Ethylene and ammonia traces measurements from the patients’ breath with renal failure via LPAS method

The application of laser photoacoustic spectroscopy (LPAS) for fast and precise measurements of breath biomarkers has opened up new promises for monitoring and diagnostics in recent years, especially because breath test is a non-invasive method, safe, rapid and acceptable to patients. Our study involved assessment of breath ethylene and breath ammonia levels in patients with renal failure receiving haemodialysis (HD) treatment. Breath samples from healthy subjects and from patients with renal failure were collected using chemically inert aluminized bags and were subsequently analyzed using the LPAS technique. We have found out that the composition of exhaled breath in patients with renal failure contains not only ethylene, but also ammonia and gives valuable information for determining efficacy and endpoint of HD.     

Sensitive detection of ammonia and ethylene with a pulsed quantum cascade laser using intra and interpulse spectroscopic techniques

Spectroscopic concentration measurements of ammonia and ethylene were done with a pulsed, distributed feedback (DFB) quantum cascade (QC) laser centered at 970 cm⁻¹. An astigmatic Herriot cell with 150 m path length was employed, and we compare the results from experiments using inter- and intrapulse techniques, respectively. The measurements include the detection of ammonia in breath with these methodologies. In the interpulse technique, the laser was excited with short current pulses (5–10 ns), and the pulse amplitude was modulated with an external current ramp resulting in a ∼0.3 cm⁻¹ frequency scan. A standard amplitude demodulation technique was implemented for extracting the absorption line, thus avoiding the need for a fast digitizer or a gated integrator. In the intrapulse technique, a linear frequency down-chirp is used for sweeping across the absorption line. A 200 ns long current pulse was used for these measurements which resulted in a spectral window of ∼1.74 cm⁻¹ during the down-chirp. The use of a room temperature mercury-cadmium-telluride detector resulted in a completely cryogen free spectrometer. We demonstrate detection limits of ∼3 ppb for ammonia and ∼5 ppb for ethylene with less than 10 s averaging time with the intrapulse method and ∼4 ppb for ammonia and ∼7 ppb for ethylene with the interpulse technique with an integration time of ∼5 s.     

Evaluation of ammonia absorption coefficients by photoacoustic spectroscopy for detection of ammonia levels in human breath

Photoacoustic spectroscopy represents a powerful technique for measuring extremely low absorptions independent of the path length and offers a degree of parameter control that cannot be attained by other methods. We report precise measurements of the ammonia absorption coefficients at the CO₂ laser wavelengths by using a photoacoustic (PA) cell in an extracavity configuration and we compare our results with other values reported in the literature. Ammonia presents a clear fingerprint spectrum and high absorption strengths in the CO₂ wavelengths region. Because more than 250 molecular gases of environmental concern for atmospheric, industrial, medical, military, and scientific spheres exhibit strong absorption bands in the region 9.2–10.8 μm, we have chosen a frequency tunable CO₂ laser. In the present work, ammonia absorption coefficients were measured at both branches of the CO₂ laser lines by using a calibrated mixture of 10 ppm NH₃ in N₂. We found the maximum absorption in the 9 μm region, at 9R(30) line of the CO₂ laser. One of the applications based on the ammonia absorption coefficients is used to measure the ammonia levels in exhaled human breath. This can be used to determine the exact time necessary at every session for an optimal degree of dialysis at patients with end-stage renal disease.     

Infrared laser-spectroscopic analysis of <Superscript>14</Superscript>NO and <Superscript>15</Superscript>NO in human breath

We report on monitoring of nitric oxide (NO) traces in human breath via infrared cavity leak-out spectroscopy. Using a CO sideband laser near 5 μm wavelength and an optical cavity with two high-reflectivity mirrors (R=99.98%), the minimum detectable absorption is 2×10⁻¹⁰ cm⁻¹ Hz^1/2. This allows for spectroscopic analysis of rare NO isotopologues with unprecedented sensitivity. Application to simultaneous online detection of ¹⁴NO and ¹⁵NO in breath samples collected in the nasal cavity is described for the first time. We achieved a noise-equivalent detection limit of 7 parts per trillion for nasal ¹⁵NO (integration time: 70 s).     

Laser-based systems for trace gas detection in life sciences

Infrared gas phase spectroscopy is becoming very common in many life science applications. Here we present three types of trace gas detection systems based on CO₂ laser and continuous wave (cw) optical parametric oscillator (OPO) in combination with photoacoustic spectroscopy and cw quantum cascade laser (QCL) in combination with wavelength modulation spectroscopy. Examples are included to illustrate the suitability of CO₂ laser system to monitor in real time ethylene emission from various dynamic processes in plants and microorganisms as well as from car exhausts. The versatility of an OPO-based detector is demonstrated by simultaneous detection of ¹³C-methane and ¹²C-methane (at 3240 nm) at similar detection limits of 0.1 parts per billion by volume. Recent progress on a QCL-based spectrometer using a continuous wave QCL (output power 25 mW, tuning range of 1891–1908 cm^-1) is presented and a comparison is made to a standard chemiluminescence instrument for analysis of NO in exhaled breath. PACS  42.62 Be; 42.62 Fi     

A calibration-free ammonia breath sensor using a quantum cascade laser with WMS 2f/1f

The amount of ammonia in exhaled breath has been linked to a variety of adverse medical conditions, including chronic kidney disease (CKD). The development of accurate, reliable breath sensors has the potential to improve medical care. Wavelength modulation spectroscopy with second harmonic normalized by the first harmonic (WMS 2f/1f) is a sensitive technique used in the development of calibration-free sensors. An ammonia gas sensor is designed and developed that uses a quantum cascade laser operating near 1,103.44 cm^?1 and a multi-pass cell with an effective path length of 76.45 m. The sensor has a 7 ppbv detection limit and 5 % total uncertainty for breath measurements. The sensor was successfully used to detect ammonia in exhaled breath and compare healthy patients to patients diagnosed with CKD.     

Telecom-grade fiber laser-based difference-frequency generation and ppb-level detection of benzene vapor in air around 3 μm

We report on the development of a field deployable compact laser instrument tunable over ∼232 cm⁻¹ from 3.16 to 3.41 μm (2932.5–3164.5 cm⁻¹) for chemical species monitoring at the ppb-level. The laser instrument is based on widely tunable continuous-wave difference-frequency generation (DFG), pumped by two telecom-grade fiber lasers. DFG power of ∼0.3 mW near 3.3 μm with a spectral purity of ∼3.3 MHz was achieved by using moderate pumping powers: 408 mW at 1062 nm and 636 mW at 1570 nm. Spectroscopic performance of the developed DFG-based instrument was evaluated with direct absorption spectra of ethylene at 3.23 μm (∼3094.31 cm⁻¹). Absorption spectra of vapor-phase benzene near 3.28 μm (∼3043.82 cm⁻¹) were recorded with Doppler-limited resolution. Line intensities of the most intense absorption lines of the ν ₁₂ band near 3043.8 cm⁻¹ were determined to support development of sensitive mid-infrared trace gas detection of benzene vapor in the atmosphere. Detection of benzene vapor in air at different concentration levels has been performed for the first time using multi-pass cell enhanced direct absorption spectroscopy at ∼3.28 μm with a minimum detectable concentration of 50 ppb (1σ).     

Small-size resonant photoacoustic cell with reduced window background for laser detection of gases

A photoacoustic resonant cell with the inner volume is ∼0.5 cm³ is presented. The shape of the cell cavity is chosen such as to minimize a background signal arising due to the absorption of laser beam in the cell windows. The experimental setup, the measurement procedure, and the design of the cell are described. The results of detecting ammonia in a nitrogen flow using the R(30) oscillating line of a CO₂ laser are represented. The minimal detectable absorption achieved in experiment is ∼3.2 × 10⁻⁸ cm⁻¹ W Hz^−1/2.     

Simultaneous detection of ammonia, methane and ethylene at 1.63 μm with diode laser photoacoustic spectroscopy

Spectral investigation around 6115 cm^-1 for simultaneous detection of ammonia, methane and ethylene in gas samples is presented. Experimental data on the ν₂+ν₃+ν₄ combination band of ammonia are reported with a resolution of 1.5 GHz. A trace gas analyzer based on a resonant photoacoustic cell and an external cavity diode laser has been used for detection. A data fitting procedure has been developed in order to improve the system sensitivity and to limit the need of a reference cell. The selected spectral region allows a sensitivity of about 60 ppm for ammonia, 6 ppm for methane and 30 ppm for ethylene with 0.3 mW laser power. An application of simultaneous detection of such molecules in a mixture reproducing their typical abundances in real gas samples from biomass gasification is discussed. PACS 42.62.Fi; 42.55.Px; 82.80.Ch     

Infrared-microwave two-photon spectroscopy with13C16O2 and12C18O2 lasers of the ν2-band of ammonia

A tunable microwave frequency was added to, or subtracted from fixed frequency¹³C¹⁶O₂ and¹²C¹⁸O₂ laser lines using the nonlinearity of molecular absorptions. In this way the frequency of 30 transitions of the ν₂-band of ammonia were measured with an accuracy of ±0.0005 cm⁻¹. A further four transitions were measured with an accuracy of ±0.0001 cm⁻¹ by saturating the two-photon transition and observing the Lamb dip. For laser lines up to 11 GHz off-resonant with in frared transitions Doppler-limited signals were observed with microwave power densities of only 10 mW/cm² using a wide-band intracavity cell.     

Isotope analysis by infrared laser absorption spectroscopy

The feasibility of IR laser spectroscopy as a technique for the measurement of small abundances of stable and radioactive isotopes has been examined. Theoretical considerations and first experimental results with two laser systems are presented: 1) Coincidences between emission lines of a CO₂-laser and absorption lines of¹³C-substituted ethylene can be used to determine the¹³C-concentration of C₂H₄. 2) A tunable PbS-diode laser emitting in the 4.3 μm-spectral region of the rotation-vibration bands of CO₂ can be used to determine abundances of¹²C,¹³C,¹⁶O,¹⁷O and¹⁸O in small samples of CO₂. With optimized performance, sensitivities up to 10⁻⁹–10⁻¹⁰ seem possible, and for higher abundances an accuracy of 10⁻³. This should allow geophysical isotope studies to be performed and it is hoped that the technique will eventually be applicable to measuring the activity of long-lived radioisotopes.     

Laser oscillation by the F3− and F2− color centers in LiF crystal at room temperature

A stable, free-running LiF:F₃ ⁻ and LiF:F₂ ⁻ color center laser oscillation is achieved at room temperature by pumping with the 930-nm laser radiation. The LiF:F₃ ⁻ laser radiation has a peak at 1100 nm and shifts from the peak wavelength of the F₃ ⁻ luminescence band because of the absorption of the F₃ ⁻ luminescence by the F₂ ⁻ center, which co-exists with the F₃ ⁻ center. Received: 2 March 1999 / Revised version: 16 March 1999 / Published online: 12 May 1999     

Optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) in a ring cavity

Optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) has been demonstrated by coupling a distributed feedback diode laser to a ring cavity. Frequency-selected light decaying from the ring cavity is retro-reflected, inducing a counter-propagating intra-cavity beam, and providing optical feedback to the laser. At specific laser-to-cavity distances, all cavity mode frequencies return to the diode laser with the same phase, allowing spectra to be accumulated across the range of frequencies of the current-tuned laser. OF-CEAS has been used to measure very weak oxygen isotopologue (¹⁶O¹⁸O and ¹⁶O¹⁷O) absorptions in ambient air at wavelengths near 762 nm using the electric-dipole forbidden O₂ A-band. A bandwidth reduced minimum detectable absorption coefficient of 2.2×10⁻⁹ cm⁻¹ Hz^−1/2 is demonstrated.     

Optoacoustic gas analyzer based on a13C16O2 laser for multicomponent pollution of air

A computer-aided optoacoustic gas analyzer based on a continuous¹³C¹⁶C₂ laser for multicomponent pollution of atmospheric air is described. The analyzer has the ability to detect absorption of radiation by detected substances at the level of ∼1·10⁻⁹ cm⁻¹ at a time resolution of 30 sec. Results of an experiment on simultaneous detection of H₂O, CO₂, NO₂, NH₃, HNO₃, OCS, and C₂H₄ in the atmospheric air using 40 laser lines are presented. B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68, F. Skorina Ave., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 3, pp. 345–350, May–June, 1999.     

Cavity enhanced absorption spectroscopy measurements of?pressure-induced broadening and shift coefficients in?the?υ1+υ3 combination band of ammonia

Cavity enhanced absorption spectroscopy is performed using an external cavity diode laser operating around 1516 nm. We demonstrate a sensitivity of 6×10⁻⁸ cm⁻¹ Hz^−1/2 and utilise a simple method to measure pressure-induced broadening and shift coefficients. The broadening and shift coefficients for six gases (helium, neon, argon, xenon, oxygen and nitrogen) have been determined at room temperature for four transitions in the υ ₁+υ ₃ combination band of ammonia. Comparisons of the broadening coefficients with previous work in this region, where it exists, show good agreement. The broadening and shift coefficients of nitrogen and oxygen are also in good agreement with calculated values using the Robert and Bonamy theory. Both the broadening and shift coefficients show a clear trend through the rare gases, which can be explained in terms of the varying magnitude of the long range attractive forces operating between the colliding partners. We also demonstrate the application of the Parmenter–Seaver formalism to estimate the potential well depth of the ammonia dimer from the obtained broadening coefficients. The obtained well depth agrees well with theoretical calculations.     

CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector, and a Fourier spectrometer

We report a prototype CO₂ gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260 cm⁻¹ (4.4 μm) and has a full width at half maximum of 200 cm⁻¹ (25 meV). It covers strong absorption bands of two spectrally overlapping CO₂ isotopomers, namely the P-branch of ¹²CO₂ and the R-branch of ¹³CO₂. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO₂ concentrations corresponding to the different gas mixtures. A detection limit of 500 ppb was obtained in these experiments.     

Narrow resonances (Δv=2·10−2 cm−1) of multiple-photon IR absorption in ethylene

Narrow peaks of multiple-photon absorption in thev ₇ band of ethylene with the spectral width 0.02 cm⁻¹ and the contrast factor of up to 10² have been observed at the intensity of laser radiation 0.04 and 0.6 MW/cm². The multiple-photon spectra of ethylene in intense IR field have been studied with the use of a quasi-single-mode continuously tunable CO₂ laser. The results of the experiments are interpreted within the model of spepwise molecular excitation due to weak transitions.     

Tunable diode laser spectroscopy of ethylene oxide near 1693 nm

We describe the application of the vertical-cavity surface-emitting laser (VCSEL) to absorption spectroscopy of the ethylene oxide (EO) Q-branch near 1693 nm. The VCSEL was electrically scanned over spectral intervals of up to ∼13 cm⁻¹ at a 1 kHz repetition rate. A methane absorption line at 5903.3 cm⁻¹ and Fabry–Pérot etalon with a free spectral range of 0.4 cm⁻¹ were used to calibrate frequency scale. The EO was mixed with ambient air, and total gas pressure and mixing ratios were varied from a few mbar to 1 bar and from ∼10² to 10⁵ parts per million, respectively. A rapid roll off of EO absorbance at 5903.7 cm⁻¹ was observed at gas pressures below ∼0.5 bar. A linear dependence of EO peak absorbance on mixing ratio was found at a total gas pressure of 1 bar. We conclude that monolithic VCSELs operating near 1693 nm could be used in EO sensors with a detection limit in the ppb range.     

Negative ion effects in CO2 convection laser discharges

Negative ions are computed to be formed on a time scale and in quantities such that they may be a cause of plasma instability observed in low pressure electrical discharge convection CO₂ lasers. In a typical CO₂−N₂−He−H₂O laser mixture the principal ions are CO ₃ ⁻ , CO ₄ ⁻ and H⁻ with the total negative ion densityn ₋ given by 0.1n _e <n ₋<n _e , wheren _e is the electron density: but if the gases are re-cycled or if there is an air leak NO ₂ ⁻ and NO ₃ ⁻ are formed in significant amounts andn ₋ can become greater thann _e in a time considerably less than the gas dwell time in the electrical excitation discharge. CO is effective in reducingn ₋ in a system without re-cycling, but is ineffective in a re-cycled system with the oxides of nitrogen present.     

External-cavity quantum cascade lasers with fast wavelength scanning

Fast wavelength scanning of an external-cavity quantum cascade laser (EC-QCL) has been developed using a modified Littrow-type cavity configuration. Scan rates up to 5 kHz and tuning ranges up to 7 cm⁻¹ have been realized using folded cavity arrangement with a fast piezoactuated intracavity mirror to vary the effective diffraction grating angle. High-resolution molecular spectroscopy, limited primarily by the effective laser linewidth, has been demonstrated in pulsed and CW mode using direct absorption spectroscopy of ammonia and ethylene in the 10-μm wavelength region. A high-resolution broadband spectral scan employing mode-hop-free tuning of a set of discrete laser modes within the fast frequency scan has been developed. The method has been demonstrated by performing high-resolution broadband spectrum of ethylene under atmospheric and reduced pressures.