Adaptation of continuous‐flow cavity ring‐down spectroscopy for batch analysis of δ¹³C of CO₂ and comparison with isotope ratio mass spectrometry

Abstract

Measurements of δ(13)C in CO(2) have traditionally relied on samples stored in sealed vessels and subsequently analyzed using magnetic sector isotope ratio mass spectrometry (IRMS), an accurate but expensive and high-maintenance analytical method. Recent developments in optical spectroscopy have yielded instruments that can measure δ(13)CO(2) in continuous streams of air with precision and accuracy approaching those of IRMS, but at a fraction of the cost. However, continuous sampling is unsuited for certain applications, creating a need for conversion of these instruments for batch operation. Here, we present a flask (syringe) adaptor that allows the collection and storage of small aliquots (20-30 mL air) for injection into the cavity ring-down spectroscopy (CRDS) instrument. We demonstrate that the adaptor's precision is similar to that of traditional IRMS (standard deviation of 0.3‰ for 385 ppm CO(2) standard gas). In addition, the concentration precision (±0.3% of sample concentration) was higher for CRDS than for IRMS (±7% of sample concentration). Using the adaptor in conjunction with CRDS, we sampled soil chambers and found that soil-respired δ(13)C varied between two different locations in a piñon-juniper woodland. In a second experiment, we found no significant discrimination between the respiration of a small beetle (~5 mm) and its diet. Our work shows that the CRDS system is flexible enough to be used for the analysis of batch samples as well as for continuous sampling. This flexibility broadens the range of applications for which CRDS has the potential to replace magnetic sector IRMS.

Evaluating calibration strategies for isotope ratio infrared spectroscopy for atmospheric <sup>13</sup>CO<sub>2</sub> / <sup>12</sup>CO<sub>2</sub> measurement

“…With proper calibration, it can achieve precision similar to that of isotope ratio mass spectrometry (IRMS) (Kerstel and Gianfrani, 2008;Berryman et al, 2011;Werner et al, 2012). At least five types of IRIS instruments are available for field measurement of δ 13 C, including tunable diode laser absorption spectroscopy (Campbell Scientific Inc., Logan, UT; e.g., Bowling et al, 2003;Griffis et al, 2008;Wingate et al, 2010;Santos et al, 2012), quantum cascade laser absorption spectroscopy (Aerodyne Research, Inc., Billerica, MA; e.g., Wada et al, 2011;Kammer et al, 2011;Sturm et al, 2012), wavelength-scanned cavity ring-down spectroscopy (Picarro Inc., Sunnyvale, CA; e.g., Friedrichs et al, 2010;Bai et al, 2011;Berryman et al, 2011), off-axis integrated cavity output spectroscopy (Los Gatos Research, Mountain View, CA;e.g., McAlexander et al, 2011;Guillon et al, 2012), and Fourier transform infrared spectroscopy (e.g., Mohn et al, 2008;Griffith et al, 2012;Hammer et al, 2013). All the IRIS instruments should maintain accuracy traceable to the international PDB-CO 2 or VPDB-CO 2 scale.…”

“…Method 2 removes the instrument drift and concentration dependence by interpolating the measured delta value using the mixing ratio of the major isotopologue; this method is used in field measurements of water vapor isotopes Wen et al, 2008Wen et al, , 2012Welp et al, 2012) and has yet to be applied to δ 13 C measurements. Recommended by Picarro Inc., Method 3 is a variation to Method 2 whereby the interpolation is carried out using two or more delta values (Berryman et al, 2011;Wada et al, 2011;Vogel et al, 2013). Recommended by Los Gatos Inc., Method 4 corrects the measured delta with a single offset value and thus requiring only one calibration gas.…”

System for <i>δ</i><sup>13</sup>C–CO<sub>2</sub> and <i>x</i>CO<sub>2</sub> analysis of discrete gas samples by cavity ring-down spectroscopy

“…However, realising these benefits requires regular batch analysis of discrete samples -existing arrangements that couple CRDS instruments directly to soil headspace chambers are generally constrained to measuring just one experiment at a time (Albanito et al, 2012;Bai et al, 2011;Midwood et al, 2008). Berryman et al (2011) describe a syringe sample delivery system for isotope ratio CRDS that allows small air samples (20-30 mL NTP) to be analysed. In their method, the optical cavity of the CRDS analyser is flushed and completely evacuated prior to direct sample injection to ensure consistency and prevent sample-to-sample contamination.…”

“…As well as returning high-resolution mole fraction measurements (Crosson, 2008), CRDS is used for stable isotope analysis of CO 2 , CH 4 , H 2 O, and N 2 O (Crosson et al, 2002;Dahnke et al, 2001;Kerstel et al, 2006;Sigrist et al, 2008). Commercial deployment of CRDS has created novel analytical possibilities with greater stability, precision, instrument portability, and a lower cost basis compared with many traditional spectroscopic, chromatographic, and mass spectrometric techniques (Berryman et al, 2011;Hancock and OrrEwing, 2010;Mürtz and Hering, 2010;Picarro, 2009). Crosson et al (2002) provide a description of the working principles for taking isotopic measurements by CRDS.…”

“…Like Berryman et al (2011), this method was conceived for isotopic-CO 2 CRDS to provide δ 13 C-CO 2 and CO 2 mole fraction (xCO 2 ) analysis in soil respiration studies, but remains general enough to be used in other contexts and adjusted for other gas species. Instead of evacuating the cavity prior to sample introduction, our process intersperses samples against background measurements of a fixed reference air and post-corrects for bias in the measurements.…”