GEMOC ARC National Key CentreNorman J. Pearson, William L. Griffin, Simon E. Jackson and Suzanne
Y. O'Reilly
GEMOC, School of Earth and Planetary Sciences, Macquarie University,
NSW 2109 Australia
Correction procedures for mass fractionation in plasma source mass spectrometers have followed the well-established internal normalisation procedures used in thermal ionisation mass spectrometry (TIMS) or the external method used in gas-source analysis of bracketed measurements of standards.
A new method developed for ICPMS applications involves spiking the sample solution with an element of similar mass that can be used to measure instrument mass fractionation and correct the element of interest. Examples of this approach using multicollector ICPMS include Tl to correct Pb (Walder & Furuta, 1993; Belshaw et al., 1998; Rehkämper & Halliday, 1998); Cu for Zn, and Zn for Cu (Maréchal et al., 1999).
The critical assumption in this method is that the mass fractionation coefficients of the two elements are equal. Plasma operating conditions such as nebuliser gas flow and torch position, as well as cone cleanliness and plasma loading will affect the mass fractionation and the ratio of mass fractionation coefficients of the element pairs. Results obtained on the Nu Plasma will be presented for Cr-Fe, Fe-Ni, Cu-Zn, Se-Br, Rb-Sr, Ag-Cd, Sb-Sn, Nd-Sm, Re-Ir, Os-Ir and Tl-Pb. The results show that for a range of normal operating conditions the ratio of mass fractionation coefficients for a pair of elements remains constant, although it may not be equal to one. The longterm consistency of the ratios allows the adjustment of the ėtrue' isotopic ratio of elements used for mass bias normalisation or isobaric overlap correction.
References
Belshaw, N.S., Freedman, P.A., O'Nions, R.K., Frank, M. Guo, Y., 1998.
Int. J. Mass Spec. Ion Proc. 181, 51-58.
Maréchal, C., Télouk, P., Albarède, F., 1999. Chem. Geol., 156, 251-273.
Rehkämper, M. & Halliday, A.N., 1998. Int. J. Mass Spec. Ion Proc. 181, 123-133.