Comparative performance study of ICP mass spectrometers by means of U "isotopic measurements'"

The performance of four commercially available ICPMS instruments of three d ifferent types was compared by means of uranium "isotopic measurements". Ex amined were two quadrupole sector (different generation, different manufact urer), one single detector double focusing magnetic sector and one multiple collector double focusing magnetic sector instruments. The same samples of the IRMM-072 series were used under routine conditions to measure the U-23 3/U-235 and the U-233/U-238 ratios which, in these samples, vary over almos t three orders of magnitude from similar to 1 to similar to 2 (.) 10(-3). W ithin expanded (k = 2) uncertainties, good agreement was observed between t he certified values and the data internally corrected for mass-discriminati on effects. The magnitude of the evaluated uncertainties was different for each type of instrument. With the multiple collector instrument, expanded u ncertainties varied from +/- 0.04% to+/- 0.24% for the U-233/U-235 ratio, a nd from +/- 0.08% to +/- 0.27% for the U-233/U-238 ratio. They were similar to 1 to 5 times larger with the single detector magnetic sector instrument , and similar to 10 to 25 times larger with both quadrupole sector instrume nts. With the multiple collector instrument, repeatability of the measureme nts seemed to be limited by the difficulty of correcting properly for instr umental background, whereas with the single detector magnetic sector instru ment the counting statistics was the only limitation (on smallest ratios). Apparent mass-discrimination was clearly found to be larger but more reprod ucible (and hence easier to correct for) in the case of magnetic sector ins truments than for both quadrupole: sector instruments. If space charge effe cts were the main source of mass-discrimination for all instruments, these results are in contradiction with the hypothesis of the size of mass-discri mination decreasing with the acceleration voltage. With the single detector magnetic sector instrument in particular (when operated by changing the io n energy only), our results pointed at more than only one major source of m ass-discrimination with variable size depending on the ratios measured.