Spectral contaminant identifier for off-axis integrated cavity output spectroscopy measurements of liquid water isotopes

Developments in cavity-enhanced absorption spectrometry have made it possible to measure water isotopes using faster, more cost-effective field-deployable instrumentation. Several groups have attempted to extend this technology to measure water extracted from plants and found that other extracted organics absorb light at frequencies similar to that absorbed by the water isotopomers, leading to δ 2 H and δ 18 O measurement errors (Δδ 2 H and Δδ 18 O). In this note, the off-axis integrated cavity output spectroscopy (ICOS) spectra of stable isotopes in liquid water is analyzed to determine the presence of interfering absorbers that lead to erroneous isotope measurements. The baseline offset of the spectra is used to calculate a broadband spectral metric, m BB , and the mean subtracted fit residuals in two regions of interest are used to determine a narrowband metric, m NB . These metrics are used to correct for Δδ 2 H and Δδ 18 O. The method was tested on 14 instruments and Δδ 18 O was found to scale linearly with contaminant concentration for both narrowband (e.g., methanol) and broadband (e.g., ethanol) absorbers, while Δδ 2 H scaled linearly with narrowband and as a polynomial with broadband absorbers. Additionally, the isotope errors scaled logarithmically with m NB . Using the isotope error versus m NB and m BB curves, Δδ 2 H and Δδ 18 O resulting from methanol contamination were corrected to a maximum mean absolute error of 0.93 ‰ and 0.25 ‰ respectively, while Δδ 2 H and Δδ 18 O from ethanol contamination were corrected to a maximum mean absolute error of 1.22 ‰ and 0.22 ‰. Large variation between instruments indicates that the sensitivities must be calibrated for each individual isotope analyzer. These results suggest that the properly calibrated interference metrics can be used to correct for polluted samples and extend off-axis ICOS measurements of liquid water to include plant waters, soil extracts, wastewater, and alcoholic beverages. The general technique may also be extended to other laser-based analyzers including methane and carbon dioxide isotope sensors.