Discrepancies between isotope ratio infrared spectroscopy and isotope ratio mass spectrometry for the stable isotope analysis of plant and soil waters

Abstract

The use of isotope ratio infrared spectroscopy (IRIS) for the stable hydrogen and oxygen isotope analysis of water is increasing. While IRIS has many advantages over traditional isotope ratio mass spectrometry (IRMS), it may also be prone to errors that do not impact upon IRMS analyses. Of particular concern is the potential for contaminants in the water sample to interfere with the spectroscopy, thus leading to erroneous stable isotope data. Water extracted from plant and soil samples may often contain organic contaminants. The extent to which contaminants may interfere with IRIS and thus impact upon data quality is presently unknown. We tested the performance of IRIS relative to IRMS for water extracted from 11 plant species and one organic soil horizon. IRIS deviated considerably from IRMS for over half of the samples tested, with deviations as large as 46 per thousand (delta(2)H) and 15.4 per thousand (delta(18)O) being measured. This effect was reduced somewhat by using activated charcoal to remove organics from the water; however, deviations as large as 35 per thousand (delta(2)H) and 11.8 per thousand (delta(18)O) were still measured for these cleaned samples. Interestingly, the use of activated charcoal to clean water samples had less effect than previously thought for IRMS analyses. Our data show that extreme caution is required when using IRIS to analyse water samples that may contain organic contaminants. We suggest that the development of new cleaning techniques for removing organic contaminants together with instrument-based software to flag potentially problematic samples are necessary to ensure accurate plant and soil water analyses using IRIS.

System for High Throughput Water Extraction from Soil Material for Stable Isotope Analysis of Water

“…Extraction times may vary based on the soil type, e.g., clay vs. sand, and as such calibration should be performed for differing soil types. The system described here can also be used to extract water form plant material such as leaves and stems; however, some concern has been noted about the interference of organics that can co-distill with the water causing errors in the spectroscopic analysis of these water samples [14,15]. More recently, a study has shown that it is possible to calibrate the instrument for these types of contaminants and remove the interference from the measurement [16].…”

Dryland ecohydrology and climate change: critical issues and technical advances

“…This new type of instruments does not usually require pretreatment and have precisions similar to traditional cryogenic based mass spectrometry methods (Lee et al, 2005;Wen et al, 2008;Wang et al, 2009d;Griffis et al, 2010). Recent research has indicated that plant derived volatile compound induced spectral contamination in leaf and stem water measurements could affect the accuracy of water stable isotopic compositions significantly (West et al, 2010;Zhao et al, 2011), limiting the application of spectroscopy-based method to plant sample measurements. There are studies which developed post correction method for modest spectral contamination of plant tissues (Schultz et al, 2011), but this remains a topic requiring further developments.…”

Time for cotton to uptake water of a known isotopic signature as measured in leaf petioles

“…For this experiment the water was isotopically enriched using the partitioning effect caused by evaporation where light water, i.e., 16 2 H O will evaporate at a faster rate than heavy water, i.e., DH 16 O, 16 2 D O , DH 18 O, 18 2 D O , or 18 2 H O [6,7,16,26,27]. To accomplish this a 130-L plastic trashcan was filled with tap water from Lubbock, TX on 31 May 2012, and placed in a greenhouse and was allowed to enrich through evaporation over the course of 20 days.…”

“…Organic contaminants have been shown to cause spectral interference in water samples obtained by vacuum distillation of plant material [16][17][18]. However, the effects of these organic compounds can be evaluated and corrected using a procedure developed by Schultz et al [17].…”

“…One complication of using IRIS to analyze water from plant extracts is the interference caused by the -OH bonds from organic compounds that will co-distill with the plant water such as methanol (MeOH), ethanol, (EtOH) as well as organic acids (-COOH) [16]. The interference from these compounds can lead to erroneous isotope ratios.…”