Rediscovery of classical methods and their application to the measurement of stable bromine isotopes in natural samples

Isotopic characterisation of bromine could be a powerful tool for the determination of geological processes. However, no analytical procedures have been published to determine bromine stable isotope composition in natural samples, probably due to expected analytical difficulties and the expected similarity with chlorine isotopic compositions, or even because no (signifcant) isotopic fractionation was expected at all. We describe a method for measuring the stable bromine isotope composition in natural samples with a precision of better than 0.18‰. A sample containing 2–5 mg bromide is mixed with K2Cr2O7 and H2SO4. This mixture is boiled and during this process Br− is oxidised to Br2 and distilled, while Cl− is not. Thus chlorine and bromine are separated. The Br2 is reduced back to Br− and precipitated as AgBr. This is reacted with CH3I and CH3Br is formed. The gas mixture is separated by gas chromatography, and the pure CH3Br is measured by Isotope Ratio Mass Spectrometry. The technique has been tested on a set of natural samples (oil field formation waters) with a known chlorine isotope range between 0‰ and −5‰SMOC. The bromine isotope range in these sample was found to be between 0‰ and +1.8‰SMOB, indicating that processes fractionating bromine isotopes are different from those fractionating chlorine isotopes.