Coincidence laser spectroscopy (CLS) for the detection of ions in ICP-MS (ICP-MS-CLS). A feasibility study

This paper reports a theoretical study of the feasibility of using laser-excited ionic fluorescence in time correlation with ion counting, termed coincidence laser spectroscopy (CLS), for improved specificity in the detection of ions in ICP-MS. The technique is here named ICP-MS-CLS. A number of factors are considered including: the preferred instrumental configuration, simulation of the performance of the optical detector and correlation step in reducing background, the spectroscopy of the selected of isotopes, 10Be+, 55Fe+, 63Ni+, 90Sr+, 99Tc+, 147Pm+, 238U+, 238Pu+ and 241Am+, which might be appropriate candidates for ICP-MS-CLS detection, the laser power required to attain saturation, the effects of ion energy and energy spread on pumping efficiency, the optical abundance sensitivity for adjacent isotopes of the same element, and the detection limits obtainable under a variety of scenarios. The ICP is established as an ideal ion source for elemental mass spectrometry, but as shown here, the ion energy spread produced is too large for optimum optical pumping because the ions are Doppler shifted to an extent that not all of them would be excited efficiently by a narrow-line laser source. This necessitates the inclusion of an ion cooler into the instrumental configuration so that ions maybe brought into resonance with the laser with 100% efficiency. The calculations show that for ions with simple spectra, such as 90Sr+ which can be repetitively pumped by the laser to produce a photon burst, ICP-MS-CLS can reduce the effect of very high backgrounds, 10 6 cps on mass and 10 10 cps at adjacent mass, to low levels and improve detection limits by 2–3 orders of magnitude compared with the normal technique. Optical abundances of 10-5 –10-9 are achievable which, combined with the mass abundance sensitivity of 10-5, yields overall abundance sensitivities of 10-10–10-14. This is of the same order as techniques such as accelerator mass spectrometry (AMS) or resonance ionisation mass spectrometry (RIMS). The technique is much less efficient for ions that undergo optical trapping and emit only one photon when pumped and/or exhibit hyperfine structure which distributes the oscillator strength over several hyperfine components. These factors significantly degrade performance and indicate a requirement for further refinement in terms of using twocolour excitation, or quenching of meta-stable levels, to enable the recycling of ions for further pumping.