Neighbor effects

  • Motivation: Another one of the key challenges in applying laser absorption spectrometers for physiological, ecological, and related types of research is making accurate trace gas concentration and isotope ratio measurements under conditions where neighboring absorption features directly interfere with analysis of the target features.

  • Background: Neighboring absorption features can impact target features in different ways. Adjacent features with very broad peak geometry can perturb that baseline of a target feature, whereas adjacent features with very narrow peak geometry can distort the apparent symmetry of the target line profile around its central frequency.

  • Approach: I collaborated with Shirley Papuga and colleagues to develop methods to: (a) detect spectral artifacts, (b) correctly attribute the artifacts to the underlying mechanism(s), and (c) then correct the distortions. We again focused on the spectroscopy of the water molecule, and here used both L2120-i and L2130-i analyzers. Our study is outlined in the slides below.

  • 1. Rationale

    Induction module cavity ring-down spectroscopy (IM-CRDS) has been proposed as a rapid and cost-effective alternative to cryogenic vacuum distillation (CVD) and isotope ratio mass spectrometry (IRMS) for the measurement of δ18O and δ2H values in matrix-bound waters.

  • 2. Specific aims

    In the current study, we characterized the performance of IM-CRDS relative to CVD and IRMS and investigated the mechanisms responsible for differences between the methods.

  • 3. Sample collection and measurement

    We collected a set of 75 soil, stem, and leaf water samples, and measured the δ18O and δ2H values of each sample with four techniques: CVD and IRMS, CVD and CRDS, CVD and IM-CRDS, and IM-CRDS alone.

  • 4. Data analysis

    We then calculated the isotopic errors for each of the three CRDS methods relative to CVD and IRMS, and analyzed the relationships among these errors and suites of diagnostic spectral parameters that are indicative of organic contamination.

  • 5. Patterns in the errors

    The IM-CRDS technique accurately assessed the δ18O and δ2H values of pure waters, but exhibited progressively increasing errors for soil waters, stem waters, and leaf waters.

  • 6. Attribution of the errors

    For soils, the errors were attributable to subsampling of isotopically heterogeneous source material, whereas for stems and leaves, they were attributable to spectral interference.

  • 7. A surprising result

    Unexpectedly, the magnitude of spectral interference was higher for the solid samples analyzed directly via IM-CRDS than for those originally extracted via CVD and then analyzed by IM-CRDS.

  • 8. Conclusions from this study

    There are many types of matrix-bound water samples for which IM-CRDS measurements include significant errors from spectral interference. As a result, spectral analysis and validation should be incorporated into IM-CRDS post-processing procedures.

  • 9. Recommendations

    In the future, IM-CRDS performance could be improved through: (i) identification of the compounds that cause spectral interference, and either (ii) modification of the combustion step to completely oxidize these compounds to CO2, and/or (iii) incorporation of corrections for these compounds into the spectral fitting models used by the CRDS analyzers.

The full story, and its significance

  • Full story: The details of our study can be found in this reference:

    • Reference: Johnson, J. E., L. Hamann†, D. L. Dettman, D. Kim-Hak, S. W. Leavitt, R. K. Monson, and S. A. Papuga. 2017. Performance of induction module-cavity ring-down spectroscopy (IM-CRDS) for measuring δ18O and δ2H values of soil, stem, and leaf waters. Rapid Communications in Mass Spectrometry 31(6): 547-560.

    • Article DOI: 10.1002/rcm.7813

    • Other links: summary in Picarro library

  • Significance: This study has primarily been used by researchers in ecohydrology who are seeking methods for analysis of plant and soil water samples that contain complex mixtures of organic contaminants. Prior to our work, researchers in this area were interested in high-throughput methods for online extraction, combustion, and analysis, but skeptical of the performance of the existing technologies (i.e., micro-combustion module and induction module). Our study diagnosed the strengths and weaknesses of these approaches, and provided clear guidance about the conditions under which they can provide precise and accurate results.