Photoacoustic Spectrometry Research Articles

Photoacoustic spectrometry and resonant frequency tracking based dual-mode gas sensor

Photoacoustic spectrometry has excellent advantages in trace gas detection. However, due to the influence of gas saturation absorption, when the gas concentration spikes to a certain level, this technology is difficult to achieve accurate detection. Therefore, a dual-mode gas sensor based on photoacoustic spectroscopy and resonant frequency tracking is developed by the features of both detection technologies. A tunable DFB diode laser in the 1654 nm wavelength range and a loudspeaker are used for the photoacoustic detection, and the resonant frequency tracking, respectively. A resonant photoacoustic cell is used as the sensing element to work in dual-mode by time-division multiplexing technology. The wavelength-modulated photoacoustic detection is enabled in the methane concentration below 30%. When the gas concentration exceeds 30%, it will be automatically switched to the measurement-mode of resonant frequency tracking by the fast Fourier transform. The double-range gas detection is realized by a single sensor with dual-mode detecting functionality. With the wavelength modulation and lock-in harmonic detection method, the minimum detectable limit (1σ) of 3.5 ppm CH4 was demonstrated under the photoacoustic detection mode. The response time of the sensor is about 10 s within the measurement range of double-range.

Multi-component Gas Photoacoustic Detection

Based on the basic principles of photoacoustic spectrometry, this paper uses methane and acetylene as examples to conduct a research for the multi-component gas photoacoustic detection. The photoacoustic spectrometer designed in this paper has the advantages of good selectivity and large detection range, and the proposed multi-component gas photoacoustic quantification method based on least squares regression does not require cell constants and gas absorption coefficient parameters, thus avoiding errors introduced by these parameters. The method proposed in this paper provides a method reference for the engineering application of photoacoustic spectrometry.

Sensing Technologies for Detection of Acetone in Human Breath for Diabetes Diagnosis and Monitoring.

The review describes the technologies used in the field of breath analysis to diagnose and monitor diabetes mellitus. Currently the diagnosis and monitoring of blood glucose and ketone bodies that are used in clinical studies involve the use of blood tests. This method entails pricking fingers for a drop of blood and placing a drop on a sensitive area of a strip which is pre-inserted into an electronic reading instrument. Furthermore, it is painful, invasive and expensive, and can be unsafe if proper handling is not undertaken. Human breath analysis offers a non-invasive and rapid method for detecting various volatile organic compounds thatare indicators for different diseases. In patients with diabetes mellitus, the body produces excess amounts of ketones such as acetoacetate, beta-hydroxybutyrate and acetone. Acetone is exhaled during respiration. The production of acetone is a result of the body metabolising fats instead of glucose to produce energy. There are various techniques that are used to analyse exhaled breath including Gas Chromatography Mass Spectrometry (GC–MS), Proton Transfer Reaction Mass Spectrometry (PTR–MS), Selected Ion Flow Tube-Mass Spectrometry (SIFT–MS), laser photoacoustic spectrometry and so on. All these techniques are not portable, therefore this review places emphasis on how nanotechnology, through semiconductor sensing nanomaterials, has the potential to help individuals living with diabetes mellitus monitor their disease with cheap and portable devices.

Application of photoacoustic spectroscopy to characterize thermal diffusivity and porosity of caprocks

Photoacoustic spectrometry (PAS) was used to determine thermal diffusivity and porosity of three sealing caprocks originating from oil and gas fields in Brazil. In the petroleum industry developments in the low permeability rocks characterization are a key issue for safe exploitation. Also to assess the potential for using depleted reservoirs as a geologic repository for radioactive waste disposal or CO2 sequestration involves the knowledge of thermal properties parameters in caprocks. The experimental PAS technique using the open photoacoustic cell (OPC) method was employed to determine the porosity through correlation based on the thermal diffusivity values for consolidated and unconsolidated caprocks using small amounts of rocks and a few adjustments of empirical equations. The values obtained for thermal diffusivity were between 0.01667 (±0.0013) and 0.09298±(0.00018) cm2/s while the porosity ranged from 1.42% to 9%. It was observed that thermal diffusivity declines as the apparent porosity of the samples decreases. The variations in thermal diffusivity between the analyzed gas and oil fields were supported by field emission scanning electron microscopy and X-ray diffraction analyses, while porosity values were in a good agreement from those measured using the combination of nitrogen gas adsorption and mercury intrusion capillary pressure techniques. The use of PAS data on determining thermal diffusivity and subsequently low-porosity of porous media, such as caprocks, has not, to the best of our knowledge, been previously reported in the literature.

Cantilever enhanced photoacoustic spectrometry: Quantitative analysis of the trace H2S produced by SF6 decomposition

As one of the key characteristic components that result from sulfur hexafluoride (SF6) decomposition in SF6 gas-insulated equipment, hydrogen sulfide (H2S) can reflect the severity of the internal insulation faults and indicate whether or not such faults involve solid insulation material effectively. The decomposition of SF6 and its reaction with other impurities to form H2S are simulated in this study via Materials Studio. The simulation verifies that H2S is generated only when serious faults occur in the equipment; thus, the online monitoring of the trace H2S is highly necessary. To achieve a high detection accuracy and avoid cross interference, the spectral line R (8) of the H2S ν1+ν2+ν3 co-frequency absorption band is taken as the absorption line for the gas detection by online simulation based on the HITRAN on the Web. In addition, this study develops a cantilever-enhanced photoacoustic spectrometry trace gas detection platform and conducts experimental research on the quantitative detection of trace H2S/SF6 and H2S/N2. Experimental results show that the detection sensitivity of the detection platform to trace H2S under the background gas N2 and SF6 is 0.84 and 1.75μL/L, respectively, and a strong linear relationship exists between the trace H2S concentration and its corresponding PA signal. Moreover, based on both the theoretical simulation and experiment, the influence of temperature and pressure on the detection platform is discussed and analyzed. The results indicate that the change in the PA signal amplitude decreases with an increase in the pressure or temperature of the PA cell, and the detection platform is more sensitive to pressure.

Detection of SF6Decomposition Components Under Partial Discharge by Photoacoustic Spectrometry and its Temperature Characteristic

Photoacoustic (PA) spectrum is a promising method in SF6 decomposition product detection. To develop an on-line monitoring device, a gas detection system based on PA effect was introduced. Three kinds of gases, SO2, CO2, and CF4, were chosen to test the device performance. Some experiments were conducted to get the relationship of PA signal and gas concentration. The minimum detection limit of each gas was also measured. After that, the influence of temperature on PA detection was studied under 30 °C−50 °C. It is found that the PA signals of SO2 (1360 cm $^{-1}$ ), CO2 (2347 cm $^{-1}$ ), and CF4 (1285 cm $^{-1}$ ) all have negative temperature characteristics within that temperature range. Then, a temperature linear correction model was proposed. The maximum error between correction result and standard PA signal under reference temperature was <4%. The temperature correction method lays a foundation for manufacturing SF6 decomposition components on-line monitoring device.

Photoacoustic Compound Fire Alarm System for Detecting Particles and Carbon Monoxide in Smoke

In view of the high sensitivity and high selectivity of photoacoustic technology, a compound fire alarm system used for detecting smoke particles and carbon monoxide was designed on the basis of the photoacoustic spectrometry principle. The system uses an infrared laser with a central wavelength of 1579 nm as the light source, adopts wavelength modulation technology to modulate the continuous light source, and moves the inspection frequency to high frequency with lower noise by integrating harmonic detection technology, thus realizing a measurement of the photoacoustic signal. Concurrently, the system obtains the CO concentration and extinction coefficient of smoke particles via inversion by using the measured scattered light intensity data combined with a certain algorithm. A test was carried out on the system by using three small-scale experiments, namely, cotton rope smoldering fire, polyurethane foam fire and sandalwood smoldering fire. The results show that the CO gas concentrations and extinction coefficient of smoke particles produced by these three experiments are well detected. The CO detection limit of the system was 2.98 ppm. The test results also show that pure powder (used for simulating particles) or pure CO gas will not trigger the system alarm, so unnecessary alarms caused by traditional fire detectors can be avoided and the reliability of the fire alarm can be improved.

Research on On-Line Calibration Based Photoacoustic Spectrometry System for Monitoring the Concentration of CO2 in Atmosphere

Resonate frequency and cell constant of photoacoustic spectrum system are usually calibrated by using standard gas in laboratory, whereas the resonate frequency and cell constant will be changed in-situ, leading to measurement accuracy errors, caused by uncertainties of standard gas, differences between standard and measured gas components and changes in environmental condition, such as temperature and humidity. As to overcome the above problems, we have proposed an on-line atmospheric oxygen-based calibration technology for photoacoustic spectrum system and used in measurement of concentration of carbon dioxide in atmosphere. As the concentration of atmospheric oxygen is kept as constant as 20.96%, the on-line calibration for the photoacoustic spectrum system can be realized by detecting the swept-frequency and peak signal at 763.73 nm. The cell of the PAS has a cavity with length of 100 mm and an inner diameter of 6 mm, and worked in a first longitudinal resonant mode. The influence of environmental temperature and humidity, gas components on the photoacoustic cell's performance has been theoretically analyzed, and meanwhile the resonant frequencies and cell constants were calibrated and acquired respectively using standard gas, indoor air and outdoor air. Compared with calibrated gas analyzer, concentration of carbon dioxide is more accurate by using the resonant frequency and cell constant calculated by oxygen in tested air, of which the relative error is less than 1%, much smaller than that calculated by the standard gas in laboratory. The innovation of this paper is that using atmospheric oxygen as photoacoustic spectrum system's calibration gas effectively reduces the error caused by using standard gas and environmental condition changes, and thus improves the on-line measuring accuracy and reliability of the photoacoustic spectrum system.

Clinical utility of breath ammonia for evaluation of ammonia physiology in healthy and cirrhotic adults

Blood ammonia is routinely used in clinical settings to assess systemic ammonia in hepatic encephalopathy and urea cycle disorders. Despite its drawbacks, blood measurement is often used as a comparator in breath studies because it is a standard clinical test. We sought to evaluate sources of measurement error and potential clinical utility of breath ammonia compared to blood ammonia.We measured breath ammonia in real time by quartz enhanced photoacoustic spectrometry and blood ammonia in 10 healthy and 10 cirrhotic participants. Each participant contributed 5 breath samples and blood for ammonia measurement within 1 h. We calculated the coefficient of variation (CV) for 5 breath ammonia values, reported medians of healthy and cirrhotic participants, and used scatterplots to display breath and blood ammonia.For healthy participants, mean age was 22 years (±4), 70% were men, and body mass index (BMI) was 27 (±5). For cirrhotic participants, mean age was 61 years (±8), 60% were men, and BMI was 31 (±7). Median blood ammonia for healthy participants was within normal range, 10 μmol L−1 (interquartile range (IQR), 3–18) versus 46 μmol L−1 (IQR, 23–66) for cirrhotic participants. Median breath ammonia was 379 pmol mL−1 CO2 (IQR, 265–765) for healthy versus 350 pmol mL−1 CO2 (IQR, 180–1013) for cirrhotic participants. CV was 17 ± 6%.There remains an important unmet need in the evaluation of systemic ammonia, and breath measurement continues to demonstrate promise to fulfill this need. Given the many differences between breath and blood ammonia measurement, we examined biological explanations for our findings in healthy and cirrhotic participants. We conclude that based upon these preliminary data breath may offer clinically important information this is not provided by blood ammonia.

Identification and Measurement of Intermolecular Interaction in Polyester/Polystyrene Blends by FTIR-Photoacoustic Spectrometry

Fourier transform infrared photoacoustic spectrometry was used to reveal and identify n–π type intermolecular interaction formed in plastic comprising binary blends of polystyrene and a biodegradable polymer, either polylactic acid, polycaprolactone or poly(tetramethyleneadipate-co-terephthalate). This was the first reported direct and definitive experimental evidence of n–π type bond formation in thermoplastic blends of these polyesters with polystyrene. Also, an infrared method was devised that permits quantitative measurement of the degree of interaction between the two polymers at various concentrations in the blends. The method employs spectral deconvolution by least squares curve fitting of the polymer carbonyl band into its underlying peaks. In a new algorithm the method compares deconvoluted Gaussian/Lorentzian peaks of the polymer blends with deconvoluted peaks in the neat polymers and computes both the magnitude and direction of change in the n–π bond formation with change in polymer concentration. The results indicated that the degree of interaction was dependent on the type and concentration of the biodegradable polymer in the blend. These findings are supported by differential scanning calorimetry and thermogravimetric analyses. Unlike conventional spectral deconvolution methods, this technique with its new algorithm approximates infrared absorptivities of all the underlying peaks, and is thus a superior method that should be applicable to multicomponent polymer blends in general.

Relationship between Oxidation Level and Optical Properties of Secondary Organic Aerosol

Brown carbon (BrC), which may include secondary organic aerosol (SOA), can be a significant climate-forcing agent via its optical absorption properties. However, the overall contribution of SOA to BrC remains poorly understood. Here, correlations between oxidation level and optical properties of SOA are examined. SOA was generated in a flow reactor in the absence of NOx by OH oxidation of gas-phase precursors used as surrogates for anthropogenic (naphthalene, tricyclo[5.2.1.0(2,6)]decane), biomass burning (guaiacol), and biogenic (α-pinene) emissions. SOA chemical composition was characterized with a time-of-flight aerosol mass spectrometer. SOA mass-specific absorption cross sections (MAC) and refractive indices were calculated from real-time cavity ring-down photoacoustic spectrometry measurements at 405 and 532 nm and from UV-vis spectrometry measurements of methanol extracts of filter-collected particles (300 to 600 nm). At 405 nm, SOA MAC values and imaginary refractive indices increased with increasing oxidation level and decreased with increasing wavelength, leading to negligible absorption at 532 nm. Real refractive indices of SOA decreased with increasing oxidation level. Comparison with literature studies suggests that under typical polluted conditions the effect of NOx on SOA absorption is small. SOA may contribute significantly to atmospheric BrC, with the magnitude dependent on both precursor type and oxidation level.

Vacuum-enhanced nickel-induced crystallization of hydrogenated amorphous silicon

We report the results of enhanced nickel induced crystallization of intrinsic hydrogenated amorphous silicon thin films under vacuum conditions. Crystallization was performed by conventional furnace annealing at both atmospheric pressure and vacuum or low pressure conditions (≈10–6 Torr) for comparison. We have investigated the influence of low pressure during annealing on the resulting polycrystalline films by means of optical microscopy, ultraviolet reflectance, and photoacoustic spectrometry measurements. A faster crystallization and a smaller grain size were observed when the process is carried out under vacuum, with an annealing time reduction of more than 50%. We discuss, from a thermodynamical viewpoint, some possible causes by which vacuum annealing influences incubation and nucleation stages due to the presence of mobile hydrogen atoms inside the amorphous silicon matrix. Large grains with diameters of 30 and 100 μm were obtained at vacuum and atmospheric pressure, respectively.

Determination of cobalt in water samples by photoacoustic spectroscopy with a solid-phase spectrophotometry approach using 3-(2-pyridyl)-5,6-bis(4-sulfophenyl)-1,2,4-triazine

This work forms part of an investigation which seeks to determine metal complexes in a solid phase by photoacoustic spectrometry (PAS). Results of spectral determination of Co(II) complexes are introduced. The method is based on Co(II) colorimetric reaction with 3-(2-pyridyl)-5,6-bis(4-sulfophenyl)-1,2,4-triazine (FST, ferrozine) retained on an anion-exchange resin, DEAE Sephadex A-25. The immobilization of Co(II) on the solid phase is combined with PAS measurement. A numerical approach is applied to reduce noise in the spectral data. At λ = 477 nm, the calibration for Co(II) is linear over the range 14–300 μgL − 1 . The Co(II) concentration in water samples was determined by conventional photoacoustic measurement. The relative standard deviation (R.S.D.) of the method for the calibration is < 5%. Under optimized conditions, the obtained analytical features were LOD 14 μgL − 1 and LOQ 45 μgL − 1 .

Lead ion detection in turbid media by pulsed photoacoustic spectrometry based on dissolution of gold nanoparticles

In this paper, we present a pulsed photoacoustic configuration for the detection of lead ion in aqueous solution based on the dissolution of gold nanoparticle. The photoacoustic signal is generated by laser-induced nanobubble. This technique allows for a sensitive measurement of size changes of the gold nanoparticles in the presence of lead ions even in turbid samples. As a first step, the best gold nanoparticle size and concentration were determined. Using 1.25 nM of gold nanoparticle with a diameter of 11 nm containing 1 mM of 2-mercaptoethanol, a linear range from 1 nM to 2.5 μM and a limit of detection of 0.5 nM were obtained. Then, to study the response of our system to strong light scattering in the medium, the scattering coefficient of samples containing lead ions was incremented using spherical silica particles (diameter 500 nm). The results corroborate that the technique used in this work is almost insensitive to light scattering.

Cerebral blood flow and oxygen metabolism measured with the Kety–Schmidt method using nitrous oxide

The Kety-Schmidt method is the reference method for measuring global cerebral blood flow (CBF), cerebral metabolic rates (CMR) and flux, especially where scanners are unavailable or impractical. Our primary objective was to assess the repeatability of the Kety-Schmidt method in a variety of different approaches using inhaled nitrous oxide (N2O) as the tracer, combined with photoacoustic spectrometry. A secondary objective was to assess the impact of this tracer on the systemic vascular concentration of nitrite (NO2(-)). Twenty-nine healthy male volunteers underwent 61 CBF measurements by breathing a normoxic gas mixture containing 5% N2O until tension equilibrium. Paired blood samples were collected from an arterial and a jugular bulb catheter in the saturation or desaturation phase, by continuous or the discontinuous sampling. N2O concentration was measured with photoacoustic spectrometry after equilibration of blood samples with air. CBF was calculated by the Kety-Schmidt equation. CMR of oxygen (CMRO2) was determined by the Fick principle. NO2(-) in plasma and red blood cells (RBC) was measured by ozone-based chemiluminescence. The most robust approach for CBF measurement was achieved by discontinuous sampling in the desaturation phase [CBF, 64 (95% confidence interval, 59-71 ml)] 100 g/min; CMRO2 1.8 (1.7-2.0) micromol/g/min). The tracer did not influence plasma or RBC NO2(-) (P>0.05 vs. baseline). These findings confirm the reliability and robustness of the Kety-Schmidt method using inhaled N2O for the measurement of global CBF and CMR. At the low tracer concentration used, altered NO metabolism is unlikely to have affected cerebral haemodynamic function.

The potential of CO2 laser photoacoustic spectrometry for detection of methanol in alcoholic beverage

The first use of CO2 laser photoacoustic measurements for detecting the methanol contents in alcohol-like solutions is presented. With an intracavity cell configuration, the minimum detectable concentration was ∼200 ppm for methanol and the linear range of the calibration curve for methanol was from 200 to 70000 ppm. For demonstrating the reliability of analysis in alcoholic beverages, a series of different concentrations of two-component samples was prepared and measured by the same procedures. The results showed the feasibility on determining methanol and ethanol contents accurately within a specific tolerance, limited mainly by background signal and laser stability. This potential method with no pre-treatment of samples takes only ∼10 min to finish one single measurement. It suggests that the PA detection is suitable for routine diagnosis of adulterated wines in commercial products.

Monitoring NH3, N2O, CO2 and CH4 Emissions During Pig Solid Manure Storage — Effect of Turning

In France, as in other countries, there is a need to improve knowledge of greenhouse gas and ammonia emissions from livestock production with various management options. The aim of this study was to quantify the NH3, N2O, CO2 and CH4 emissions from two pig solid manure heaps, turned and not turned, in order to examine the effect of turning on gaseous emissions and to obtain additional information about emissions during composting in commercial conditions. The manure produced from the fattening of 72 pigs on straw bedding was divided between two identical heaps and stored outside on a concrete area for three months during autumn 2004. One heap was turned at day 0, 11 and 27. Gaseous emissions (NH3, N2O, CH4, CO2, H2O) of both heaps, covered for the purpose by ventilated greenhouses, were continuously measured over four periods (57 days in total). The tracer method, using SF6, was used to measure the ventilation rate in the greenhouses. Concentrations inside and outside the greenhouses (to have a differential) were measured by photoacoustic infrared absorption spectrometry with a gas analyser coupled to a multipoint sampler and doser. Emissions were calculated by combining various flows and differential concentrations and interpolated between the different measuring periods. Results show that emission kinetics were very similar for the two heaps, except after turning, when emissions are given a new impetus. On average, the measured gas emissions for unturned and turned heaps represent a nitrogen loss (mainly in the form of NH3) of 10.5 % and 9 % of the initial nitrogen, and a carbon loss (mainly in the form of CO2) of 24 et 28 % of the initial carbon. For both heaps, N2O is the primary greenhouse gas emitted (3% of the initial nitrogen) and not CO2 or CH4. Considering the uncertainty on the measurements and the representativity of the experiment, these results did not show any clear difference between the two heaps. They underlined the relationship between gaseous emissions and the initial product (dry matter > 30 %) and the diversity of the litters, which will be studied on the farm.

Photoacoustic spectrum of a new, 12-fold-coordinated Ho(III) hydrazone complex

A new, 12-fold-coordinated holmium(III) compound [Ho(NO3)3(PBH)2]NO3·1.5H2O has been prepared and studied with high-resolution photoacoustic spectrometry (PAS). The spectroscopic parameters of the obtained photoacoustic lines attributed to the f–f transitions were analyzed and compared with the parameters for a similar 10-fold-coordinated holmium(III) complex [Ho(NO3)2(PicBH)2]. Computer fitting of the obtained spectra allowed decomposing them into Gaussian components and calculating the spectral parameters of each transition. It was found that of the five observed f–f transitions, the 5I8→5G6 and the 5I8→5G3 transitions were found to be the most sensitive to the change of ligand coordination. The charge transfer π→π∗ and n→π∗ transitions registered in the UV part of the photoacoustic spectrum showed a significant variation due to the change in the coordination number and the shift of ligands toward greater wavenumbers.

Photoacoustic detection of dissolved gases in transformer oil

AbstractThis paper investigates the applications of photoacoustic spectrometry (PAS) to analyze dissolved gases in transformer oil. An inexpensive experimental device with an electric pulse infrared light and six filters has been developed to detect six main fault gases in transformer oil, namely, CH4, C2H6, C2H4, C2H2, CO, and CO2. The method of choosing the characteristic wavelengths for the above six fault gases is discussed in detail. A weighted least‐square error method is proposed to analyze the measured data and to determine the component concentrations of dissolved gas in oil. Two sample sets of gas mixtures are analyzed by the experimental device for the purpose of verification. The PAS measurements are compared with the true values and the values measured by a conventional gas chromatograph (GC) method. The comparison results show that PAS is an effective way of analyzing dissolved gases in transformer oil, which exhibits some advantages in the gas detection such as consuming no gas, separating no gas, high accuracy, high stability, and rapid measurements. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Model and real pollutant dispersion: concentration studies by conventional analytics and by laser spectrometry

The differential absorption LIDAR together with spot analyzers (infrared absorption and chemiluminescence method) was used for concentration measurements in an urban street canyon in Prague. Measurements in the real atmosphere were compared with measurements of concentration distribution inside a model. Experiments in a wind-tunnel-simulated atmosphere were carried out using laser photoacoustic spectrometry and the laser visualization method. A comparison of concentrations on the windward and leeward sides of the model and real street canyon was performed.