Boron isotope measurements in polar snow and ice using Negative Thermal Ionization Mass Spectrometry

초록

Polar snow and ice provide very valuable records for characterizing environmental changes in the atmosphere over time. The measured range of boron isotopic variations in nature is about 100‰, making boron being a very powerful geochemical tracer. However, measurements of boron isotope ratios in snow and ice at a nano gram per gram level are an analytical challenge because the sample is very subject to contamination. And the low mass numbers of boron is lead to severe isotope fractionation during analysis. To minimize contamination, boron-free HEPA filter and acid-cleaned labware are mandatory to be utilized. PFA beakers are cleaned with chloroforom and soaked in 25% nitric acid for 3 months. Also the samples are loaded on outgassed zone-refined rhenium filaments using a boron-free seawater activator. The samples are then analyzed by negative-thermal ionization mass spectrometry (N-TIMS). The ionization efficiency of N-TIMS is much higher than positive-TIMS and allows samples containing nano gram per gram level of boron to be analyzed. Blanks are measured by N-TIMS isotope dilution using spike solution prepared from NIST SRM 952. The prepared spike solution is verified by measuring the boron concentration of NIST SRM 1643e water standard with certified boron concentration.