Applicability of High-Resolution ICP−Mass Spectrometry for Isotope Ratio Measurements

Abstract

The present paper reports on the capability of high-resolution inductively coupled plasma mass spectrometry (HR-ICPMS) for the accurate and precise determination of isotope ratios of which at least one of the isotopes involved is spectrally interfered when measured at low resolution, using Cu as a typical example. When a commercially available Finnigan MAT Element high-resolution ICP−mass spectrometer operated at a resolution setting of 3000 is used, careful selection of the measurement conditions allows a 63Cu/65Cu or 206Pb/207Pb isotope ratio precision of ∼0.1% (RSD for n = 10) at sufficiently high count rates (≥100 000 counts/s). In this work, Cu isotope ratios were determined both in (i) an Antarctic sediment digest and in (ii) a human serum reference material. Even at the low Cu concentration level in the Antarctic sediment digests (∼10 μg/L), the 63Cu/65Cu isotope ratio could be measured with an RSD of ≤0.6% (n = 5). Although both isotopes involved were severely spectrally interfered when measured at low resolution, measurement at R = 3000 resulted in an isotope ratio that agreed within the measurement uncertainty with the expected natural isotopic abundance ratio. For the human serum reference material, the sample preparation was limited to 10-fold dilution with 0.14 M HNO3, resulting in solutions containing ∼100 μg/L Cu. The 63Cu/65Cu isotope ratio was measured with an RSD of 0.3% (n = 5) and agreed within the measurement uncertainty with the value expected on the basis of the natural isotopic abundances. Finally, on the basis of the corresponding mass spectra at R = 3000, for both samples an attempt was made to identify the polyatomic ions that spectrally interfere with the Cu ion signals at low resolution. For the Antarctic sediment digest, these interfering ions were speculatively identified as 40Ar23Na+, 28Si35Cl+, and 23Na216O1H+ for 63Cu+ and 30Si35Cl+, 28Si37Cl+, and 23Na219F+ for 65Cu+. For the human serum reference material, the interfering signals were attributed to 40Ar23Na+, 31P16O2+, and 23Na216O1H+ for 63Cu+ and to 32S16O21H+ for 65Cu+.