Organic contamination detection for isotopic analysis of water by laser spectroscopy

Abstract

RationaleHydrogen and oxygen stable isotope ratios (δ2H, δ17O, and δ18O values) are commonly used tracers of water. These ratios can be measured by isotope ratio infrared spectroscopy (IRIS). However, IRIS approaches are prone to errors induced by organic compounds present in plant, soil, and natural water samples. A novel approach using 17O‐excess values has shown promise for flagging spectrally contaminated plant samples during IRIS analysis. A systematic assessment of this flagging system is needed to prove it useful.MethodsErrors induced by methanol and ethanol water mixtures on measured IRIS and isotope ratio mass spectrometry (IRMS) results were evaluated. For IRIS analyses both liquid‐ and vapour‐mode (via direct vapour equilibration) methods are used. The δ2H, δ17O, and δ18O values were measured and compared with known reference values to determine the errors induced by methanol and ethanol contamination. In addition, the 17O‐excess contamination detection approach was tested. This is a post‐processing detection tool for both liquid and vapour IRIS triple‐isotope analyses, utilizing calculated 17O‐excess values to flag contaminated samples.ResultsOrganic contamination induced significant errors in IRIS results, not seen in IRMS results. Methanol caused larger errors than ethanol. Results from vapour‐IRIS analyses had larger errors than those from liquid‐IRIS analyses. The 17O‐excess approach identified methanol driven error in liquid‐ and vapour‐mode IRIS samples at levels where isotope results became unacceptably erroneous. For ethanol contaminated samples, a mix of erroneous and correct flagging occurred with the 17O‐excess method. Our results indicate that methanol is the more problematic contaminant for data corruption. The 17O‐excess method was therefore useful for data quality control.ConclusionsOrganic contamination caused significant errors in IRIS stable isotope results. These errors were larger during vapour analyses than during liquid IRIS analyses, and larger for methanol than ethanol contamination. The 17O‐excess method is highly sensitive for detecting narrowband (methanol) contamination error in vapour and liquid analysis modes in IRIS.

Drought reduces water uptake in beech from the drying topsoil, but no compensatory uptake occurs from deeper soil layers

“…We did not observe such a method-dependent d 18 O difference in either soil , 1995). This lack of oVOC might explain the negligible organic interference observed in this study as especially alcohols are known to exert strong spectral effects (Brand et al, 2009;Mart ın G omez et al, 2015;Millar et al, 2021).…”

“…Beech is known to produce monoterpenes (van Meeningen et al ., 2016) but only low amounts of oxygenated volatile organic compounds (oVOCs; such as alcohols, ketones and aldehydes)(König et al ., 1995). This lack of oVOC might explain the negligible organic interference observed in this study as especially alcohols are known to exert strong spectral effects (Brand et al ., 2009; Martín Gómez et al ., 2015; Millar et al ., 2021).…”

On the urgent need for standardization in isotope‐based ecohydrological investigations

“…However, the term analyte is more appropriate, especially in the case of plant, clay soil, and organic soil extracts as there are often organic compounds co‐extracted with the water. Those compounds can cause errors during isotopic analyses (Brand et al, 2009; Martín‐Gómez et al, 2015; Millar et al, 2021; West et al, 2010). Differentiation between the terms ‘water’ and ‘analyte’ is essential, as ‘water’ implies no co‐extracted organics, leading to mistreatment further down the process chain.…”

17O‐excess and d‐excess of atmospheric water vapor measured by cavity ring‐down spectrometry: Evidence of a matrix effect and implication for the calibration procedure

“…The shaded area indicates the optimum integration time window effect is not related to the removal of gas trace amounts during the drying procedure but rather the result of trace gases in the atmosphere, such as carbon dioxide, argon, methane, and other hydrocarbons or volatile organic compounds, that are not present in the synthetic air type used in this study. [48][49][50][60][61][62] Gralher et al 48 showed that changes in the mixing ratios of N 2 /CO 2 or CO 2 /O 2 can significantly influence the laser-spectroscopic isotope analysis using a Picarro L2120-i. However, our results show that the difference in the CO 2 concentration between the synthetic air and the atmosphere can account for only a small part of the observed matrix effect (Figure 1), indicating the interference of other molecules with the water absorption spectra.…”

“…Nevertheless, the contribution of other hydrocarbons may be also infrared waves in the same region as water and therefore cause spectral interferences. 49 Millar et al 61 showed that a change in 0.01% methanol can bias the 17 O-excess by >1‰ and d-excess by >10‰.…”