The influence of the double spike proportion effect on stable isotope (Zn, Mo, Cd, and Sn) measurements by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS)

Abstract

This study investigates the double spike (DS) proportion effect on measurements of stable Zn, Mo, Cd, and Sn isotopes by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS). The effect of DS proportion values between 0.1 and 0.9 (i.e., 10–90% DS) on the compositions of measured references materials was determined. The δ^66/64Zn, δ^114/110Cd, and δ^120/118Sn values positively correlate with the proportion of the DS within a mixture, whereas δ^98/95Mo values negatively correlate with the proportion of DS. Stable Mo, Cd, and Sn isotopes have a range of DS values around the optimum where the resulting delta values are insensitive to DS proportion values and reflect the values of the reference materials with the optimum DS proportion values. However, Zn isotopes do not have a stable DS proportion range, indicating that δ^66/64Zn values are much more sensitive to DS proportion values than those of the other three elements. The measured δ^98/95Mo values of a mixture of the IAPSO seawater reference with variable amounts of DS solution also show a negative correlation with DS proportions. This indicates that although it is difficult to explain why DS proportion values have a significant influence on isotopic analyses, we suspect that the iterative calculations involved in the three algebraic equations used to resolve DS data generate correlations between delta and DS proportion values. This study highlights the importance of assessing DS proportion effects when establishing analytical protocols for stable isotope measurements using the DS technique. Any correlation between the resulting measured isotopic compositions and the associated DS proportion values means that DS solutions must be precisely added to unknown samples to ensure that appropriate proportions of DS are used.