Matrix effects

  • Motivation: 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 the background gas matrix changes dynamically.

  • Background: Dynamic matrices are challenging because collisions between the target molecule and the background impact the frequencies at which the target absorption line profile exhibits maximum absorption intensities (i.e., peak shifting) and the shapes of the absorption line profiles around those central frequencies (i.e., peak broadening or narrowing).

  • Approach: In order to explore this problem systematically and evaluate potential solutions, I collaborated with Chris Rella to study background gas effects on a commercially available cavity ring-down spectrometer that targets water isotopologue absorption features, the L2120-i manufactured by Picarro, Inc. Our study is outlined in the slides below.

  • 1. Ideal backgrounds

    Cavity ring-down spectrometers have generally been designed to operate under conditions in which the background gas has a constant composition, such as controlled laboratory experiments or observations of well-mixed natural reservoirs like the atmosphere.

  • 2. Real backgrounds

    However, there are a number of situations of interest in which the background gas has a variable composition, such as: calibrations for atmospheric observations; observations of confined atmospheres; and experiments with active control of the matrix.

  • 3. Our study of background effects

    In this study, we examine the effect of background gas composition on a cavity ring-down spectrometer that measures δ18O–H2O and δ2H–H2O values based on the amplitude of water isotopologue absorption features around 7184 cm−1 (L2120-i, Picarro, Inc.).

  • 4. Patterns of background effects

    For background mixtures balanced with N2, the apparent δ18O values deviate from true values by −0.50 ± 0.001 ‰ O2 %−1 and −0.57 ± 0.001 ‰ Ar%−1, and apparent δ2H values deviate from true values by 0.26±0.004‰ O2 %−1 and 0.42±0.004‰ Ar %−1.

  • 5. Mechanisms of background effects

    The artifacts are the result of broadening, narrowing, and shifting of the target absorption lines and strong neighboring lines. The upper panels illustrate these effects on single absorption lines; the lower panels illustrate the spectral region targeted by the L2120-i.

  • 6. Possible corrections

    While the background-induced isotopic artifacts can largely be corrected with simple empirical or semi- mechanistic models, neither type of model is capable of completely correcting the isotopic artifacts to within the inherent instrument precision.

  • 7. Recommendations

    The development of strategies for dynamically detecting and accommodating background variation in N2 , O2 , and/or Ar would facilitate the application of cavity ring-down spectrometers to a new class of observations and experiments.

The full story, and its significance

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

    • Reference: J. E. Johnson and C. W. Rella. 2017. Effects of variation in background gas mixing ratios of N2, O2, and Ar on the measurement of δ18O-H2O and δ2H-H2O values by cavity ring-down spectroscopy. Atmospheric Measurement Techniques 10(8): 3073-3091.

    • Article DOI: 10.5194/amt-10-3073-2017

    • Data DOI: 10.17605/osf.io/c7nsg

    • Other links: summary in Picarro library

  • Significance: To date, our analysis has been used by researchers in diverse fields as a point of reference for the development of semi-empirical calibration techniques. However, it also indicates there is potential for fundamentally re-designing the spectral acquisition and fitting strategies to accommodate dynamic background gas matrices. Looking to the future, this is a pathway that has potential to open up novel applications for these powerful instruments.