Quartz-enhanced photoacoustic spectroscopy

A new approach to detecting a weak photoacoustic signal in a gas medium is described. Instead of a gas-filled resonant acoustic cavity, the sound energy is accumulated in a high- Q crystal element. Feasibility experiments utilizing a quartz-watch tuning fork demonstrate a sensitivity of 1.2x10(-7) cm(-1) W/ radicalHz . Potential further developments and applications of this technique are discussed.     

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.     

Theory of X-ray photoacoustic spectroscopy

PhotoAcoustic Spectroscopy (PAS) in the X-ray region is becoming a new field in PAS research and poses some new problems, such as heat production mechanisms and non-exponential heat distribution due to multi-excitation processes, additional signals caused by escaped fluorescence and electrons, special backing and fronting setup for comparison with absorption experiment, etc. In this paper we treat these problems and include them in an extended theoretical model which can be reduced to the RG model in a special case. The results indicate that the non-exponential heat distribution has no effect on the PA phase. The contribution by gas heating of the escaped electrons can be neglected. The thermally thin gas layers between the sample and the Be windows are sensitive factors to both amplitude and phase. The PA signal generated in the backing gas layer by the backing surface of the sample should be considered to understand the frequency dependence of the PA signal. The expansion of the backing gas layer is the reason for the phase change in PA-EXAFS.     

Retrieval of chromophore concentration in a tissue phantom by diffuse reflectance spectroscopy

This study describes a method for analysis of back reflectance spectroscopic data to estimate the concentration of endogenous or exogenous chromophores of tissue, noninvasively and in real time. In the study, tissue phantoms were prepared using intralipid and two chromophores, indocyanine green and methylene blue. Reduced scattering and absorption coefficients ranges of tissue phantoms were kept in the range of biological tissues. Spectroscopic measurements on the tissue phantoms were carried out using a miniature spectrometer, an optical fiber probe, a halogen tungsten light source and a laptop. Monte Carlo simulations of the experiments were run, and an average optical path lengths of the detected photons were obtained for different absorption and scattering coefficients. The average optical path lengths of the photons were used to estimate concentrations of the chromophores in the tissue phantoms. Scattering and absorption coefficients were estimated with an average error of 4.7 and 5.4%, respectively. The developed method has the potential to be used in diagnosis of pathologic tissues based on the variation of biochemical composition of tissues and in photodynamic therapy to estimate the concentration of photosensitizers.     

聚丁二炔Langmuir—Blodgett薄膜的光声谱研究：（Ⅰ）结构表征与光声？…

本在用Ｘ－射线小角衍射聚合与转载先后次序不同的ＰＤＡ ＬＢ膜进行结构表征的基础上,不同厚度的ＰＤＡ ＬＢ膜的光声新斩波频率效应说明,随着ＰＤＡ ＬＢ膜厚度增加,ＬＢ膜的热学性质逐渐明显。此外,还探讨了如何从ＰＤＡ ＬＢ膜的光声谱求得光声吸收系数问题,本的研究结果表明,光声谱技术是研究ＬＢ膜热学特性的有效手段。     

全光型石英增强光声光谱

设计并演示了一种全光型石英增强光声光谱技术, 该技术在传统的石英增强光声光谱系统中增加了另一束探测光束, 把与气体浓度成正比的石英晶振振臂的振动幅值转化为探测光束的强度变化, 实现了探测气体处无电子元件的全光学系统. 如此的设计使该系统具有较强的抗电磁干扰能力和非常小的传感头体积, 能够用于探测空间受限或探测环境恶劣的情况下, 并实现远距离探测. 在这种配置下, 探测大气压下的水汽, 获得的噪声等效吸收系数为1.13×10^-6 cm^-1W/√Hz. 进一步讨论了优化系统和提升其探测灵敏度的途径. 关键词： 石英增强光声光谱 音叉式石英晶振 气体传感     

Laser-induced breakdown spectroscopy in Asia

Laser-induced breakdown spectroscopy (LIBS) is an analytical detection technique based on atomic emission spectroscopy to measure the elemental composition. LIBS has been extensively studied and developed due to the non-contact, fast response, high sensitivity, real-time and multi-elemental detection features. The development and applications of LIBS technique in Asia are summarized and discussed in this review paper. The researchers in Asia work on different aspects of the LIBS study in fundamentals, data processing and modeling, applications and instrumentations. According to the current research status, the challenges, opportunities and further development of LIBS technique in Asia are also evaluated to promote LIBS research and its applications.     

Laser-induced breakdown spectroscopy in China

Laser-induced breakdown spectroscopy （LIBS） has been regarded as a future superstar for chemical analysis for years due to its unique features such as little or no sample preparation, remote sensing, and fast and multi-element analysis. Chinese LIBS comnmnity is one of the most dynamically developing communities in the World. The aim of the work is to inspect what have been done in China for LIBS development and, based on the understanding of the overall status, to identify the challenges and opportunities for the future development. In this paper, the scientific contributions from Chinese LIBS comlnunity are reviewed for the following four aspects： fimdamentals, instrumentation, data processing and modeling, and applications; and the driving force of LIBS development in China is analyzed, the critical issues for successful LIBS application are discussed, and in our opinion, the potential direction to improve the technology and to realize large scale commercialization in China is proposed.     

Off-beam quartz-enhanced photoacoustic spectroscopy with LEDs

A photoacoustic ozone sensor based on quartz-enhanced photoacoustic spectroscopy is presented. Instead of a laser, a UV-LED at 285 nm is utilized as light source. Using an acoustical resonator in off-beam configuration, it is feasible to align the light of the LED through the resonator tube. The sensor is integrated in a miniaturized flow-through gas cell which allows fast gas exchange. The sensor performance and the influence of the speed of sound on the measurement signal are investigated. For the detection of ozone in the Hartley band, a detection limit of S = 1.27 ± 0.08 ppmv (1σ) and a noise equivalent absorption sensitivity of D = (3.02 × 10^?8 ± 1 × 10^?10) cm^?1W(Hz)^?1/2 have been achieved.     

光纤倏逝波型石英增强光声光谱技术

采用块状光学准直聚焦透镜组的传统石英增强光声光谱(QEPAS)技术存在体积难以缩减,结构稳定性不佳,无法适应空间狭小、振动复杂的特殊环境等缺点.基于此,将光纤倏逝波技术与QEPAS技术相结合,提出了一种新型微纳结构光纤QEPAS痕量气体检测技术.实验中,为了提高QEPAS系统信号幅值,优化了石英音叉与激光束的空间位置、激光波长调制深度,同时对比了两种不同共振频率的石英音叉,最终采用共振频率较低的30.720 kHz石英音叉作为声波探测元件,获得的检测极限为6.25×10~(-4)(体积分数),归一化噪声等效吸收系数为4.18×10~(-7)cm~(-1).W·Hz~(-1/2).     

Laser-induced fluorescence spectroscopy of the secondary cataract

Excitation–emission matrices of laser-induced fluorescence of lens capsule epithelium, the lens nucleus, and the lens capsule are investigated. A solid-state laser in combination with an optical parametric generator tunable in the range from 210 to 350 nm was used for excitation of fluorescence. The spectra of fluorescence of all three types of tissues exhibit typical features that are specific to them and drastically differ from one another. This effect can be used for intrasurgical control of presence of residual lens capsule epithelium cells in the capsular bag after surgical treatment of a cataract.     

High-stability reference setup for photoacoustic spectroscopy

2 laser at 9.676 μm. Received: 2 December 1996/Revised version: 2 April 1997     

Temperature-dependent photoacoustic spectroscopy with a Helmholtz resonator

We describe and example the Beam Propagation Method (BPM) used to model and simulate nonlinear refractive and absorptive effects in materials with applications to optical limiting and switching. Various scenarios including laser-beam trapping and laser-beam division are investigated, in order to demonstrate the power of the BPM. A novel technique is also described for efficiently modelling the external far-field propagation from nonlinear media, including the propagation of non-Gaussian-shaped spatial profiles. The methods are finally combined with the phenomenon of nonlinear absorption to demonstrate enhanced power limiting in the presence of self-refraction. Optimal parameters for high-fluence power-limiting are subsequently discussed.     

利用激光诱导击穿光谱分析土壤成份

搭建了一套激光诱导击穿光谱实验装置,并通过配置特定样品,开展了一系列激光诱导击穿光谱探测实验。根据含有不同质量分数的同种元素样品的激光诱导击穿光谱实验结果,获得元素质量分数与谱线强度的关系曲线(定标曲线)。对中南民族大学附近的土壤进行激光诱导击穿光谱实验,发现土壤中含有Mg,Ca,Na等18种元素,对河南云台山茱萸峰岩石的激光诱导击穿光谱实验结果仅获得Fe,Mg,Ca 3种金属元素。比较这2种实际样品的激光诱导击穿光谱结果表明,检测样品的物理结构影响激光诱导光谱的实验结果。     

High-resolution infrared and laser photoacoustic spectroscopy of monochloroacetylene

High-resolution vibration—rotation spectra of monochloroacetylene (HCCCl) have been recorded in two different wavenumber regions. Fourier transform infrared interferometer measurements in the 4000–10000cm^?1 range have been performed with a resolution of 0.005–0.016cm^?1 and titanium: sapphire ring laser intracavity photoacoustic measurements in the 12500–12600cm^?1 and 12800–12860cm^?1 regions have been carried out with a Doppler-limited resolution of about 0.02 cm^?1. Altogether, 40 vibrational bands belonging to the HCC³⁵ Cl and 15 bands belonging to the HCC³⁷ Cl isotopic species have been rotationally assigned. A vibrational model, based on the conventional rectilinear normal coordinate theory including anharmonic Fermi, Darling—Dennison and vibrational l-type doubling resonances, has been used to attach vibrational labels to the bands. This approach is found to describe well the rovibrational energy level structure at least in the energy range covered in this work. The model shows that the CH oscillator becomes decoupled from the rest of the molecule as vibrational energy increases.     

Longitudinal resonant spectrophone for CO-laser photoacoustic spectroscopy

A photoacoustic (PA) system for monitoring gaseous air pollutants absorbing in the CO-laser range is presented. The characteristics of the CO laser and the interference caused by water-vapor absorption demand a special design of the PA cell and experimental setup. The optimum cell design was found by numerical simulation of the acoustic properties of various cell geometries. For this purpose a model using infinitesimal analogue acoustic impedances was developed. Based on a matrix formalism for fourterminals, a computer program was applied that permits the calculation of the frequency response of the PA signal amplitude at any position within a one-dimensional PA cell. Excellent agreement with experimental data is obtained. As a result, a new design for an acoustically resonant spectrophone with improved properties is presented. The response of the cell with aQ-factor of 52, operated at 555 Hz, is 2000 Pa cm/W.     

光声光谱技术在测量光谱响应特性方面的应用

用光声光谱技术测量光电探测器的相对光谱响应特性,以高灵敏度的光声池(池内装有吸收系数约为1的碳黑)作为参考探测器,具有较好的波长无选择性。用碳黑监测光源功率,减少光源功率起伏误差。采用二次测量归一化方法避免了分束器的影响。实验测得了比较准确的相对光谱响应特性曲线,为研究光电探测器的特性提供了一种可行的方法。     

Theoretical analysis of a quartz-enhanced photoacoustic spectroscopy sensor

Quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors are based on a recent approach to photoacoustic detection which employs a quartz tuning fork as an acoustic transducer. These sensors enable detection of trace gases for air quality monitoring, industrial process control, and medical diagnostics. To detect a trace gas, modulated laser radiation is directed between the tines of a tuning fork. The optical energy absorbed by the gas results in a periodic thermal expansion which gives rise to a weak acoustic pressure wave. This pressure wave excites a resonant vibration of the tuning fork thereby generating an electrical signal via the piezoelectric effect. This paper describes a theoretical model of a QEPAS sensor. By deriving analytical solutions for the partial differential equations in the model, we obtain a formula for the piezoelectric current in terms of the optical, mechanical, and electrical parameters of the system. We use the model to calculate the optimal position of the laser beam with respect to the tuning fork and the phase of the piezoelectric current. We also show that a QEPAS transducer with a particular 32.8 kHz tuning fork is 2–3 times as sensitive as one with a 4.25 kHz tuning fork. These simulation results closely match experimental data.     

Laser-induced fluorescence of cardiac tissue with calcinosis

Investigations of laser-induced fluorescence spectra for human cardiac valve tissue with calcinosis are performed as compared to similar spectra of bone and myocardial tissue of an animal that are excited by an excimer laser with a 248-nm wavelength. It is shown that a healthy tissue has a laser-induced fluorescence maximum in the region of 300–400 nm which corresponds to protein tissue luminescence. For tissue affected by calcinosis, the laser-induced fluorescence spectra differ significantly from the spectra of healthy tissue and have a maximum in the region of 400–500 nm. The obtained results offer the prospects for using laser-induced fluorescence to diagnose tissue with calcinosis in open-heart surgery. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 4, pp. 539–541, July–August, 1997.     

利用激光诱导击穿光谱分析土壤成份

 搭建了一套激光诱导击穿光谱实验装置,并通过配置特定样品,开展了一系列激光诱导击穿光谱探测实验。根据含有不同质量分数的同种元素样品的激光诱导击穿光谱实验结果,获得元素质量分数与谱线强度的关系曲线(定标曲线)。对中南民族大学附近的土壤进行激光诱导击穿光谱实验,发现土壤中含有Mg,Ca,Na等18种元素,对河南云台山茱萸峰岩石的激光诱导击穿光谱实验结果仅获得Fe,Mg,Ca 3种金属元素。比较这2种实际样品的激光诱导击穿光谱结果表明,检测样品的物理结构影响激光诱导光谱的实验结果。