We have fabricated single-frequency diode lasers from a number of III-V semiconducting compounds. These diode lasers were specifically designed for laser absorption spectroscopy. Their emission wavelengths span the internal of 0.76 to 2.7 micrometers . Water vapor, CO, CO₂, NH₃, CH₄ HF, and O₂ have been detected using them. After a brief review of their physical structure and principles of operation, we present representative output characteristics of these lasers, along with a discussion of several important applications.

It is well known that DFB lasers tune by a factor of at least 10 times more with temperature than with current. The problem, however, is that the electrical modulation of the laser is easier and much faster than temperature modulation. This paper describes a novel technique to temperature modulate a DFB laser. A 1393 nm DFB laser chip is mounted directly on a single element thermo-electric cooler (TEC) which allows temperature modulation of the chip by passing a current in both the forward and reverse direction through the TEC. A +/- 40 mA modulation through the TEC at a rate of up to 30 Hz provides a frequency sweep of 46 GHz of the laser output frequency. The time constant of the setup is 10 ms.

A 1530nm multimode diode laser with an external resonator containing a grating achieved a single-mode tuning range of 100nm. Etalon effects and modehops during the frequency tuning of the laser were minimized by special design features. Absorptions of water vapor were recorded intracavity and extracavity. The intracavity experiments were impeded by modehops induced by the absorption itself. This effect could be reduced by increasing the selectivity of the resonator. Modeling of the optical properties of the resonator allows the quantitative specification of the optical parameters for a singlemode intracavity tunable diode laser (TDL) operation. This enables a resonator design to be realized which achieves an optimum stability and selectivity for a singlemode TDL for both extracavity applications.

A standard grating-tuned extended-cavity diode laser is used for injection seeding of a tapered semiconductor laser/amplifier. With sufficient injection power the output of the amplifier takes on the spectral characteristics of the master laser. We have constructed master-oscillator power-amplifier systems that operator near 657 nm, 675 nm, 795 nm, and 850 nm. Although the characteristics vary from system to system, we have demonstrated output powers of greater than 700 mW in a single spatial mode, linewidths less than 1 kHz, coarse tuning greater than 20 nm, and continuous single-frequency scanning greater than 150 GHz. We discuss the spectroscopic applications of these high power, highly coherent, tunable diode lasers as applied to Ca, Hg⁺, I₂, and two-photon transitions in Cs.

Visible/near-infrared diode lasers are well-suited for use as spectroscopic light sources in detection of a wide variety of gases by optical absorption. The high spectral resolution of these devices permits the selective detection of targeted species, while their characteristics of low cost, room temperature operation, and compatibility with fiber optics make them attractive for instrument development. A partial list of industrially or environmentally significant gases that may be measured by near-IR diode laser spectroscopy includes oxygen, water vapor, methane, acetylene, carbon monoxide, carbon dioxide, hydrogen halides, ammonia, hydrogen sulfide, and nitrogen oxides. This paper describes recent work at Southwest Sciences in development of diode laser-based instrumentation for industrial or environmental monitoring applications. Instrumentation utilizing a 1.393 micrometers DFB diode laser for measurement of trace moisture contamination in high purity process gases is described. In addition, recent laboratory studies to characterize the performance of new types of diode lasers in gas sensing applications are discussed, including vertical cavity surface emitting lasers in the 650 to 960 nm region and antimonide-based lasers in the 2.6 micrometers region.

Tuning and spectral characteristics of 3 micrometers wavelength InAsSb lasers were considered in viewpoint of requirements of tunable diode laser absorption spectroscopy. The spectra of lasing, far-field patterns, long term stability, current and temperature tuning performance of CW lasers which are operated at 3.4-3.6 micrometers and designed for formaldehyde detection were investigated versus various ambient conditions. A specific characteristic for III-V compound lasers was observed: InAsSb lasers are tuned by direct current to the short wavelength side whereas mode jumps occurred towards longer wavelengths. The lasers showed a good current and temperature tuning. The tuning rate is determined by competition of the electronic and thermal mechanisms of tunability. The thermal shift of the band gap of the active area leads to an average mode tuning rate of about 40 GHz/K. The tuning characteristics were compared with mode jump values to calculate the whole spectral region covered by tunable laser emission. A good stability of the emitted wavelength during a long time period of operating and a lot of cooling-heating cycles could not be found, which can be explained by the higher stability of the III-V compound compared to lead-salts. The lasers were suitable for detection of ambient formaldehyde concentration level of 200 pptv at 1 Hz electronic bandwidth.

Recent advances in room-temperature tunable diode lasers and ultrasensitive electronic noise quieting detection techniques now enable a new generation of compact, optoelectronic, ultrasensitive trace gas sensors. These advances are key to producing sensors capable of routine and extended field use. We achieve near shot noise-limited signal detection using a novel, balanced ratiometric detector (BRD) which permits measurements of absorbances of 1:10⁶. High sensitivity is achieved by coupling this technology with an extended optical pathlength. The BRD is characterized by a wide linear dynamic range. A 10 Hz measurement rate enables ground level flux measurements or airborne concentration measurements. We will present an overview of two applications of our ultrasensitive detection technology to in situ atmospheric sensing. The first sensor is being developed to monitor boundary layer NO₂ fluxes. This sensor operates at 670 nm, utilizes an open multipass optical cell, and has a sub-ppbv detection sensitivity. The second sensor is an airborne, near IR diode laser hygrometer. The sensor uses an in-situ air measurement probe housing a 50 cm, open optical path to circumvent problems inherent in extractive sampling. The sensor is capable of measuring water vapor throughout the troposphere and has a sensitivity of 0.5 ppmv at the tropopause.

Substantial improvements of tunable diode laser absorption spectroscopy (TDLAS) with respect to detection speed and detection limits were obtained by introducing high frequency modulation schemes, but the expected quantum limited performance with optical multipass cells has not been attained yet on a routine basis. TDLAS measurements are usually limited by signal instabilities due to fluctuations, which can be divided generally into slow and fast fluctuations according to their time scale. While slow fluctuation or drift effects can be attributed to thermal effects, the question is now, what are the short term fluctuations that limit sensitivity. Therefore, in this paper we will identify phase noise and refractive index fluctuations caused by a turbulent gas flow in optical multipass cells as one limiting factor for the achievement of quantum limited sensitivities. Additionally some new results for a laser sample double modulation experiment based on the Stark-effect in molecular spectra will be presented and we will show that a significant suppression of background fluctuations caused by drift effects can be obtained with this technique.

We report on an experimental investigation of the possibility to correlate diode laser gas transmission spectra to computed or previously measured spectra. Spectral correlation of this kind could profitably be applied in automated gas analyzers in order to retrieve optimum laser emission characteristics after laser degradation. The full procedure reported here includes experimental gas transmission spectroscopy by lead-chalcogenide lasers recorded simultaneously with the transmission of a Mach- Zehnder interferometer within a reference channel. The interferometer fringes are used for identification of a single-mode laser operating range and for wavenumber linearization and background intensity correction of the gas transmission spectra within this range. Direct correlation of these corrected spectra turns out to be inadequate for spectra with strong quasi-periodic absorption lines as is the case for carbon dioxide. In particular in the presence of noise, a number of nearly indistinguishable correlation factor maxima are found. Better results were obtained by generating a barcode pattern from the corrected transmission before performing the numerical correlation with one unambiguous correlation factor maximum. This barcode technique was also verified to be more reliable for irregular spectra as for methane, and to be less sensitive to noise and background intensity variations.

Supersonic molecular expansions provide a unique environment in which to acquire the infrared spectra of many molecular systems. Tunable diode lasers offer high spectral brightness, low noise, rapid tuning, and contiguous spectral coverage in the infrared, from 3000 to 500 cm^-1. Molecular beams combined with tunable diode lasers represents a powerful tool for reducing or removing spectral congestion arising from both pressure broadening and rotational excitation. Depending on expansion conditions, rotational temperatures on the order of 10K can be routinely obtained. With the appropriate pulsed slit nozzle, Doppler broadening is reduced six-fold or more over that of room temperature, static cells. In addition, by employing modified White cell optics, effective optical path lengths of 3 meters can be realized. A description of Pacific Northwest National Laboratory's beam-diode laser spectrometer will be given along with recent results for studies involving the fully rotationally resolved spectra of several molecules of atmospheric interest including chlorine nitrate and carbon tetrachloride.

We fabricated and tested a low temperature cell which is mounted directly on the second stage of a CTI-Cryogenics Model 22C CRYODYNE CRYOCOOLER. The vacuum system consists of a room temperature vacuum shroud, a radiation shield maintained at 77K and the cell which is mounted directly to the second stage of the cryocooler. The ultimate cell temperature is 12.4 Kelvin, and the low temperature limit increases at a rate of 5.6 Kelvin/Watt. We achieve a cell temperature of 22 Kelvin under typical experimental conditions of approximately 29 milli Torr helium, slow flowing gas, and a heated injector. The absorption path length of the cell is 3.35 cm, and the window clear aperture is 1.27 cm. We preformed a series of experiments in which we determined the translational temperatures of vibration- rotation transitions in the band of CO for different cell temperatures. The results of our tests are discussed in this paper.

A time-resolved flash photolysis/tunable diode laser absorption spectroscopy system has been constructed to study reaction products of gas-phase chemical reactions. The use of FP to initiate the chemistry results in essentially a wall-less reactor due to the time of reaction being much shorter than the time of diffusion. The use of TDLAS allows the time-resolved detection of HCl by monitoring individual lines in the rovibrational spectrum. While the system is being developed for study of the products of radical-radical reactions, preliminary studies involved detection of HCl from reaction of Cl-atoms with organic species. The reaction Cl + C₂H₆ yields HCl + C₂H₅ (1) was studied to characterize the system and determine the detection limit for HCl. The rate coefficient, k₁, was determined to be (5.9 +/- 0.4) X 10^-11 cm³ molec^-1s^-1 in agreement with the recommended values. HCl (v equals 0) and HCl (V equals 1) were simultaneously measured as products of the reaction Cl + CH₃CHO yields HCl + CH₃CO (2) at 2906.247 cm^-1 and 2906.337 cm^-1 respectively. It was determined for this reaction that k₂ equals (7.5 +/- 0.6) X 10^-11 cm³molec^-1s^-1.

Tunable lead-salt diode laser absorption spectroscopy (TDLAS) provides a sensitive and versatile probe for the study of the kinetics and mechanisms of atmospheric reactions. In our laboratory, the combination of laser flash photolysis with TDLAS detection of reactant and/or product species has proven useful in several studies of the gas phase oxidation of the atmospheric sulfur compound dimethylsulfide, a process which may play an important role in global climate modification/regulation. Typically a radical species is produced by UV laser photolysis of a stable precursor in a slowly flowing mixture of reactant and buffer gases. The concentration of this radical or a selected reaction product is then followed by TDLAS on a time scale of microseconds to milliseconds. This method allows direct determination of reaction rates and product branching ratios over a range of temperature, pressure and reactant concentrations in complete isolation from reactor surfaces.

Tunable diode laser absorption spectroscopy (TDLAS) meets the major requirements for atmospheric trace gas monitoring, which are sub-ppbv sensitivity, high detection speed and the potential for simultaneous in-situ measurements of several compounds. In recent years, several multi-laser TDLAS systems have been developed at the Max Planck Institute for Chemistry and used in a number of ground based, shipboard and airborne field campaigns to measure the concentrations of atmospheric trace species, e.g. N₂O, CH₄, CO, HCHO, H₂O₂ and NO₂, from the boundary layer up to the lower stratosphere at 14 km altitude. During these field measurement on various platforms, detailed comparisons of TDLAS with other techniques have been performed for CO, HCHO, and NO₂, yielding an agreement between the various instruments on the order of 10-20 percent. In addition, a TDLAS instrument has been used to measure N₂O fluxes from soils by eddy correlation and flux gradient techniques. A particular TDLAS instrument has been developed, which is capable of high-precision direct measurements of ¹³CH₄/¹²CH₄ and ¹²CH₃D/¹²CH₄ ratios. An intercomparison between this instrument and conventional mass spectrometry yielded a mean deviation of (delta) ¹³C equals 0.5 percent and (delta) D equals 5 percent.

Infrared laser beam as suffer from absorption form various gas species not to say of water vapor or carbon dioxide. If the tunable diode laser is employed as source, we can obtain more information from the fluctuating absorption signal than ever by virtue of TDL's spectral purity and its quick tunability. By scanning an absorption spectrum around an absorption line, not sticking to the center of a line, spectral interference from an adjacent line or scintillation can be eliminated with a real-time digital signal processing. Based on the actual data about temporal sequence of transmittance for TDL beam through open atmosphere, it is found that the scintillation is frozen if the spectral scanning is done within 0.1 second, and the more the less. In order to facilitate this principle, received electronic signal is treated by a high speed digital signal processing system. A special integrated circuit device, DSP, is exploited of its high performance in numerical calculation speed. Elimination of spectral interference is performed with a personal computer based on an algorithm named as 'adjoint spectrum' invented by the authors. This is also implemented by DSP. Function of the test equipment the authors have built is shown.

This paper discusses recent advances in our techniques for monitoring atmospheric trace gases using lead salt liquid nitrogen cooled diode lasers. Our approach employs an optical system with all reflective optics. Our closed path systems rely on a proprietary astigmatic multipass cell to achieve long optical path lengths in a low volume sampling cell. We have also developed open path systems which we have used for remote sensing of automobile and aircraft engine exhaust. Our data acquisition method uses rapid frequency sweeping followed by nonlinear least squares analysis of the retrieved spectrum. Recent advances include an emphasis on multi-laser multi-species detection systems, such as simultaneously monitoring the nitric oxide, carbon monoxide and carbon dioxide concentrations in automobile exhaust. Other advances are focused on achieving improved detection sensitivity. In support of this goal, we have demonstrated astigmatic multipass cells with very long optical paths, we have improved the nonlinear least squares spectral fitting routines allowing them to fit complex multi-peak spectra and we have introduced photolytic modulation as a method to discriminate spectra of photolytically active species from background absorption and optical interference fringes. These techniques are being applied to the monitoring of a wide variety of atmospheric molecules including CH₄, CO, CO₂, N₂O, NO, NO₂, HONO, HNO₃, O₃ and HOCl.

Formaldehyde (HCHO) is an important reactive intermediate in atmospheric studies. Accurate measurements of HCHO are required to constrain and validate photochemical models. Despite this importance, there is still considerable uncertainty in present ambient measurements of this gas as well as in measurement-model relationships. The present paper discusses the long-term effort at NCAR to develop, employ, and validate a highly sensitive tunable diode laser absorption spectrometer (TDLAS) for ambient measurements of HCHO. A detailed analysis of measurement precision will be presented and performance improvements using rapid background subtraction, FFT filtering, and scan-by-scan demeaning will be discussed. This paper will conclude with a brief discussion of recent photochemistry and intercomparison field campaigns employing the TDLAS.

A ground based tunable diode laser absorption spectrometer (TDLAS) developed at NCAR for the measurement of formaldehyde (HCHO) has been modified for use aboard tropospheric aircraft. Measurements of HCHO are essential to comprehensive investigations of atmospheric oxidation processes, and aircraft platforms provide the advantage of vertically and spatially resolved measured. Initial deployment of the aircraft system occurred during the spring and summer of 1996 as part of the NARE and STERAO campaigns. Data coverage exceeded 95 percent out of a sum total of 175 flight hours. Sensitivities achieved during STERAO were approximately 40-60 pptv for 4.5 min of measurement and 80- 120 pptv for 55s; NARE sensitivities were slightly worse. For both campaigns, post-flight fitting of background spectra indicated periodic outgassing and contamination of the background matrix air. Analysis of data collected during the May 1995 SOS intercomparison suggests that background subtraction largely nullifies any outgassing effect. Background matrix gas HCHO concentrations were determined by fitting background spectra and were then used to correct the associated ambient data sets. Finally, fits of the difference of successive backgrounds appear to approximate measurement replicate precisions and are more informative than calculated fit precisions.

Tunable diode laser spectroscopy has, in the last few years, become an important technique for the measurement of atmospheric trace gases and pollutants. This technique is useful not only for concentration monitoring of a particular species, but can be used to measure the flux of trace gas species released or absorbed by an area. At the present time, mainly tow types of instruments are being manufactured and used. In the summer of 1993, the Center for Laser Analytical Studies of Trace Gas Dynamics at the University of Nebraska-Lincoln made an intercomparison of these two types of tunable diode laser spectrometers as used to measure the flux and concentration of naturally produced methane. Fluxes were measured using the eddy correlation method. This method requires both a high sensitivity to the species being measured, and a fast instrument response and so was deemed a stringent test of instrument performance. The study was conducted at a prairie wetland site near Valentine, Nebraska in late July and early August, 1993. The two instruments compared were built by Unisearch Associates Inc. of Concord, Ontario, and by Campbell Scientific Inc. of Logan, Utah and were representative of the two main types in general use. The study showed that both instruments exhibited similar detectabilities, and had almost identical noise characteristics when measuring both fluxes and concentrations. The study also brought out the need for careful calibration procedures and precise thermal management of the instruments to achieve optimal performance.

An instrument which will measure the atmospheric concentration of carbon dioxide with a resolution of one part in 10⁴ will be realized in the frame of 'environment and climate' program of the European Community. The aims are the evaluation of global climate changes and the measurement of the flux at the interface air-sea. The requirements for such an instrument apart form resolution are stability and insensitivity to water. This instrument is based on room temperature tunable diode lasers at about 6350 cm(superscript - by laser sources with the required spectral characteristics. The atmospheric concentration of carbon dioxide is about 350 ppm and its absorbance along a path of 100 m is of the order of 1 percent. The measurement will be performed in a multipass cell. A second cell will supply the references for frequency stabilization and laser power monitoring. The detection is realized through frequency modulation techniques. The various aspects of the work presently in progress will be presented: spectroscopy, optical layout, detection method and so on.

The LASIR system of instruments provide a way of measuring specific pollutants with no interferences form other gases. Since they are based on laser diodes operating in the communication region of the near infrared, they are rugged, easy to operate and relatively small and inexpensive. The instrument can be located in any desired location where it is free of environmental constraints and is readily available for servicing if required. The monitoring beam is transmitted to the measuring location by fiber optics. In some configurations the delivery optics contain no electrical components and can be placed in explosive or hazardous locations. These measurements locations can be up to several kilometers distant from the control. Three versions of the LASIR are available, (1) the LASIR-R for perimeter or area ambient air monitoring, (2) the LASIR-P for point source measurements, and (3) the LASIR-S for in stack monitoring. Multiplexing can be used to operate, simultaneously, a combination of a number of these versions for mt he same instrument at a number of locations, and if desired, with a variety of different lasers. Examples of the application of these system for measuring HF in aluminum smelters have shown that there is ample sensitivity to measure HF both in the stacks and in the pot-rooms of these smelters.

A multiplexed diode-laser sensor system, based on absorption spectroscopy techniques and comprised of two InGaAsP diode lasers and fiber-optic components, has been developed to measure temperature and species concentration non- intrusively over a single path for closed-loop process control. The system was applied to measure and control the gas temperature in the post-flame gases 6 mm above the surface of a Hencken burner. The wavelengths of the lasers were independently current-tuned across H₂O transitions near 1343 nm and 1392 nm. Temperature was determined from the ratio of measured peak absorbances, and H₂O concentration was determined from the measured peak absorbance of one transition set at the measured temperature. A closed-loop feedback system was demonstrated to control the mean temperature and the amplitude of temperature fluctuations at particular frequencies by appropriately modulating the fuel flow rate. The results obtained demonstrate the potential of multiplexed diode lasers for rapid, continuous, in situ measurements and control of gas dynamic parameters in high-temperature combustion flowfields and other environments with difficult optical access.

Tunable diode laser absorption spectroscopy (TDLAS) shows promise for a number of environmental monitoring applications. This technique is advantageous over more classical methods because of excellent dynamic range, signal to noise ratios, and narrow bandwidth detection. With the rapid advances in the communications industry, lasers and optical components necessary for sensor technology are becoming affordable as well. One serious obstacle towards this effort is the paucity of spectroscopic data for the most useful, albeit weak, transitions near fiber optic communications wavelengths, especially at elevated temperatures. This data is important not only for species of monitoring interest, but also for those of possible interferants. In the near infrared, these are typically overtone and combination bands and hence accurate prediction of location, linestrength, and broadening coefficients is non-trivial. This is especially true for transitions arising from a highly excited rotational lower energy state. These are lines which may not be observable at room temperature but can play an important role for in-situ monitoring of a hot duct or smokestack. We have developed a system comprised of an extended cavity tunable diode laser and high temperature oven to characterize absorption spectra as a function of temperature and pressure. The experimental apparatus is described and data presented for water near 1.55 microns form 100 to 300 degrees C and 30 Torr.

An in situ gas filter correlation spectrometer is applied to measure gas emission in smoke stacks. Experiments were performed at a simulated stack with NO at parts per million level for various elevated temperatures. A line-by-line spectroscopic computational model, originally developed at NASA Langley Research Center, was applied to the analysis. This paper summarizes recent development work at GE CRD and GE Reuter-Stokes to understand and to improve the sensitivity and efficiency of the analyzer for in situ monitoring nitrogen monoxide (NO) in the fossil fuel-fired power plants. The model calculation agreed very well with the experimental results. Special consideration was given to analyzing the detector response and light source effect on the performance of the system. The report concludes with a discussion of several design improvements and their potential impact on the signal to noise ratio and, therefore, the NO detection sensitivity.

Trace contents of ammonia in outdoor, indoor and exhaled air were measured on the base of high resolution absorption spectra. Tunable diode laser system developed for this purpose possesses approximately one second time constant, approximately 200 cm3 sample volume, 5 ppb sensitivity. The calibration of unit was based on measurements of relative intensities of sQ(3,1)...sQ(3,3) absorption lines of v_2s and following calculation on the base of a priori data on strength and broadening coefficients of detected lines. Measured indoor contents of ammonia was in 5-10 times higher than outdoor contents. Approximately two times drop in NH₃ room content after 6 p.m. was detected. Obtained behavior of ammonia content in respiration right after the smoking demonstrates that the removing of ammonia from lungs has the ventilation character. Measured contents of NH₃ in human respiration was ranged between 120 and 220 ppb. The absence of ammonia content differences from respiration of smoking and non smoking persons demonstrates that the accumulation of NH₃ by human organism seems to be rather negligible for a short time exposure, e.g. like smoking.

For the simplification of molecular spectra and of the increase of spectral line intensity the enclosive-flow cooling technique was developed. The vertical cell arrangement needs only one warm window and proved to be robust and easy to handle. Compared with supersonic jet cooling, the present method provides an absorption efficiency higher by several orders of magnitude. In this paper, an improved flow cooling cell including a multireflection optics is described. Compared to the prototype cell, the multireflection cell has only 50 percent of the mass. Optical paths up to 20 m for FTIR applications and up to 40 m for TDLAS applications can be chosen. The pressure range extends from below 0.001 mbar up to 3 bar. The temperature is adjustable from 65 to 350 K. The new cooling technique offers promising applications in trace gas analysis, in the generation and spectroscopy of molecular clusters, especially of water, and in the simulation of the conditions and processes that occur in the atmosphere of Earth and other planets.

The medium non-linear dispersion influence for complex form of quasimonochromatic signal propagating through environment is described. It is shown, that a non-linear dispersion disturbs both a real amplitude and an imaginary phase of signal. The proofs are adduced for AM-signal. The results of a computer simulation of a process coroborate the theoretic conclusions. Noted effects may be important for the signal propagating within spectral absorption line at the TDLS.

Two cw-single mode diode-lasers with powers of 30 and 50 mW at the center wavelengths 682 and 791 nm are applied as signal and pump sources for difference frequency generation (DFG) in an AgGaS₂ crystal with a length of 30 mm. For 90 degree type I phase matching tunable mid-infrared laser radiation is obtained in the spectral range between 4.9 and 5.1 micrometers , while the DFG-output power is 0.2 (mu) W. The performance of this diode-laser MIR-DFG spectrometer is shown as the absorption of CO for the P(28) rotational line around 2023 cm^-1 is probed in a cell and on-line in the exhaust of an engine.

A family of multicomponent gas analyzers based on tunable diode lasers (TDL) are presented including an open optical path 3-component gas analyzer for monitoring atmosphere pollution; a 4-gas component analyzer with a multipass cell, a multichannel TDL system with multipass cell and MIR fiber optics delivery, and a multichannel TDL analyzer for breath content gas analysis. A4B6 tunable diode lasers of different spectral region form 4 to 12 micron were used in every channel of the analyzers to obtain atmospheric transmission spectra and to measure concentration of studied gases. Main gaseous atmospheric pollutants like CO, NO, NH₃, CH₄, SO₂, etc. and some of breath gases could be measured at ppb concentration level in real time with these systems depending on customers requirements. The analyzers are driven by IBM compatible PC. The first results on field monitoring tests as well as on their application to human exhalation content analysis and human exposure monitoring are presented.