Photoacoustic Analysis Research Articles

Continuously wavelength tuned optical parametric oscillator based on fan-out periodically poled lithium niobate

A PC-controlled optical parametric oscillator (OPO) based on fan-out periodically poled lithium niobate (MgO : PPLN) structures was developed. Continuous wavelength tuning (2.40–3.85 μm) was realized via linear displacements of fan-out MgO : PPLN structures using a PC-controlled precision motorized translation stage. The total wavelength scanning time in the range of 2.40–3.85 μm was ≤1 min. The OPO was developed as a source of tunable radiation for use in a laser photo-acoustic gas analyzer. Studies of the methane absorption spectrum showed a good coincidence of the experimental and theoretical data.

NO2 measurement artifacts in the presence of soot

Nitrogen dioxide is a regulated pollutant, which is measured routinely. Since it can be formed during combustion processes, it is often measured in the presence of soot. This study investigates the possible artifact formation due to the interaction of soot and NO2 in the sampling lines and instrument prefilters. The transfer of varying NO2 concentrations through filters and tubes coated with different kinds of soot was investigated by using a dedicated photoacoustic soot and NO2 analyzer (TwinPAS). The effects of flow rate, temperature, relative humidity, tubing respectively filter material, soot reactivity, and passivation on the NO2 measurement artifacts have been investigated. We found significant lags (up to 2 min) of the NO2 transfer as well as total NO2 losses of up 10 %.

Ammonia, Greenhouse Gas, and Particulate Matter Emissions of Aviary Layer Houses in the Midwestern U.S.

<abstract><title><italic>Abstract.</italic></title> There has been an increased interest in alternative housing for laying hens in certain parts of the world, including the U.S. Associated with the movement are many questions concerning sustainability of such systems. This study continually quantified concentrations and emissions of ammonia (NH₃), carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and particulate matter (PM₁₀ and PM_2.5) for two side-by-side aviary barns each housing 50,000 Hy-Line brown laying hens, located in the Midwestern U.S. The gaseous concentrations were continually monitored using an infrared photoacoustic multi-gas analyzer, while the PM concentrations were measured with tapered element oscillating microbalances (TEOMs). Barn ventilation rate was determined by monitoring the operation time of ventilation fans that had been calibrated in situ. Nineteen consecutive months of monitored data (June 2010 to December 2011) are analyzed and presented. Daily indoor aerial concentrations (mean ±SD) were 8.7 (±8.4) ppm NH₃, 1,636 (±1,022) ppm CO₂, 10.0 (±6.8) ppm CH₄, 2.3 (±1.6) mg m^-3 PM₁₀, and 0.25 (±0.26) mg m^-3 PM_2.5. The aerial emissions are expressed as quantities per hen, per animal unit (AU, 500 kg body weight), and per kg egg output. Daily emission rates (g bird^-1) were 0.15 (±0.08) NH₃, 75 (±15) CO₂, 0.09 (±0.08) CH₄, 0.11 (±0.04) PM₁₀, and 0.008 (±0.006) PM_2.5. The results were compared to reported emission values for conventional (high-rise and manure-belt cage) U.S. laying-hen housing systems. Data from this study provide baseline concentration and emission values for the aviary housing system in the Midwestern U.S., which will also contribute to improvement of the U.S. national air emissions inventory for farm animal operations.

Colorimetric polymer-metal nanocomposite sensor of ammonia for the agricultural industry of confined animal feeding operations

The proposed colorimetric sensor of ammonia for the confined animal feeding industry uses the method of optoelectronic spectroscopic measurement of the reversible change of the color of a nanocomposite reagent film in response to ammonia. The film is made of a gold nanocolloid in a polymer matrix with an ammonia-sensitive indicator dye additive. The response of the indicator dye (increase of the optical absorption between 550 and 650 nm) is enhanced by the nanoparticles (∼8 nm in size) in two ways: (a) concentration of the optical field near the nanoparticle due to the plasmon resonance and (b) catalytic acceleration of the chemical reaction of deprotonization of the indicator dye in the presence of ammonia and water vapor. This enhancement helps to miniaturize the sensing element without compromising its sensitivity of <1 parts per million (ppm) for the range 0 to 100 ppm. The sensor underwent field tests in commercial poultry farms in Georgia and Arkansas and was compared against a scientific-grade photoacoustic gas analyzer. The coefficient of correlation between the sensor and the photoacoustic data for several weeks of continuous side-by-side operation in a commercial poultry house was ∼0.9 and the linear regression slope was 1.0. The conclusions on the necessary improvements were made.

Spatial laser beam determination by pulsed photoacoustics: detection radius/signal wavelength approximation

Utilizing a mathematical algorithm developed for photoacoustic tomography, it is possible to deduce the laser beam spatial profile within pulsed photoacoustic measurements. This method is based on the temporal shape of the photoacoustic signal analysis. The exact solution for the laser beam spatial profile obtained with this method involves summation of a series and may take much time to compute. Sometimes the simplification of this solution can be done if the detection radius is much larger than the photoacoustic signal wavelength. This simplification is the so-called detection radius/signal wavelength approximation. Our numerical results obtained with this approximation are presented here. These results are compared with the one obtained using a different method and an exact solution. Experimental conditions for photoacoustic signal generation used in our numerical analysis correspond to the multiphoton absorption in SF6–Ar mixtures. Together with the laser beam spatial profile, the molecular vibrational to relaxational time of the excited molecules is calculated simultaneously. Application of neural computation and genetic optimization in pulsed photoacoustics is also discussed.

Genetic Algorithms Application for the Photoacoustic Signal Temporal Shape Analysis and Energy Density Spatial Distribution Calculation

Recently, a few numerical methods based on the photoacoustic (PA) signal temporal shape analysis and energy density spatial distribution calculation, which is directly related to the laser beam spatial profile, have been presented. It has been shown that these methods allow a precise reproduction of the spatial profile and the radius of the laser beam, determining the vibrational-to-translational (V–T) relaxation time with good accuracy. Their applicability has been shown and confirmed for the analysis of an arbitrary symmetric laser beam spatial profile in cylindrical geometry. Here, the application of genetic optimization for solving the problem of a simultaneous laser beam spatial profile and V–T relaxation time determination by pulsed PAs is presented. Real-coded genetic algorithms are used to calculate the mentioned relaxation time by fitting the experimental signal \({\delta }{p}(\mathbf{\textit{r} }, t)\) with the theoretical one. The aim is to find combinations of PA signal parameters, namely, the radius of the laser beam and the V–T relaxation time that provide the best match with the given signal. A calculated PA signal with a known profile is used to simulate an experimental signal, and the sum of the square deviations representing deviations of the given and fitted signals is minimized by means of genetic optimization. In that way, the genetic algorithms are used to simultaneously estimate the radius of the laser beam and the V–T relaxation time efficiently and with high accuracy. Compared to previous methods, the presented method is much simpler and requires less time to compute.

Photoacoustic analysis of the ultrasonic irradiation effect in the photosynthetic activity in aquatic lirium plants

We report, the application of the photoacoustic technique for monitoring the photosynthesis evolution in aquatic lirium (Eichhornia Crassipes), before and after it was exposed to ultrasonic irradiations. We obtained the disappearance of the phototobaric contribution in the PA signal measured for the irradiated samples with ultrasound of 17kHz, and therefore of a possible damage in the centers producing the photosynthesis, due to the irradiation. These results show the utility of the ultrasonic irradiation, as well as, of the photosynthesis monitoring by means of the photoacoustic technique, for the elaboration and establishment of methodologies in the control of this aquatic plant, whose propagation causes many consequences extremely unfavorable for the environment, as well as for the diverse human activities that are developed in the bodies of water in the tropical and sub-tropical regions of the world.

Photoacoustic Detection and Analysis of Acetylene in Transformer Oil Gases

Acetylene in transformer oil is an important feature gas which reflects early discharge faults. Tunable fiber laser-based second harmonic photoacoustic spectroscopy techniques, with their fast response and good noise immunity, can be well applied to detect trace gases. In this work, acetylene gas measurement with a tunable laser photoacoustic spectrometer was demonstrated at the 1530.37 nm transition line and a detectable limit of about 1.12 ppb (Vol/Vol,SNR = 1) was achieved. After the interference in background gas was further removed with the proposed BSS model based on overcomplete ICA basis and five-point sampling method based on the created weight-truncation equation, a method detection limit about 0.71 ppb was obtained. The experiment for detecting acetylene in transformer oil gases shows that the discharge acetylene productivity can achieve 308.859 ppb/s with a dynamic response time of less than 10 seconds, relative error is about 1.79 %, and relevance coefficient is up to approximate 0.99946. At room temperature and atmospheric pressure, this can meet the demands of acetylene detection and analysis in early or ultra early prediction for transformer discharge faults.

Nitrous oxide losses from untreated and digested slurry as influenced by soil moisture and application method

Effects of injection depth, slurry composition and soil moisture content on N2O emissions were investigated at laboratory scale. A loamy sand soil was moistened to 61, 64 or 71% of water-filled pore space (WFPS) and packed to the field bulk density. Using a mechanical device, untreated (UN) or anaerobically digested (AD) pig slurry (122 kg N ha−1) was injected to 30 or 60 mm below the soil surface to simulate closed slot injection. Soil with no slurry applied was included as reference. Incubations were carried out at 20 °C for 30 days; emissions of N2O and CO2 were determined several times by a photo-acoustic analyser. Pre-trials were conducted to validate soil packing and gas analysis methods. The experimental setup was found suitable for studying the regulation of N2O emissions from soil. In some cases, increasing WFPS stimulated N2O emissions while in other cases the increase in emission was not statistically significant with increasing WFPS. There were no effects of injection depth and slurry treatment, except that at 71% WFPS N2O emissions were lower from UN compared to AD slurry and the untreated control.

Numerical modeling of SF6 photoacoustic gas analyzer in the atmosphere with frequency modulation of thermal radiation

A simple design of a photoacoustic gas analyzer with a thermal source and an interference filter is suggested. The photoacoustic gas analyzer operation was modeled in the LabView visual programming environment while measuring the sulfur hexafluoride (SF6) concentration in atmospheric air. The optimal parameters of its elements were determined, i.e., a photoacoustic cell, an interference filter, an infrared thermal source, and microphones. The influence of the main absorbing components of the air on the PA signal amplitude was considered. It is shown that the use of low-cost commercial elements of the gas analyzer allows detection of atmospheric SF6 to a threshold level of about 0.06 ppm.

Determination of Tequila Quality by Photoacoustic Analysis

A pulsed laser photoacoustic (PLPA) technique is proposed to distinguish original from adulterated tequila. In fact, it brings a reliable cheaper and more sensible method in adulteration detection, in comparison with traditional techniques. The method proposed is comparative and non-destructive, and it is based on a correlation analysis of photoacoustic signals, obtained by exciting tequila samples with short laser pulses (7 ns), in the UV region (355 nm). Eleven samples of tequila were analyzed. From a reference sample, all other samples were classified.

Double spectrum analysis of photoacoustic signal

Photoacoustic spectroscopy based on the photoacoustic effect is the combination of optical imaging and acoustical imaging, which has become a powerful medical diagnosis tool to distinguish different tissues and components with several different wavelength photoacoustic images. But photoacoustic spectroscopy is limited by the scanning speed, system stability and signal accuracy. To solve these limitation problems, in this paper we propose a new method called photoacoustic double spectrum analysis which can greatly improve the image contrast and identification capability with quantitative analysis of the detected photoacoustic signal frequency. The final experimental results indicate that this method has the feasibility to distinguish different tissues quickly and easily with better contrast, which will be helpful for improving the applications of photoacoustic imaging in various branches of physics, biology, engineering, medicine, etc. We also expect the theoretical and experimental research proposed in this paper to establish the foundation and method for photoacoustic frequency imaging.

Photoacoustic spectrum analysis for microstructure characterization in biological tissue: A feasibility study

This study investigates the feasibility of characterizing the microstructures within a biological tissue by analyzing the frequency spectrum of the photoacoustic signal from the tissue. Hypotheses are derived from theoretical analyses on the relationships between the dimensions/concentrations of the photoacoustic sources within the region-of-interest and the linear model fitted to the power spectra of photoacoustic signals. The hypotheses are validated, following the procedures of ultrasound spectrum analysis, by simulations and experiments with phantoms fabricated by embedding the polyethylene microspheres in porcine gelatin, indicating that photoacoustic spectrum analysis could be a potential tool for characterizing microstructures in biological samples.

Photoacoustic Spectroscopy Analysis of Traditional Chinese Medicine

Chinese medicine is a historic cultural legacy of China. It has made a significant contribution to medicine and healthcare for generations. The development of Chinese herbal medicine analysis is emphasized by the Chinese pharmaceutical industry. This study has carried out the experimental analysis of ten kinds of Chinese herbal powder including Fritillaria powder, etc., based on the photoacoustic spectroscopy (PAS) method. First, a photoacoustic spectroscopy system was designed and constructed, especially a highly sensitive solid photoacoustic cell was established. Second, the experimental setup was verified through the characteristic emission spectrum of the light source, obtained by using carbon as a sample in the photoacoustic cell. Finally, as the photoacoustic spectroscopy analysis of Fritillaria, etc., was completed, the specificity of the Chinese herb medicine analysis was verified. This study shows that the PAS can provide a valid, highly sensitive analytical method for the specificity of Chinese herb medicine without preparing and damaging samples.

Validation of the Innocor Device for Noninvasive Measurement of Oxygen Consumption in Children and Adults

Outpatient measurements of oxygen consumption (VO2) and cardiac output (CO) are valuable in the management of pediatric cardiac disease. Current methods are inaccurate and cumbersome or require invasive procedures. New devices to measure these variables in adults have not been rigorously tested for children. The Innocor system uses a photoacoustic analyzer to measure gas content for noninvasive measurement of VO2 and CO. This study sought to validate Innocor-derived VO2 measurements in children and adults by comparing them against the gold standard Douglas bag method. Subjects were tested in an outpatient setting. Adaptations were made for pediatric patients based on weight. Resting VO2 measurements were obtained simultaneously by the Innocor system and Douglas bag during 3 min. The study enrolled 31 children (mean age, 12.2 years; range, 7-17 years, 17 girls) and 29 adults (mean age, 36.7 years; range, 19-57 years; 17 women). Strong correlation between the two techniques was seen for both the adults (R (2) = 0.88) and the children (R (2) = 0.82). The average discrepancy between the Innocor and Douglas bag measurements was 1.7 % (range, 0.6-19.1 %) for the adults, and 5.4 % (range, 0.1-32.2 %) for the children. The discrepancy was more than 15 % for 17 % of the adults and 22 % of the children, with the Innocor device tending to overestimate VO2 in children compared with the Douglas bag. This trend was not seen in adults. The Innocor system has excellent correlation with the Douglas bag and shows promise for noninvasive measurement of VO2 and CO in the school-age pediatric population.

Photoacoustic Analyzer for the Artifact-Free Parallel Detection of Soot and NO2 in Engine Exhaust

Soot particles and NO(2) are among the most hazardous emissions from diesel combustion engines. Currently, no analytical system exists which allows for the simultaneous, time-resolved online analysis of these two components. Furthermore, state-of-the-art NO(2) analyzers for exhaust gas require particle filtration prior to the analysis, which may induce artifacts and measurement errors. We present a photoacoustic instrument which overcomes these drawbacks. The sensitivity of the instrument (LOD(NO(2)) = 0.3 ppm, LOD(soot) = 0.54 μg m(-3), limit of detection/quantification) as well as the temporal resolutions are dictated by the needs of typical automotive applications. Also required for this specific application, we developed PA cells which can be heated to 80 °C, while the microphones maintain a temperature of 45 °C. Setup and specific parameters of the instrument are discussed, and the results of typical engine tests are compared to reference analytical instrumentation.

Structural phototransformation of WO3 thin films detected by photoacoustic analysis

The photoacoustic technique (PA) was used to detect the phase transformation from amorphous to crystalline state of tungsten oxide (WO3) thin films induced by UV pulsed laser radiation at low energy (<1.5mJ). The evolution of photoacoustic signal was studied by a correlation analysis, comparing successive signals at fluences ranging from 0 to 20mJ/cm2. In this interval, it was possible to observe structural changes and the ablation threshold in films due to incident laser fluence effect. Thin films of WO3 were deposited by DC reactive magnetron sputtering over glass substrates at different deposition times. The results obtained by correlation analysis were compared with Raman spectroscopy data.

Laser photo-acoustic analysis of multicomponent gas mixtures

Laser photo-acoustic analysis of multicomponent gas mixtures

Estimation of the Interference in Multi-Gas Measurements Using Infrared Photoacoustic Analyzers

Two methods were described to estimate interference in the measurements of infrared (IR) photoacoustic multi-gas analyzer (PAMGA). One is IR spectroscopic analysis (IRSA) and the other is mathematical simulation. An Innova 1412 analyzer (AirTech Instruments, Ballerup, Denmark) with two different filter configurations was used to provide examples that demonstrate the two methods. The filter configuration in Example #1 consists of methane (CH4), methanol (MeOH), ethanol (EtOH), nitrous oxide (N2O), carbon dioxide (CO2), and water vapor (H2O), and in Example #2 of ammonia (NH3), MeOH, EtOH, N2O, CO2, and H2O. The interferences of NH3 as a non-target gas in Example #1 were measured to validate the two methods. The interferences of H2O and NH3 as target gases in Example #2 were also measured to evaluate the analyzer’s internal cross compensation algorithm. Both simulation and experimental results showed that the interference between the target gases could be eliminated by the internal cross compensation algorithm. But the interferences of non-target gases on target gases could not be addressed by the internal cross compensation, while they could be assessed by the IRSA and mathematical simulation methods. If the IR spectrum of a non-target gas overlaps with that of target gas A at filter A, it could affect not only gas A (primary interference), but also other target gases by secondary interference (because the IR spectrum of gas A overlaps with gas B at filter B and thus affects gas B measurements). The IRSA and mathematical simulation methods can be used to estimate the interference in IR PAMGA measurements prior to purchase or calibration of the unit.

Laser Photoacoustic Method for Disc Tree-Ring Gas Analysis

A laser photoacoustic gas analysis was used to study the annual CO2 and H2O trends in tree rings and their comparison with the tree ring width of dry discs was made. The measurements show that 1) the CO2 concentration in gas samples vacuum-extracted from annual tree rings correlates with atmospheric CO2 rise in certain cases; approximately 4- year cycles in CO2 distribution were readily fixed; 2) for the Scots pine discs, years with small tree ring width are charac- terized by high CO2 concentration, while those with large tree ring width are associated with small CO2 concentration. Abrupt changes in atmospheric conditions are assumed to favour increased amplitudes of one of natural cycles of annual CO2.