High precision selenium isotope analysis using a Nu Sapphire collision–reaction cell MC-ICP-MS

Selenium is a redox-sensitive trace element that is both an essential nutrient and toxin. Studying selenium cycling in nature is of great interest to the fields of environmental health, geomicrobiology, chemical oceanography and volcanology. The six stable isotopes of selenium are fractionated during redox reactions, leaving fingerprints of redox conditions and micronutrient dynamics in modern and ancient environments. However, the study of selenium isotope variability in nature is plagued by analytical difficulties, including its low natural abundance and the prevalence of argon-based interferences in plasma-based mass spectrometers. Here we present a new approach to selenium isotopic analysis using a collision–reaction cell multiple collector inductively coupled plasma mass spectrometer. By using a He–N₂ gas mixture, we can achieve near-complete removal of argon dimers from the beam, allowing precise analysis of all selenium isotopes. This new method enables greater analytical precision per nanogram of selenium analyzed and is also less sensitive to concentration mismatch between samples and standards. Future work can leverage CRC-equipped mass spectrometers to study subtle isotopic effects in low-selenium reservoirs.