Ppm Methane Research Articles

Hot-wire-type micromachined chemiresistive gas sensors for battery-powered city gas alarms

Abstract Metal oxide semiconductor (MOX) chemiresistive gas sensors used in gas alarms have contributed to the safe use of city gas and liquid petroleum gas. In this study, we successfully fabricated hot-wire-type MOX sensors using micro-electro-mechanical systems (MEMS) technology. The hot-wire type structure, in which an electrode plays dual roles in detecting and heating, was adopted for efficient production. Owing to the miniaturization together with the thermal insulation, the sensors exhibited a fast thermal response. The average power consumption of the sensor in the pulsed operation was less than 100 μW. The sensor exhibited high sensitivity of more than 100 mV to 3000 ppm methane and showed low cross-sensitivity to interference gases such as ethanol and hydrogen. These sensing properties were retained for more than five years, demonstrating excellent long-term stability of the sensors.

Design and evaluation of a low-cost sensor node for near-background methane measurement

Abstract. We developed a low-cost methane sensing node incorporating two metal oxide (MOx) sensors (Figaro Engineering TGS2611-E00 and TGS2600), humidity and temperature sensing, data storage, and telemetry. We deployed the prototype sensor alongside a reference methane analyzer at two sites: one outdoors and one indoors. We collected data at each site for several months across a range of environmental conditions (particularly temperature and humidity) and methane levels. We explored calibration models to investigate the performance of our system and its suitability for methane background monitoring and enhancement detection, first selecting a linear regression to fit a sensor baseline response and then fitting methane response by the sensor deviation from baseline. We achieved moderate accuracy in a 2 to 10 ppm methane range compared to data from the reference analyzer (RMSE < 0.6 ppm), but we found that the sensor response varied over time, possibly as a result of changes in non-targeted gas concentrations. We suggest that this cross sensitivity may be responsible for mixed results in previous studies. We discuss the implications of our results for the use of these and similar inexpensive MOx sensors for methane monitoring in the 2 to 10 ppm range.

Development of a fast‐response system with integrated calibration for high‐resolution mapping of dissolved methane concentration in surface waters

AbstractDissolved gas concentrations in surface waters can have sharp gradients across marine and freshwater environments, which often prove challenging to capture with analytical measurement. Collecting discrete samples for laboratory analysis provides accurate results, but suffers from poor spatial resolution. To overcome this limitation, water equilibrators and gas membrane contactors (GMCs) have been used for the automated underway measurement of dissolved gas concentrations in surface water. However, while water equilibrators can provide continuous measurements, their analytical response times to changes in surface water concentration can be slow, lasting tens of minutes. This leads to spatial imprecisions in the dissolved gas concentration data. Conversely, while GMCs have proven to have much faster analytical response times, often lasting only a few minutes or less, they suffer from poor accuracy and thus require routine calibration. Here we present an analytical system for the high accuracy and high precision spatial mapping of dissolved methane concentration in surface waters. The system integrates a GMC with a cavity ringdown spectrometer for fast analytical response times, with a calibration method involving two Weiss‐style equilibrators and discrete measurements in vials. Data from both the GMC and equilibrators are collected simultaneously, with discrete vial samples collected periodically throughout data collection. We also present a mathematical algorithm integrating all data collected for the routine calibration of the GMC dataset. The algorithm facilitates comparison between the GMC and equilibrator datasets despite the substantial differences in response times (0.7–2.1 and 4.1–17.6 min, respectively). This measurement system was tested with both systematic laboratory experiments and field data collected on a research cruise along the US Atlantic margin. Once calibrated, this system identified numerous sharp peaks of dissolved methane concentration in the US Atlantic margin dataset that would be poorly resolved, or outright missed with previous measurement techniques. Overall, the precision and accuracy for the technique presented here were determined to be 11.2% and 10.4%, respectively, the operating range was 0–1000 ppm methane, and the e‐folding response time to changes in dissolved methane concentration was 0.7–2.1 min.

Near-Infrared Dual Greenhouse Gas Sensor Based on Hollow-Core Photonic Crystal Fiber for Gas-Cell In-Situ Applications.

A greenhouse gas sensor has been developed to simultaneously detect multiple gas species within a hollow-core photonic bandgap fiber (HC-PBF) structure entirely composed of fibers. To enhance sensitivity, the gas cell consists of HC-PBF enclosed between two single-mode fibers fused with a reflective end surface to double the absorption length. The incorporation of side holes for gas diffusion allows for analysis of the relationship between gas diffusion speed, number of drilled side holes, and energy loss. As the number of drilled holes increases, the response time decreases to less than 3 min at the expense of energy loss. Gas experiments demonstrated detection limits of 0.1 ppm for methane and 2 ppm for carbon dioxide, with an average time of 50 s. In-situ testing conducted in rice fields validates the effectiveness of the developed gas detection system using HC-PBF cells, establishing all-fiber sensors with high sensitivity and rapid response.

Calibration of a Low-Cost Methane Sensor Using Machine Learning.

In order to combat greenhouse gas emissions, the sources of these emissions must be understood. Environmental monitoring using low-cost wireless devices is one method of measuring emissions in crucial but remote settings, such as peatlands. The Figaro NGM2611-E13 is a low-cost methane detection module based around the TGS2611-E00 sensor. The manufacturer provides sensitivity characteristics for methane concentrations above 300 ppm, but lower concentrations are typical in outdoor settings. This study investigates the potential to calibrate these sensors for lower methane concentrations using machine learning. Models of varying complexity, accounting for temperature and humidity variations, were trained on over 50,000 calibration datapoints, spanning 0-200 ppm methane, 5-30 °C and 40-80% relative humidity. Interaction terms were shown to improve model performance. The final selected model achieved a root-mean-square error of 5.1 ppm and an R2 of 0.997, demonstrating the potential for the NGM2611-E13 sensor to measure methane concentrations below 200 ppm.

Evaluation of the applicability of a metal oxide semiconductor gas sensor for methane emissions from agriculture

This work investigated the potential of metal oxide semiconductor (MOS) gas sensors for environmental monitoring of methane. Calibrations were performed under controlled conditions in the lab, and under semi-controlled conditions in the field, using a modified head space chamber set-up. Concentrations up to ±300 ppm methane were tested. The relationship between sensor conductance and methane concentrations could be very well described using principles from adsorption theory. The adjustable parameters were background conductance G0, a sensitivity constant S and a non-ideality coefficient n, where n has a non-rational value between 0 and 1. Sensor behaviour was very different in dry air than in humid air, with the background conductance increasing approximately tenfold and sensitivity decreasing between 20 fold and 80 fold, while the non-ideality coefficient increased from ±0.4 to ±0.6. Nevertheless, at high methane concentrations comparable conductance values were recorded in dry and humid air. The standard deviation of predicted values was 1.6 μS.for the least well described dataset. Using the corresponding calibration curve, a detection limit of 11 ppm is calculated for humid ambient air. This values suggests that MOS sensor are adequately sensitive to be used for methane detection in an agricultural context.

Catalytic Decomposition of an Organic Electrolyte to Methane by a Cu Complex-Derived In Situ CO2 Reduction Catalyst.

Metal complexes are often transformed to metal complex-derived catalysts during electrochemical CO2 reduction, enhancing the catalytic performance of CO2 reduction or changing product selectivity. To date, it has not been investigated whether metal-complex derived catalysts also enhance the decomposition of the solvent/electrolyte components as compared to an uncoated electrode. Here, we tested the electrochemical stability of five organic solvent-based electrolytes with and without a Cu complex-derived catalyst on carbon paper in an inert atmosphere. The amount of methane and hydrogen produced was monitored using gas chromatography. Importantly, the onset potential for methane production was reduced by 300 mV in the presence of a Cu complex-derived catalyst leading to a significant amount of methane (417.7 ppm) produced at -2.17 V vs Fc/Fc+ in acetonitrile. This suggests that the Cu complex-derived catalyst accelerated not only CO2 reduction but also the reduction of the electrolyte components. This means that Faradaic efficiency (FE) measurements under CO2 in acetonitrile may significantly overestimate the amount of CH4. Only 28.8 ppm of methane was produced in dimethylformamide under an inert atmosphere, much lower than that produced under CO2 (506 ppm under CO2) at the same potential, suggesting that dimethylformamide is a more suitable solvent. Measurements in propylene carbonate produced mostly hydrogen gas while in dimethyl sulfoxide and 3-methoxypropionitrile neither methane nor hydrogen was detected. A strong linear correlation between the measured current and the amount of methane produced with and without the Cu complex-derived catalyst confirmed that the origin of methane production is solvent/electrolyte decomposition and not the decomposition of the catalyst itself. The study highlights that in a nonaqueous system, highly active catalyst in situ deposited during electrochemical testing can significantly influence background measurements as compared to uncoated electrodes, therefore the choice of solvent is paramount for reliable testing.

Effect of Ni doping on methane removal from hydrogen in ZrMnFe alloy and mechanism analysis

Getter alloy has the advantages of low cost and high hydrogen purity in small and medium-scale hydrogen purification. However, the commonly used ZrMnFe alloy is reactive to various impurities in hydrogen, and requires a temperature above 600 °C to remove methane. To improve reactivity with methane and ensure a high hydrogen adsorption platform pressure, partial substitution of Fe with Ni, which has a smaller bonding energy with methane ligand, is beneficial. In this work, we prepared ZrMnFeNi alloys with different Ni stoichiometric ratios by suspension induction melting and investigated their performance in purifying hydrogen containing trace amounts of methane. All alloys maintain C14 Laves phase, and the interplanar spacing of the (103) crystal planes decreases with the replacement of Ni. It is proved that ZrMnFeNi alloys can significantly improve the absorb efficiency of methane in hydrogen and reduce the removal temperature. At 450 °C, ZrMnFe0.7Ni0.3 can remove 10 ppm methane to 0.124 ppm. The purification efficiency of hydrogen at 450 °C can reach 96.94%, while the purification efficiency of ZrMnFe alloy is only 27.12%. The ZrMnFe0.7Ni0.3 alloy decomposes methane into carbon and hydrogen during purification, and the carbon diffuses into the alloy at high temperature to form a metal carbide with zirconium. The XPS results show that the electron-rich state of Ni is more beneficial for activating the C–H bond breaking of methane.

A methanotrophic bacterium to enable methane removal for climate mitigation

The rapid increase of the potent greenhouse gas methane in the atmosphere creates great urgency to develop and deploy technologies for methane mitigation. One approach to removing methane is to use bacteria for which methane is their carbon and energy source (methanotrophs). Such bacteria naturally convert methane to CO2 and biomass, a value-added product and a cobenefit of methane removal. Typically, methanotrophs grow best at around 5,000 to 10,000 ppm methane, but methane in the atmosphere is 1.9 ppm. Air above emission sites such as landfills, anaerobic digestor effluents, rice paddy effluents, and oil and gas wells contains elevated methane in the 500 ppm range. If such sites are targeted for methane removal, technology harnessing aerobic methanotroph metabolism has the potential to become economically and environmentally viable. The first step in developing such methane removal technology is to identify methanotrophs with enhanced ability to grow and consume methane at 500 ppm and lower. We report here that some existing methanotrophic strains grow well at 500 ppm methane, and one of them, Methylotuvimicrobium buryatense 5GB1C, consumes such low methane at enhanced rates compared to previously published values. Analyses of bioreactor-based performance and RNAseq-based transcriptomics suggest that this ability to utilize low methane is based at least in part on extremely low non-growth-associated maintenance energy and on high methane specific affinity. This bacterium is a candidate to develop technology for methane removal at emission sites. If appropriately scaled, such technology has the potential to slow global warming by 2050.

Methane and Hydrocarbon Emission Rates from Oil and Gas Production in the Province of Basra, South of Iraq

Petroleum deposits typically contain a gaseous component known as “associated gas”. If associated gas is not collected or used on-site, it may be flared or vented as a byproduct of oil production. Flaring is preferential to venting in terms of global warming. Flaring oxidizes carbon and produces CO2, but by destroying methane and other hydrocarbons, it decreases the overall risk of global warming emissions. On a 100-year time scale, fossil methane is 36 times more potent than CO2. The measurements were taken at six oil fields (AlTuba, Allhais, Artawi, North Rumaila, Nahran Omar, and Majnoon) and two power plants (Alnajebia power plant and Shatt Al-Basra). To measure the concentrations of hydrocarbons, a direct measurement was taken for six months. ANOVA test was used to analyze the results at p ≤ 0.05 and a correlation between hydrocarbon concentrations, wind speed, and temperature was done. The result showed that the hydrocarbons were found in all stations, in all measurements with different concentrations except in Alnajebia and Shatt Al-Basra power plant at first measurements, the device did not detect hydrocarbons concentration. The highest average of hydrocarbon concentrations was at Allhais which was recorded as 995, 598.15, 418.7, 358.89 and 279.13 ppm for methane, ethane, propane, butane and pentane respectively. The lowest average was found at the majnoon oil field for methane, ethane, propane, butane and pentane 395, 238.8, 167.16, 142.2 and 111.4 ppm respectively. The relationship between the average hydrocarbon concentrations with the average monthly wind speed and with average monthly temperature was positive.

Calibration-free near-infrared methane sensor system based on BF-QEPAS

Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) is a sensitive and selective analytical technique widely used for gas sensing. To improve the response time of the methane sensing system based on QEPAS, a Beat Frequency Quartz-Enhanced Photoacoustic Spectroscopy (BF-QEPAS) system was developed. Theoretical model based on the classical spring-mass oscillator model to explain the generation of beat-frequency signal in the quartz tuning fork (QTF) was established. The BF-QEPAS system parameters were optimized and evaluated using the developed system. The performance of the system was analyzed through Allan variance and gas calibration. Experimental results showed that the BF-QEPAS system achieved a minimum detectable limit of 28.35 ppm for methane with an integration time of 114 s. The corresponding minimum detectable normalized noise equivalent absorption (NNEA) coefficient was 1.93 × 10-9 cm-1WHz1/2, which demonstrates the excellent sensitivity of the BF-QEPAS system for methane detection.

Room-temperature methane sensors based on ZnO with different exposed facets: A combined experimental and first-principle study

Methane is a flammable and explosive gas, which is difficult to detect at room temperature. In this work, the photo-activation route was explored to realize the monitoring of methane at room temperature, and the strategy of surface morphology engineering was used to boost excellent methane gas-sensing performance. Three types of ZnO structures: rods, plates, and spheres with exposed (001) and (100) crystal facets in different ratios, were obtained using solvothermal methods. Such characteristics of samples were confirmed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption-desorption analyzer. The gas-sensing performances of sensors based on as-prepared materials were tested under UV light and in the dark. The results indicate that sensors exhibit excellent gas-sensing performances toward methane under UV light at room temperature. ZnO spheres showed a much better response of 20.577 toward 5000 ppm methane and a faster response time of 6 s compared to ZnO plates and rods. The enhanced gas sensing properties were ascribed to high-energy exposed facets and unique hollow structures. In addition, density functional theory (DFT) was investigated to support the effect of crystal facets

Dual Absorption Broadband Photoacoustic Technique to Eliminate Interference in Gas Mixtures

This article presents a dual-wavelength band broadband photoacoustic spectroscopy (BPAS) technique ideally suited to low-energy broadband sources for industrial and environmental multigas sensing applications. The broad spectral range of the supercontinuum source is used to cover a wide range of absorption bands of various gases. The major limitation in most of the gas sensors with broadband sources is the effect of interference between gases. This article proposes a dual-wavelength technique to eliminate the impact of cross sensitivity among gases. This sensor exhibits a comprehensive concentration measurement from parts per million (ppm) to % levels. The proposed technique showed a ±1.2% error between the calculated and calibrated concentration values. We achieved a minimum detection limit (MDL) of 0.4 and 0.7 ppm for methane (CH4) with an integration time of 142 and 200 s using 2315- and 1650-nm band, respectively, 0.3 and 1.8 ppm of ammonia (NH3) with an integration time of 100 and 48 s using 2315- and 2017-nm band, respectively, and also 2.6 ppm of carbon monoxide (CO) with an integration time of 71 s using the 2315-nm band.

Coupling a biochar filter to a leach bed reactor for anaerobic digestion of chicken litter

In this study, coupling a biochar-packed anaerobic filter (AF) to a batch leach bed reactor (LBR) is investigated as a pragmatic method for biochar application to enhance high-solids anaerobic digestion of chicken litter. Recirculating leachate through a biochar filter reduced solid retention time by 45 % compared with a batch system without recirculation. The key benefit of biochar over inert filter media was the capability to reduce hydrogen sulfide production by 37 % overall; 25 % in the LBR, and 93 % in the AF. No significant differences in methane yield were observed. Biogas produced in the biochar AF was of higher quality with a maximum hydrogen sulfide content of 188 ppm and peak methane content of 73 %. However, majority of the biogas was produced in the LBR due to poor phase separation. Further research is required to increase biogas production in the AF and/or reduce hydrogen sulfide production in the LBR.

Electrostatically functionalized CVD grown multiwalled carbon nanotube/ palladium polymer nanocomposite (MWCNT/Pd) for methane detection at room temperature

By reducing the aqueous mixture of electrostatically modified or functionalized multi-walled carbon nanotubes (f-MWCNT) and palladium (Pd) using sodium borohydride (NaBH4), a nanocomposite was synthesized. The morphology of the f-MWCNT and f-MWCNT/ Pd nanocomposite was investigated by analytical tools FESEM and TEM respectively. f-MWCNTs matrix reinforced with Pd nanoparticles are utilized for sensing methane with its dilution varying from 0.5 to 100 ppm in air at room temperature (RT = 27 °C). The f-MWNT/Pd nanocomposite-based sensor for methane gas show several merits over conventional catalytic beads and metal oxide semiconductor (MOS) based sensors in context of reduced size, reduced power consumption and ease of fabrication. The temporal electrical responses of the nanocomposite to methane were measured at RT. It exhibited a response magnitude of ∼19.5–41.71 % with a small variation of ± 2 % towards 0.5–500 ppm methane. Furthermore, the responses were extremely reversible and repeatable, implying that it may be used to detect methane at ambient temperature. It has also been experimentally observed that an excellent response time (≈ 20 s) and the recovery time (≈ 25 s) for these devices were recorded for methane. The effect of environmental temperature and humidity were also studied on the methane sensor for the analysis of their long-term stability and self-life. The results showed that very small change is observed over a wide of temperature from 25 to 70 °C and that for %Rh from 20 to 90 %. Therefore, the reported methane sensor absolutely demonstrated long-term stability at ambient conditions and hence, these devices can prove to be ideal for methane leakage detection for practical real time applications. A sensing mechanism by which methane is detected by f-MWCNT/Pd nanocomposite is also elaborately discussed.

Synthesis of Porous Hierarchical In2O3 Nanostructures with High Methane Sensing Property at Low Working Temperature.

Different hierarchical porous In2O3 nanostructures were synthesized by regulating the hydrothermal time and combining it with a self-pore-forming method. The gas-sensing test results show that the response of the sensor based on In2O3 obtained after hydrothermal reaction for 48 h is about 10.4 to 500 ppm methane. Meanwhile, it possesses good reproducibility, stability, selectivity and moisture resistance as well as a good exponential linear relationship between the response to methane and its concentration. In particular, the sensor based on In2O3 can detect a wide range of methane (10~2000 ppm) at near-room temperature (30 °C). The excellent methane sensitivity of the In2O3 sensor is mainly due to its unique nanostructure, which has the advantages of both porous and hierarchical structures. Combined with the DFT calculation, it is considered that the sensitive mechanism is mainly controlled by the surface adsorbed oxygen model. This work provides a feasible strategy for enhancing the gas sensitivity of In2O3 toward methane at low temperatures.

Impact of Shale Gas Exploration and Exploitation Activities on the Quality of Ambient Air—The Case Study of Wysin, Poland

The continuous two-year monitoring of a set of air pollutants, as well as gases directly related to shale gas exploration processes (methane, non-methane hydrocarbons, carbon dioxide), was carried out at Stary Wiec village in the vicinity (1100 m) of the shale gas wells area in Wysin (Pomeranian voivodeship, north of Poland), covering the stages of preparation, drilling, hydrofracturing and closing of wells. The results of analysis of air pollution data from Stary Wiec and nearby urban and rural stations, over the period 2012–2017 (starting three years before preparations for hydraulic fracturing) indicated that Stary Wiec represents a clean rural environment with an average concentration of nitrogen oxides, carbon monoxide and particulate matter that is one of the lowest in the Pomeranian region. The aim of this study was to explore the range of potential impact of shale gas exploration on local ambient air quality. Analysis of dependence of the concentration level of pollutants on the wind direction indicated that during the drilling period, when the air was coming directly from the area of the wells, nitrogen oxide concentration increased by 13%. Increases of concentration during the hydro-fracturing period, recorded at the Stary Wiec station, were equal to 108%, 21%, 18%, 12%, 7%, 4%, 1% for nitrogen oxide, non-methane hydrocarbons, carbon monoxide, nitrogen dioxide, particulate matter, carbon dioxide and methane. The results of one-minute concentration values for the period 1–4 September 2016 showed a series of short peaks up to 7.45 ppm for methane and up to 3.03 ppm for non-methane hydrocarbons, being probably the result of operations carried out at the area of the wells.

Enhanced Methane Sensing Properties of SnO2 Nanoflowers With Ag Doping: A Combined Experimental and First-Principle Study

Ag-doped SnO2 nanoflowers (NFs) are prepared via a simple one-step hydrothermal method and subsequent Ag-doping using AgNO3 solution as precursors. The crystal structure, morphology and elements of the Ag-doped SnO2 NFs are investigated in detail. The maximal response of Ag-doped SnO2 sensor with 1.0 wt.% Ag doping to 500 ppm methane is increased by a factor of 1.6 as compared to that of pure SnO2 sensor. The long-term stability and the selectivity of the SnO2-based sensor are improved by Ag-doping as well. Meanwhile, pre-adsorption of oxygen molecules on the surface of the SnO2-based substrate is studied in-depth with the method of first-principle calculation. Ag doping lowers adsorption energy for all the adsorption sites, demonstrating a higher adsorption stability of chemisorbed oxygen ions on the Ag-doped SnO2 as compared to the pristine phase. More electron transfer from the semiconductor to the chemisorbed oxygen is revealed by the Mulliken charge analysis and the electron density difference for the Ag-doped SnO2, explaining the better sensitivity of gas sensors based on Ag-doped SnO2 nanostructures as compared to its undoped counterpart.

The Effect of Cobalt Catalyst Loading at Very High Pressure Plasma-Catalysis in Fischer-Tropsch Synthesis

The influence of different catalyst cobalt loadings on the C1–C3 hydrocarbon product yields and energy consumption in plasma-catalytic Fischer-Tropsch synthesis (FTS) was investigated from the standpoint of various reactor operating conditions: pressure (0.5 to 10 MPa), current (250 to 450 mA) and inter-electrode gap (0.5 to 2 mm). This was accomplished by introducing a mullite substrate, coated with 2 wt%-Co/5 wt%-Al2O3, 6 wt%-Co/5 wt%-Al2O3 or 0 wt%-Co/5 wt%-Al2O3 (blank catalyst), into a recently developed high pressure arc discharge reactor. The blank catalyst was ineffective in synthesizing hydrocarbons. Between the blank catalyst, 2 wt%, and the 6 wt% Co catalyst, the 6 wt% improved C1–C3 hydrocarbon production at all conditions, with higher yields and relatively lower energy consumption at (i) 10 MPa at 10 s, and 2 MPa at 60 s, for the pressure variation study; (ii) 250 mA for the current variation study; and (iii) 2 mm for the inter-electrode gap variation study. The inter-electrode gap of 2 mm, using the 6 wt% Co catalyst, led to the overall highest methane, ethane, ethylene, propane and propylene yields of 22 424, 517, 101, 79 and 19 ppm, respectively, compared to 40 ppm of methane and <1 ppm of C1–C3 hydrocarbons for the blank catalyst, while consuming 660 times less energy for the production of a mole of methane. Furthermore, the 6 wt% Co catalyst produced carbon nanotubes (CNTs), detected via transmission electron microscopy (TEM). In addition, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD) showed that the cobalt catalyst was modified by plasma treatment.

Tunable dual-comb spectrometer for mid-infrared trace gas analysis from 3 to 4.7 µm.

Dual-frequency comb spectroscopy has emerged as a disruptive technique for measuring wide-spanning spectra with high resolution, yielding a particularly powerful technique for sensitive multi-component gas analysis. We present a spectrometer based on two electro-optical combs with subsequent conversion to the mid-infrared via tunable difference frequency generation, operating in the range from 3 to 4.7 µm. The repetition rate of the combs can be tuned from 250 to 500 MHz. For 500 MHz, the number of detected comb modes is 440 with a signal-to-noise ratio exceeding 105 in 1 s. The conversion preserves the coherence of the combs within 3 s measurement time. Concentration measurements of 5 ppm methane at 3.3 µm, 100 ppm nitrous oxide at 3.9 µm and a mixture of 15 ppm carbon monoxide and 5% carbon dioxide at 4.5 µm are demonstrated with a noise-equivalent absorption coefficient of 6.4(3) x 10-6 cm-1 Hz-1/2.