Resonance ionization method

Resonance ionization methods (RIMS) have also been explored for improving Th ionization efficiency for mass spectrometric measurement (Johnson and Fearey 1993). As shown in Figure 3, two lasers are required, a continuous resonant dye laser for resonance of thorium atoms, and a continuous UV argon laser for transition from resonance to ionization. Consequently, sophisticated laser instrumentation is required for these methods. 

A resonance ionization mass spectrometer (RIMS) uses a tunable, narrow bandwidth laser to excite an atom or molecule to a selected energy level that is then analyzed by MS. The selective ionization often is accomplished by absorption of more photons from the exciting laser, but can also be effected by a second laser or a broadband photon source. Multiple photon absorption can result in direct ionization or in production of excited species that can then be ionized with a low-energy photon source (IR laser) or by a strong electric field. Resonance ionization methods have been applied to nearly all elements in the periodic table and to many radionuclides, including Cs (Pibida et al., 2001), Th (Fearey et al., 1992), U (Herrmann et al., 1991), Np (Riegel et al., 1993), Pu (Smith, 2000 Trautmann et al., 2004 Wendt et al., 2000), radioxenon and radiokrypton (Watanabe et al., 2001 Wendt et al., 2000), and 41Ca (Wendt et al., 1999). 

The impact of gas-phase radical chain reactions in heterogeneous catalysis was evaluated not only through kinetic analysis, but also by experimental detection of free radicals formed in heterogeneous processes. Such isolation could be done via some specific procedures, such as matrix isolation of radicals, combined with IR and EPR spectra, photoelectron spectroscopy, multiphoton and resonance ionization methods, to name a few. 

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. 

Resonance ionization spectroscopy is a photophysical process in which one electron can be removed from each of the atoms of a selected type. Since the saturated RIS process can be carried out with a pulsed laser beam, the method has both time and space resolution along with excellent (spectroscopic) selectivity. In a recent article [2] we showed, for example, that all of the elements except helium, neon, argon, and fluorine can be detected with the RIS technique. However, with commercial lasers, improved in the last year, argon and fluorine can be added to the RIS periodic table (see figure 2). 

With few exceptions, magnetic sector instruments are comparatively large devices capable of high resolution and accurate mass determination, and suited for a wide variety of ionization methods. Double-focusing sector instruments are the choice of MS laboratories with a large chemical diversity of samples. In recent years, there is a tendency to substitute these machines by TOE or by Fourier transform ion cyclotron resonance (FT-ICR) instruments. 

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... 

Resonance ionization mass spectrometry as a combination of resonance laser ionization with mass spectrometry can be performed on gas atoms only. Therefore, in RIMS of solid samples, before resonance ionization, a neutral gas has to be produced using several methods known from solid state mass spectrometry. During the evaporation of solid material, e.g., by laser evaporation, thermal evaporation or by sputtering with a primary ion beam, the formation of ions should to be avoided. In RIMS, mostly the thermal evaporation of sample from a heated W or Re filament is applied. 

As the preceding discussion describes thermal ionization from hot surfaces has limited scope A new and comprehensive method of ionization is evolving that appears capable of ionizing nearly every element in the periodic table (11 >38-42) Coupled with thermal vaporization processes and mass spectrometry resonance ionization has already been used in our laboratory to demonstrate ionization feasibility for more than one-fourth of the elements in the periodic table ( ) The current status of feasibility demonstration for atomic resonance ionization mass spectrometry in this and other labora torles Is summarized in Figure 1C (39-41>44-52) This summary Includes experiments performed with thermal and ion sputter-initiated techniques ... 

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