Determination of the resolution of a multichannel Raman spectrometer using Fourier transform Raman spectra

The resolution of a grating polychromator for Raman spectroscopy has been simulated by measuring spectra on a Fourier transform (FT) Raman spectrometer and selecting the FT of the apodization function so that the instrument line shape function mimics the triangular spectral slit function of the polychromator. To this end, FT-Raman spectra measured with a nominal resolution of 0.5 cm-1 were modified through the application of sinc2 apodization functions of various widths to simulate spectra measured on a polychromator at lower resolution. The success of this approach was validated using the 1085 cm-1 band of calcite. When the modified FT-Raman spectra were compared with spectra measured on a grating polychromator equipped with slits of widths 100 and 150 microns, the resolution of the polychromator was estimated to be 6.3 and 7.8 cm-1, respectively. This conclusion was verified experimentally by measuring the separation of two bands in the Raman spectrum of BaSO4 at approximately 460 cm-1.     

Performance enhancements of compound semiconductor radiation detectors using digital pulse processing techniques

The potential benefits of using compound semiconductors for X-ray and gamma ray spectroscopy are already well known. Radiation detectors based on high atomic number and wide band gap compound semiconductors show high detection efficiency and good spectroscopic performance even at room temperature. Despite these appealing properties, incomplete charge collection is a critical issue. Generally, incomplete charge collection, mainly due to the poor transport properties of the holes, produces energy resolution worsening and the well known hole tailing in the measured spectra. In this work, we present a digital pulse processing (DPP) system for high resolution spectroscopy with compound semiconductor radiation detectors. The DPP method, implemented on a PC platform, performs a height and shape analysis of the detector pulses (preamplifier output pulses), digitized by a 14-bit, 100 MHz ADC. Fast and slow shaping, automatic pole-zero adjustment, baseline restoration and pile-up rejection allow precise pulse height measurements both at low and high counting rate environments. Pulse shape analysis techniques (pulse shape discrimination, linear and nonlinear pulse shape corrections) to compensate for incomplete charge collection were also implemented. The results of spectroscopic measurements on a planar CdTe detector show the high potentialities of the system, obtaining low tailing in the measured spectra and energy resolution quite close to the theoretical limit. High-rate measurements (up to 820 kcps) exhibit the excellent performance of the pulse height analysis and the benefits of pulse shape techniques for peak pile-up reduction in the measured spectra. This work was carried out in the framework of the development of portable X-ray spectrometers for both laboratory research and medical applications.     

Real-time fourier transform-infrared analysis of carbon monoxide and nitric oxide in sidestream cigarette smoke

Fourier transform infrared (FT-IR) spectroscopy was compared directly to independent standard analytical techniques for the routine measurement of carbon monoxide (CO) and nitric oxide (NO) yields from cigarette sidestream smoke. The FT-IR instrument was configured in-line with a nondispersive infrared (NDIR) analyzer for CO analysis and a chemiluminescence (CL) analyzer for NO analysis to monitor the sidestream smoke from a single port of a linear smoking machine. A cold trap was inserted prior to the FT-IR to minimize the levels of vapor phase interferents, such as water. Univariate and multivariate regression analysis were evaluated for the prediction of cigarette yield from time-resolved spectral data at 1, 2, 4 and 8 cm-1 spectral resolution. Regressions were developed using three different spectral ranges including unique rotation-vibration lines, the R-branch, and the entire absorption band. As per standard methods, yields were calculated from the concentration traces generated during the smoke runs for five different cigarettes spanning the expected range of mainstream total particulate matter deliveries. The FT-IR traces for the smoke runs revealed improved temporal resolution yielding analytical information from smoke generated in between puffs. The performance between the validation methods and the FT-IR calibrations was statistically compared. In general, for the determination of CO, the FT-IR calibrations underestimated the yield measured by NDIR by less than 10%. For the NO measurement, the univariate FT-IR calibrations overestimated the NO yield measured by the CL analyzer, whereas the partial least squares (PLS) calibrations showed good agreement. PLS calibrations were developed for both analytes providing no significant difference when compared to the respective standard analytical techniques. Results for sidestream CO and NO yields for Kentucky reference cigarette 1R4F utilizing 8 cm-1 calibrations compared favorably to values reported elsewhere in the literature. Hence, calibration of the FT-IR system at 8 cm-1 spectral resolution clearly revealed the potential of this method, providing enhanced temporal resolution, simultaneous determination of several smoke components, and reduced complexity of the experimental setup in contrast to the standard techniques.     

Measurement of the Am–Li radionuclide neutron source spectrum

The neutron energy spectrum of an ²⁴¹Am–Li radionuclide source has been measured in a low scatter room at the National Physical Laboratory, using a variety of spectrometers namely: a Bonner sphere set, a ³He ionisation chamber, and a set of proton recoil proportional counters. Calculations with the Monte Carlo multi-particle transport code MCNP have been used to explain some of the features seen in the spectrum. The data have been analysed using different unfolding techniques, and a final spectrum has been derived based on all the available measurements.     

Rapid limit tests for metal impurities in pharmaceutical materials by X-ray fluorescence spectroscopy using wavelet transform filtering

We introduce a new method for analysis of X-ray fluorescence (XRF) spectra based on continuous wavelet transform filters, and the method is applied to the determination of toxic metals in pharmaceutical materials using hand-held XRF spectrometers. The method uses the continuous wavelet transform to filter the signal and noise components of the spectrum. We present a limit test that compares the wavelet domain signal-to-noise ratios at the energies of the elements of interest to an empirically determined signal-to-noise decision threshold. The limit test is advantageous because it does not require the user to measure calibration samples prior to measurement, though system suitability tests are still recommended. The limit test was evaluated in a collaborative study that involved five different hand-held XRF spectrometers used by multiple analysts in six separate laboratories across the United States. In total, more than 1200 measurements were performed. The detection limits estimated for arsenic, lead, mercury, and chromium were 8, 14, 20, and 150 μg/g, respectively.     

Temperature-dependent near-infrared spectra of bovine serum albumin in aqueous solutions: spectral analysis by principal component analysis and evolving factor analysis

Fourier transform near-infrared (FT-NIR) spectra have been measured for bovine serum albumin (BSA) in an aqueous solution (pH 6.8) with a concentration of 5.0 wt% over a temperature range of 45-85 degrees C. Not only conventional spectral analysis methods, such as second-derivative spectra and difference spectra, but also chemometrics, such as principal component analysis (PCA) and evolving factor analysis (EFA), have been employed to analyze the temperature-dependent NIR spectra in the 7500-5500 and 4900-4200 cm-1 regions of the BSA aqueous solution. Intensity changes of bands in the 7200-6600 cm-1 and 4650-4500 cm-1 regions in the difference spectra indicate variations of the hydration and secondary structure of BSA in the aqueous solution, respectively. The plot of a band intensity at 7080 cm-1 in the different spectra shows a clear turning point at 63 degrees C, revealing that a significant change in the hydration occurs at about 63 degrees C. The forward and backward eigenvalues (EVs) from EFA suggest that marked changes in the hydration and secondary structure of BSA take place in the temperature ranges of 61-65 degrees C and 59-63 degrees C, respectively. In addition, the temperature of 71 degrees C marked in the EFA plots may correspond to the onset temperature of increase in the intermolecular beta-sheet structure.     

Monitoring uranium, hydrogen, and lithium and their isotopes using a compact laser-induced breakdown spectroscopy (LIBS) probe and high-resolution spectrometer

The development of field-deployable instruments to monitor radiological, nuclear, and explosive (RNE) threats is of current interest for a number of assessment needs such as the on-site screening of suspect facilities and nuclear forensics. The presence of uranium and plutonium and radiological materials can be determined through monitoring the elemental emission spectrum using relatively low-resolution spectrometers. In addition, uranium compounds, explosives, and chemicals used in nuclear fuel processing (e.g., tributyl-phosphate) can be identified by applying chemometric analysis to the laser-induced breakdown (LIBS) spectrum recorded by these spectrometers. For nuclear forensic applications, however, isotopes of U and Pu and other elements (e.g., H and Li) must also be determined, requiring higher resolution spectrometers given the small magnitude of the isotope shifts for some of these elements (e.g., 25 pm for U and 13 pm for Pu). High-resolution spectrometers will be preferred for several reasons but these must fit into realistic field-based analysis scenarios. To address the need for field instrumentation, we evaluated a previously developed field-deployable hand-held LIBS interrogation probe combined with two relatively new high-resolution spectrometers (λ/Δλ ~75,000 and ~44,000) that have the potential to meet field-based analysis needs. These spectrometers are significantly smaller and lighter in weight than those previously used for isotopic analysis and one unit can provide simultaneous wide spectral coverage and high resolution in a relatively small package. The LIBS interrogation probe was developed initially for use with low resolution compact spectrometers in a person-portable backpack LIBS instrument. Here we present the results of an evaluation of the LIBS probe combined with a high-resolution spectrometer and demonstrate rapid detection of isotopes of uranium and hydrogen and highly enriched samples of (6)Li and (7)Li.     

Neutron transmission measurements of zinc and lead single crystals

Neutron transmission measurements of zinc and lead single crystals have been carried out in a neutron wavelength band from 0.03 to 0.55 nm at different orientations of the crystal with regard to the beam direction. The measurements were performed using both time-of-flight and fixed-angle scattering spectrometers installed in front of the ET-RR-1 reactor horizontal channels. It was found that the position of the observed dips in the neutron transmission measurements corresponded to the reflections from the (hkl) planes of the hexagonal zinc single crystal which was cut along the (002) plane, while in the case of lead, the single crystal was cut perpendicular to the (311) plane. The reflectivity from the (002) plane of zinc was determined using both transmission and reflection methods. The maximum reflectivity was found to be 55% when the zinc crystal was orientated at 45° to the beam direction. The wavelength spread of the observed reflectivity curve was found to be in agreement with the calculated one, taking into consideration the spectrometer's resolution and the crystal mosaic spread.     

Observation of Fourier transform near-infrared vibrational circular dichroism to 6150 cm-1

We report here the first measurement of near-infrared (NIR) vibrational circular dichroism (VCD) with Fourier transform (FT) instrumentation. The measurements were carried out using a commercial rapid-scan FT-IR VCD spectrometer modified for operation in the NIR region between 3750 and 6150 cm-1. Overtone and combination band VCD intensities, first reported by Keiderling and Stephens using dispersive VCD instrumentation in 1976, were reproduced with equivalent signal quality and spectral resolution using collection times of only a few minutes. These results offer substantial promise for the routine use of rapid-scan FT instrumentation for the measurement of overtone and combination band VCD spectra in the NIR region.     

Retrieval of trace gases from aerosol-influenced infrared transmission spectra observed by low-spectral-resolution Fourier-transform spectrometers

A method for the simultaneous retrieval of gas concentrations and an extinction spectrum of aerosols and polar stratospheric clouds from infrared transmission spectra observed in the solar occultation geometry is described. It is particularly suited to measurements by Fourier-transform spectrometers with relatively low spectral resolution (0.1-1 cm(-1)). The method does not require a priori assumptions on aerosol properties; it utilizes only the fact that the wave-number dependence of aerosol extinction is much weaker than that of gas absorption. In this method, an aerosol extinction spectrum is approximated by a straight line within a relatively wide spectral range defined as mediumwindow.     

Measurement of the energy spectrum of cosmic-ray induced neutrons aboard an ER-2 high-altitude airplane

Crews working on present-day jet aircraft are a large occupationally exposed group with a relatively high average effective dose from galactic cosmic radiation. Crews of future high-speed commercial aircraft flying at higher altitudes would be even more exposed. To help reduce the significant uncertainties in calculations of such exposures, the atmospheric ionizing radiation (AIR) project, an international collaboration of 15 laboratories, made simultaneous radiation measurements with 14 instruments on five flights of a NASA ER-2 high-altitude aircraft. The primary AIR instrument was a highly sensitive extended-energy multisphere neutron spectrometer with lead and steel shells placed within the moderators of two of its 14 detectors to enhance response at high energies. Detector responses were calculated for neutrons and charged hadrons at energies up to 100 GeV using MCNPX. Neutron spectra were unfolded from the measured count rates using the new MAXED code. We have measured the cosmic-ray neutron spectrum (thermal to >10 GeV), total neutron fluence rate, and neutron effective dose and dose equivalent rates and their dependence on altitude and geomagnetic cutoff. The measured cosmic-ray neutron spectra have almost no thermal neutrons, a large "evaporation" peak near 1 MeV and a second broad peak near 100 MeV which contributes about 69% of the neutron effective dose. At high altitude, geomagnetic latitude has very little effect on the shape of the spectrum, but it is the dominant variable affecting neutron fluence rate, which was eight times higher at the northernmost measurement location than it was at the southernmost. The shape of the spectrum varied only slightly with altitude from 21 km down to 12 km (56-201 g cm-2 atmospheric depth), but was significantly different on the ground. In all cases, ambient dose equivalent was greater than effective dose for cosmic-ray neutrons.     

Spectral calibration requirement for Earth-looking imaging spectrometers in the solar-reflected spectrum

Earth-looking imaging spectrometers operating in the solar-reflected spectrum measure spectra of the total upwelling radiance for each spatial element in an image. These measurements are used to derive physical parameters of the Earth's surface and atmosphere from the energy, molecular absorption, and constituent scattering characteristics expressed in each spectrum. To achieve these quantitative objectives, the measured spectra must be spectrally, radiometrically, and spatially calibrated. The ubiquitous presence of numerous, strong, narrow atmosphere and solar absorptions in the upwelling spectral radiance in conjunction with the narrow spectral channels of imaging spectrometers forms the basis for a general spectral calibration requirement. In order to determine the requirement for spectral calibration accuracy, a sensitivity analysis has been completed for imaging spectrometers with contiguously sampled spectral channel response functions of 5, 10, and 20 nm full width at half-maximum from 400 to 2500 nm. This sensitivity analysis shows that spectral calibration errors of 10% and 5% cause significant, spectrally distinct errors in the measured radiance throughout the solar-reflected spectrum. These errors result from the sensitivity of the measured radiance to the exact convolution of the narrow channels of imaging spectrometers with the upwelling spectral radiance that contains narrow atmosphere and solar absorptions. These errors are systematic and add directly to the radiometric calibration uncertainty for every spectrum in the image. This analysis establishes that a spectral calibration accuracy approaching 1% of the full width at half-maximum throughput of the spectral response function for both spectral channel position and shape is necessary to suppress these errors in the measured radiance spectrum.     

Stability test for amorphous materials in humidity controlled 96-well plates by near-infrared spectroscopy

The purpose of this research is to apply near infrared spectrometry (NIR) with chemoinformetrics to predict the change of crystalline properties of indomethacin (IMC) amorphous under various levels of relative humidity storage conditions. Stability test for amorphous and meta-stable polymorphic forms was performed in humidity controlled the modified 96-well quartz plates containing various kinds of saturated salt solutions (0-100% of relative humidity (RH)) by NIR spectroscopy. Amorphous form was obtained melt product to pour into liquid nitrogen and after then ground. Samples were stored at 25°C in the 6-well plates at various levels of RH. The spectra of the powder samples were measured by the reflectance FT-NIR spectrometer. The second derivative spectra of form α showed specific absorption peaks at 4980, 6036, 7296 and 8616 cm-1 and that of form γ showed those at 5020, 5028, 7344, 7428 and 8436 cm-1. After storage at less than 50% RH, the peak intensities at 5020, 5028, 7344, 7428 and 8436 cm-1 of the amorphous solid increased with increasing of storage time. However, the peak intensity at 4980, 6036 and 7296 cm-1 increased at more than 50% RH Please check and confirm the edit. The results suggested that at lower humidity, the IMC amorphous solid transformed into form γ, but it transformed into form α at more than high humidity. It is possible that crystalline stability of the pharmaceutical preparations could be predicted by using humidity controlled 96-well plates and reflectance NIR-chemoinformetric methods.     

Two types of multi-moderator neutron spectrometers: Gamma-ray insensitive type and high-efficiency type

Two types of multi-moderator neutron spectrometers were developed; one is a gamma-ray insensitive type, and the other is a high-efficiency type. An indium activation detector is loaded in the former spectrometer, which can measure the photon-dominant pulsed neutron field such as in the primary photon beam of a high-energy medical electron accelerator. The latter, in which a ³He counter is loaded, is so sensitive that it can measure leakage neutrons from a well shielded facility or even the skyshine neutrons. The response functions of the spectrometers were measured by thermal and mono-energetic neutron standard fields, and were also calculated by the one-dimensional discrete ordinates transport code, ANISN. The measured and calculated responses showed generally good agreement. A benchmark measurement of ²⁵²Cf fission neutrons by using these two spectrometers agreed well with the calculated spectrum. The spectrometers were used in the measurements of neutrons produced by a medical electron accelerator and of skyshine neutrons from an intense 14 MeV neutron source facility.     

Prediction of ethylene content in melt-state random and block polypropylene by near-infrared spectroscopy and chemometrics: influence of a change in sample temperature and its compensation method

This paper reports on the influence of a change in sample temperature, and a method for its compensation, for the prediction of ethylene (C2) content in melt-state random polypropylene (RPP) and block polypropylene (BPP) by near-infrared (NIR) spectroscopy and chemometrics. Near-infrared (NIR) spectra of RPP in the melt and solid states were measured by a Fourier transform near-infrared (FT-NIR) on-line monitoring system and an FT-NIR laboratory system. There are some significant differences between the solid and melt-state RPP spectra. Moreover, we investigated the predicted values of the C2 content from the RPP or BPP spectra measured at 190 degrees C and 250 degrees C using the calibration model for the C2 content developed using the RPP or BPP spectra measured at 230 degrees C. The errors in the predicted values of the C2 content depend on the pretreatment methods for each calibration model. It was found that multiplicative signal correction (MSC) is very effective in compensating for the influence of the change of temperature for the RPP or BPP samples on the predicted C2 content. From the suggestion of principal component analysis (PCA) and difference spectrum analysis, we propose a new compensation method for the temperature change that uses the difference spectra between two spectra sets measured at different temperatures. We achieved good results using the difference spectra between the RPP/BPP spectra sets measured at 190 degrees C and 250 degrees C after correction and the calibration model developed with the spectra measured at 230 degrees C. The comparison between the method using MSC and the proposed method showed that the predicted error in the latter is slightly better than those in the former.     

Multivariate calibration and instrument standardization for the rapid detection of diethylene glycol in glycerin by Raman spectroscopy

The transfer of a multivariate calibration model for quantitative determination of diethylene glycol (DEG) contaminant in pharmaceutical-grade glycerin between five portable Raman spectrometers was accomplished using piecewise direct standardization (PDS). The calibration set was developed using a multi-range ternary mixture design with successively reduced impurity concentration ranges. It was found that optimal selection of calibration transfer standards using the Kennard-Stone algorithm also required application of the algorithm to multiple successively reduced impurity concentration ranges. Partial least squares (PLS) calibration models were developed using the calibration set measured independently on each of the five spectrometers. The performance of the models was evaluated based on the root mean square error of prediction (RMSEP), calculated using independent validation samples. An F-test showed that no statistical differences in the variances were observed between models developed on different instruments. Direct cross-instrument prediction without standardization was performed between a single primary instrument and each of the four secondary instruments to evaluate the robustness of the primary instrument calibration model. Significant increases in the RMSEP values for the secondary instruments were observed due to instrument variability. Application of piecewise direct standardization using the optimal calibration transfer subset resulted in the lowest values of RMSEP for the secondary instruments. Using the optimal calibration transfer subset, an optimized calibration model was developed using a subset of the original calibration set, resulting in a DEG detection limit of 0.32% across all five instruments.     

Fast neutron spectrometry with organic scintillators applied to magnetic fusion experiments

Neutron spectrometry with NE213 liquid scintillators is commonly used in thermonuclear fusion experiments to measure the 2.45 and 14.1 MeV neutron flux. We present the unfolded neutron spectrum, which was accumulated during several ohmic deuterium plasma discharges in the Frascati Tokamak Upgrade using a 2″×2″ NE213 scintillator. In this paper, we review the application of organic scintillator neutron spectrometers to tokamaks, focusing in particular on the comparison between NE213 and stilbene scintillators. Various aspects of the calibration technique and neutron spectra unfolding procedure are considered in the context of their application for fusion neutron spectrometry. Testing and calibration measurements have been carried out using D–D and D–T neutron generator facilities with both NE213 and stilbene scintillators. The main result from these measurements is that stilbene scintillator has better neutron energy resolution than NE213. Our stilbene detector could be used for the determination of the ion temperature (T_i) from neutron spectrum broadening in tokamak thermonuclear plasmas with T_i=4 keV and higher.     

Optimum sensors for color constancy in scenes illuminated by daylight

The apparent color of an object within a scene depends on the spectrum of the light illuminating the object. However, recording an object's color independent of the illuminant spectrum is important in many machine vision applications. In this paper the performance of a blackbody-model-based color constancy algorithm that requires four sensors with different spectral responses is investigated under daylight illumination. In this investigation sensor noise was modeled as gaussian noise, and the responses were quantized using different numbers of bits. A projection-based algorithm whose output is invariant to illuminant is investigated to improve the results that are obtained. The performance of both of these algorithms is then improved by optimizing the spectral sensitivities of the four sensors using freely available CIE standard daylight spectra and a set of lightness-normalized Munsell reflectance data. With the optimized sensors the performance of both algorithms is shown to be comparable to the human visual system. However, results obtained with measured daylight spectra show that the standard daylights may not be sufficiently representative of measured daylight for optimization with the standard daylight to lead to a reliable set of optimum sensor characteristics.     

基于不同加工工艺的微小型结构件边缘识别

针对不同加工工艺微小型结构件的不同边缘特征,提出了一种基于工艺匹配思想的微小型结构件边缘识别算法．该算法通过计算有效平均梯度,提取不同加工工艺微小型结构件的边缘过渡区,建立边缘过渡区的多项式回归模型,求导确定边缘点精确位置．通过对4种常用微细加工工艺建模分析可以看出,加工工艺对微小型结构件边缘区域影响较大,边缘精确识别时应加入工艺匹配的思想．该算法考虑了实际加工工艺的影响,算法上加入统计学方法,通过建立过渡区数学模型,使边缘检测结果达到亚像素级．     

Accurate and precise wavelength calibration for wide bandwidth array spectrometers

Calibrating the wavelength scale of an array spectrometer typically involves measurements of lines at well-known wavelengths from a calibration lamp such as a mercury-argon source. This process is relatively straightforward when the lines are well separated, relative to the bandwidth of the spectrometer. When the spectrometer's bandwidth is large, compared with the distance between calibration wavelengths, it becomes increasingly difficult to accurately locate lines in the calibration spectrum. Even calibrations for instruments with a modest bandwidth of 12 nm can be difficult. Here we present results from a simple approach to improve the accuracy of wavelength calibration for an instrument with a large bandwidth (12 nm, center-to-center pixel spacing 3.3 nm). A monochromator has been used to filter the source so that each calibration line can be measured separately. For ten spectrometers, we were able to achieve accuracy better than 0.12 nm, or 0.09 nm on average; this is less than 3% of the pixel spacing. We anticipate this approach will be useful for improving the accuracy of measurements on array spectrometers and particularly in transferring multivariate calibrations between instruments.