A comparative study to analyze O and H water isotopes in organic enriched solutions

The stable isotopes of hydrogen and oxygen of the water molecule are widely used in ecohydrological-process studies to understand water uptake, redistribution by plants or to partition evaporation and transpiration. In this regard, within the last decade, the demands for a high spatio-temporal resolution of stable isotope data from xylem water has risen to better understand the water interactions between the Critical Zone compartments.

The arrival of isotope ratio infrared spectroscopy (IRIS) for analysing stable water isotopes based on the different adsorption spectra of water molecules with different isotopic composition allowed much faster sampling processing, in-situ measurements in the field, and lower costs per sample. Currently two IRIS instruments are available on the market with measurement technology based on i) off-axis integrated cavity output spectroscopy (OA-ICOS) and ii) wavelength scanned cavity ring-down spectroscopy (WS-CRDS). However, IRIS measurements of water samples can be seriously compromised by interference of some specific organic compounds in the sample with the absorption spectrum of water isotopologues. The impact of contamination issue by organic compounds on the IRIS measured isotopic composition has led to a range of measures to support usability of IRIS instruments in ecohydrological studies.

Until recently isotope ratio mass spectrometry (IRMS) was the standard in isotope hydrology studies and it is still considered as the reference in ecohydrology to mitigate the effects of organic contamination. However, IRMS analysis of water samples include the oxygen and hydrogen isotopes of organic compounds that are present in the water before entering the furnace of the spectrometer. Those compounds are burned at the same time as the oxygen and hydrogen of the water molecule and can dissipate together with those resulting in joint signal detection in the mass spectrometer analysis. This contribution to the measured oxygen and hydrogen isotopic composition from the organic compounds has the potential to compromise the IRMS results depending on the concentration, species, and the isotopic composition of the organic compounds present in the water sample.

In this study, we assess the type and concentration of organic compounds in extracted xylem water and evaluate their impact on stable water isotope analysis with IRMS and IRIS. Our working hypothesis is that samples that are currently analysed in ecohydrology, such as xylem samples heavily enriched in organic compounds, decrease more the analytical precision of IRMS than that of IRIS. We perform an intercomparison study between IRMS and two IRIS instruments with different configurations (with and without combustion module; old and new catalyst) on water without organic compounds, water spiked using different organic molecules (glucose, ethanol, methanol) and beech sap samples.