Application of Infrared Photoacoustic Spectroscopy in Soil Analysis

Abstract

Soil analysis has become routine work for soil management and crop production. However, laboratory analysis–based determination of soil properties is expensive and time consuming, which is not suitable for precision agriculture. Infrared spectroscopy (IR) appears as an alternative and fast technique to measure soil properties and has had wide application; in particular, a new method called infrared photoacoustic spectroscopy (FTIR-PAS) has been applied in soil analysis. The soil infrared photoacoustic spectrum is more convenient to record; the spectra contain more useful information versus conventional reflectance spectroscopy, and it appears promising for identification of soil types and measure soil properties. The step-scan function of FTIR-PAS makes it possible to explore the soil microstructure in situ; furthermore, more sensible photoacoustic cells (PA), such as a quartz-enhanced PA cell, will make FTIR-PAS a strong tool for the study of soil science. The application of infrared photoacoutic spectroscopy in soil analysis is largely dependent on spectra pretreatment and chemometrics methods due to strong interferences, and more mathematical tools models will benefit or optimize the prediction performance. To make full use of soil infrared spectra, soil spectra library construction is required in the future, which will play an important role in the application of soil analysis.     

The photoacoustic effect and non-radiative transitions OF F-centers in alkali halides

The common photoacoustic spectrum (PAS) is wider than the optical absorption spectrum in KCl crystals. A photoacoustic experiment was performed with a solid KCl sample that was additively colored according to the Van Doorn technique. The sample had about 10 17 v [F/cm 3 ]. It was determined that the PAS is composed of many bands of the F and aggregate centers. It is explained here, using the configuration coordinate model, that the photoacoustic effect in alkali halides with F-centers shows experimental broadening of the photoacoustic band relative to the F -band. Furthermore, experimental data is added resolving the photoacoustic bands of the agglomerates and also the bands of higher excited states of the F-center. Finally, assuming a configuration coordinate model and the F-centers as sources of heat, an estimation is made for the increase of the sample temperature.     

一种高信噪比的双光束激光光声光谱仪原理及应用

作为一种新兴的检测手段 ,激光光声光谱技术与其他检测技术相比具有很多优点。本文设计的光声光谱仪用激光做光源 ,有两个光声池 ,分别用来放置参考样品和待测样品 ,输出结果为二者光声信号的比值。利用这一比值 ,可以由参考样品的性能参数方便地求出待测样品的相关性能参数。该光声光谱仪有效地减小了本底吸收噪声的影响 ,提高了信噪比 ,扩大了固体光声理论的应用范围。本文还阐述了该光声光谱仪在定性定量分析中的一些应用     

Photoacoustic and ultrasonic coimage with a linear transducer array

A technique is developed to simultaneously acquire ultrasound and photoacoustic (PA) images based on a linear transducer array. The system uses conventional ultrasound for rapid identification of potential targets. After a target is identified, the ultrasound echo and PA signals can be simultaneously obtained with optimized excitation and a signal collection sequence. The corresponding ultrasound impedance and optical absorption images are reconstructed with a phase-controlled algorithm. The approach can effectively reduce the artifacts associated with a conventional filter backprojection algorithm used in PA imaging by linear scanning. The technique provides a new approach for practical applications.     

Nonlinear photoacoustic imaging dedicated to thermal-nonlinearity characterization

We proposed a nonlinear photoacoustic(PA) technique as a new imaging contrast mechanism for tissue thermalnonlinearity characterization. When a sine-modulated Gaussian temperature field is introduced by a laser beam, in view of the temperature dependence of the thermal diffusivity, the nonlinear PA effect occurs, which leads to the production of second-harmonic PA(SHPA) signals. By extracting the fundamental frequency PA and SHPA signal amplitudes of samples through the lock-in technique, a parameter that only reflects nonlinear thermal-diffusivity characteristics of the sample then can be obtained. The feasibility of the technique for thermal-nonlinearity characterization has been studied on phantom samples. In vitro biological tissues have been studied by this method to demonstrate its medical imaging capability,prefiguring great potential of this new method in medical imaging applications.     

光声光谱检测装置中光声池的数值计算及优化

利用光声光谱技术进行痕量气体的检测具有独特的优势,光声池是系统装置中最为重要的核心部件,它决定着整机性能的优劣.以一圆柱形共振型光声池为研究对象,基于声学与吸收光谱学的基本理论,建立了光声池声场激发的数学模型;利用数值模拟方法对光声池空腔结构进行了声学模态仿真,获得了前8阶声学模态值以及声压可视化振型;在考虑热黏性声学损耗的作用下,对光声池进行了热-声耦合多物理场仿真计算;将仿真结果与解析计算和实验结果进行对比,明确了利用数值模拟方法来计算光声池有关指标的可靠性与可行性;针对光声池的优化问题,提出了一种将响应面代理模型与遗传算法相结合的优化算法,在将原光声池中的谐振腔两端形貌更改为喇叭口形的情况下,通过优化算法获得了以光声池品质因数Q及池常数C_(cell)为最大值寻优的Pareto最优解集;选取一组解进行考察,结果表明,代理模型预测值与数值模拟值指标最大误差仅为1.3%,优化后的新型光声池Q较之前增长了48.9%, C_(cell)增长了34.4%.研究方法可为光声光谱中光声池的优化设计提供参考借鉴.     

稀土氧化物粉末样品的光声光谱研究

利用光声光谱方法对几种常见的稀土氧化物粉末样品在紫外-可见、近红外, 以及中红外波段的光声光谱进行了测量与分析。根据光声光谱图和稀土离子的能级图, 对稀土离子的激发态电子的辐射和无辐射跃迁进行了探讨。与传统的透射谱和吸收谱相比较, 光声光谱具有分辨率高、快速、无损检测等优点, 特别适合于研究不透明、高反射和高散射粉末样品的光学性质。     

Photoacoustic spectroscopy of Baru – <Emphasis Type="Italic">Dipteryx alata</Emphasis> Vog

Photoacoustic spectroscopy (PAS) was used to investigate samples produced from Baru (Dipteryx alata Vog.), a typical fruit from the Brazilian Cerrado, which is considered as a good source of nutrients. The photoacoustic (PA) spectra of samples prepared from Baru seeds present three different absorption bands in the wavelength range 0.3–1.0 μm, named Band-C, Band-S, and Band-L. We found that PAS can be useful for monitoring the strong absorption of visible light by Baru's seeds constituents mainly in the shorter wavelength measured range. This study is important from both agricultural and commercial point of view once it introduces PAS as a potential characterization technique allowing better control of fruit and seed selection, storage and transportation.     

Studies of defect structure effects on the transport properties of pure crystalline n-CdS VIA the temperature dependence of photoacoustic and photocurrent spectra

Microphone gas-coupled photoacoustic spectroscopy (PAS) and photocurrent spectroscopy (PCS) have been applied simultaneously to pure high resistivity single crystals of n-CdS at room and low temperatures. The PA and PC spectra obtained at open-circuit and with an applied AC or DC transverse electric field were consistent with an enhancement of the non-radiative recombination quantum efficiency at subbandgap wavelengths at room temperature, accompanied by increasing domination of the open circuit PA spectrum by this parameter at lower temperatures. Further evidence of the importance of non-radiative carrier recombinations at intrabandgap defect centers in the generation of the PA and PC signals was obtained upon introducting deliberate mechanical damage to a single crystal of CdS. The PA technique was found to have an advantage over PC spectroscopy in its ability to spectrally resolve completely the observed peaks attributed to the intrinsic band-to-band transition and to the wavelength dependence of the non-radiative quantum yield.     

Identification of standard explosive traces by infrared laser spectroscopy: PCA on LPAS data

Infrared spectroscopy based methods are promising in the design of an integrated optical system for the real time detection and identification of explosive species to support homeland security. Infrared laser photoacoustic spectroscopy (LPAS) analysis of explosive compounds (DNT; TNT; RDX; HMX; TATP; PETN; TETRYL) and TOLUENE was reported. Standard commercial samples of the explosive species were analyzed in solid phase by a home made PAS apparatus equipped with a stabilized 10 W continuous wave CO₂ laser source. A very small amount (less than 100 ??g) of the sample was sufficient to produce a clear signal in a 3 cc volume PA cell. The spectral data were treated by a principal component analysis (PCA) based algorithm. In a three principal components representation of the results, every explosive species resulted to be very well distinguishable from each other, as well as from toluene and other solvents. The reported experimental activity was performed in the Molecular Spectroscopy Laboratory of ENEA Research Centre in Frascati in the frame of ISOTREX European Project.     

Photoacoustic Spectroscopy Study on TB3+-GD3+-Sal Complexes

The photoacoustic spectra(PAS) of the coprecipitates Tb³⁺-Gd³⁺-Sal complexes are reported. With the increase of the concentration of Gd³⁺ ion, the PA intensity of complexes decrease firstly, then increase. It is indicated that the addition of the second rare earth ion Gd³⁺ changes the relaxation processes of the complexes. The changes of the fluorescence spectra turn out complementary to the PAS. The intramolecular energy transfer and the intermolecular energy transfer processes in coprecipitates are discussed.     

Sensitive harmonic detection of ammonia trace using a compact photoacoustic resonator at double-pass configuration and a wavelength-modulated distributed feedback diode laser

A sensitive detection of ammonia in parts per billion by volume is described. The system based on photoacoustic spectroscopy (PAS) consists of distributed feedback laser diode emitting near 1,531.7 nm and a compact PA cell at double-pass configuration. In order to optimize the signal background ratio of the system, two types of modulations were tested, amplitude and wavelength modulations (WM). Using a digital lock-in amplifier, the 1f and 2f detection in WM could be investigated. A detection limit of 470 parts per million by volume could be achieved at WM-2f. In the sense of quantifing the adsorption–desorption process, the response time of the system and detection accuracy was performed in different flows. Response times between 10 and 49 s, depending on the flow rate, were obtained which enables the PA system to measure low concentrations of ammonia with high accuracy in real time.     

Infrared-laser photoacoustic spectroscopy

This paper reviews the applications of IR-laser photoacoustics to trace-gas monitoring as well as to spectroscopic studies on absorbing liquids.In the first part we present a stationary, dual-beam CO-laser and a mobile CO₂-laser photoacoustic system which have both been applied to the monitoring of various gaseous pollutants. Emphasis is put on selectivity, sensitivity and on temporal resolution. Novel cell designs and experimental techniques and an iterative procedure for the analysis of photoacoustic spectra of multicomponent mixtures are introduced. New results are presented for measurements on car and industrial exhausts as well as on ambient air.The second part is devoted to theoretical and experimental photoacoustic studies on strongly absorbing liquids, in particular on the investigation of different boundary conditions. A characteristic enhancement of the photoacoustic signal in the liquid is obtained if a liquid or solid surface layer is present. This new phenomenon permits the analysis of surface films with a thickness of ⩾ 1 μm. Furthermore, the photoacoustic in-situ monitoring of the polymerization process on a liquid surface is presented for the first time.     

Infrared photoacoustic spectroscopy in catalysis and surface science

The analysis of solid samples can often be a difficult problem for the researchers dealing with infrared (IR) spectroscopy. In conventional absorption spectroscopy the measurement of absorption is transferred to that of the radiation transmitted through the sample. Three methods stand out as being more suitable for studying solid materials. These methods are: diffuse reflectance (DR), photoacoustic spectroscopy (PAS), and Fourier transform (FT) Raman. All three methods require little or no sample preparation, and therefore are ideal for the samples that may change during the preparation as mineral oil mulls or KBr disks. In the case of PAS, the adsorbed radiation is determined directly via its heat and hence the sound produced in the sample. Fourier transform infrared PAS (FT-IR/PAS) is one of the main IR techniques which can be successfully applied in catalysis and surface science research. Recent examples of this spectroscopic technique application will be presented.     

光声光谱气体传感技术研究进展

光声光谱是通过光声效应把样品吸收光谱转换成声波探测,实现样品成分、浓度分析检测的一种光谱传感技术,是光谱学的一个重要分支。光声光谱除了具有吸收光谱的高选择性、高灵敏度外,还具有信号只跟样品光吸收有关,不受散射光影响,零背景, 信号与光功率成正比以及信号探测器不受光波长影响等诸多优点。在环境监测、工业过程控制与检测、医学诊断和国防危化品检测等领域得到了越来越多的应用,呈现出快速发展的趋势。除了传统的共振光声光谱技术,近年来先后出现了悬臂增强型光声光谱、石英音叉谐振增强型光声光谱、多通道光声光谱等各具特色的新技术。对光声光谱气体传感技术的研究进展进行了介绍,并分析了其应用前景和未来发展趋势。     

Determination of the low absorption coefficients of thick semi-conductor samples using rear side photoacoustic spectroscopy detection

Based on the Rosencwaig and Gersho theory, we have theoretically derived the photoacoustic signal in the gaz behind the unilluminated surface of thick samples and presented a simple analytical procedure for determining the low optical absorption spectra of these samples.The rear side photoacoustic spectroscopy (PAS) detection has been employed on thick samples of GaP (0,2-1mm) and optical absorption coefficients in the region between 15 to 200 cm^?1 were measured.Our results are discussed in comparison with those obtained by usual front side PAS detection and spectroscopic ellipsometry.     

结合小波分析法与导数光声光谱技术测量弱光谱信号的实验研究

结合导数光声光谱技术与小波分析方法,精确测量了光声光谱中的弱光谱信号.首先利用自己设计的仪器装置实现光声光谱的一阶导数,在此基础上,根据导数光声光谱的数据选择合理的分析小波,将光声光谱信号分解为不同频率信号的叠加,被分解的信号满足线性性质且原信号的峰值信息保持不变,由频率的差异可区分出光声信号和噪声信号,从而提取出光声光谱中的弱光谱信号.结合物理方法与软件方法的分析结果,准确地测量了氙灯的输出光声光谱中3个不明显的弱峰值,位置分别为699.7nm、753.4nm和776.5nm.该方法可以更准确地提取出光声光谱的峰值信息,有效地提高了光声光谱的测量精度,为光声光谱分析法在生物医学及化学分析中的应用提供了一种更精确的分析方法.     

In vitro determination of the sun protection factor of sunscreens through photoacoustic spectroscopy: A new approach

The investigation of the sun protection factor (SPF) and the efficacy of ultraviolet absorber ingredients are extremely important items for the cosmetic industry. The development of new efficient in vitro methodologies for these investigations is an essential condition for the evolution of sunscreens. The most accepted test for assessing the efficacy of sunscreens is the in vivo (in humans) SPF determination, following validated and worldwide adopted methodologies like those regulated by FDA (USA), COLIPA (Europe) and JCIA (Japan). However, due to the high costs and time consumption of in vivo SPF determination, the constant development of in vitro methods able to predict the in vivo SPF is very important. The present study evaluates different experimental methods for in vitro SPF determination – photoacoustic spectroscopy (PAS) and transmission spectroscopy (TS), employing different substrates (PMMA and VitroSkin^?) and concentrations (0.8 mg/cm² and 2 mg/cm²). Results are compared to the labeled SPF. The high correlation between the labeled SPF and the SPF determined by PAS expands the application of this technique in the photobiology field.     

Theoretical analysis of off beam quartz-enhanced photoacoustic spectroscopy sensor

Off beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS) sensors are based on a recently developed approach to off-beam photoacoustic (PA) detection which employs a quartz tuning fork (QTF) as an acoustic transducer. A microresonator (mR) with a side slit in the middle is used to enhance PA signal. This paper describes a theoretical model of an OB-QEPAS-based sensor. By deriving the acoustic impedances of the mR at two ends and the side slit in the middle in the model, we obtain a formula for numerically calculating the optimal mRs' parameters of OB-QEPAS-based sensor. We use the model to calculate the optimal mRs' lengths with respect to the resonant frequency of the QTF, acoustic velocities inside mRs, inner diameters of mRs, and acoustic conductivities of the mRs' side slits, and found out that the calculated results closely match experimental data. We also investigated the relationship between the mR selected in “on beam” QEPAS, OB-QEPAS, and an acoustic resonator (AR) excited in its first longitudinal mode used in conventional photoacoustic spectroscopy (PAS).     

Complexation of Molecular Iodine by Linear Poly (Ethylene Glycol)

Abstract

Linear poly (ethylene glycol) (PEG) is able to complex with neutral molecular Iodine in the solution state and in solid state, to produce I^? ₃ ions detectable by conventional spectroscopy and by photoacoustic spectroscopy (PAS). This behavior of PEG is another instance of its ability to mimic the complexation features of the macrocyclic polyether 18-crown-6.