Direct determination of carbon dioxide in aqueous solution using mid-infrared quantum cascade lasers

A method for the direct determination of carbon dioxide in aqueous solutions using a room-temperature mid-infrared (MIR) quantum cascade laser at 2330 cm(-1) is reported. The absorption values of different carbon dioxide concentrations were measured in a 119 microm CaF2 flow-through cell. An optical system made of parabolic mirrors was used to probe the flow cell and to focus the laser beam on the mercury cadmium telluride (MCT) detector. Aqueous carbon dioxide standards were prepared by feeding different mixtures of gaseous N2 and CO2 through wash bottles at controlled temperature. The concentration of the dissolved CO2 was calculated according to Henry's law, taking into account the temperature and the partial pressure of CO2. The carbon dioxide standards were connected via a selection valve to a peristaltic pump for subsequent, automated measurement in the flow-through cell. A calibration curve was obtained in the range of 0.338 to 1.350 g/L CO2 with a standard deviation of the method sxo equal to 19.4 mg/L CO2. The limit of detection was calculated as three times the baseline noise over time and was determined to be 39 mg/L.     

Carrier loss mechanisms in type II quantum wells for the active region of GaSb-based mid-infrared interband cascade lasers

Thermal quenching of photoluminescence has been measured for InAs/GaInSb broken gap system quantum wells as a function of the thickness of the conduction and valence band wells. The primary activation energy appears too small for the electrons to leave the well, and insensitive to even significant changes in the InAs layer thickness and hence electron confinement potential. On the contrary, it varies strongly with the modification of the confinement of holes in the GaInSb layer. Thus, tunneling-assisted escape of holes into the GaSb layers has been recognized as the dominant carrier loss mechanism in these type II quantum wells.     

External cavity tunable quantum cascade lasers and their applications to trace gas monitoring

Since the first quantum cascade laser (QCL) was demonstrated approximately 16 years ago, we have witnessed an explosion of interesting developments in QCL technology and QCL-based trace gas sensors. QCLs operate in the mid-IR region (3-24?μm) and can directly access the rotational vibrational bands of most molecular species and, therefore, are ideally suited for trace gas detection with high specificity and sensitivity. These sensors have applications in a wide range of fields, including environmental monitoring, atmospheric chemistry, medical diagnostics, homeland security, detection of explosive compounds, and industrial process control, to name a few. Tunable external cavity (EC)-QCLs in particular offer narrow linewidths, wide ranges of tunability, and stable power outputs, which open up new possibilities for sensor development. These features allow for the simultaneous detection of multiple species and the study of large molecules, free radicals, ions, and reaction kinetics. In this article, we review the current status of EC-QCLs and sensor developments based on them and speculate on possible future developments.     

分布反馈量子级联激光器的光栅制备

利用全息曝光方法制备了分布反馈量子级联激光器的光栅掩模,选择和发展了恰当的用于InGaAs/InP材料的光栅腐蚀优化工艺,得到腐蚀规律,讨论了腐蚀机制。在量子级联激光器的InGaAs/InP层上制备光栅得到分布反馈量子级联激光器,其单模特性较好,信噪比大于30dB。     

Nonlinear optical interactions of laser modes in quantum cascade lasers

We overview the results of recent experimental and theoretical studies of nonlinear dynamics of mid-infrared quantum cascade lasers (QCLs) associated with nonlinear interactions of laser modes. Particular attention is paid to phase-sensitive nonlinear mode mixing which turns out to be quite prominent in QCLs of different kinds and which gives rise to frequency and phase locking of laser modes. Nonlinear phase coupling of laser modes in QCLs leads to a variety of ultrafast and coherent phenomena: synchronization of transverse modes, beam steering, the RNGH multimode instability, and generation of mode-locked ultrashort pulses.     

Mid-infrared photothermal heterodyne spectroscopy in a liquid crystal using a quantum cascade laser

We report a technique to measure the mid-infrared photothermal response induced by a tunable quantum cascade laser in the neat liquid crystal 4-octyl-4'-cyanobiphenyl (8CB), without any intercalated dye. Heterodyne detection using a Ti:sapphire laser of the response in the solid, smectic, nematic and isotropic liquid crystal phases allows direct detection of a weak mid-infrared normal mode absorption using an inexpensive photodetector. At high pump power in the nematic phase, we observe an interesting peak splitting in the photothermal response. Tunable lasers that can access still stronger modes will facilitate photothermal heterodyne mid-infrared vibrational spectroscopy.     

Quantitative in situ monitoring of an elevated temperature reaction using a water-cooled mid-infrared fiber-optic probe

A novel water-cooled mid-infrared fiber-optic probe is described which is heatable to 230 °C. The probe has chalcogenide fibers and a ZnSe internal reflection element and is compact and fully flexible, allowing access to a wide range of standard laboratory reaction vessels and fume cupboard arrangements. Performance is demonstrated via the in situ analysis of an acid-catalyzed esterification reaction in toluene at 110 °C, and the results are compared with those from a conventional extractive sampling loop flow cell arrangement. Particular emphasis is given to the quantitative interpretation of the spectroscopic data, using gas chromatographic reference data. Calibration data are presented for univariate and partial least squares models, with an emphasis on procedures for improving the quality of interpreparation calibration and prediction through the use of focused reference analysis regimes. Subset univariate procedures are presented that yield relative errors of <5%, and bias-corrected partial least squares procedures are described that result in relative errors of interpreparation calibration and prediction consistently <3%. This paper demonstrates the considerable power of fiber-optic mid-IR spectroscopy combined with bias correction partial least squares procedures for the efficient in situ quantitative analysis of laboratory scale reactions.     

On-line velocity measurements using phase probes at the SuperHILAC

Phase probes have been placed in several external beam lines at the LBL heavy ion linear accelerator (SuperHILAC) to provide nondestructive velocity measurements independent of the ion being accelerated [B. Leemann, D. Brodzik, B. Feinberg and D. Howard, IEEE Trans. Nucl. Sci. NS-32 (1985)1982]. The system uses three probes in each line to obtain accurate velocity measurements at all beam energies. Automatic gain control and signal analysis are performed so that the energy/nucleon along with up to three probe signals are displayed on a vector graphics display with a refresh rate better than twice per second. The system uses a sensitive pseudocorrelation technique to pick out the signal from the noise, features simultaneous measurements of up to four ion velocities when more than one beam is being accelerated, and is controlled by a touch-screen operator interface. It is accurate to within ±0.25% and has provisions for on-line calibration tests. The phase probes thus provide a velocity measurement independent of the mass defect associated with the use of crystal detectors, which can become significant for heavy elements. They are now used as a routine tuning aid to ensure proper bunch structure, and as a beam velocity monitor.     

Energy transfer between lead sulfide quantum dots in the liquid phase

Förster resonant energy transfer (FRET) from smaller sized PbS Quantum Dots (QDs) to larger sized ones occurs in the liquid phase when adjacent QDs are brought in close proximity by the bridging action of the two SH groups in aromatic or aliphatic dithiols. Signatures of FRET were observed by the lowering of the fluorescence peak corresponding to the smaller particle group and intensification of the peak corresponding to the larger particle group in a mixture of the two in the liquid phase. The suspensions of the two QDs size distributions were mixed in a ratio such that total surface area of the smaller QDs was equal to that of the larger QDs. A 10–13% size deviation in each PbS QDs group also allows this phenomenon to be observed in these samples and is manifested by red-shift and broadening of the fluorescence peak. The ratio of the absorption peak intensity corresponding to the two groups of PbS QDs in the mixture remains the same.     

Evaluation techniques for two-way data from in situ fourier transform mid-infrared reaction monitoring in aqueous solution

A model chemical reaction was monitored with in situ Fourier transform mid-infrared spectroscopy using an attenuated total reflectance probe. The evaluation of the IR spectra is complicated by the fact that the reaction runs in nonisothermal aqueous solution with large variations in pH. Despite this, it was possible to extract large amounts of useful information on the reaction after suitable pretreatment of the spectra. Alternating least-squares (ALS) multivariate curve resolution is shown to be a useful technique for obtaining pure component spectra and concentrations if suitable spectral regions are analyzed. Rank mapping methods are used as the basis for this sectioning into smaller regions. Techniques for finding and analyzing selective spectral regions are also shown to be applicable to this type of data. Partial least-squares (PLS) regression models based on spectral data were used to verify the results where possible. The correlation between the concentrations predicted from PLS and ALS is excellent.     

Middle infrared, quantum cascade laser optoelectronic absorption system for monitoring glucose in serum

Advances in middle infrared technology are leading researchers beyond the Fourier transform infrared spectrometer and to the quantum cascade laser. While most research focuses on gas-phase detection, recent research explores its use for condensed-phase matter studies. This work investigates its use for monitoring biologically relevant samples of glucose in serum. Samples with physiological glucose concentrations were monitored with a laser at 1036 cm-1. A 0.992 R2 linearity value was observed. In addition, using another laser at 1194 cm-1 as a measure of the background spectroscopic characteristics, a linearity of 0.998 R2 was observed. The average predictive standard errors of the mean (SEM) were 32.5 and 24.7 mg/dL, respectively, for each method. Quantum cascade lasers could be used to develop middle infrared devices for uses beyond the confines of the laboratory.     

On-line temperature monitoring in a capillary electrochromatography frit using microcoil NMR

A new nuclear magnetic resonance (NMR) spectroscopy probe has been designed to measure the temperature of the water inside a capillary. The probe provides the ability to measure the temperature in a several hundred micrometer long capillary section, corresponding to liquid volumes in the picoliter to nanoliter range with a temperature monitoring accuracy of 0.2 degrees C. The NMR probe is based on a novel two-turn vertical solenoidal design, and its performance for capillary-scale temperature measurements is characterized. The temperature rise in a chromatographic frit of the type used in capillary electrochromatography is measured as a function of applied power, and temperature rises of more than 50 degrees C are observed. The temperature of the electrolyte cools rapidly after exiting the frit and can be followed as a function of distance from the frit. The ability to accurately monitor the temperature of water as it moves through porous materials such as packed chromatographic beds and frits is important to allow the effects of temperature on CEC separation performance to be determined.     

Aircraft and balloon in situ measurements of methane and hydrochloric acid using interband cascade lasers

Aircraft and balloon in situ measurements of CH4 and HCl using cw distributed feedback (DFB) interband cascade (IC) lasers are reported. In the stratosphere and upper troposphere, sensitivity toward CH4 and HCl is better than 10 ppbv (1 s) and 90 pptv (50 s), respectively. These are the first flight measurements of trace gas-phase species using cw DFB IC lasers.     

On-line condition monitoring of tool wear in end milling using acoustic emission

The monitoring of tool wear is important in maintaining the quality of workpieces produced. This paper presents a methodology to monitor on-line tool wear in end milling using acoustic emission. It is well known that the root-mean-square (RMS) value of the acoustic emission is directly proportional to the power expended in turning. A mathematical model has been developed to predict the RMS value of the acoustic-emission signal in milling. This mathematical model incorporates the machining parameters as variables. The accuracy of the model has been verified by a series of experiments. The experiments were carried out on a Bridgeport milling machine and data was collected and analysed by using an on-line computer data acquisition system. A comparison of the experimental and theoretical RMS values indicates a very good agreement between them. A control strategy similar to the moving average/moving range charts has been developed for monitoring on-line tool wear. The limits for the control charts were obtained from the theoretical equation of statistical quality control. An observation of the control charts clearly indicates the region of tool failure and the time at which the tool failed. The philosophy behind the use of control charts is based on the ease of implementation and widespread use in industry.     

On-line tool condition monitoring in face milling using current and power signals

The vast majority of tool condition monitoring systems use the cutting force as the predictor signal. However, due to prohibitive cost to performance ratios and maintenance and operational problems, such methods are not favoured by industries. In this paper, a method for continuous on-line estimation of tool wear, based on the inexpensive spindle motor current and voltage measurements, is proposed for the complex and intermittent cutting face milling operation. Sensors for these signals are free from problems associated with the cutting forces and the vibration signals. Novel signal processing strategies have been proposed for on-line computation of useful features from the measured signals. Feature space filtering is introduced to obtain robust and improved predictors from the extracted features. A multiple linear regression model, built on the filtered features, is then used to estimate tool wear in real-time. Very accurate predictions are achieved for both laboratory and industrial experiments, surpassing earlier results using cutting forces and estimation methods based on complex methodologies such as artificial neural networks.     

On-line in vivo monitoring of endogenous quinones using microdialysis coupled with electrochemical detection

The continuous on-line monitoring of endogenous quinones has been realized for the first time in an animal model of brain ischemia induced by a vasoconstrictor peptide, endothelin-1. A microdialysis probe, implanted in the striatum of a freely moving rat, was coupled on-line with an amperometric thin-layer cross-flow detector with a glassy carbon working electrode operating at -200 mV vs Ag/AgCl. The instrumental setup comprised a syringe pump pulse-damper consisting of an air bubble and a silica capillary, which permitted considerable reduction of background current fluctuations and allowed improved detection limits. This original configuration allowed the quantitation of micromolar amounts of total quinones, generated from dopamine during the reperfusion period, to be readily monitored. Several operational parameters have been investigated: flow rate, presence of oxygen in the perfusion fluid, and the working potential. The selectivity of the assay toward quinones was confirmed by studying possible interfering species such as ascorbate, hydrogen peroxide, riboflavin, and thiols. The results on freely moving rats have shown that the endogenous quinone amount was directly related to dopamine concentrations. The latter was determined by HPLC from dialysate samples collected at the outlet of the on-line system. HPLC studies showed that the primary quinone, generated from dopamine by bulk electrolysis, was also found in dialysates from ischemic brain.     

Quantitative Raman reaction monitoring using the solvent as internal standard

Despite its potential, the use of Raman spectroscopy for real-time quantitative reaction monitoring is still rather limited. The problems of fluorescence, laser instability, low intensities, and the inner filter effect often outscore the advantages as narrow bands, the use of glass fibers, and low scattering of water and glass. In this paper, we present real-time quantitative monitoring of the catalyzed Heck reaction by using the solvent as internal standard. In this way, all multiplicative distortions, e.g., laser intensity variations or absorbance of the laser light, can be corrected for. We also show that a limited amount of fluorescence does not hamper the analysis. Finally, we present a new method to correct for the inner filter effect, i.e., the absorbance of Raman scattered light by the reaction medium. Simultaneous absorption measurements of the reaction mixture enable accurate correction of Raman signals for the inner filter effect. Thus, for reaction monitoring applications, a Raman spectrometer should be equipped with an absorbance measurement device.     

Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.     

Ultrasonic trapping of microparticles in suspension and reaction monitoring using Raman microspectroscopy

An ultrasonic standing wave around 2 MHz has been used for trapping and concentration of suspended micrometer-size particles in a flow cell, whereas Raman microspectroscopy was used as a nondestructive technique to provide molecular information about the trapped particles. With this approach, detection and discrimination of different polymer microparticles based on their characteristic Raman spectra was performed. Dextran, poly(vinyl alcohol), and melamine resin-based beads, with and without functionalization, were used for this purpose. Furthermore, taking advantage of the flow-through characteristics of the cell and the versatility of the employed flow system, full control over the media surrounding the trapped particles was achieved. This allowed us to perform chemical reactions on the trapped particles and to monitor spectral changes in real time. Here retention of cation-exchanger beads loaded with silver ions and subsequent reduction of the silver ions was demonstrated. In this way, surface-enhanced Raman (SER) active beads were prepared and retained in the focus of the Raman microscope by means of the ultrasonic field. Injection of analytes in the flow system thus allowed recording of their SER spectra. Using 9-aminoacridine, a linear dependence of the found SER signal in the range from 1 to 10 microM has been achieved. The repeatability in the recorded SER intensities was on the order of 4-5%. This included bead retention, surface-enhanced Raman layer synthesis, and analyte detection.     

液相等离子沉积非晶态碳膜的试验研究

应用简单的电镀槽，通过阴极/电解液界面前沿气泡鞘的形成而建立辉光放电，促成非晶态碳薄膜在电极表面的沉积。沉积物宏观上分布不均匀，在阴极浸入电解液的末端，由于电流的尖端效应，导致生成物为石墨。而远离末端为正常沉积区，表面平整光滑，生成物为非。晶态碳膜。结论：液相等离子沉积行为可发生在辉光放电条件下的阴极过程，从而可应用于非晶态薄膜如非晶态碳的制备。