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Impact of nitrate contamination on the analysis of carbon and oxygen isotopes in carbonate and a low-temperature phosphoric acid digestion approach for online sample preparation.

RATIONALE
High-temperature phosphoric acid digestion combined with continuous-flow isotope ratio mass spectrometry (CF-IRMS) is one of the standard methods for the determination of carbon and oxygen isotopes in carbonate. However, the routine purification protocol has not been tested by samples with nitrate contamination. Here we show that the standard protocol is not capable of measuring such samples with satisfactory accuracy and precision.


METHODS
We tested the low-temperature (25°C) phosphoric acid digestion method combined with CF-IRMS for the measurement of natural carbonate samples with ~1% nitrate concentration and nitrate-doped carbonate standards. We also recorded the Raman spectra of the gases extracted from the nitrate-doped carbonate standards to identify the gas species that affect the results.


RESULTS
The accuracy and precision of δ18 O values for nitrate-doped carbonate standards digested at 72°C are much poorer than routine measurements. For the measurements of doped standards reacted at 25°C, the accuracy and precision of the δ13 C and δ18 O values are indistinguishable from normal results. Raman spectra demonstrate that NO2 generated from high-temperature digestion is the possible interference species for oxygen isotope measurement.


CONCLUSIONS
Low-temperature digestion is a reliable method for the measurement of carbon and oxygen isotopes in nitrate-contaminated carbonate samples because the formation of NO2 can be largely reduced. This approach can be applied to some nitrate-rich speleothems, evaporites and other samples with nitrate contamination.