Improved Methods for High Precision Isotope-Ratio Infrared Laser Spectroscopy of Ecohydrological Waters With Organic...

Applications of high-precision isotope analysis of ecohydrological water samples such as soil and plant waters with infrared (IR) laser spectrometry have been hampered by spectral interferences of volatile organic compounds.

In this study, three methods were tested to remove methanol (MeOH) and/or ethanol (EtOH) added to water for δ²H, δ¹⁸O, and ¹⁷O-excess analysis with Picarro L2140-i: microcombustion module (MCM), pre-treatment with solid-phase extraction (SPE), and pre-treatment with simple MeOH/EtOH combustion (SMEC) method.

The Picarro MCM was not able to combust the alcohols of high concentrations. Hydrophilic-lipophilic balanced (HLB) SPE could adsorb MeOH/EtOH at low concentrations (e.g., 0.1% EtOH). However, its adsorption capacity could be readily exhausted, and adsorbed water on the SPE adsorbents co-elutes and mix with a sample water. The SMEC can successfully combust highly concentrated waters (0.5% MeOH and 1% EtOH) at 450-500 °C, but the alcohol combustion and oxygen isotope exchange with added O₂ could significantly increase δ¹⁸O values (up to 6-7‰) and decrease ¹⁷O-excess (up to 110 per meg) in the presence of a catalyst. The combination of post-analysis corrections of Picarro MCM data (δ²H and δ¹⁸O) and pre-treatment with SMEC method (δ²H and ¹⁷O-excess) can reach the accuracy and precision of ≤3 and ≤2 ‰ (δ²H), ≤0.2 and ≤0.3‰ (δ¹⁸O), and ≤20 and ≤47 per meg (¹⁷O-excess), respectively.

A strategy of combining the two methods is proposed for high-precision isotope analysis of organic contaminated samples: post-analysis corrections of Picarro MCM data (δ²H and δ¹⁸O) and pre-treatment with SMEC method (δ²H and ¹⁷O-excess). The improved methods in this study can provide sufficient accuracy and precision for isotope analysis of plant and soil waters, which can contribute to the fields of ecohydrology and agricultural sciences.