Precise measurement of selenium isotopes by HG-MC-ICPMS using a 76–78 double-spike

Abstract

Selenium (Se) stable isotopes are a new geochemical tool with great potential as a tracer of redox processes and chemical cycling of chalcophiles and volatile elements. However, Se isotope measurements in low-Se samples present a formidable analytical challenge. In this study, we report a new method to measure Se stable isotopes (δ^82/78Se; per mil deviation relative to Se NIST SRM 3149) with extremely high precision. Selenium has six stable isotopes and therefore is a good candidate for isotope analysis using a double spike approach, which has the advantage that it can correct for any stable isotope fractionation that may occur during sample processing. We have calibrated a novel ⁷⁶Se–⁷⁸Se double spike and have developed a rapid and precise analytical protocol on a multi-collector inductively coupled plasma mass spectrometer using an ESI hydride generation introduction system. Sensitivity is over 1000 V per ppm for the total Se signal; a measurement typically requires 25 ng of natural Se. Argon dimer interferences on masses 76, 78 and 80 were corrected in situ by measuring mass 80. Germanium interferences on masses 74 and 76 were corrected by measuring mass 73 and mass 75 was monitored to correct for arsenic hydride on mass 76. Wash-out times were in the order of 180 s, greatly reduced compared to previous studies that rely on on peak zero argon dimer corrections (wash-out times of up to one hour). The 2 s.e. error for a single analysis typically ranges from 0.01 to 0.025‰ (n = 80) for δ^82/78Se. Long-term reproducibility and accuracy were estimated by multiple analyses of the Se Merck standard over numerous different analytical sessions, resulting in a mean δ^82/78Se value of −0.989 ± 0.034‰ (n = 93; 2 s.d.), which is in excellent agreement with previous studies.