Laser mass spectrometry

Extremely small amounts of long lived radioactive isotopes and their isotope composition, e.g. 239/240Pu, 89/90Sr and 41Ca, in environmental, technical and biomedical samples can be determined by atom counting using selective laser based resonance ionization in combination with conventional mass spectrometry. In the field of ultra-trace analysis of radioisotopes, the radiometrical determination of low concentrations of α and β emitters can have severe limitations: interferences from other radionuclides may exist, and the level of detection is dependent on the lifetime. For isotopes with long half-lives, measuring times of the order of up to a few days are often required, which might not be acceptable, for example in the event of an emergency. Here selective atom counting of the individual species of interest using laser mass spectrometric techniques has a number of advantages. Laser mass spectrometry applying resonant excitation and ionization by laser light and subsequent mass analysis (RIMS) provides a number of striking features: almost complete isobaric suppression, high to ultra-high isotopic selectivity and good overall efficiency. This allows the realization of extremely low detection limits, which presently lie in the range of about 106 atoms per sample, corresponding to less than 1 fg for most radioisotopes studied so far. Additionally, RIMS has the advantage of being a rapid technique, with measuring times in the range of a few hours, which makes it attractive for fast surveillance and for the case of an emergency. In comparison with most competitive techniques, only minor chemical treatments are needed for RIMS sample preparation. At Mainz University different versions of RIMS for highly selective ultra-trace analysis have been developed that combine multistep resonance ionization in an atomic beam or vapour with well adapted mass spectrometry. By taking advantage of isotope shifts and hyperfine structures in the different atomic transitions involved, and by using narrowband continuous lasers, an extremely high isotopic selectivity can be obtained. The widespread analytical potential and applicability of the laser mass spectrometric method are utilized for the determination of long lived radioactive ultra-trace isotopes, in particular 239/240Pu, 99Tc, 89/90Sr and 41Ca