APPLICABILITY OF HIGH-RESOLUTION ICP MASS-SPECTROMETRY FOR ISOTOPE RATIO MEASUREMENTS

The present paper reports on the capability of high-resolution inducti vely coupled plasma mass spectrometry (HR-ICPMS) for the accurate and precise determination of isotope ratios of which at least one of the i sotopes involved is spectrally interfered when measured at low resolut ion, using Cu as a typical example. When a commercially available Finn igan MAT Element high-resolution ICP-mass spectrometer operated at a r esolution setting of 3000 is used, careful selection of the measuremen t conditions allows a Cu-63/Cu-65 Or Pb-206/Pb-207 isotope ratio preci sion of similar to 0.1% (RSD for n = 10) at sufficiently high count ra tes (greater than or equal to 100 000 counts/s), In this work, Cu isot ope ratios were determined both in (i) an Antarctic sediment digest an d in (ii) a human serum reference material, Even at the low Cu concent ration level in the Antarctic sediment digests (similar to 10 mu g/L), the Cu-63/Cu-65 isotope ratio could be measured with an RSD of less t han or equal to 0.6% (n = 5), Although both isotopes involved were sev erely spectrally interfered when measured at low resolution, measureme nt at R = 3000 resulted in an isotope ratio that agreed within the mea surement uncertainty with the expected natural isotopic abundance rati o. For the human serum reference material, the sample preparation was limited to 10-fold dilution with 0.14 M HNO3, resulting in solutions c ontaining similar to 100 mu g/L Cu. Rte Cu-63/Cu-65 isotope ra-tio was measured with an RSD of 0.3% (n = 5) and agreed within the measuremen t uncertainty with the value expected an the basis of the natural isot opic abundances, Finally, on the basis of the corresponding mass spect ra at R = 3000, for both samples an attempt was made to identify the p olyatomic ions that spectrally interfere with the Cu ion signals at lo w resolution. For the Antarctic sediment digest, these interfering ion s were speculatively identified as (ArNa+)-Ar-40-Na-23, (SiCl+)-Si-28- Cl-35, and (Na2OH+)-Na-23-O-16-H-1 for Cu-63(+) and (SiCl+)-Si-30-Cl-3 5, (SiCl+)-Si-28-Cl-37, and (Na2F+)-Na-23-F-19 for Cu-65(+). For the h uman serum reference material, the interfering signals were attributed to (ArNa+)-Ar-40-Na-23, (PO2+)-P-31-O-16, and (Na2OH+)-Na-23-O-16-H-1 for Cu-63(+) and to (SO2H+)-S-32-O-16-H-1 for Cu-85(+).