High-precision direct measurements of (13)CH(4)/(12)CH(4) and (12)CH(3)D/(12)CH(4) ratios in atmospheric methane sources by means of a long-path tunable diode laser absorption spectrometer

Simultaneous Sensitive Determination of δ13C, δ18O, and δ17O in Human Breath CO2 Based on ICL Direct Absorption Spectroscopy

Previous research revealed that isotopes ¹³C and ¹⁸O of exhaled CO₂ have the potential link with Helicobacter pylori; however, the ¹⁷O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous δ¹³C, δ¹⁸O, and δ¹⁷O analysis of human breath CO₂ based on mid-infrared laser direct absorption spectroscopy with an interband cascade laser (ICL) at 4.33 μm. There was a gas cell with a small volume of less than 5 mL, and the pressure in the gas cell was precisely controlled with a standard deviation of 0.0035 Torr. Moreover, real-time breath sampling and batch operation were achieved in gas inlets. The theoretical drifts for δ¹³C, δ¹⁸O, and δ¹⁷O measurement caused by temperature were minimized to 0.017‰, 0.024‰, and 0.021‰, respectively, thanks to the precise temperature control with a standard deviation of 0.0013 °C. After absolute temperature correction, the error between the system responded δ-value and the reference is less than 0.3‰. According to Allan variance analysis, the system precisions for δ¹³C, δ¹⁸O, and δ¹⁷O were 0.12‰, 0.18‰, and 0.47‰, respectively, at 1 s integration time, which were close to the real-time measurement errors of six repeated exhalations.

Quasi-Simultaneous Sensitive Detection of Two Gas Species by Cavity-Ringdown Spectroscopy with Two Lasers

We developed a cavity ringdown spectrometer by utilizing a step-scanning and dithering method for matching laser wavelengths to optical resonances of an optical cavity. Our approach is capable of working with two and more lasers for quasi-simultaneous measurements of multiple gas species. The developed system was tested with two lasers operating around 1654 nm and 1658 nm for spectral detections of 12CH4 and its isotope 13CH4 in air, respectively. The ringdown time of the empty cavity was about 340 µs. The achieved high detection sensitivity of a noise-equivalent absorption coefficient was 2.8 × 10-11 cm-1 Hz-1/2 or 1 × 10-11 cm-1 by averaging for 30 s. The uncertainty of the high precision determination of δ13CH4 in air is about 1.3‰. Such a system will be useful for future applications such as environmental monitoring.

Raman Laser Spectrometer: Application to 12C/13C Isotope Identification in CH4 and CO2 Greenhouse Gases

A compact Raman laser gas spectrometer is developed. It comprises a high-power green laser at 532.123 nm as an excitation source and a specially designed gas cell with an internal volume of less than 0.6 cm3 that can withstand gas pressures up to 100 atm. The resolution of the spectrometer is ~1 cm−1. The Raman spectra of chemically pure isotopically enriched carbon dioxide (12CO2, 13CO2) and methane (12CH4, 13CH4) gases are studied. The expected limit of detection (LOD) is less than 100 ppm for the isotopologues of CO2 and less than 25 ppm for those of CH4 (at a gas pressure of 50 atm.), making the developed spectrometer promising for studying the sources of emissions of greenhouse gases by resolving their isotopologue composition. We also show the suitability of the spectrometer for Raman spectroscopy of human exhalation.

Hollow waveguide integrated laser spectrometer for 13CO2/12CO2 analysis

Using hollow waveguide hybrid optical integration, a miniaturized mid-infrared laser absorption spectrometer for ¹³CO₂/¹²CO₂ isotopologue ratio analysis is presented. The laser analyzer described focuses on applications where samples contain a few percent of CO₂, such as breath analysis and characterization of geo-carbon fluxes, where miniaturization facilitates deployment. As part of the spectrometer design, hollow waveguide mode coupling and propagation is analyzed to inform the arrangement of the integrated optical system. The encapsulated optical system of the spectrometer occupies a volume of 158 × 60 × 30 mm³ and requires a low sample volume (56 µL) for analysis, while integrating a quantum cascade laser, coupling lens, hollow waveguide cell and optical detector into a single copper alloy substrate. The isotopic analyzer performance is characterized through robust error propagation analysis, from spectral inversion to calibration errors. The analyzer achieves a precision of 0.2‰ in 500 s integration. A stability time greater than 500 s was established to allow two-point calibration. The accuracy achieved is 1.5‰, including a contribution of 0.7‰ from calibrant gases that can be addressed with improved calibration mixtures.

Development of a field-deployable method for simultaneous, real-time measurements of the four most abundant N2O isotopocules

Understanding and quantifying the biogeochemical cycle of N₂O is essential to develop effective N₂O emission mitigation strategies. This study presents a novel, fully automated measurement technique that allows simultaneous, high-precision quantification of the four main N₂O isotopocules (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O) in ambient air. The instrumentation consists of a trace gas extractor (TREX) coupled to a quantum cascade laser absorption spectrometer, designed for autonomous operation at remote measurement sites. The main advantages this system has over its predecessors are a compact spectrometer design with improved temperature control and a more compact and powerful TREX device. The adopted TREX device enhances the flexibility of the preconcentration technique for higher adsorption volumes to target rare isotope species and lower adsorption temperatures for highly volatile substances. All system components have been integrated into a standardized instrument rack to improve portability and accessibility for maintenance. With an average sampling frequency of approximately 1 h^-1, this instrumentation achieves a repeatability of 0.09, 0.13, 0.17 and 0.12 ‰ for δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O and site preference of N₂O, respectively, for pressurized ambient air. The repeatability for N₂O mole fraction measurements is better than 1 ppb (parts per billion, 10^-9 moles per mole of dry air).

Determining Biogenic Content of Biogas by Measuring Stable Isotopologues 12CH₄, 13CH₄, and CH₃D with a Mid-Infrared Direct Absorption Laser Spectrometer

A tunable laser absorption spectrometer (TLAS) was developed for the simultaneous measurement of δ¹³C and δD values of methane (CH₄). A mid-infrared interband cascade laser (ICL) emitting around 3.27 µm was used to measure the absorption of the three most abundant isotopologues in CH₄ with a single, mode-hop free current sweep. The instrument was validated against methane samples of fossil and biogenic origin with known isotopic composition. Three blended mixtures with varied biogenic content were prepared volumetrically, and their δ¹³C and δD values were determined. Analysis demonstrated that, provided the isotopic composition of the source materials was known, the δ¹³C and δD values alone were sufficient to determine the biogenic content of the blended samples to within 1.5%.

Hydrogen Isotopic Composition of Arctic and Atmospheric CH4 Determined by a Portable Near-Infrared Cavity Ring-Down Spectrometer with a Cryogenic Pre-Concentrator

In this study, near-infrared continuous wave cavity ring-down spectroscopy was applied to the measurement of the δ²H of methane (CH₄). The cavity ring-down spectrometer (CRDS) system consisted of multiple DFB laser diodes to optimize selection of spectral line pairs. By rapidly switching measurements between spectral line peaks and the baseline regions, the long-term instrumental drift was minimized, substantially increasing measurement precision. The CRDS system coupled with a cryogenic pre-concentrator measured the δ²H of terrestrial atmospheric CH₄ from 3 standard liters of air with a precision of ±1.7‰. The rapidity with which both C and H isotopic measurements of CH₄ can be made with the CRDS will enable hourly monitoring of diurnal variations in terrestrial atmospheric CH₄ signatures that can be used to increase the resolution of global climate models for the CH₄ cycle. Although the current instrument is not capable of measuring the δ²H of 10 ppbv of martian CH₄, current technology does exist that could make this feasible for future spaceflight missions. As biological and abiotic CH₄ sources have overlapping carbon isotope signatures, dual-element (C and H) analysis is key to reliable differentiation of these sources. Such an instrument package would therefore offer improved ability to determine whether or not the CH₄ recently detected in the martian atmosphere is biogenic in origin. Key Words: Arctic-Hydrogen isotopes-Atmospheric CH₄-CRDS-Laser. Astrobiology 16, 787-797.

Dual wavelength 3.2-μm source for isotope ratio measurements of (13)CH(4)/(12)CH(4)

Difference frequency generation using one 1.58-μm and two 1.06-μm distributed feedback Bragg-grating laser diodes and a ridge-type PPLN alternately provide two 3.2-μm coaxial waves resonant with individual isotopic transitions separated by 13 cm(-1). The ν(3) band R(6) A(2) allowed transition of (13)CH(4) and the ν(3) band R(6) A(2) weakly allowed transition of (12)CH(4) are an ideal pair for isotope ratio measurements. The (13)CH(4)/(12)CH(4) isotope ratio is determined for three sample gases with a relative uncertainty of 0.7 ‰, and it is confirmed that the temperature dependence is smaller than the uncertainty.

Site-selective nitrogen isotopic ratio measurement of nitrous oxide using a TE-cooled CW-RT-QCL based spectrometer

The feasibility of laser spectroscopic isotopic composition measurements of atmospheric N2O was demonstrated, although making them useful will require further improvements. The system relies on a thermoelectrically (TE) cooled continuous-wave (CW) room temperature (RT) quantum cascade laser source emitting wavelength of around 4.6μm, where strong fundamental absorption bands occur for the considered specie and its isotopomers. The analysis technique is based on wavelength modulation spectroscopy with second-harmonic detection and the combination of long-path absorption cell. Primary laboratory tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of (15)N/(14)N and (18)O/(16)O isotope ratios. The experiment results showed that the site-selective (15)N/(14)N ratio can be measured with a precision of 3‰ with 90s averaging time using natural-abundance N2O sample of 12.7ppm.

High sensitivity trace gas detection by cantilever-enhanced photoacoustic spectroscopy using a mid-infrared continuous-wave optical parametric oscillator

Highly sensitive cantilever-enhanced photoacoustic detection of hydrogen cyanide and methane in the mid-infrared region is demonstrated. A mid-infrared continuous-wave frequency tunable optical parametric oscillator was used as a light source in the experimental setup. Noise equivalent detection limits of 190 ppt (1 s) and 65 ppt (30 s) were achieved for HCN and CH(4), respectively. The normalized noise equivalent absorption coefficient is 1.8 × 10(-9) W cm(-1) Hz(-1/2).

Modulation cancellation method for isotope 18O/16O ratio measurements in water

The application of an innovative spectroscopic balancing technique to measure the isotope 18O/16O ratio in water vapor is reported. Quartz enhanced photoacoustic spectroscopy has been employed as the absorption sensing technique. Two isotope absorption lines with the same quantum numbers, with very close lower energy levels, have been selected to limit the sensitivity to temperature variations and guarantee identical broadening as well as relaxation properties. The sensitivity in measuring the deviation from a standard sample δ18O is 1.4‰, in 200 sec of integration time.

Robotic reconnaissance platform. I. Spectroscopic instruments with rangefinders

In this paper, basic principles of the design and implementation of a portable, multi-functional scientific instrument, operating from a robotic reconnaissance mobile platform are discussed. The current version of the instrument includes a multi-gas laser sensor, multi-functional spectrometer, isotopes identifier, cameras, and rangefinder. An additional set of sensors monitors temperature, pressure, humidity, and background radiation. All components are installed on a mini-robotic platform, which provides data acquisition, processing, and transmittance. The design focuses on the development of calibration-free, reliable, low power-consumption devices. To create a highly survivable, accurate, and reliable instrument, a concept of an inhomogeneous sensory network has been developed. Such a network combines non-identical sensors and provides cross-use of information received from different sensors to describe environmental conditions, to choose appropriate algorithms of data processing, and to achieve high accuracy gas-concentration measurements. The system uses the same lasers to operate different optical devices such as sensors, rangefinders, spectrometers, and isotopes identifiers. Among the innovative elements described in this paper, are a calibration-free, laser multi-gas sensor with range-finding option; a high signal/noise ratio transmittance spectrometer; a single-frequency laser with nano-selector; and low repetition-rate femtosecond fiber lasers operating in near- and middle- infrared spectral ranges. New detailed analyses of absorption spectroscopy theoretical approximations made it possible to achieve high-accuracy gas-concentration measurements with miniature optical sensors.

An efficient and compact difference-frequency-generation spectrometer and its application to (12)CH(3)D/(12)CH(4) isotope ratio measurements

We have developed an efficient and compact 3.4 μm difference-frequency-generation spectrometer using a 1.55 μm distributed feedback (DFB) laser diode, a 1.06 μm DFB laser diode, and a ridge-waveguide periodically poled lithium niobate. It is continuously tunable in the 30 cm(-1) span and is applied to (12)CH(3)D/(12)CH(4) isotope ratio measurements. The suitable pair of (12)CH(3)D ν(4) (p)P(7,6) and (12)CH(4) ν(2)+ν(4) R(6) F(1)((1)) lines enabled us to determine their isotope ratio with a precision repeatability of 0.8‰ using a sample and a working standard of pure methane with an effective signal averaging time of 100 ms.

Measurements of 13C/12C methane from anaerobic digesters: comparison of optical spectrometry with continuous-flow isotope ratio mass spectrometry

Methane production by anaerobic digestion of biomass has recently become more attractive because of its potential for renewable energy production. Analytical tools are needed to study and optimize the ongoing processes in biogas reactors. It is considered that optical methods providing continuous measurements at high temporal resolution of carbon isotope ratios of methane (delta(13)C(CH4)) might be of great help for this purpose. In this study we have tested near-infrared laser optical spectrometry and compared it with conventional continuous-flow isotope ratio mass spectrometry (CF-IRMS) using several methane carbon isotope standards and a large number of biogas samples from batch anaerobic reactors. Results from measurements on these samples were used to determine and compare the precision of the two techniques and to quantify for systematic offsets. With pure standards analytical precision of measurements for delta(13)C(CH4) was found to be in the range of 0.33 and 0.48 per thousand, and 0.09 and 0.27 per thousand for the optical method and CF-IRMS, respectively. Biogas samples showed an average mean deviation of delta(13)C(CH4) of 0.38 per thousand and 0.08 per thousand for the optical method and CF-IRMS, respectively. Although the tested laser optical spectrometer showed a dependence of delta(13)C(CH4) on CH(4) mixing ratio in the range of 500 to 8000 ppm this could be easily corrected. After correction, the delta(13)C(CH4) values usually varied within 0.7 per thousand from those measured by conventional CF-IRMS and thus results from both methods agreed within the given analytical uncertainties. Although the precision of the conventional CF-IRMS is higher than the tested optical system, both instruments were well within the acceptable delta(13)C(CH4) precision required for biogas methane measurements. The advantages of the optical system are its simplicity of operation, speed of analysis, good precision, reduced costs in comparison to IRMS, and the potential for field applications.

Laser-based human breath analysis: D/H isotope ratio increase following heavy water intake

Following the ingestion of only 5.1 mL of D2O, a mid-infrared laser spectrometer determines the D/H isotope ratio increase in exhaled water vapor for the first time, to the best of our knowledge. This increase is still detectable several weeks after the heavy water intake. Collected breath samples are directly transferred into a high-temperature multipass cell operated at 373 K. No breath sample preparation is required. Aside from the capability to hinder unwanted condensation, measurements at elevated temperatures offer other advantages such as a lower temperature dependence of the delta value or the possibility to vary the intensity of absorption lines. We lay the foundation for many laser-based clinical applications. As an example, we measure a total body water weight of 55.2%+/-1.8% with respect to the total body weight, in agreement with the normal value of the male population.

Precise measurements of the total concentration of atmospheric CO2 and 13CO2/12CO2 isotopic ratio using a lead-salt laser diode spectrometer

We have developed a tunable diode laser spectrometer, called SIMCO (spectrometer for isotopic measurements of CO(2)), for determining the concentrations of (12)CO(2) and (13)CO(2) in atmospheric air, from which the total concentration of CO(2) and the isotopic composition (expressed in delta units) delta(13)CO(2) are calculated. The two concentrations are measured using a pair of lines around 2290.1 cm(-1), by fitting a line profile model, taking into account the confinement narrowing effect to achieve a better accuracy. Using the Allan variance, we have demonstrated (for an integration time of 25 s) a precision of 0.1 ppmv for the total CO(2) concentration and of 0.3[per thousand] for delta(13)CO(2). The performances on atmospheric air have been tested during a 3 days campaign by comparing the SIMCO instrument with a gas chromatograph (GC) for the measurement of the total CO(2) concentration and with an isotopic ratio mass spectrometer (MS) for the isotopic composition. The CO(2) concentration measurements of SIMCO are in very good agreement with the GC data with a mean difference of Delta(CO(2))=0.16+/-1.20 ppmv for a comparison period of 45 h and the linearity of the concentration between the two instruments is also very good (slope of correlation: 0.9996+/-0.0003) over the range between 380 and 415 ppmv. For delta(13)CO(2), the comparison with the MS data shows a larger mean difference of Delta(delta(13)CO(2))=(-1.9+/-1.2)[per thousand], which could be partly related to small residual fluctuations of the overall SIMCO instrument response.

Aircraft and balloon in situ measurements of methane and hydrochloric acid using interband cascade lasers

Aircraft and balloon in situ measurements of CH4 and HCl using cw distributed feedback (DFB) interband cascade (IC) lasers are reported. In the stratosphere and upper troposphere, sensitivity toward CH4 and HCl is better than 10 ppbv (1 s) and 90 pptv (50 s), respectively. These are the first flight measurements of trace gas-phase species using cw DFB IC lasers.

Stable isotopic analysis of atmospheric methane by infrared spectroscopy by use of diode laser difference-frequency generation

An infrared absorption spectrometer has been constructed to measure the stable isotopic composition of atmospheric methane samples. The spectrometer employs periodically poled lithium niobate to generate 15 microW of tunable difference-frequency radiation from two near-infrared diode lasers that probe the nu3 rotational-vibrational band of methane at 3.4 microm. To enhance the signal, methane is extracted from 25 l of air by use of a cryogenic chromatographic column and is expanded into the multipass cell for analysis. A measurement precision of 12 per thousand is demonstrated for both delta13C and deltaD.

Simultaneous 13C/12C and (18)O/(16)O isotope ratio measurements on CO2 based on off-axis integrated cavity output spectroscopy

A prototype off-axis integrated cavity output spectrometer (OA-ICOS) utilizing two identical cavities together with a near-infrared (1.63 microm) external cavity tunable diode laser is described. The two-cavity design-one for a reference gas and one for a sample gas-takes advantage of classical double-beam infrared spectrometer characteristics in reducing uncertainties due to laser scan or power instabilities and major temperature variations by a factor of three or better compared with a single-cavity scheme. This is the first OA-ICOS instrument designed to determine 13C/12C and (18)O/(16)O ratios from CO2 rotation/vibration fine structure in three different combination bands. Preliminary results indicate that at 0.8 Hz a precision of 3.3 and 2.8 per thousand is obtained for delta13C and delta(18)O, respectively, over a period of 10 h and a pure CO2 gas sample at 26 hPa. By averaging 100 spectra over a subset of the data, we achieved a precision of 1.6 and 0.8 \permil\ for delta13C and delta(18)O, respectively.

Measuring the 13C/12C isotope ratio in atmospheric CO2 by means of laser absorption spectrometry: a new perspective based on a 2.05-microm diode laser

The potential use of high sensitivity laser absorption spectroscopy for measuring the 13C/12C isotope ratio in atmospheric CO2 has been demonstrated, using a GaSb-based diode laser at 2.05 microm. In this spectral region, the overlapping between relatively strong 12CO2 and 13CO2 absorption features gives rise to several line pairs which are well suitable for a spectroscopic determination of the isotope ratio. Preliminary results have demonstrated that a short-term precision better than 1 per thousand can be easily obtained, for a CO2 concentration of 1000 ppm. We extensively discuss the influence of a possible non-linearity in the detectors' response on the delta-value and suggest an instrumental development that would allow to eliminate this effect.

A frequency-modulated quantum-cascade laser for spectroscopy of CH4 and N2O isotopomers

We report the development of a novel laser spectrometer for high-sensitivity detection of methane and nitrous oxide. The system relies on a quantum-cascade laser source emitting wavelength of around 8.06 microm, where strong fundamental absorption bands occur for the considered species and their isotopomers. The detection technique is based on audio-frequency and radio-frequency modulation of laser radiation. First experimental tests have been performed to estimate the achievable detection limits and the signal reproducibility levels in view of possible measurements of (13)C/(12)C, (18)O/(16)O, (17)O/(16)O and (15)N/(14)N isotope ratios.

Carbon isotopomers measurement using mid-IR tunable laser sources

Recent developments of two mid-infrared tunable laser spectrometers dedicated to carbon isotope ratio determination are presented. First, a field deployable quantum cascade laser-based sensor is described, along with line selection strategy for (13/12)CO(2) ratio measurements. Secondly, an instrument architecture based on difference frequency generation is presented. The analyses of fundamental limitations, specifically temperature and pressure stability, and water vapor collision broadening, are detailed.

Measuring methane and its isotopes 12CH4, 13CH4, and CH3D on the surface of Mars with in situ laser spectroscopy

In light of the recent discovery of methane on Mars and its possible biological origin, a strategy is described for making in situ measurements of methane and its isotopes on the surface of Mars by laser spectroscopy in the 3.3-microm wavelength region. An instrument of reasonable mass (approximately 1 lb) and power (few watts) is capable of measuring mixing ratios down to 0.1 part per 10(9) by volume, a hundred times lower than recently reported observations. Making accurate measurements of 13CH4 and CH3D will be more difficult. For measuring delta13C to 10/1000 and deltaD to 50/1000, sample preconcentration will be required to approximately 3 parts per 10(6) by volume for delta13C and to approximately 40 parts per 10(6) by volume for deltaD. This need would be mitigated by the discovery of larger local abundances of methane near the source regions.

High-precision isotopic ratio measurement system for methane (12CH3D/12CH4,13CH4/12CH4) by using near-infrared diode laser absorption spectroscopy

We demonstrate that the absorption spectroscopic method can be applied to a precise deltaD (an index of 12CH3D/12CH4) and delta13C (an index of 13CH4/12CH4) analysis for methane samples of natural isotopic abundance. We chose an appropriate absorption line pair whose absorption coefficients have nearly the same temperature dependences so as to minimize the temperature effect in absorbance ratio measurements. We measured 12CH3D/12CH4 ratio by using near-infrared external cavity diode lasers and a new type multi-pass cell. The deltaD value can be determined from the 12CH3D/12CH4 signal-intensity ratio with a fine correction by taking account of the interference of 13CH4 lines. Similarly, the delta13C value is determined from the 13CH4/12CH4 signal-intensity ratio, which is measured by using distributed-feedback laser and a modified Herriot-type cell and corrected for the abundance of 12CH3D. The precision was +/-0.7 and +/-0.027/1000 for deltaD and delta13C, respectively.

Determination of the 2H/1H, 17O/16O, and 18O/16O isotope ratios in water by means of tunable diode laser spectroscopy at 1.39 microm

We demonstrate the feasibility of the accurate and simultaneous measurement of the 2H/1H, 17O/16O, and 18O/16O isotope ratios in water vapor by means of tunable diode laser spectroscopy. The absorptions are due to the v1 + v3 combination band, observed using a room temperature, distributed feedback (DFB) diode laser at 1.39 microm. The precision of the instrument is approximately 3, 1, and 0.5/1000 for the 2H, 17O, and 18O isotope ratios, respectively, and is at present limited by residual optical feedback to the laser. The signal-to-noise, however, is superior to that obtained in a similar experiment using a color center laser at 2.7 microm. Replacing the current laser with a better unit, we are confident that a precision well below 1/1000 is attainable for all three isotope ratios. The diode laser apparatus is ideally suited for applications demanding a reliable, cheap, and/or portable instrument, such as the biomedical doubly labeled water method and atmospheric sensing.

Infrared laser spectrometer with balanced absorption for measurement of isotopic ratios of carbon gases

Measurement of the isotopic compositions of carbon dioxide and methane is a powerful tool for quantifying their atmospheric sources and sinks, which is especially important considering the dramatic increase in these greenhouse gases during the industrial era. Laser absorption spectroscopy is a technique which has demonstrated the high sensitivity needed for isotopic measurement. A significant problem in the spectroscopic measurement of isotopic abundances is the large difference in concentrations of the major and minor isotopic constituents. The measurement of two isotopic species using lines of similar strength but very unequal concentrations leads to low precision, with either the minor constituent having too small an absorption depth, or the major constituent having too great an absorption depth. If lines with unequal strength are chosen to compensate for the absorption depth imbalance, then precision tends to suffer due to the greater temperature sensitivity of the weaker line strength. We describe the development of a compact instrument for isotopic analysis CO2 and CH4 using tunable infrared laser absorption spectroscopy which combines novel optical design and signal processing methods to address this problem. The design compensates for the large difference in concentration between major and minor isotopes by measuring them with pathlengths which differ by a factor of 72 within the same multipass cell. We have demonstrated the basic optical design and signal processing by determining delta13C (CO2) isotopic ratios with precision as small as 0.2/1000 using a lead salt diode laser based spectroscopic instrument.

Pressure broadening study at low temperature: application to methane

We studied the R(0) line profile in the CH4 v4 band from room temperature to 188 K with N2 as a perturber, to 100 K with O2 as perturber, and from room temperature to 15 K using He as a perturber. The N2 and O2 measurements were performed over a total pressure range of 15-110 mbar, and for the He measurements the maximum sample pressure at 15 K was 1.10 mbar. Broadening parameters were determined, taking into account the confinement narrowing (Dicke effect), and narrowing parameters, deduced from the soft or hard collision model, are compared with the dynamic friction coefficient calculated values. We also obtained preliminary values for the temperature dependence of the N2, O2 and He line broadening parameters for this transition.

Real-time detection of 13CH4 in ambient air by use of mid-infrared cavity leak-out spectroscopy

We report on spectroscopic real-time detection of (13)CH(4) in ambient air. Our measurements were carried out by means of cavity leak-out absorption spectroscopy employing a tunable cw laser in the mid-infrared spectral region near lambda = 3 microm. A CO laser in combination with tunable microwave sideband generation was used as the light source. Using a 50-cm-long ringdown cell with R = 99.98% mirrors, we achieved a detection limit of 290 parts in 10(12) (ppt) (13)CH(4) in ambient air (integration time, 100 s). The corresponding noise-equivalent absorption coefficient was 5 x 10(-9)/cm.

Effective utilization of quantum-cascade distributed-feedback lasers in absorption spectroscopy

A variable duty cycle quasi-cw frequency scanning technique was applied to reduce thermal effects resulting from the high heat dissipation of type I quantum-cascade lasers. This technique was combined with a 100-m path-length multipass cell and a zero-air background-subtraction technique to enhance detection sensitivity to a parts-in-10(9) (ppb) concentration level for spectroscopic trace-gas detection of CH4, N2O, H2O, and C2H5OH in ambient air at 7.9 micrometers. A new technique for analysis of dense high resolution absorption spectra was applied to detection of ethanol in ambient air, yielding a 125-ppb detection limit.

Precision trace gas analysis by FT-IR spectroscopy. 2. The 13C/12C isotope ratio of CO2

We report the development of a method of carbon stable isotope ratio analysis based on 1-cm-1 resolution Fourier transform infrared (FT-IR) spectroscopy, deployable in both laboratory and field applications. We demonstrate the determination of the 13C/12C ratio of CO2 (i.e., delta 13CO2) in air with an analytical precision of the order of +/- 0.1/1000 (i.e., +/- 0.01%). The FT-IR method relies on calibration using synthetically calculated absorbance spectra and a multivariate calibration algorithm. The method requires no sample preparation other than optional drying of the sample and may be applied directly to ambient air samples containing approximately 350 mumol mol-1 CO2 (molar mixing ratio). It may also be applied to samples more concentrated in CO2, such as human breath, approximately 5% CO2. We demonstrate the utility of the technique to the analysis of delta 13CO2 in air during an experimental field campaign and to the laboratory-based analysis of human breath. A similar method could also be used to determine the H/D ratio in atmospheric water vapor.

Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of 8.1 micrometers was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm-1, and absorption spectra of the CH4 nu 4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) ( ppm) in 50 Torr of air was measured in a 43-cm path length with +/- 0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of 13CH4 and CH3D species in a CH4 sample was determined.

Infrared absorption by methane isotopes near 2999 cm(-1): an analytical method using the Zeeman split He-Ne laser line at 2999.24 cm(-1)

We report the strengths and locations of the absorption lines of CH(3)D, (13)CH(4), and (12)CH(4) in the vicinity of the helium-neon laser line at 2999.24 cm(-1). In this region we find intrinsically strong lines of the rare species overlapping, but not precisely coincident with, intrinsically weak lines of the major species. In methane of normal isotopic composition, the lines of all three species have comparable strengths. We show how absorption measurements of the He-Ne laser, with Zeeman splitting, can be used to determine variations of the concentrations of the rare species from standard concentrations at a calculated precision of 0.5%0.