The detection of extremely small quantities of toxic or radioactive substances is an important task for "Physical Measurement Technology in Medicine and Environmental Protection”. This is the research area in which the physicist Prof. Dr. Heinz-Jürgen Kluge and the chemist Prof. Dr. Norbert Trautmann, both from the University of Mainz, were honoured with the 1990 Helmholtz Prize for their interdisciplinary work "A resonance ionization mass spectrometer as an analytical instrument for trace analysis".
The instrument they developed combined the advantages of mass spectrometry and the resonant excitation and ionization of atoms by laser light for highly sensitive element- and isotope-specific detection. In this way, a detection limit of 2 ∙ 106 atoms could be achieved for the radioactive isotope plutonium-239, corresponding to 8 ∙ 10-16 grams. Thus, this method was two orders of magnitude more sensitive than α-spectroscopy, which detects plutonium-239 via its α-decay.
The work of Kluge and Trautmann was based on the idea of first exciting and then ionizing the atoms of the element or isotope under investigation in three successive steps, so that the resulting ions could then be separated and detected with a mass spectrometer. For the excitation and ionization of the atoms, three pulsed and tunable dye lasers were used. Since the atoms were excited resonantly in the first two steps, the probability of ionizing interfering substances was very small (about 10-10).
The atoms, ionized in pulses, could be separated isotopically using a time-of-flight mass spectrometer. The two researchers demonstrated this using the example of the seven naturally occurring isotopes of gadolinium, where the mass resolution M/ΔM was 2700. If the resonant excitation was tuned to a specific isotope, this isotope could be selectively ionized and detected, while other isotopes were strongly suppressed, as verified with plutonium. Thus, the determination of the isotopic composition of plutonium in an environmental sample made it possible to distinguish between reactor plutonium and nuclear fallout.
Since the 1990s, resonance ionization mass spectrometry has found many applications as a sensitive trace detection technique for actinides and other elements contained in environmental or biological samples and for determining nuclear ground state properties of isotopes produced by accelerators, for example at CERN.
