The use of Fluorinated Graphite Polymer in the Oxygen-Isotope Analysis of Silicates by Continuous Flow Isotope Ratio Mass Spectrometry (CF-IRMS)

To date the majority of analytical procedures for δ18O analysis of silicates require relatively expensive apparatus involving the use of hazardous fluorine or halogen fluoride gases. Carbon reduction offers the potential to avoid the use of hazardous gases; however, it has been plagued by low yields and variable isotopic fractionation leading to significant differences in isotopic composition compared to conventional fluorination techniques (CFT). We adopt a carbon reduction method, which includes a fluorinated graphite polymer (F>60%) to help oxidize the silicate, thus increasing the percent yield as well as the precision of the analyses. Three silicate standards (NBS-28; UWG-2; GBW044190) were analyzed to compare this method against CFT. Silicate samples of 0.2mg are combined with 2mg of fluorinated graphite and sealed in a silver cup. The cups are then heated instantaneously to 1470°C in a high temperature conversion elemental analyzer (TC/EA). The evolved oxygen reacts with carbon in the furnace resulting in carbon monoxide which is transported via a helium carrier gas to a series of chemical and cold traps prior to introduction into the gas chromatograph. An Ascarite II and Mg(ClO4)2 chemical trap removes acids (e.g. HF) and water, followed by an LN2 cold trap to remove SiF4. The He carrier gas flow is kept low (0.7bar) to maximize reaction time, whereas He flow to the open split is increased (0.7bar) in order to reduce peak tailing in the IRMS. Using this method individual analysis can be completed in ~7 minutes. Oxygen yields range from 54-92%; however reproducibility better than 0.5‰ is normally achievable. Measured values are consistently enriched in 18O compared with accepted values; however, this offset is systematic and can be corrected for using a linear correction factor. Corrected values are: 9.11±0.3‰, 5.95±0.5‰, and -1.78±0.4‰ for NBS-28, UWG-2, and GBW044190 respectively. As the number of laboratories with CF-IRMS and TC/EA apparatus increases the development of this method has the potential to make δ18O analysis of silicates more widely available.