An improved method for high precision measurement of chromium isotopes using double spike MC-ICP-MS
Abstract
Chormium(Cr) isotopes have been used to trace pollution processes and reconstruct paleo-redox conditions. However, the precise determination of Cr isotopes has still been challenged due to difficulties in purifying Cr from samples with low Cr content and complex matrices. Here we report an improved four-step column chromatographic procedure to separate Cr from matrix elements. Firstly, Cr in sample solution was mixed with 50Cr-54Cr double spike (the optimized 54Crspike/52Crsample = 0.4 and (50Cr/54Cr)spike = 1.3:1) was completely converted into Cr (III) in 8.5mol/L HCl and loaded onto 2ml of AG50W-X8(200-400m) resin conditioned with 11 mol/L HCl. The 2.65ml of eluent was adjusted to 4.5ml of 6mol/L HCl and immediately loaded onto a Bio-Rad column filled with 2ml of AG1-X8 anion resin (100-200m). These two steps can remove at least 99% of Ca, Fe and most matrix elements. Secondly, the 7.5ml of eluent was dried down and dissolved in 0.1ml of 0.5mol/L HNO3.before adding 2ml 4mol/L HF, which was then loaded onto 1ml of AG1-X8 anion resin (100-200m) to remove Ti and V. Finally, sample was dissolved in 0.1ml of 0.5 mol/L HNO3 and oxidized by 0.5mL 0.2 mol/L (NH4)2S2O8 and 4.4mL H2O, which was then centrifuged to remove Mn oxide, and supernatant was loaded onto AG1-X8 resin to remove SO42-, Ni, Al, Na and some Mg using 8ml H2O and 3ml 2mol/L HCl. Cr was eluted by 2 mol/L HNO3 containing 5% H2O2 and the dried Cr was dissolved in 3% HNO3for isotopic analysis. The total yield to Cr is great than 80% even for samples with low Cr content. Chromium isotopes was measured on a Neptune plus MC-ICP-MS in China University of Geosciences(Beijing). Using our improved method, the δ53/52CrSRM979 values of USGS reference materials BHVO-2, BCR-2 and SGR-1b are -0.12±0.06‰(n=15), -0.09±0.06‰ (n=5), and 0.30±0.06‰ (n=12), respectively, which agree well with previously reported values. The δ53/52CrSRM979 of carbonaceous shale CP0-1 and CP0-12 collected from Hubei, China are 2.05±0.08‰ (n=3) and 2.08±0.07‰ (n=3), respectively, identical to 2.17±0.08‰ and 2.14±0.08‰ determined at the Department of Geology, UIUC. This work was supported by the National Key Basic Research Program of China (2014CB238903) and the National Natural Science Foundation of China (41473028, 41273029).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V33C0536Z
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 1094 Instruments and techniques;
- GEOCHEMISTRY