Multipass Gas Cell Research Articles

High-Precision 13CO2/12CO2 Isotopic Ratio Measurement Using Tunable Diode Laser Absorption Spectroscopy at 4.3 μm for Deep-Sea Natural Gas Hydrate Exploration

For the detection of deep-sea natural gas hydrates, it is very important to accurately detect the 13CO2/12CO2 isotope ratio of dissolved gas in seawater. In this paper, a 13CO2/12CO2 isotope ratio sensor is investigated, which uses a tunable diode laser absorption spectroscopy (TDLAS) technique at 4.3 μm. The proposed sensor consists of a mid-infrared interband cascade laser (ICL) operating in continuous wave mode, a long optical path multi-pass gas cell (MPGC) of 24 m, and a mid-infrared mercury cadmium telluride (MCT) detector. Aiming at the problem of the strong absorption intensity of the two absorption lines of 13CO2 and 12CO2 being affected by temperature, a high-precision temperature control system for the MPGC was fabricated. Five different concentrations of CO2 gas were configured to calibrate the sensor, and the response linearity could reach 0.9992 for 12CO2 and 0.9996 for 13CO2. The data show that the carbon isotope measurement precision was assessed to be 0.0139‰ when the integration time was 92 s and the optical path length was 24 m. The sensor is combined with a gas–liquid separator to detect the 13CO2/12CO2 isotope ratio of CO2 gas extracted from water. Results validate the reported sensor system’s potential application in deep-sea natural gas hydrate exploration.

Highly sensitive acetylene detection based on a compact multi-pass gas cell and optimized wavelength modulation technique

A highly sensitive acetylene (C2H2) sensor system was demonstrated using tunable diode laser absorption spectroscopy (TDLAS) based on a compact multi-pass gas cell with a 10 m optical path. The sensor system can work in two measurement modes: direct measurements and wavelength modulation (WMS). The incident laser beam with a small spot radius (0.5 mm) effectively eliminated interference fringes and improved the signal-to-noise ratio. The minimum detection limit (MDL) of the direct detection mode was 4.9 ppm. As for the wavelength modulation method, after optimization the modulation depth, a MDL of 14.3 ppb was obtained. The corresponding noise-equivalent absorbance (NEA) was calculated and obtained an excellent value of 1.8 × 10-8 cm-1 Hz−1/2. An Allan deviation analysis was used to analyze the system stability. The MDL was improved to 1.3 ppb when the averaging time was 270 s.

Hydrogen sulphide detection using near-infrared diode laser and compact dense-pattern multipass cell**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFC0303900 and 2017YFC0209700).

Sub-ppmv level detection of hydrogen sulphide (H2S) using a 1.578- distributed feedback tunable diode laser combining with wavelength modulation spectroscopy and second harmonic detection scheme is reported. A home-developed novel compact dense-pattern multipass gas cell with an effective optical path length of 29.37 m is used to improve sensitivity and reduce sample volume. Detection parameters are optimized, including modulation frequency and amplitude. The analysis of Allan variance shows that a minimum detectable concentration 60 ppbv is obtained with a lock-in time constant of 10 ms, and a detection limit of 13 ppbv can be achieved by average in 300 s. The demonstrated H2S sensor has a strong penitential application in natural gas process for regulating and controlling H2S concentration.

Development of a compact tunable diode laser absorption spectroscopy based system for continuous measurements of dissolved carbon dioxide in seawater

Robust marine carbon sensors with small size, low power consumption, and high sensitivity provide greater insight into the carbon cycle studies and resolve environmental variability. We report here the development of a diminutively integrated tunable diode laser absorption spectroscopy (TDLAS) system with a specially designed multipass gas cell for small amounts of dissolved gas extractions and measurements. It was used to detect and monitor carbon dioxide (CO2) dissolved in water and seawater. Systematic experiments have been carried out for system evaluation in the lab. Extracted CO2 was determined via its 4989.9 cm-1 optical absorption line. The achieved TDLAS measurement precision was 4.18 ppm for CO2, measured by averaging up to 88 s. The integrated absorbance was found to be linear to gas concentrations over a wide range. Comparison measurements of the atmospheric CO2 values with a commercial instrument confirmed a good accuracy of our TDLAS-based system. The first test campaign was also accomplished with a hollow fiber membrane contactor, and concentrations of CO2 were quantitatively detected with partial degasification operations. The results clearly show the ability to continuously measure dissolved gases and highlight the potential of the system to help us better understand physical and geochemical processes in a marine environment.

A Lagrangian interpolation-assisted direct laser absorption spectrum analyzer based on digital signal processor for methane detection

A novel Lagrangian interpolation-based direct laser absorption spectroscopy (LI-DLAS) technique was presented to suppress noise in infrared gas detection by incorporating Lagrangian interpolation and nonlinear least-square fitting (NLLSF). An LI-DLAS analyzer was reported for methane (CH4) detection using a 1654 nm distributed feedback (DFB) laser, a compact digital signal processor (DSP), and a multi-pass gas cell (MPGC) with a 16 m optical path length. The performance of the developed LI-DLAS CH4 analyzer was evaluated by means of laboratory experiments. Compared with the traditional DLAS-based sensor without Lagrangian interpolation, the detection sensitivity was improved from 6 ppmv to 2 ppmv, and the detection stability was enhanced as the Allan–Werle deviation was dropped from 1.514 to 0.531 ppmv for a 1 s averaging time. Compared with a DLAS analyzer based on LabVIEW platform, the DSP-based CH4 analyzer shows the merits of compact size and low cost with potential filed-deployable applications in industrial monitoring and control.

ICL-based mid-infrared carbon dioxide sensor system for deep-sea natural gas hydrate exploration.

For deep-sea natural gas hydrate exploration, highly sensitive detection of the dissolved gas in seawater near the seabed is significant because it requires the sensor system to be small in size, low in power consumption, and high in sensitivity. A mid-infrared sensor system was developed to detect dissolved carbon dioxide (CO2) in sea-water, while employing a 4319 nm continuous-wave interband cascade laser (ICL) and a multi-pass gas cell (MPGC) with a 29.8 m optical path length. A compact rectilinear optical structure was proposed by using the free-space-emitting ICL and tunable laser absorption spectroscopy (TLAS). This leads to a minimized sensor size and a simple optical alignment for deep-sea operation. A strong CO2 absorption line, located at 2315.19 cm-1 and a weak 2315.28 cm-1 line and at a low pressure of 40 Torr, was targeted for low- and high-concentration CO2 detection within a concentration range of 0-1000 parts per billion by volume (ppbv) and 0-40 parts per million by volume (ppmv), respectively. The limit of detection (LoD) was assessed to be 0.72 ppbv at an averaging time of 2 s, and the response time was measured to be ~30 s at a flow rate of ~180 standard cubic centimeters per minute (sccm). Deployment of the CO2 sensor combined with a gas-liquid separator was carried out for the CO2 detection in the gas extracted from water, which validated the reported sensor system's potential application for deep-sea natural gas hydrate exploration.

Far-infrared self-continuum absorption of H216O and H218O (15–500 cm−1)

The water vapor continuum absorption is studied in a spectral range covering most of the pure rotational spectrum of water molecule up to 500 cm−1. The continuum absorption was derived from the broadband water vapor spectra recorded by Fourier transform spectrometer equipped with the 151-m multipass gas cell at the AILES beam line of the SOLEIL synchrotron. The coherent (10–35 cm−1) and standard (40–500 cm−1) radiation modes of the synchrotron were used. In order to refine the magnitude and clarify the physical origin of the continuum, spectra of the two major water isotopologues, H216O and H218O, were considered. Recordings at several water vapor pressures were used to check the expected quadratic pressure dependence of the continuum.The new data extend and supplement previous measurements filling, in particular, the gap between 200 and 350 cm−1, which was never studied before. The H216O and H218O absorption continua in the range of 50–650 cm−1 show similar frequency dependence and magnitude. In particular, both continua exhibit a clear water dimer spectral signature near 15 cm−1, in good agreement with previous ab initio calculations. The present data confirm that the MT-CKD empirical continuum model widely used in atmospheric applications, overestimates importantly the continuum magnitude in the whole range of the rotational band. The observed irregular frequency dependence of the retrieved self-continuum cross-section values is tentatively interpreted as due to uncertainties on the resonance lines of the water monomer spectrum which is subtracted from the recorded spectra. On the basis of spectra simulations, the inadequate description of the line shapes in the range of the intermediate wings (detuning of 5–10 cm−1 from line center) and the uncertainties on the self-broadening coefficients of water monomer lines are identified as possible mechanisms responsible of the observed irregular fluctuations.

Multi-gas detection in power transformer oil based on tunable diode laser absorption spectrum

Online dissolved gas analysis (DGA) is an effective technique to obtain the health status information of insulation oil in power transformers. The typical hydrocarbon gases (methane, ethyne, ethene, ethane) decomposed from insulation oil, are the key indicators to diagnose the fault type and severity. To realize contactless and field measurement, a multi-gas detection system based on tunable diode laser absorption spectrum (TDLAS) is proposed in this paper. Critical technique problems on the light source, long path gas cell, and the topology for multi-gas detection are investigated. Individual central wavelengths of hydrocarbon gases are set at as 1653.72 nm for methane, 1530.37 nm for ethyne, 1620.04 nm for ethene and 1679.06 nm for ethane in the near infrared band. A 10.13 m long multi-pass gas cell is developed with the advantage of anti-vibration design. To share the optical absorption path, an optical switch is adopted to merge the multi-gas detection system. The high sensitivity of the TDLAS multi-gas detection system was demonstrated in the laboratory calibrations, with ethyne detection reaching sub-parts per million level, and the other hydrocarbon gases achieving ppm level. Furthermore, the comparison detection in the real 220 kV power transformer gave evidence to the effective monitoring of DGA, proving it an alternative approach to online detection of multiple gases in power transformer oil.

Interband cascade laser absorption sensor for real-time monitoring of formaldehyde filtration by a nanofiber membrane.

It is important to reduce the indoor formaldehyde (H2CO) level to improve indoor air quality. To investigate the H2CO filtration by a novel nanofiber membrane made from metal-organic frameworks (MOFs), we developed a laser absorption gas sensor for real-time H2CO monitoring using a room-temperature interband cascade laser (ICL) emitting at 3.6µm. Wavelength modulation spectroscopy combined with a multipass gas cell (36m path length) was implemented to achieve a detection sensitivity of 3ppb H2CO at 1-s averaging time. We custom-designed a permeation H2CO generator that produces reference H2CO/N2 mixtures with an uncertainty of 6.4% in concentration. With the time-resolved continuous measurements, we observed a high filtration efficiency of 83% for the MOF filter, which, however, decreases linearly to 30% after operating for 3h. Hence, the ICL-based gas sensor has proved to be a promising technique to assess novel nanomaterials for indoor air purification and pollutant control applications.

Highly sensitive and selective CO sensor using a 2.33 μm diode laser and wavelength modulation spectroscopy.

A ppm-level CO sensor based on a 2f wavelength modulation spectroscopy (2f-WMS) technique was developed for the application of SF6 decomposition analysis in an electric power system. A detailed investigation of the optimum target line selection was carried out to avoid spectral interference from high purity SF6 in a wide wavelength range. A diode laser emitting at 2.33 μm and a 14.5-m multipass gas cell (MGC) was employed to target the R(6) line of the CO first overtone band and increase the optical path, respectively, thus resulting in a minimum detection sensitivity of 1 ppm. A Levenberg-Marquardt nonlinear least-squares fit algorithm makes full use of the information from all data points of the 2f spectrum and as a result, a measurement precision of ~40 ppb was achieved with a data update rate of 0.6 s. The sensor performance was also evaluated in terms of the gas flow rate, stability, and linearity. The results showed that the best operating condition with a precision of 6 ppb can be achieved by increasing the gas flow rate to the value that matches the optimum averaging time of 48 s.

Interband cascade laser-based ppbv-level mid-infrared methane detection using two digital lock-in amplifier schemes

A parts-per-billion in volume (ppbv) level mid-infrared methane (CH4) sensor system was demonstrated using second-harmonic wavelength modulation spectroscopy (2f-WMS). A 3291 nm interband cascade laser (ICL) and a multi-pass gas cell (MPGC) with a 16 m optical path length were adopted in the reported sensor system. Two digital lock-in amplifier (DLIA) schemes, a digital signal processor (DSP)-based DLIA and a LabVIEW-based DLIA, were used for harmonic signal extraction. A limit of detection (LoD) of ~ 13.07 ppbv with an averaging time of 2 s was achieved using the DSP-based DLIA and a LoD of ~ 5.84 ppbv was obtained using the LabVIEW-based DLIA with the same averaging time. A rise time of 0→2 parts-per-million in volume (ppmv) and fall time of 2→0 ppmv were observed. Outdoor atmospheric CH4 concentration measurements were carried out to evaluate the sensor performance using the two DLIA schemes.

Refractive index retrievals for polystyrene latex spheres in the spectral range 220–420 nm

ABSTRACTThe recent development of an Aerosol Extinction Differential Optical Absorption Spectrometer (AE-DOAS) has allowed for the retrieval of wavelength dependent complex refractive indices for polystyrene latex spheres (PSL). The AE-DOAS is a white-type multi-pass gas cell coupled to a UV-Vis spectrometer. Refractive index values are retrieved for wavelengths between 220 and 420 nm by minimizing the χ2 goodness-of-fit between measured extinction for five diameters of PSL and model Mie Theory predictions. Comparison to literature shows agreement at wavelengths >360 nm demonstrating the validity of this new instrumental approach while expanding the known refractive index range for PSLs further into the UV where it is previously unreported. In the studied wavelength range, coefficients for the general Cauchy dispersion relationship (A = 1.538(11); B = 0.0043(16) μm2; C = 0.00094(5) μm4) for PSLs were determined using the retrieved real portion of the refractive index and the wavelength in microns. In addi...

Development and field deployment of a mid-infrared methane sensor without pressure control using interband cascade laser absorption spectroscopy

A mid-infrared methane (CH4) sensor without pressure control was developed using a continuous-wave (CW) interband cascade laser (ICL) for targeting a CH4 absorption line located at 3038.5cm−1. A multi-pass gas cell with an absorption path length of 54.6m was utilized for enhancing gas absorption. The pressure inside the MPGC was measured using direct Lorentzian absorption fitting for the compensation of CH4 concentration changes resulting from pressure variations. Laboratory pressure calibration was conducted in the range of 25–800Torr using 1.3-, 1.5-, 1.7- and 2.1-ppmv CH4 samples. A pressure precision of ∼1.65Torr with a ∼2.5-s averaging time was achieved based on the measurement of a 2.1-ppmv CH4 sample at 700-Torr. Concentration level measurements of a 2.1-ppmv CH4 sample at a 700-Torr pressure yielded an Allan deviation of 2.25 ppbv for an averaging time of 2.5s. The sensor functioned normally with CH4 samples at 1.0, 1.2, 1.4, 1.6 and 2.1ppmv concentration levels as the pressure changes from 25 to 800Torr. Indoor/outdoor CH4 concentration measurements on the Rice University campus and a field campaign in the Greater Houston Area (GHA) were conducted to evaluate the performance of the sensor system.

Tracing methane dissolved in transformer oil by tunable diode laser absorption spectrum

Dissolved gas analysis (DGA)in power transformers is related to the degradation of insulation materials and dissolved methane (CH 4 )is a symbol gas of low energy electrical or thermal faults in power transformer oil. Conventional online DGA equipment suffers from problems such as cross sensitivity, electromagnetic compatibility and reliability. Based on tunable diode laser absorption spectroscopy (TDLAS) technique, a novel optical method is proposed to detect dissolved methane in transformer oil. Oil samples with fault gases are prepared and dissolved methane detection has been carried out in the laboratory. 1653.72 nm is selected as the central wavelength of tunable diode laser, moreover, a multipass gas cell and wavelength modulation strategy is utilized to trace methane dissolved in oil. Results show that the resolution of detection reaches 0.28 µL/L, and accuracy is less than 2 µL/L at low concentration. In addition, the detection cycle is so time-saving, less than 5 minutes. The proposed measurement method shows tremendous advantages, such as immediate response to concentration changes, no cross interference, no carrier gases, which is proved to be a promising maintain-free technique instead of conventional DGA equipment.

Spectral Apparatus with a Cryogenic, High-Throughput, Multipass Gas Cell for Studies of Absorption of Radiation by Gaseous Media

Spectral systems with an MKhK-6 cryogenic, high-throughput, multipass gas cell for studying the absorption spectra of gaseous media with high spectral resolution in the 0.1–6 μm range at pressures of 100 to 5·106 Pa and temperatures of 180–300 K are discussed. Their use in measurements of spectral absorption coefficients, temperature dependences of the spectral transmission function, and parameters of spectral absorption lines is examined.

A handheld mid-infrared methane sensor using a dual-step differential method for additive/multiplicative noise suppression

A miniature mid-infrared (mid-IR) methane (CH4) sensor system was developed by employing a wide-band wire-source and a semi-ellipsoid multi-pass gas cell. A dual-step differential method instead of the traditional one-step differential method was adopted by this sensor to tune measuring range/zero point and to suppress the additive/multiplicative noise. This method included a first subtraction operation between the two output signals (including a detection signal and a reference signal) from the dual-channel detector and a second subtraction operation on the amplitudes of the first-subtraction signal and the reference signal, followed by a ratio operation between the amplitude of the second-subtraction signal and the reference signal. Detailed experiments were performed to assess the performance of the sensor system. The detection range is 0–50kppm, and as the concentration gets larger than 12kppm, the relative detection error falls into the range of −3% to +3%. The Allan deviation is about 4.65ppm with an averaging time of 1s, and such value can be further improved to 0.45ppm with an averaging time of 124s. Due to the cost-effective incandescence wire-source, the small-size ellipsoid multi-pass gas cell and the miniature structure of the sensor, the developed standalone device shows potential applications of CH4 detection under coal-mine environment.

Development of a mid-infrared nitrogen dioxide sensor based on Faraday rotation spectroscopy

A sensitive nitrogen dioxide (NO2) sensor system based on Faraday Rotation Spectroscopy (FRS) was developed. The advantages of FRS include high detection sensitivity, zero background and free from the influence of diamagnetic species, such as humidity and carbon dioxide. A widely tunable external cavity quantum cascade laser (EC-QCL) was used as the excitation source. The tunable EC-QCL operates mode-hop free between 1600cm−1 and 1650cm−1 and allows targeting the optimum 441<−440 Q-branch transition of NO2 at 1613.2cm−1 with an optical power of ∼135mW. A custom made 22.47cm long multipass gas cell (MPGC) was implemented in which the EC-QCL radiation was passed 45 times and provided an effective optical path length of 10.1m in order to design a compact sensor system with high detection sensitivity. An air core solenoid surrounding the MPGC was used for generating a magnetic field of 200G. Performance details of a FRS based sensor and its application for long time continuous NO2 measurements are reported. The long time stability of the sensor system was evaluated. A NO2 detection sensitivity based on an Allan deviation plot of ∼95ppt was obtained by averaging the associated data for 300s.

Ppb-level mid-infrared ethane detection based on three measurement schemes using a 3.34-μm continuous-wave interband cascade laser

A ppb-level mid-infrared ethane (C2H6) sensor was developed using a continuous-wave, thermoelectrically cooled, distributed feedback interband cascade laser emitting at 3.34 μm and a miniature dense patterned multipass gas cell with a 54.6-m optical path length. The performance of the sensor was investigated using two different techniques based on the tunable interband cascade laser: direct absorption spectroscopy (DAS) and second-harmonic wavelength modulation spectroscopy (2f-WMS). Three measurement schemes, DAS, WMS and quasi-simultaneous DAS and WMS, were realized based on the same optical sensor core. A detection limit of ~7.92 ppbv with a precision of ±30 ppbv for the separate DAS scheme with an averaging time of 1 s and a detection limit of ~1.19 ppbv with a precision of about ±4 ppbv for the separate WMS scheme with a 4-s averaging time were achieved. An Allan–Werle variance analysis indicated that the precisions can be further improved to 777 pptv @ 166 s for the separate DAS scheme and 269 pptv @ 108 s for the WMS scheme, respectively. For the quasi-simultaneous DAS and WMS scheme, both the 2f signal and the direct absorption signal were simultaneously extracted using a LabVIEW platform, and four C2H6 samples (0, 30, 60 and 90 ppbv with nitrogen as the balance gas) were used as the target gases to assess the sensor performance. A detailed comparison of the three measurement schemes is reported. Atmospheric C2H6 measurements on the Rice University campus and a field test at a compressed natural gas station in Houston, TX, were conducted to evaluate the performance of the sensor system as a robust and reliable field-deployable sensor system.

CW EC-QCL-based sensor for simultaneous detection of H2O, HDO, N2O and CH4 using multi-pass absorption spectroscopy.

A sensor system based on a continuous wave, external-cavity quantum-cascade laser (CW EC-QCL) was demonstrated for simultaneous detection of atmospheric H2O, HDO, N2O and CH4 using a compact, dense pattern multi-pass gas cell with an effective path-length of 57.6 m. The EC-QCL with a mode-hop-free spectral range of 1225-1285 cm-1 operating at ~7.8 µm was scanned covering four neighboring absorption lines, for H2O at 1281.161 cm-1, HDO at 1281.455 cm-1, N2O at 1281.53 cm-1 and CH4 at 1281.61 cm-1. A first-harmonic-normalized wavelength modulation spectroscopy with second-harmonic detection (WMS-2f/1f) strategy was employed for data processing. An Allan-Werle deviation analysis indicated that minimum detection limits of 1.77 ppmv for H2O, 3.92 ppbv for HDO, 1.43 ppbv for N2O, and 2.2 ppbv for CH4 were achieved with integration times of 50-s, 50-s, 100-s and 129-s, respectively. Experimental measurements of ambient air are also reported.

Compact TDLAS based optical sensor for ppb-level ethane detection by use of a 3.34 μm room-temperature CW interband cascade laser

A mid-infrared ethane (C2H6) sensor based on a wavelength modulation spectroscopy (WMS) technique was developed using a thermoelectrically cooled (TEC), continuous-wave (CW) interband cascade laser (ICL) emitting at 3.34μm and a dense multi-pass gas cell (MPGC, 17×6.5×5.5cm3) with a 54.6m optical path length. A compact optical sensor system with a physical size of 35.5×18×12.5cm3 was designed and constructed. An ICL was employed for targeting a strong C2H6 line at 2996.88cm−1 at <100Torr gas pressure in the fundamental absorption band of C2H6. The sensor performance, including the minimum detection limit (MDL) and the stability were improved by reducing the effect of laser power drift by means of the 2f/1f-WMS technique. A MDL of ∼1.2 parts per billion (ppbv) for 2f-WMS and ∼1.0ppbv for 2f/1f-WMS were achieved, respectively, with a measurement time of 4s. The MDL was further improved from 299pptv (@108s for 2f-WMS) to 239pptv (@208s for 2f/1f-WMS), based on an Allan deviation analysis. The rise time (@0→100ppbv) and fall time (@100→0ppbv) were determined to be ∼64s and ∼48s, respectively, at a gas pressure of <100Torr for the C2H6 sensor operation.