Raman Natural Gas Analyzer: Effects of Composition on Measurement Precision

13CH4/12CH4 sensing using Raman spectroscopy

The paper presents a technique for measuring the concentration of 13CH4 in natural methane using Raman spectroscopy. The peak positions and the relative scattering cross-sections of the Q-branches for the most intense vibrational bands of 13CH4 are determined. Features of the 13CH4/12CH4 ratio measurement methods using Q-branches of the ν1 and ν3 bands were considered. It was shown that the 13CH4/12CH4 ratio can be determined by simulation of the ν3 bands of these molecules without the use of experimental spectra. In our experiments the measurement error of δ13C value was 10 ‰ using the 100-s exposure spectrum at a gas pressure close to 1 atm recorded on the developed Raman spectrometer. In addition, the Raman spectra of alkanes (up to n-hexane) in the range of 2850-3050 cm-1 at a resolution of 0.4 cm-1 are presented, and their integrated intensities in the ranges of the characteristic bands of 13CH4 and 12CH4 are provided. The data obtained make it possible to expand the capabilities of Raman gas analyzers in the mud gas logging industry.

PAS-based analysis of natural gas samples

Photoacoustic spectroscopy (PAS) is well known for the detection of short-chain hydrocarbons, such as methane, ethane and propane, in the ppm (parts per million) or ppb (parts per billion) range. However, in the production process of natural gas and its combustion in gas-fired devices the composition, especially the concentrations of the main alkanes, plays a decisive role. Gas chromatography (GC) is considered the gold standard for natural gas analysis. We present a method to analyze natural gas samples by PAS. Furthermore, we describe a method to prepare storage gas samples, which are usually under atmospheric pressure, for PAS analysis. All measurements are validated by means of GC. The investigation allows conclusions to be drawn to what extent PAS is suitable for the investigation of natural gas samples.

Natural Gas Analysis Using Polarized Raman Spectroscopy

Raman spectroscopy is a promising tool for measuring the composition of natural gas. However, to obtain high measurement accuracy, it is necessary to take into account changes in the spectral characteristics of methane, since its spectrum overlaps the characteristic bands of other species. In this study we present a technique for natural gas analysis based on polarized Raman spectroscopy. It is shown that the use of only isotropic components of spectra simplifies the procedure for extracting concentrations and improves the measurement accuracy of components whose spectral bands are significantly overlapped in conventional Raman spectra. The presented technique will be very useful both in the field of analysis of various multicomponent gas mixtures and in the field of measuring the isotopic composition of molecules.

Raman Spectroscopy for Urea Breath Test

The urea breath test is a non-invasive diagnostic method for Helicobacter pylori infections, which relies on the change in the proportion of ¹³CO₂ in exhaled air. Nondispersive infrared sensors are commonly used for the urea breath test in laboratory equipment, but Raman spectroscopy demonstrated potential for more accurate measurements. The accuracy of the Helicobacter pylori detection via the urea breath test using ¹³CO₂ as a biomarker is affected by measurement errors, including equipment error and δ¹³C measurement uncertainty. We present a Raman scattering-based gas analyzer capable of δ¹³C measurements in exhaled air. The technical details of the various measurement conditions have been discussed. Standard gas samples were measured. ¹²CO₂ and ¹³CO₂ calibration coefficients were determined. The Raman spectrum of the exhaled air was measured and the δ¹³C change (in the process of the urea breath test) was calculated. The total error measured was 6% and does not exceed the limit of 10% that was analytically calculated.

Pressure broadening in Raman spectra of CH4-N2, CH4-CO2, and CH4-C2H6 gas mixtures

Different molecular environments change the spectrum of a given gas sample involved in a mixture compared to the spectrum of a pure gas. It is necessary to account for this effect to improve the accuracy of the analysis of the natural gas composition by Raman spectroscopy. First, the change in the main components of natural gas (methane, nitrogen, carbon dioxide, and ethane) must be considered. This work is devoted to the mutual influence of CH₄-N₂, CH₄-CO₂, and CH₄-C₂H₆ on their characteristic Raman bands in the range of 300-2500 cm^-1. The half-width and asymmetry of the Q branches of N₂, CO₂, and C₂H₆ as a function of methane concentration were obtained in the range of 1-50 bar. The averaged broadening coefficients of the rotational-vibrational lines of the ν₂ band of CH₄ perturbed by N₂, CO₂, and C₂H₆ are measured. A high-sensitivity spectrometer with a resolution of 0.5 cm^-1 based on spontaneous Raman scattering was used to obtain reliable results. The algorithm and all the necessary parameters for simulating the effect of various molecular environments on the Raman bands of the main components of natural gas are presented.

Broadening of the ν2 Raman Band of CH4 by C3H8 and C4H10

Raman spectroscopy is a promising method for the analysis of natural gas. It is necessary to account for the broadening effects on spectral lines to improve measurement accuracy. In this study, the broadening coefficients for methane lines in the region of the ν₂ band perturbed by propane, n-butane, and isobutane at room temperature were measured. We estimated the measurement errors of the concentration of oxygen and carbon dioxide in the case of neglecting the broadening effects on the methane spectrum by the pressure of C₂-C₆ alkanes. The obtained data are suited for the correct simulation of the methane spectrum in the hydrocarbon-bearing gases and can be used to improve the accuracy of the analysis of natural gas by Raman spectroscopy.

Multiple Gas Detection by Cavity-Enhanced Raman Spectroscopy with Sub-ppm Sensitivity

Accurate and sensitive detection of multicomponent trace gases below the parts-per-million (ppm) level is needed in a variety of medical, industrial, and environmental applications. Raman spectroscopy can identify multiple molecules in the sample simultaneously and has excellent potential for fast diagnosis of various samples, but applications are often limited by its sensitivity. In this contribution, we report the development of a cavity-enhanced Raman spectroscopy instrument using a narrow-line width 532 nm laser locked with a high-finesse cavity through a Pound-Drever-Hall locking servo, which allows continuous measurement in a broad spectral range. An intracavity laser power of up to 1 kW was achieved with an incident laser power of about 240 mW, resulting in a significant enhancement of the Raman signal in the range of 200-5000 cm^-1 and a sub-ppm sensitivity for various molecules. The technique is applied in the detection of different samples, including ambient air, natural gas, and reference gas of sulfur hexafluoride, demonstrating its capability for the quantitative measurement of various trace components.

Optical Sensors Technology and Applications

Optical methods are non-invasive tools, and their use in various fields, including sensing applications, is continuously increasing, which is thanks to the continuous development of innovative low-cost sources and detectors [...].