Position-Specific Isotope Analysis of Propane by Mid-IR Laser Absorption Spectroscopy

Abstract

Intramolecular or position-specific carbon isotope a n a l y s i s o f p r o p a n e ( 1 3 C H 3 − 1 2 C H 2 − 1 2 C H 3 a n d 12 CH 3 − 13 CH 2 − 12 CH 3 ) provides unique insights into its formation mechanism and temperature history. The unambiguous detection of such carbon isotopic distributions with currently established methods is challenging due to the complexity of the technique and the tedious sample preparation. We present a direct and nondestructive analytical technique to quantify the two singly substituted, terminal ( 13 C t ) and central ( 13 C c ), propane isotopomers, based on quantum cascade laser absorption spectroscopy. The required spectral information on the propane isotopomers was first obtained using a high-resolution Fourier-transform infrared (FTIR) spectrometer and then used to select suitable mid-infrared regions with minimal spectral interference to obtain the optimum sensitivity and selectivity. We then measured high-resolution spectra around 1384 cm −1 of both singly substituted isotopomers by mid-IR quantum cascade laser absorption spectroscopy using a Stirling-cooled segmented circular multipass cell (SC-MPC). The spectra of the pure propane isotopomers were acquired at both 300 and 155 K and served as spectral templates to quantify samples with different levels of 13 C at the central (c) and terminal (t) positions. A prerequisite for the precision using this reference template fitting method is a good match of amount fraction and pressure between the sample and templates. For samples at natural abundance, we achieved a precision of 0.33 ‰ for δ 13 C t and 0.73 ‰ for δ 13 C c values within 100 s integration time. This is the first demonstration of sitespecific high-precision measurements of isotopically substituted non-methane hydrocarbons using laser absorption spectroscopy. The versatility of this analytical approach may open up new opportunities for the study of isotopic distribution of other organic compounds.

Measurement of CO2 Isotopologue Ratios Using a Hollow Waveguide-Based Mid-Infrared Dispersion Spectrometer

“…CO 2 isotopes have strong line intensities near 4.329 μm, which permits sensitive CO 2 isotope measurements in this spectral region. In this region, the absorption line intensities of the three isotopes 13 C 16 O 2 , 12 C 16 O 2 , and 18 O 12 C 16 O are all of the same order of magnitude (>10 −21 cm −1 / (molec•cm −2 )), and the spectral distances of the three absorption lines are within one wavelength scan of the available ICL. To avoid the interference of H 2 O and its isotopic lines, Figure 2 shows a spectral simulation of 1% CO 2 and 1.2% H 2 O for the spectral region from 2309.6 to 2310.8 cm −1 at 298.15 K and 230 mbar with an absorption length of 1 m based on the HITRAN2020 database.…”

“…O 12 C 16 O, and 12 C 16 O 2 , respectively, at a modulation frequency of 160 MHz and a pressure of 230 mbar. The measurement precisions of δ 13 C and δ 18 O are 0.89 and 0.88 ‰, respectively, corresponding to an integration time of 167 s. An experimental comparison between a HPSDS and a built wavelength modulation system with second-harmonic detection (WMS-2f) is conducted.…”

“…6 Precision in the range of 0.01 to 0.3‰ is of interest in atmospheric chemistry. In the medical field, as established molecular biomarkers, changes in the contents of 13 CO 2 , 18 O 12 C 16 O, and 12 CO 2 in exhaled breath can be used to monitor multiple disease patterns, such as the diagnosis of Helicobacter pylori infections and noninvasive monitoring of glucose metabolism. 7,8 In the 13 C-urea breath test, the delta over baseline value relative to standards can be used to demonstrate the enrichment of 13 C. 9 18 O isotope analysis in breath gas can also be used to assist in the diagnosis of Helicobacter pylori pathogenesis.…”

“…The gas concentration can be thus determined from the phase of the beat note signal. The 13 C/ 12 C and 18 O/ 16 O ratios in CO 2 are usually expressed as a deviation from the Vienna PDB (VPDB) standard, denoted as per mil (‰), the so-called δ-value:…”

“…However, commercial instruments typically do not support simultaneous multicomponent analysis, and a rather large sample volume (over 100 mL) is required. Compared to IRMS and NDIRS, LAS technology − has received increasing attention owing to its advantages such as high selectivity, fast response, small instrument size, simple operation, real-time online analysis, and the ability to determine multiple analytes simultaneously. In recent years, various techniques based on LAS for measuring carbon isotope ratios have been investigated, including off-axis integral cavity spectroscopy (OA-ICOS), cavity ring-down spectroscopy (CRDS), and wavelength modulation spectroscopy (WMS) .…”