A Spectral Analysis Method to Eliminate Methanol and Ethanol Interference with CRDS Measurements of Water Isotope Ratios: Insights, Opportunities, and Challenges
Abstract
The application of infrared spectroscopy for measurement of δ18O and δ2H in natural water samples has been limited by the fact that organic compounds often interfere either directly or indirectly with the target absorption spectra of pure water. In the current study, we evaluated the performance of a mechanistic spectral fitting algorithm ('organic fitter') combined with a statistical correction of residual errors ('post-processing correction') for analysis of δ18O and δ2H in organic-contaminated waters via cavity ring-down spectroscopy (CRDS).
We generated two sets of test samples: a set of simple synthetic mixtures of methanol, ethanol, and waters of different isotopic compositions (n = 24), and a set of more complex natural mixtures of waters and organic contaminants extracted from plant leaf and stem samples (n = 48). We then measured the δ18O and δ2H of the synthetic and natural mixtures using three methods: (i) CRDS with organic fitter alone (two different schemes, based on different regions of the water spectrum); (ii) CRDS with organic fitter and post-processing correction; and (iii) temperature conversion/elemental analyzer isotope ratio mass spectrometry (TC-EA-IRMS). For the synthetic mixtures with the highest levels of alcohols, the organic fitters alone made a significant improvement in minimizing large positive biases for both δ18O and δ2H, compared to when no fitter was used. Even greater agreement with the true values was obtained when adding the post-processing correction. When compared to TC-EA-IRMS results, post-processing correction of the CRDS measurements did little to minimize the difference between the two methods for the plant extracts, especially for δ18O. Potential causes for the differences and their implications will be discussed.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.H41Q1962D
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1813 Eco-hydrology;
- HYDROLOGY