ARS | Publication request: Identification and correction of spectral contamination in 2H/1H and 18O/16O measured in leaf, stem, and soil water

Submitted to: Rapid Communications in Mass Spectrometry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 24, 2011
Publication Date: October 3, 2011
Citation: Schultz, N., Griffis, T.J., Lee, X., Baker, J.M. 2011. Identification and correction of spectral contamination in 2H/1H and 18O/16O measured in leaf, stem, and soil water. Rapid Communications in Mass Spectrometry. 25:3360-3368.

Interpretive Summary: In the past, the analysis of 18O/1631 O and D/H in liquid water has been exclusively conducted using stable isotope mass spectrometry (IRMS). Recently, the development of isotope ratio infrared spectroscopy (IRIS) has simplified the isotope analysis of water, allowing the simultaneous measurement of 18O/16O and D/H in liquid water. Isotope ratio infrared spectroscopy analyzers do not require the chemical conversion of compounds to their elemental constituents prior to analysis unlike IRMS. Additional benefits of IRIS analyzers include cost, speed of analysis, and portability. The analytical precision and accuracy of IRIS analyzers are similar to that of IRMS when analyzing pure water; however, it has recently been shown that there are discrepancies between the isotope ratios of plant and soil water measured with IRIS and IRMS. The conventional method of cryogenic vacuum distillation for extraction of water from plant and soil samples can co-distill organic materials (e.g. methanol and ethanol) that may interfere with the spectral signal for the IRIS methods, resulting in erroneous isotope values. Los Gatos Research Inc. has proposed a method to correct the isotope ratios of contaminated samples if the contaminants are known. The correction method consists of spiking clean water with known contaminants and measuring the degree of contamination using the proprietary software. In this paper, we test how well this correction method works. To our knowledge, this is the first study to attempt to correct the isotope ratios in plant and soil samples with known contamination with an IRIS analyzer. Using the correction method proposed by LGR, Inc., we were able to eliminate the errors in 18O/16O and greatly reduce the errors in D/H caused by spectral contamination. We suspect that the incomplete corrections in D/H resulted from the inability to create a correction curve for ethanol contamination. In theory, these correction curves should be applicable to water samples analyzed on other water isotope instruments. However, when comparing our correction curves with the example curves created by the manufacturer, large differences are evident in both magnitude and direction of the corrections. Thus, it is likely that each individual analyzer will require custom correction curves. These results will improve the accuracy of atmospheric isotope measurements, which are critical for testing the accuracy of climate and vegetation models.

Technical Abstract: Plant water extracts typically contain organic materials that may cause spectral interference using isotope ratio infrared spectroscopy (IRIS), resulting in errors in the measured isotope ratios. IRIS manufacturers have developed post-processing software to identify the degree of contamination in water samples, and potentially correct the isotope ratios of water with known contaminants. Here, the correction method proposed by IRIS manufacturer, Los Gatos Research, Inc., was conducted and compared to isotope ratio mass spectrometry (IRMS). Deionized water was spiked with methanol and ethanol to create correction curves for _1812 O and _D. The contamination effects of different sample types (leaf, stem, soil) and different species from agricultural fields, grasslands, and forests were compared. The average corrections in leaf samples ranged from 0.35 to 15.73h for _D and 0.28 to 9.27h for _1815 O. The average corrections in stem samples ranged from 1.17 to 13.70h for _D and 0.47 to 7.97h for _18 16 O. There was no contamination observed in soil water. Cleaning plant samples with activated charcoal had minimal effects on the degree of spectral contamination, reducing the corrections, on average, 0.44h for _D and 0.25h for _1819 O. The correction method eliminated the discrepancies between IRMS and IRIS for _1820 O, and greatly reduced the discrepancies for _D. The mean differences in isotope ratios between IRMS and corrected IRIS methods was 0.19h for _1822 O, and -3.54h for _D. The inability to create an ethanol correction curve for _D likely caused the larger discrepancies. We conclude that ethanol and methanol are the primary compounds causing interference in IRIS analyzers, and that is likely that each individual analyzer will require custom correction curves.