High-precision optical measurements of ¹³ C/ ¹² C isotope ratios in organic compounds at natural abundance

Abstract

A continuous-flow cavity ring-down spectroscopy (CRDS) system integrating a chromatographic separation technique, a catalytic combustor, and an isotopic 13 C/ 12 C optical analyzer is described for the isotopic analysis of a mixture of organic compounds. A demonstration of its potential is made for the geochemically important class of short-chain hydrocarbons. The system proved to be linear over a 3-fold injection volume dynamic range with an average precision of 0.95‰ and 0.67‰ for ethane and propane, respectively. The calibrated accuracy for methane, ethane, and propane is within 3‰ of the values determined using isotope ratio mass spectrometry (IRMS), which is the current method of choice for compound-specific isotope analysis. With anticipated improvements, the low-cost, portable, and easy-to-use CRDS-based instrumental setup is poised to evolve into a credible challenge to the high-cost and complex IRMS-based technique.cavity ring-down spectroscopy ͉ combustion ͉ isotopic ratio

Evaluation of a cavity ring-down spectrometer for in situ observations of <sup>13</sup>CO<sub>2</sub>

“…The consumption, however, is still not comparable to the minimal amounts needed for our classical IRMS systems (Huang et al, 2012), that allows to measure smaller samples. For our CRDS instrument the abundance of 13 CO 2 and 12 CO 2 are determined independently from two spectral lines in the nearinfrared (near 6251 cm −1 ) before calculating δ 13 C (Zare et al, 2009). Although measuring wet gas streams is possible with this system, we follow the strategy of avoiding negative impacts rather than to correct for these afterwards.…”

Evaluation of the airborne quantum cascade laser spectrometer (QCLS) measurements of the carbon and greenhouse gas suite – CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and CO – during the CalNex and HIPPO campaigns

“…Growing interest in understanding the drivers of climate change has sparked innovation in instrumentation to measure long-lived greenhouse gases and associated chemical tracers (Chen et al, 2010;Fried et al, 2009;Nelson et al, 2004;O'Shea et al, 2013;Xiang et al, 2013a;Zahniser et al, 2009;Zare et al, 2009). The improvements in measurement precision and ease of use must be matched with a corresponding increase in calibration efforts to achieve comparable gains in compatibility.…”

Cavity-based absorption spectroscopy techniques

“…Later designs Spence et al , 2000) employ two orthogonally polarised laser beams in a high-fi nesse three-mirror ring cavity, with the s -polarised beam switched by an AOM operating as in strategy (a) to record cw-CRDS spectra while the other beam ( p -polarised, with lower fi nesse) is locked to that cavity; an NEA limit of 8.8 × 10 − 12 cm − 1 Hz − 1/2 was realised . Such designs underlie commercially available cw-CRDS gas analyser systems (Fidric et al , 2003;Crosson, 2008), typically dedicated to specifi c sensing applications such as environmental gas analysis (Fidric et al , 2003;Crosson, 2008;Chen et al , 2010) or isotopic analysis of chemicals (Crosson et al , 2002;Fidric et al , 2003;Wahl et al , 2006;Brand et al , 2009;Zare et al , 2009;Johnson et al , 2011). Other notable periodically locked, high-repetition-rate cw-CRDS spectrometer designs have been presented by van Leeuwen et al (2003) and by Mart í nez et al (2006), including a critical evaluation of cw-CRDS techniques of this type.…”

“…These sub-topics refl ect the extreme detection sensitivities achievable by cavity-based absorptionspectroscopic techniques. CRDS and/or CEAS/ICOS provide optical ways to distinguish and quantitatively analyse different isotopic modifi cations of various molecular substances (Crosson et al , 2002;Kerstel et al , 2006;Wahl et al , 2006;Wehr et al , 2008;Brand et al , 2009;Zare et al , 2009) different isotopic modifi cations of various molecular substances. For example, Zare et al (2009) combined continuous-fl ow near-infrared cw-CRDS detection (of 13 CO 2 and 12 CO 2 at a wavelength of ~1.603 μ m) with catalytic conversion and chromatographic separation for 13 C/ 12 C isotopic analysis of gas-phase mixtures of the hydrocarbons CH 4 , C 2 H 6 and C 3 H 8 .…”

“…CRDS and/or CEAS/ICOS provide optical ways to distinguish and quantitatively analyse different isotopic modifi cations of various molecular substances (Crosson et al , 2002;Kerstel et al , 2006;Wahl et al , 2006;Wehr et al , 2008;Brand et al , 2009;Zare et al , 2009) different isotopic modifi cations of various molecular substances. For example, Zare et al (2009) combined continuous-fl ow near-infrared cw-CRDS detection (of 13 CO 2 and 12 CO 2 at a wavelength of ~1.603 μ m) with catalytic conversion and chromatographic separation for 13 C/ 12 C isotopic analysis of gas-phase mixtures of the hydrocarbons CH 4 , C 2 H 6 and C 3 H 8 . Recently, a saturated-absorption CRDS technique (Giusfredi et al , 2010) has been used to record radiocarbon ( 14 C) concentrations down to <0.05 ppt by exciting 14 C 16 O 2 in the mid-infrared at ~4.5 μ m (Galli et al , 2011(Galli et al , , 2012Zare, 2012).…”