ICP-MS instruments in their standard configuration are potentially capable to measure isotope ratios with a precision adequate for isotope dilution analysis. In the field of igneous rock geochemistry, ICP-MS/ID analytical procedures have been developed for the determination of lead (Campbell et al., 1991), thorium and uranium (Pin et al., 1992) and zirconium and hafnium (Xie et al., 1995). In this work we report on a new simple method for the simultaneous determination of neodymium and samarium in silicate matrix by ICP-MS/ID.
Nd and Sm concentrations can be quantified in rocks and minerals by a number of modern analytical methods (es. INAA, ICP-AES, ICP-MS, SIMS, etc.); however advanced geochemical investigations and geochronological studies require highly precise and accurate data as those attainable by isotope dilution mass spectrometry. Isotope dilution determination of Nd and Sm in igneous materials is commonly carried out by thermal ionisation mass-spectrometry (TIMS); this implies a complex and time-consuming procedure entailing, among the others, a complete separation of Nd and Sm from each other and from all the other chemical elements of the sample matrix.
As an alternative to TIMS, isotope dilution analyses can be profitably accomplished by ICP-MS. Compared to TIMS/ID, the major benefit of ICP-MS/ID is the increase in sample throughput with a relatively small decrease in precision and accuracy. This mainly arises from the sampling of the sample solution at atmospheric pressure, from the capability of measuring isotope ratios of different elements in the same rock solution without processing it through chemical separation procedures, and from the relatively short time of analysis (about ten minutes per sample).
The simultaneous determination of Nd and Sm by ICP-MS/ID is practicable; in fact, both elements have two isotopes (143Nd, 145Nd and 147Sm, 149Sm) virtually free from isobaric interferences (the interference from 130Ba17OH on 147Sm and 133Cs16O on 149Sm proved to be negligible also for very Ba-enriched samples) and 145Nd/143Nd and 149Sm/147Sm ratios can be measured with a precision adequate for ID analysis (typically 0.2-0.5% RSD); moreover, spikes enriched in 145Nd and 149Sm are currently
available.
Instrumental mass bias has been corrected for by referencing to a synthetic Nd and Sm solution, of known isotopic composition, analysed every three/five unknown samples.
The ICP-MS/ID results obtained on international and in-house geochemical reference samples with a wide range of Nd and Sm contents (Nd, 2.5-200 ppm; Sm, 1.1-33 ppm), show a precision (~ 0.7% and ~ 0.5% RSD for Nd and Sm, respectively) intermediate between those obtained by ICP-MS with conventional external calibration and by TIMS/ID, and a good agreement with recommended values.
Campbell, M.J., Vandecasteele, C. & Dams, R., In Applications of Plasma Source Mass Spectrometry (eds. Holland, G. & Eaton, A.) 130-138 (The Royal Society of Chemistry, Cambridge, 1991).
Pin, C., Lacombe, S., Telouk, P. & Imbert, J.L., Anal. Chim. Acta 256, 153-161 (1992).
Xie, Q. & Kerrich, R., Chem. Geol. 123, 17-27 (1995).