Study on the radiation and self-absorption characteristics of plasma under various background gases.

The self-absorption effect is a primary factor responsible for the decline in the precision of quantitative analysis techniques using plasma emission spectroscopy, such as laser-induced breakdown spectroscopy (LIBS). In this study, based on the thermal ablation and hydrodynamics models, the radiation characteristics and self-absorption of laser-induced plasmas under different background gases were theoretically simulated and experimentally verified to investigate ways of weakening the self-absorption effect in plasma. The results reveal that the plasma temperature and density increase with higher molecular weight and pressure of the background gas, leading to stronger species emission line intensity. To reduce the self-absorption effect in the later stages of plasma evolution, we can decrease the gas pressure or substitute the background gas with a lower molecular weight. As the excitation energy of the species increases, the impact of the background gas type on the spectral line intensity becomes more pronounced. Moreover, we accurately calculated the optically thin moments under various conditions using theoretical models, which are consistent with the experimental results. From the temporal evolution of the doublet intensity ratio of species, it is deduced that the optically thin moment appears later with higher molecular weight and pressure of the background gas and lower upper energy of the species. This theoretical research is essential in selecting the appropriate background gas type and pressure and doublets in self-absorption-free LIBS (SAF-LIBS) experiments to weaken the self-absorption effect.

Non-contact bacterial identification and decontamination based on laser-induced breakdown spectroscopy.

As a new kind of modern military biological weapon, bacterial agents pose a serious threat to the public health security of human beings. Existing bacterial identification requires manual sampling and testing, which is time-consuming, and may also introduce secondary contamination or radioactive hazards during decontamination. In this paper, a non-contact, nondestructive and "green" bacterial identification and decontamination technology based on laser-induced breakdown spectroscopy (LIBS) is proposed. The principal component analysis (PCA) combined with support vector machine (SVM) based on radial basis kernel function is used to establish the classification model of bacteria, and the two-dimensional decontamination test of bacteria is carried out using laser-induced low-temperature plasma combined with a vibration mirror. The experimental results show that the average identification rate of the seven types of bacteria, including Escherichia coli, Bacillus subtilis, Pseudomonas fluorescens, Bacillus megatherium, Pseudomonas aeruginosa, Bacillus thuringiensis and Enterococcus faecalis reaches 98.93%, and the corresponding true positive rate, precision, recall and F1-score reaches 0.9714, 0.9718, 0.9714 and 0.9716, respectively. The optimal decontamination parameters are laser defocusing amount of -50 mm, laser repetition rate of 15-20 kHz, scanning speed of 150 mm/s and number of scans of 10. In this way, the decontamination speed can reach 25.6 mm2/min, and the inactivation rates for both Escherichia coli and Bacillus subtilis are higher than 98%. In addition, it is confirmed that the inactivation rate of plasma is 4 times higher than that of thermal ablation, meaning that the decontamination ability of LIBS mainly relies on the plasma rather than the thermal ablation effect. The new non-contact bacterial identification and decontamination technology does not require sample pretreatment, and can quickly identify bacteria in situ and decontaminate the surfaces of precision instruments, sensitive materials, etc., which has potential application value in modern military, medical and public health fields.

Ultra-repeatability measurement of calorific value of coal by NIRS-XRF.

Calorific value is an important indicator to evaluate the comprehensive quality of coal, and its real-time and rapid analysis is of great significance for optimizing the coal blending process and improving boiler combustion efficiency. Traditional assays are time-consuming, and prompt gamma neutron activation analysis (PGNAA) and laser-induced breakdown spectroscopy (LIBS) have certain limitations. In this paper, a novel technique for ultra-repeatability measurement of coal calorific value by combining near-infrared spectroscopy (NIRS) and X-ray fluorescence (XRF) is proposed. In this NIRS-XRF technology, the former can stably measure organic components such as C-H and N-H that are positively correlated with the calorific value, while the latter can stably measure inorganic elements such as Na, Al, Si, Ca, Fe, and Mn that are negatively correlated with the calorific value. The combination of the two can greatly improve the measurement repeatability of coal calorific value. In the quantitative analysis algorithm, a holistic-segmented prediction model based on partial least squares (PLS) is proposed, that is, the holistic model is used to roughly predict the calorific value and determine the segment accordingly, and then the corresponding segmented model is used to accurately predict the calorific value. The experimental results show that the root mean square error of prediction (RMSEP), the average relative error (ARE), and the standard deviation (SD) of this method for predicting the calorific value of coal are 0.71 MJ kg-1, 1.18% and 0.07 MJ kg-1 respectively. The measurement repeatability meets the requirements of the Chinese national standard. This calorific value measurement technology based on NIRS-XRF is safe, fast, and stable, providing a new way to optimize and control the utilization process of coal in coal washing plants, power plants, coking, and other industries.

Study on direct identification of bacteria by laser-induced breakdown spectroscopy.

Bacteria are everywhere in the natural environment. Although most of them are harmless, there are still some hazardous bacteria that will harm human health, so it is particularly important to identify bacteria quickly. Compared with traditional time-consuming and complicated identification methods, laser-induced breakdown spectroscopy (LIBS) is one of the potential technologies for rapid identification of bacteria. In this paper, six weakly active bacteria, including Escherichia coli, Enterococcus faecalis, Bacillus megaterium, Bacillus thuringiensis, Pseudomonas aeruginosa and Bacillus subtilis, are taken as analysis samples. The thawed bacteria are placed in deionized water, and then uniformly smeared on five kinds of substrates to verify the feasibility of using LIBS to identify these bacteria. Spectrum filtering, normalization and principal component analysis (PCA) are used to preprocess the spectra, and a multi-class identification method based on the one-against-all linear kernel function of support vector machine (SVM) is proposed to establish the prediction model. The identification performance is evaluated by using precision and recall. The experimental results show that high-purity graphite is the best substrate with the least interference to the LIBS spectrum of bacteria. The prediction precision of these six bacteria is 77.27%, 92.86%, 84.21%, 94.12%, 81.82% and 76.92%, respectively, recall is 85%, 100%, 94.12%, 80%, 81.82% and 75% respectively, and the identification rate is 84.17%. It can be seen that the direct identification of bacteria can be preliminarily realized by smearing bacteria on the graphite substrate and analyzing its LIBS spectra, which provides a feasible way for simple, rapid and on-site bacterial identification.

ppb-Level SO2 Photoacoustic Sensors with a Suppressed Absorption-Desorption Effect by Using a 7.41 µm External-Cavity Quantum Cascade Laser.

A sensitive photoacoustic sensor system for the detection of ppb-level sulfur dioxide (SO2) was developed by the use of a continuous-wave room-temperature, high-power quantum cascade laser (QCL) with an external diffraction grating cavity geometry. The excitation wavelength of the QCL was set to 7.41 µm for the strongest SO2 absorption line strength. A custom-made differential photoacoustic cell (PAC) with two identical resonators was designed to allow a gas flow rate up to 1200 sccm. A qualitative theoretical model was employed in order to understand the dynamic adsorption and desorption processes of SO2 in the PAC walls. A 1σ detection limit of 2.45 ppb, corresponding to a normalized noise equivalent absorption value of 3.32 × 10-9 cm-1 W/Hz1/2, was achieved after measures for suppressing the absorption-desorption effect were taken.

Simultaneous multi-gas detection between 3 and 4µm based on a 2.5-m multipass cell and a tunable Fabry-Pérot filter detector.

We demonstrated a versatile and innovative gas sensing system based on a Fabry-Pérot (FP) filter detector, which operates in the spectral range from 3.1 to 4.4µm (3226-2273cm-1) with a spectral resolution of 20nm. The developed sensor system can be used to record the entire spectrum by means of a one-time scan or, alternatively, to access selected spectral regions by using the tunable FP filter detector. A multipass cell with an effective path length of 2.5m was implemented to improve the detection sensitivity. The spectra of methane, formaldehyde and carbon dioxide were simultaneously measured, with detection limits of 200ppm, 900ppm and 20ppm, respectively. A seven-day continuous measurement for indoor carbon dioxide gas was carried out demonstrating the stability and robustness of the reported sensor system.

Calculation model of dense spot pattern multi-pass cells based on a spherical mirror aberration.

We report a novel calculation model for dense spot pattern multi-pass cells consisting of two common identical spherical mirrors. A modified ABCD matrix without the paraxial approximation was developed to describe the ray propagation between two spherical mirrors and the reflection on the mirror surfaces. The intrinsic aberration from the spherical curvature creates a set of intricate variants with respect to a standard Herriot circle spot pattern. A series of detailed numerical simulations are implemented to verify that the input and output beams remain the same and, hence, retrace the same ray pattern. The set of exotic spot patterns obtained with a high fill factor improves the utilization efficiency of the mirror surfaces and produces a longer total optical path length with a low mirror cost.

Highly sensitive photoacoustic multicomponent gas sensor for SF6 decomposition online monitoring.

A ppb-level photoacoustic multicomponent gas sensor system for sulfur hexafluoride (SF6) decomposition detection was developed by the use of two near-infrared (NIR) diode lasers and an ultraviolet (UV) solid-state laser. A telecommunication fiber amplifier module was used to boost up the excitation optical power from the two NIR lasers. A dual-channel high-Q photoacoustic cell (PAC) was designed for the simultaneous detection of CO, H2S, and SO2 in SF6 buffer gas by means of a time division multiplexing (TDM) method. Feasibility and performance of the multicomponent sensor was evaluated, resulting in minimum detection limits of 435 ppbv, 89 ppbv, and 115 ppbv for CO, H2S, and SO2 detection at atmospheric pressure.

A novel methodology to directly pre-determine the relative wavelength response of DFB laser in wavelength modulation spectroscopy.

A novel methodology to directly pre-determine the relative wavelength response (RWR) of a DFB laser, in terms of a combined current linearly scanned wavelength response and current modulated wavelength response (CMWR), in wavelength modulation spectroscopy (WMS) is presented. It is shown that the assessed RWR can be used to mimic the measured response with standard deviation of discriminations that are below 3.4 × 10-3cm-1 under a variety of conditions. It is also shown that its performance supersedes two commonly used assessment models of the CMWR but is slightly worse than that of the third model, however with the benefit of solely using a single fitting parameter (the concentration) instead of more. When the novel method is applied to the assessment of CO2 concentration in a Herriot-type multipass cell by using the technique of calibration-free WMS, the results show that there is virtually no difference compared to that by use of the best of the other methods. It is concluded that the novel method is more robust and simplifies the retrieval process of gas concentration.

Highly sensitive and selective CO sensor using a 2.33 µm diode laser and wavelength modulation spectroscopy.

A ppm-level CO sensor based on a 2f wavelength modulation spectroscopy (2f-WMS) technique was developed for the application of SF6 decomposition analysis in an electric power system. A detailed investigation of the optimum target line selection was carried out to avoid spectral interference from high purity SF6 in a wide wavelength range. A diode laser emitting at 2.33 µm and a 14.5-m multipass gas cell (MGC) was employed to target the R(6) line of the CO first overtone band and increase the optical path, respectively, thus resulting in a minimum detection sensitivity of 1 ppm. A Levenberg-Marquardt nonlinear least-squares fit algorithm makes full use of the information from all data points of the 2f spectrum and as a result, a measurement precision of ~40 ppb was achieved with a data update rate of 0.6 s. The sensor performance was also evaluated in terms of the gas flow rate, stability, and linearity. The results showed that the best operating condition with a precision of 6 ppb can be achieved by increasing the gas flow rate to the value that matches the optimum averaging time of 48 s.

Homogeneous-material-based calibration method for correcting laser-induced breakdown spectroscopy measurement-error bias in the case of dust pollution.

A calibration method based on homogeneous material for correcting laser-induced breakdown spectroscopy (LIBS) measurement-error bias in the case of dust pollution under laboratory conditions is proposed. The measured plasma spectra of the sample can be corrected by measuring the spectral integral of the homogeneous material. Thus, we can effectively minimize the dust pollution effect on LIBS and guarantee its precision. Results show that the mean absolute errors of CaO, MgO, Fe2O3, Al2O3, and SiO2 in cement samples are decreased notably from 1.02%, 0.06%, 0.15%, 0.57%, and 0.80% to 0.41%, 0.02%, 0.04%, 0.35%, and 0.39%, respectively. Combination of this calibration method with the traditional optical dustproof methods will significantly extend the LIBS equipment maintenance cycle and make preliminary preparations for the next practical industrial application.

Development and performance evaluation of self-absorption-free laser-induced breakdown spectroscopy for directly capturing optically thin spectral line and realizing accurate chemical composition measurements.

A novel self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) technique is proposed to directly capture the optically thin spectral line by matching the measured doublet atomic lines intensity ratios with the theoretical one. To realize the experimental SAF-LIBS, the integration time, the fiber collection angle, and the delay time are optimized. The optically thin conditions are validated by comparing the linearity of Boltzmann plots with the traditional self-absorption (SA) correction method and evaluating the SA coefficients. The applicability and limitation of SAF-LIBS on element concentration and laser energy are also discussed. Univariate quantitative analysis results show that, compared with ordinary LIBS, the average absolute error of aluminum concentration has been reduced by an order of magnitude, which proves that this SAF-LIBS technique is qualified to realize accurate chemical composition measurements.

Beat frequency quartz-enhanced photoacoustic spectroscopy for fast and calibration-free continuous trace-gas monitoring.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a sensitive gas detection technique which requires frequent calibration and has a long response time. Here we report beat frequency (BF) QEPAS that can be used for ultra-sensitive calibration-free trace-gas detection and fast spectral scan applications. The resonance frequency and Q-factor of the quartz tuning fork (QTF) as well as the trace-gas concentration can be obtained simultaneously by detecting the beat frequency signal generated when the transient response signal of the QTF is demodulated at its non-resonance frequency. Hence, BF-QEPAS avoids a calibration process and permits continuous monitoring of a targeted trace gas. Three semiconductor lasers were selected as the excitation source to verify the performance of the BF-QEPAS technique. The BF-QEPAS method is capable of measuring lower trace-gas concentration levels with shorter averaging times as compared to conventional PAS and QEPAS techniques and determines the electrical QTF parameters precisely.

Intensity-Stabilized Fast-Scanned Direct Absorption Spectroscopy Instrumentation Based on a Distributed Feedback Laser with Detection Sensitivity down to 4 × 10-6.

A novel, intensity-stabilized, fast-scanned, direct absorption spectroscopy (IS-FS-DAS) instrumentation, based on a distributed feedback (DFB) diode laser, is developed. A fiber-coupled polarization rotator and a fiber-coupled polarizer are used to stabilize the intensity of the laser, which significantly reduces its relative intensity noise (RIN). The influence of white noise is reduced by fast scanning over the spectral feature (at 1 kHz), followed by averaging. By combining these two noise-reducing techniques, it is demonstrated that direct absorption spectroscopy (DAS) can be swiftly performed down to a limit of detection (LOD) (1σ) of 4 × 10-6, which opens up a number of new applications.

Scattered light modulation cancellation method for sub-ppb-level NO2 detection in a LD-excited QEPAS system.

A sub-ppb-level nitrogen dioxide (NO2) QEPAS sensor is developed by use of a cost-effective wide stripe laser diode (LD) emitting at 450 nm and a novel background noise suppression method called scattered light modulation cancellation method (SL-MOCAM). The SL-MOCAM is a variant of modulation spectroscopy using two light sources: excitation and balance light sources. The background noise caused by the stray light of the excitation light sources can be eliminated by exposing the QEPAS spectrophone to the modulated balance light. The noise in the LD-excited QEPAS system is investigated in detail and the results shows that > ~90% background noise can be effectively eliminated by the SL-MOCAM. For NO2 detection, a 1σ detection limit of ~60 ppb is achieved for 1 s integration time and the detection limit can be improved to 0.6 ppb with an integration time of 360 s. Moreover, the SLMOCAM shows a remote working ability in the preliminary investigation.

Single-tube on-beam quartz-enhanced photoacoustic spectroscopy.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) with a single-tube acoustic microresonator (AmR) inserted between the prongs of a custom quartz tuning fork (QTF) was developed, investigated, and optimized experimentally. Due to the high acoustic coupling efficiency between the AmR and the QTF, the single-tube on-beam QEPAS spectrophone configuration improves the detection sensitivity by 2 orders of magnitude compared to a bare QTF. This approach significantly reduces the spectrophone size with respect to the traditional on-beam spectrophone configuration, thereby facilitating the laser beam alignment. A 1σ normalized noise equivalent absorption coefficient of 1.21×10(-8) cm(-1)·W/√Hz was obtained for dry CO2 detection at normal atmospheric pressure.

Impact of Humidity on Quartz-Enhanced Photoacoustic Spectroscopy Based CO Detection Using a Near-IR Telecommunication Diode Laser.

A near-IR CO trace gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) is evaluated using humidified nitrogen samples. Relaxation processes in the CO-N2-H2O system are investigated. A simple kinetic model is used to predict the sensor performance at different gas pressures. The results show that CO has a ~3 and ~5 times slower relaxation time constant than CH4 and HCN, respectively, under dry conditions. However, with the presence of water, its relaxation time constant can be improved by three orders of magnitude. The experimentally determined normalized detection sensitivity for CO in humid gas is 1.556 × 10(-8) W ⋅ cm (-1)/Hz(1/2).

Calibration-free wavelength-modulation spectroscopy based on a swiftly determined wavelength-modulation frequency response function of a DFB laser.

A methodology for calibration-free wavelength modulation spectroscopy (CF-WMS) that is based upon an extensive empirical description of the wavelength-modulation frequency response (WMFR) of DFB laser is presented. An assessment of the WMFR of a DFB laser by the use of an etalon confirms that it consists of two parts: a 1st harmonic component with an amplitude that is linear with the sweep and a nonlinear 2nd harmonic component with a constant amplitude. Simulations show that, among the various factors that affect the line shape of a background-subtracted peak-normalized 2f signal, such as concentration, phase shifts between intensity modulation and frequency modulation, and WMFR, only the last factor has a decisive impact. Based on this and to avoid the impractical use of an etalon, a novel method to pre-determine the parameters of the WMFR by fitting to a background-subtracted peak-normalized 2f signal has been developed. The accuracy of the new scheme to determine the WMFR is demonstrated and compared with that of conventional methods in CF-WMS by detection of trace acetylene. The results show that the new method provides a four times smaller fitting error than the conventional methods and retrieves concentration more accurately.

[Development of a Laser On-Line Cement Raw Material Analysis Equipment].

In engineering construction, cement quality directly affects the safety of construction projects. So it is necessary that we use qualified cement in the engineering structure. It is of great signification that a method detects cement raw material rapidly to adjust the mixture ratio of raw ores to ensure the cement quality. Traditional detection method needs sampling, sample preparation and test, etc. With many procedures, the test results are seriously lagged behind the production process. This paper introduces a set of online analysis equipment to determinate elemental composition of cement powder timely based on laser induced breakdown spectroscopy. This equipment is composed of a LIBS detection system and a pneumatic system. The equipment can achieve the real-time measurement for it needn't sample preparation. Thus, it can guide cement raw material proportioning in time. In this paper, we have quantitatively analyzed the main components of Al2O3, CaO, Fe2O3, MgO and SiO2 in the cement raw materials using the full spectrum normalization method as well as the support vector machine. The corresponding maximum absolute errors were 0.34%, 0.35%, 0.07%, 0.14%, and 0.55%, respectively. Results showed that the measurement results of the newly developed LIBS equipment are in accord with those of the conventional chemical method. Furthermore, the measurement precision is in line with X-Ray fluorescence spectrometry. It is confirmed that the LIBS technique could be a prospect method for determination of elemental composition in the cement production industries.

[High-Sensitive Carbon Dioxide Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a 2.0 µm Distributed Feedback Laser].

A carbon dioxide (CO2) sensor is developed using quartz enhanced photoacoustic spectroscopy (QEPAS) with a 2.0 µm distributed feedback diode laser. The detection is based on a 2f wavelength-modulation spectroscopy approach by dithering and scanning the laser current. The laser modulation depth is optimized at normal atmosphere pressure and room temperature. The influence of the H2O presence in the sample gas mixture on the CO2 sensor performance is also investigated. The results show that, with 1% CO2 concentration, the H2O in the concentration ranges of 0 to 0.2% has an effect on the CO2 signal amplitude and phase, and the largest amplitude difference is ~2.1 times. When the H2O concentration is over 0.2%, the CO2 signal amplitude is saturated and remains steady. Atmospheric CO2 concentration is well measured using the optimal sensor parameters. Benefiting from the strong absorption line intensity at 4989.97 cm(-1), a detection limit of 19 ppm (1σ, 300 ms averaging time) is achieved, which corresponds to a normalized noise equivalent absorption coefficient of 4.71 × 10(-9) cm(-1) · W · Hz(-1/2).