986) An improved interface for inductively coupled plasma-mass spectrometry (ICP-MS). Spectrochim Acta 41B 197-204... 

Note Although desorption chemical ionization being the correct term, [92] DCI is sometimes called direct Cl, direct exposure Cl, in-beam Cl, or even surface ionization in the literature. 

---