Air pollution: sensitive detection of ten pollutant gases by carbon monoxide and carbon dioxide lasers

Experimental design and numerical investigation of a photoacoustic sensor for a low-power, continuous-wave, laser-based frequency-domain photoacoustic microscopy

We have developed a photoacoustic (PA) sensor using a low-power, continuous- wave laser and a kHz-range microphone. The sensor is simple, flexible, cost-effective, and compatible with commercial optical microscopes. The sensor enables noncontact PA measurements through air, whereas most current existing PA techniques require an acoustic coupling liquid for detection. The PA sensor has three main components: one is the chamber that holds the sample, the second is a resonator column used to amplify the weak PA signals generated within the sample chamber, and the third is a microphone at the end of the resonator column to detect the amplified signals. The chamber size was designed to be 8  mm  ×  3  mm as the thermal diffusion length and viscous-thermal damping of air at room pressure and temperature are 2 and 1 mm, respectively. We numerically and experimentally examined the effect of the resonator column size on the frequency response of the PA sensor. The quality factor decreased significantly when the sample chamber size was reduced from 4  mm  ×  3  mm to 2  mm  ×  3  mm due to thermos-viscous damping of the air. The quality factor decreased by 27%, demonstrating the need for optimal design for the sample chamber and resonator column size. The system exhibited noise equivalent molecular sensitivity (NEM) per unit bandwidth (NEM  /  √  Δf) of ∼19,966  Hz ^−1/2 or 33  ×  10^−21  mol or 33 zeptomol, which is an improvement of 2.2 times compared to the previous system design. This PA sensor has the potential for noncontact high-resolution PA imaging of materials without the need for coupling fluids.

Low-power noncontact photoacoustic microscope for bioimaging applications

An inexpensive noncontact photoacoustic (PA) imaging system using a low-power continuous wave laser and a kilohertz-range microphone has been developed. The system operates in both optical and PA imaging modes and is designed to be compatible with conventional optical microscopes. Aqueous coupling fluids are not required for the detection of the PA signals; air is used as the coupling medium. The main component of the PA system is a custom designed PA imaging sensor that consists of an air-filled sample chamber and a resonator chamber that isolates a standard kilohertz frequency microphone from the input laser. A sample to be examined is placed on the glass substrate inside the chamber. A laser focused to a small spot by a 40 × objective onto the substrate enables generation of PA signals from the sample. Raster scanning the laser over the sample with micrometer-sized steps enables high-resolution PA images to be generated. A lateral resolution of 1.37 ?? ? m was achieved in this proof of concept study, which can be further improved using a higher numerical aperture objective. The application of the system was investigated on a red blood cell, with a noise-equivalent detection sensitivity of 43,887 hemoglobin molecules ( 72.88 × 10 ? 21 ?? mol or 72.88 zeptomol). The minimum pressure detectable limit of the system was 19.1 ?? ? Pa . This inexpensive, compact noncontact PA sensor is easily integrated with existing commercial optical microscopes, enabling optical and PA imaging of the same sample. Applications include forensic measurements, blood coagulation tests, and monitoring the penetration of drugs into human membrane.

Graphene-based mid-infrared room-temperature pyroelectric bolometers with ultrahigh temperature coefficient of resistance

There is a growing number of applications demanding highly sensitive photodetectors in the mid-infrared. Thermal photodetectors, such as bolometers, have emerged as the technology of choice, because they do not need cooling. The performance of a bolometer is linked to its temperature coefficient of resistance (TCR, ∼2–4% K⁻¹ for state-of-the-art materials). Graphene is ideally suited for optoelectronic applications, with a variety of reported photodetectors ranging from visible to THz frequencies. For the mid-infrared, graphene-based detectors with TCRs ∼4–11% K⁻¹ have been demonstrated. Here we present an uncooled, mid-infrared photodetector, where the pyroelectric response of a LiNbO₃ crystal is transduced with high gain (up to 200) into resistivity modulation for graphene. This is achieved by fabricating a floating metallic structure that concentrates the pyroelectric charge on the top-gate capacitor of the graphene channel, leading to TCRs up to 900% K⁻¹, and the ability to resolve temperature variations down to 15 μK.

There is emerging interest in photodetectors in the mid-infrared driven by increasing need to monitor the environment for security and healthcare purposes. Sassi et al. show a thermal photodetector, based on the coupling between graphene and a pyroelectric crystal, which shows high temperature sensitivity.

Biomedical photoacoustic imaging

Photoacoustic (PA) imaging, also called optoacoustic imaging, is a new biomedical imaging modality based on the use of laser-generated ultrasound that has emerged over the last decade. It is a hybrid modality, combining the high-contrast and spectroscopic-based specificity of optical imaging with the high spatial resolution of ultrasound imaging. In essence, a PA image can be regarded as an ultrasound image in which the contrast depends not on the mechanical and elastic properties of the tissue, but its optical properties, specifically optical absorption. As a consequence, it offers greater specificity than conventional ultrasound imaging with the ability to detect haemoglobin, lipids, water and other light-absorbing chomophores, but with greater penetration depth than purely optical imaging modalities that rely on ballistic photons. As well as visualizing anatomical structures such as the microvasculature, it can also provide functional information in the form of blood oxygenation, blood flow and temperature. All of this can be achieved over a wide range of length scales from micrometres to centimetres with scalable spatial resolution. These attributes lend PA imaging to a wide variety of applications in clinical medicine, preclinical research and basic biology for studying cancer, cardiovascular disease, abnormalities of the microcirculation and other conditions. With the emergence of a variety of truly compelling in vivo images obtained by a number of groups around the world in the last 2-3 years, the technique has come of age and the promise of PA imaging is now beginning to be realized. Recent highlights include the demonstration of whole-body small-animal imaging, the first demonstrations of molecular imaging, the introduction of new microscopy modes and the first steps towards clinical breast imaging being taken as well as a myriad of in vivo preclinical imaging studies. In this article, the underlying physical principles of the technique, its practical implementation, and a range of clinical and preclinical applications are reviewed.

Photoacoustic techniques for trace gas sensing based on semiconductor laser sources

The paper provides an overview on the use of photoacoustic sensors based on semiconductor laser sources for the detection of trace gases. We review the results obtained using standard, differential and quartz enhanced photoacoustic techniques.

Real-Time Diode Laser Measurements of Vapor-Phase Benzene

An absorption spectrometer equipped with a IV-VI semiconductor tunable mid-IR diode laser was used to make sensitive measurements of benzene (C(6)H(6)) gas in the 5.1-microm spectral range. Wavelength modulation coupled with second-harmonic detection achieved accurate real-time quantification of benzene concentrations down to a minimum detection limit of 1 ppmv with an integration time of 4 s. A variety of calibrated benzene-sensing measurements were made, including the determination of the benzene concentrations in vehicle exhaust and headspace vapors from unleaded gasoline and other liquids. Kinetic phenomena, including the monitoring of benzene evaporation and absorption/desorption by granulated activated carbon were observed with the instrument. Measurements were performed that allowed experimental determination of the activation energy for desorption of benzene from activated carbon, which was found to be 198 meV/molecule (19.0 kJ/mol).

Internally excited acoustic resonator for photoacoustic trace detection

The quantum-cascade laser can be used as an infrared source for a small portable photoacoustic trace gas detector. The device that we describe uses a quantum-cascade laser without collimating optics mounted inside an acoustic resonator. The laser is positioned in the center of a longitudinal resonator at a pressure antinode and emits radiation along the length of the resonator exciting an axially symmetric longitudinal acoustic mode of an open-ended cylindrical resonator. Experiments are reported with an 8-microm, quasi-cw-modulated, room-temperature laser used to detect N2O.

Correlation of breath ammonia with blood urea nitrogen and creatinine during hemodialysis

We have spectroscopically determined breath ammonia levels in seven patients with end-stage renal disease while they were undergoing hemodialysis at the University of California, Los Angeles, dialysis center. We correlated these measurements against simultaneously taken blood samples that were analyzed for blood urea nitrogen (BUN) and creatinine, which are the accepted standards indicating the level of nitrogenous waste loading in a patient's bloodstream. Initial levels of breath ammonia, i.e., at the beginning of dialysis, are between 1,500 ppb and 2,000 ppb (parts per billion). These levels drop very sharply in the first 15-30 min as the dialysis proceeds. We found the reduction in breath ammonia concentration to be relatively slow from this point on to the end of dialysis treatment, at which point the levels tapered off at 150 to 200 ppb. For each breath ammonia measurement, taken at 15-30 min intervals during the dialysis, we also sampled the patient's blood for BUN and creatinine. The breath ammonia data were available in real time, whereas the BUN and creatinine data were available generally 24 h later from the laboratory. We found a good correlation between breath ammonia concentration and BUN and creatinine. For one of the patients, the correlation gave an R(2) of 0.95 for breath ammonia and BUN correlation and an R(2) of 0.83 for breath ammonia and creatinine correlation. These preliminary data indicate the possibility of using the real-time breath ammonia measurements for determining efficacy and endpoint of hemodialysis.

Water vapor absorption coefficients in the 8-13-microm spectral region: a critical review

Measurements of water vapor absorption coefficients in the thermal IR atmospheric window (8-13 microm) during the past 20 years obtained by a variety of techniques are reviewed for consistency and are compared with computed values based on the AFGL spectral data tapes. The methods of data collection considered were atmospheric long path absorption with a CO(2) laser or a broadband source and filters, a White cell and a CO(2) laser or a broadband source and a spectrometer, and a spectrophone with a CO(2) laser. Advantages and disadvantages of each measurement approach are given as a guide to further research. Continuum absorption has apparently been measured accurately to about the 5-10% level in five of the measurements reported. However, the effect of oxygen broadening has not been fully considered, since most laboratory measurements were made using nitrogen buffering. Oxygen could lead to a small reduction in the adopted value of the water vapor continuum absorption coefficient in air. Also, the temperature dependence does not seem to have been measured well for temperatures <20 degrees C. The rotational and v(2) line absorption coefficients do not appear to have been determined well in this spectral region except at CO(2) laser line frequencies, because the agreement between such measurements and the AFGL spectral data tapes is generally not good.

Photoacoustics in life sciences

Photoacoustic (PA) measurements provide, by the very nature of the PA effect, the possibility to obtain information on the optical and thermal properties of samples. In addition they can yield information on the enthalpy changes and characteristic times involved in photo-induced processes as the acoustic signal is proportional to the hear produced following the absorption of the modulated excitation. In the study of optical properties the relative insensitivity to scattered light of the PA signal makes such measurement an attractive way to measure biological samples in vivo, or, at least, without the need to isolate the absorbing compounds. The dependence of the PA signal on the thermal properties of the sample is particularly useful when heterogeneous samples are studied. As a photocalorimetric method the technique shows considerable promise in the study of photo-bioenergetics, especially photosynthesis. Only in special cases can analytical applications of the PA method compete with fluorescence measurements for detection, and with increasingly sophisticated optical transmission and reflectance techniques (for identification).. However, the PA method may find important uses in fundamental research and in applied areas such as biomedicine and agricultural biochemistry.

Gaseous trace analysis using pulsed photoacoustic Raman spectroscopy

A new method for trace analysis of gases, based on the pulsed photoacoustic Raman spectroscopy (PARS) technique, is described. The method has been applied to the analysis of mixtures of CH(4) in N(2), CO(2) in N(2), and N(2)O in N(2) at concentrations near 1 ppm. The apparatus used is described in some detail. Means of improving the method's sensitivity as well as sensitivity-limiting processes are evaluated. The analytical capabilities of this technique are compared with both direct (IR) absorption and other Raman techniques such as CARS and stimulated Raman gain spectroscopy (SRGS).

Optoacoustic detection using Stark modulation

Stark modulation of the absorbed laser radiation in an optoacoustic detector (or spectrophone) is reported. Measurements were made over a range of total pressure between 760 Torr and 50 Torr. Greatly enhanced molecular discrimination is suggested due to the tuning ability of the Stark-shifted absorption. The background signal obtained by operating in this mode is more than 500 times smaller than that obtained by operating the same optoacoustic detector in the conventional chopped radiation mode. The responsivity of the optoacoustic detector and the absorption coefficient of C(2)H(4) are presented as a function of total pressure.

Laser Detection of Pollution

Spectroscopic analysis is a useful technique for identifying and quantitatively determining the presence of specific gaseous constituents. Development of high-power tunable lasers has made the spectroscopic technique for detection of trace constituents in the atmosphere very attractive for practical applications. In this article three of the currently used modes for laser detection of pollution are reviewed: (i) long-path measurements, (ii) laser Raman (differential absorption) measurements, and (iii) optoacoustic detection. Progress in the field has been extremely rapid in the last few years and very useful and reliable data on air pollution can now be obtained routinely with the techniques described.

Optimization of resonant cell design for optoacoustic gas spectroscopy (H-type)

The design and certain basic characteristics of a new type of optoacoustic device (cell) are described. Initial experimental data using a CO(2) laser operating in the 9-10-microm wavelength region demonstrated sufficient sensitivity to measure the absorption due to SF(6) in the air at a concentration of one part in 10(11). Acoustic Q's were demonstrated in excess of 1800. The general expression for the optoacoustic pressure variations inside the acoustical resonant cavity is given.

High sensitivity pollution detection employing tunable diode lasers

A laser absorption spectrometer is described which employs a wavelength-tunable Pb(1-x) Sn(x) Se diode in conjunction with a multipass White cell and which is capable of measuring SO(2) concentrations in the low ppb range. We describe in some detail the modulation techniques used in signal detection which enable us to measure absorption coefficients as low as 10(-7) m(-1). In addition, calibration of the instrumentation using small sample cells is described, and the question of interference from unwanted molecular species is discussed. The instrumentation allows the measurement, basically at the same time, of a large number of other atmospheric gases which are of significance in pollution studies. For example, the present diode operates over 1050-1150 cm(-1) and can measure SO(2), O(3), N(2)O, CO(2), H(2)O, NH(3), and PAN. The addition of a second diode to the system will allow most gases of any atmospheric importance to be monitored. In general, these gases have much stronger ir absorption bands than SO(2) and hence can be detected at concentrations much less than 1 ppb.

Laser optoacoustic detection of explosive vapors

The direct detection of nitroglycerine, ethylene glycol dinitrate, and dinitrotoluene by optoacoustic spectroscopy techniques at 6 microm, 9 microm, and 11 microm is reported. The effect of interference by normal atmospheric pollutants is investigated, and it is found that of those wavelengths used in this investigation the 9-microm and 11-/microm spectral regions, using a CO(2) laser as radiation source, are the most promising for use in explosive detection.

Thermal lens technique: a new method of absorption spectroscopy

Both a single beam and a dual beam (with synchronous detection) thermal lens technique have been employed in the measurement of "colorless" organic compounds in the range 15,700 to 17,400 cm-1. Combination overtones of C-H stretching vibrations in benzene have been identified and agree with previous results obtained by conventional spectroscopy with a long optical path. Extinction coefficients as low as 1 X 10(-6) liter mole-1 cm-1 have been accurately determined. The sensitivity of the technique has been further demonstrated by measuring the So leads to T1 absorption of anthracene; the spectrum compares favorably with results obtained by conventional techniques.

Remote sensing of gas concentrations in smokestack emissions

A measurement technique is discussed that allows for remote sensing of polluting gases that are emitted together with the water droplets in a steam plume. Infrared laser radiation is backscattered from the droplets. As wavelength of the radiation is varied, resonant absorption in the gases diminishes the backscatter signal; this may be quantitatively related the Concentration of specific gases. The backscatter arrangement makes possible the construction of a compact, single-ended, remote sensing instrument. Feasibility of this concept has been demonstrated in a laboratory experiment where a laser beam at 3.391 microm and 4.217 microm was scattered from a steam plume containing controlled amounts of CH(4) and CO(2).