Resonance Ionization Mass Spectrometers RIMS

Figure 5.26 Experimental setup of a resonance ionization mass spectrometer (RIMS). (K. Wendt and N. Trautmann, Int. ]. Mass Spectrom., 242, 161 (2005). Reproduced by permission of Elsevier.)...

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 Resonance Ionization Mass Spectrometry (RIMS) is a method to detect xenon and krypton with ultra high sensitivity using a laser technique [12] developed in collaboration with the University of Tokyo and Nagoya University. The block diagram of a RIMS system with a time of flight (TOF) mass spectrometer is illustrated in Fig. 15. 

In VoL 2 of this handbook, the origin of elements has been discussed in detail. Therefore, the present authors will exclude that part, except for some comments on the importance of particular radionucKdes. In this chapter, the principles and instrumentation of accelerator mass spectrometry (AMS), the key player for detection of cosmological radionucKdes in ultra trace scale, will be discussed in detail. Detailed discussion of all the research works carried out to date with cosmogenic radionuclides is out of scope. Only the detection of million-year half-life radionucKdes in ultra trace concentration will be touched, followed by concise description of the required chemistry. Rather than giving a general description, a few of them have been chosen and described in separate sections. Inductively coupled plasma-mass spectrometry (ICP-MS), thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), or resonant laser ionization mass spectrometer (RIMS), etc. have also been used for detection of cosmogenic radionucKdes. However, these techniques have much lower sensitivity compared to AMS. Brief discussions on these instruments have been appended at the end of this chapter. This chapter ends with a conclusion. 

Apart from the galvanic detection of the ion currents, direct mass spectromet-ric detection of the ions can also be applied, as is the case with resonance ionization mass spectrometry (RIMS) [847]. In addition, ionization can be performed by multi-photon absorption, which requires very intense primary sources. 

Resonance-ionization mass-spectrometry is still in the development stage in terms of its application to cosmochemistry. The Charisma instrument, which is operated by Argonne National Laboratories, uses multiple lasers to resonantly ionize only the elements of interest, which are then analyzed with a time-of-flight mass spectrometer. The Charisma instrument has made isotopic measurements of molybdenum, zirconium, strontium, ruthenium, barium and other elements in presolar grains. These measurements are made possible by the high ionization efficiency of the RIMS technique and its ability to completely eliminate isobaric interferences. Work is now underway to build a RIMS instrument that can be operated by an individual investigator in a university laboratory. If this succeeds, RIMS will play an increasing role in analysis of extraterrestrial materials. 

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