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Coincidence laser spectroscopy (CLS) for the detection of ions in ICP-MS (ICP-MS-CLS). A feasibility study
journal contribution
posted on 2008-01-21, 12:53 authored by Barry L. Sharp, Phillip S. Goodall, Ljubinko M. Ignjatovic, Huaguo TengThis 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.
History
School
- Science
Department
- Chemistry
Citation
SHARP, B.L. ... et al, 2007. Coincidence laser spectroscopy (CLS) for the detection of ions in ICP-MS (ICP-MS-CLS). A feasibility study. Journal of analytical atomic spectrometry, 22, pp. 1447–1470Publisher
© RSC PublishingPublication date
2007Notes
This article is Restricted Access. It was published in the journal, Journal of analytical atomic spectrometry [© RSC Publishing] and is available at: http://www.rsc.org/jaasISSN
0267-9477Language
- en