Surface ionization techniques

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... 

Myers, R. L., and W. L. Fite, Electrical Detection of Airborne Particulates Using Surface Ionization Techniques, Environ. Sci. Technol., 9, 334-336 (1975). 

As new values were obtained, atomic electron affinities were reviewed periodically beginning in 1953 [1-13]. All the available experimental, extrapolated, and theoretical values were tabulated in 1984 [7]. Presently, experimental values are available at the NIST website [12]. Prior to 1970 the majority of the values for the main group elements were determined by the Born Haber cycle, electron impact, or relative and absolute equilibrium surface ionization techniques. However, values for C, O, and S had been measured by photodetachment [1-3]. By the mid-1970s virtually all the Ea of the main group elements in the first three rows had been measured by photon methods [4-7]. By the early 1980s values were obtained for the transition elements by photon techniques [7, 8]. In the 1990s the values of Ca, Sr, and Ba were measured [9-13]. Recently, experimental values have been reported for Ce, Pr, Tm, and Lu [14-17],... 

TABLE 8.2 Atomic Electron Affinities (in eV) Determined by Photodetachment, Photoelectron Spectroscopy and Surface Ionization Techniques [4, 7 12]... 

APPI) and the newer surface ionization techniques, including desorption electrospray ionization (DESI) and its derivatives, and direct analysis in real time (DART). 

MALDl is a surface ionization technique that is based on energy transfer from the absorbed laser light to the sample leading to simultaneous desorption and ionization. Energy transfer is mediated by a so-called matrix, which absorbs in the wavelength region of laser emission. Figure 4 illustrates several compounds... 

Desorption electrospray ionization (DESI) may serve as an example of the maiy atmospheric-pressure surface ionization technique that has recently been introduced [63, 76]. In DESI, the high-velocity spray of charged microdroplets from a (pneumatically assisted) electrospray needle is directed at a surface, which is mounted in front of the ion-sampling orifice of an API source (see Fig. 7.6). Surface constituents are released fiom the surface and ionized. These gas-phase ions can be introduced to and observed by MS [77]. In this way, DESI-MS enables for instance the analysis of dmgs in tablets or natural products in plant parts withont extensive sample pre-treatment or prior separation. In addition, DESI-MS and some of its related snrface ionization techniqnes enable chemical imaging of surfaces such as thin-layer chromatography (TLC) plates and tissue sections [78]. 

The first commercially available multiisotope ratio mass spectrometers offered by Finnigan MAT and VG Isotopes were fitted to thermal ionization sources. Thermal ionization of the purified target element from a metal filament, heated resistively in vacuum, is used for sensitive measurements of the alkali, alkaline earth and rare earth elements (REEs). Surface ionization techniques are typically element-specific and can produce relatively high ionization yields (> 10%) with low backgrounds. Early MC instruments were equipped with... 

Dodson, M.H. (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. 

Measurement of the high-frequency noise as described for ESI and APCI is more difficult with MALDI because the ionization event and the noise are on similar timescales. It is difficult to generate a continuous ion beam with a constant baseline from which to make comparable measurements. Since MALDI is a surface ionization technique, many parameters come into play, including depth of beam penetration, rate of raster, and crystal morphology. The most meaningful relationship that can be drawn between the ESI... 

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. 

Thermal or surface emission of ions is one of the oldest ionization techniques used for isotope ratio measurements. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

A discussion of the motivation behind doing sputtered neutral analysis versus SIMS, plus a description of the first prototype SALI instrument. A well written introduction for someone without previous surface analysis experience it also includes an historical overview of the various post-ionization techniques. 

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. 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

In ESI MS, a dissolved sample is sprayed through a capillary in an electric field which is situated in front of the vacuum inlet of the mass spectrometer [2]. Thus, in contrast to most other ionization techniques performed in high vacuum, the ionizahon process takes place at the atmospheric pressure. After leaving the capillary, the solvent forms a so-called Taylor-cone, which further forms a filament and finally, the spray of small droplets (Figure 14.2). These droplets carry charges on the surface this is frequently supported by the acidification of the solvent. The droplets shrink is caused by the evaporation of the solvent. This leads to an increase of the charge-per-surface ratio, finally... 

Different mass spectrometric techniques can be classified according to the evaporation and ionization methods applied. Evaporation of solid samples can be performed, for example, by thermal (e.g., on a hot tantalum filament or in a heated graphite furnace) or laser-induced evaporation, and by electron or ion bombardment. Electron ionizaton (El), ionization during the sputtering process with a primary ion beam, resonant or non-resonant laser ionization or thermal surface ionization... 

Thermal ionization mass spectrometry (TIMS) is one of the oldest mass spectrometric techniques, first applied by Dempster in 1918.114 The thermal emission of positivly charged ions emitted from a salt on a heated surface was first observed by Gehrcke and Reichenheim 12 years before.115 The thermal surface ionization source is a very simple ion source and operates under high vacuum conditions. TIMS is mostly useful for elements with relatively low ionization energy ( )) - in... 

In SIMSLAB from VG Scientific, both surface analytical techniques - SIMS and SNMS - have been applied (see Figure 5.34). In this mass spectrometer different types of primary ion sources are available. Ar+, Cs+, Ga+ or O) primary ions are accelerated in the secondary ion source on the solid sample surface. Similar to the CAMECA IMS-7f, with this experimental arrangement, besides depth profiling, a microlocal analysis can also be performed. The sputtered secondary ions (for SIMS) or the post-ionized sputtered neutrals (for SNMS) - the post-ionization is carried out by an electron beam in an ionizer box (right-hand schematic in Figure 5.34) - are separated... 

---