Mass-independent fractionation during Mo isotope measurement by MC-ICP-MS: implications for application of the double spike technique

This study presents our new observation on mass-independent fractionation (MIF) during Mo isotope measurement by MC-ICP-MS and investigates its implication for the application of the double spike (DS) technique. A mixture of NIST SRM 3134 Mo solution with a calibrated Mo-97-Mo-100 DS (1 : 1) was prepared for measurements over similar to 15 h. The Mo isotopic data in this study show a good linear relationship in the scatter plots of ln(Mo-97/95) vs. ln(Mo-98/95), ln(Mo-97/95) vs. ln(Mo-100/95), and ln(Mo-98/95) vs. ln(Mo-100/95). Considering measurement errors on both vertical and horizontal axes, York linear regression was applied to linear fit. The slopes derived from the York linear regression are 1.33 +/- 0.12 (2SE), 2.30 +/- 0.20 (2SE), and 1.73 +/- 0.14 (2SE), respectively. Given the error, the slopes are not exactly consistent with their theoretically expected values (1.49, 2.46, and 1.64) based on the assumption of mass-dependent fractionation (MDF). This study, for the first time, identifies MIF during Mo isotope measurement by MC-ICP-MS. DS calculation using theoretical mass fractionation factors shows that the analyzed NIST SRM 3134 Mo solution yielded delta Mo-98/95 values of 0.00 +/- 0.04 parts per thousand (2SD, n = 17) relative to the originally calibrated NIST SRM 3134 Mo. However, using experimental mass fractionation factors calculated from the slopes of line relationships, there is a deviation of about 0.46 parts per thousand for delta Mo-98/95. The results indicate that the calibrated isotope ratios of the standard and DS could not be the true values but approximate values for better matching with the MDF-based equations. After DS calculation using experimental and theoretical mass fractionation factors, respectively, an additional standard-sample bracketing (SSB) method was conducted. Both calculations gave identical and accurate delta Mo-98/95 values (0.00 +/- 0.06 parts per thousand), indicating that the SSB method is able to correct deviation from MIF during DS calculation. Accordingly, we proposed that an additional SSB method is recommended to acquire accurate isotope ratios during the application of the DS technique. This study highlights that researchers should pay attention to the possible existence of instrument-related MIF during isotope measurement.