Accurate wavelength calibration in the near-infrared for multielement analysis without the need for reference spectra

A new technique has been developed for an instant, precise, and accurate wavelength calibration over a wide pixel array for simultaneous, multielement spectral analysis based on an inverse numerical solution to the grating dispersion function. This technique is applicable to multielement analytical applications such as laser-induced breakdown spectroscopy (LIBS), particularly when using high-density gratings in the upper visible and in the near-infrared, where nonmetallic elements are detected. This application overcomes the need to use reference spectra for each window of observation and is tested on a commercially available LIBS instrument.     

Effect of topography on average surface albedo in the ultraviolet wavelength range

The reflectivity of the 22 km x 24 km region surrounding Sonnblick Observatory near Salzburg, Austria (3104-m altitude, 47.05 degrees N, 12.95 degrees E), was calculated with a three-dimensional albedo model. The average albedo of the region was calculated at 305 and 380 nm for different solar zenith angles, ground reflectances, and solar azimuth angles. To determine geometrical effects, we first carried out the simulations without taking account of the effects of the atmosphere. The ratio to the reflectivity of a corresponding flat surface area (area with the same ground characteristics) was always less than 1 and showed a decrease with increasing solar zenith angle and with diminishing ground reflectance. Even when the ground reflectance was 100%, the average albedo was less than 1. The effect of the atmosphere was then taken into consideration in these calculations and was found to diminish the reflected components. This diminishing effect was compensated for, however, by the scattered irradiance. Finally, simulations of real conditions (nonhomogeneous ground reflectivities) were performed for different snow lines in the Sonnblick region. The average albedos obtained when all the surroundings were covered with snow were 0.32-0.63 with low solar zenith angles and 0.38-0.77 with a 40 degrees solar zenith angle.     

Reflective performance of Ir film in vacuum ultraviolet wavelength region

This paper theoretically and experimentally investigates the reflective performance of Ir films in the vacuum ultraviolet wavelength region. Ir reflecting layers of different thicknesses on various substrates are calculated and fabricated by the ion-beam-sputtering technique. Their reflectance in the 115 nm to 140 nm wavelength region was measured continuously by a reflectometer located at the National Synchrotron Radiation Laboratory. The testing results show that the reflectance of Si substrates is quite sensitive to the deposited Ir layer thickness, while the reflectance on a quartz or a BK7 glass substrates is higher. The energy of the sputtering ion beam exerts a significant influence on the reflectance of the layer, and the postannealing can cause a substantial decrease in the reflectance. For normal incidence, the reflectance of an Ir film on BK7 glass can reach as high as 30%.     

Reflectance spectra of extreme ultraviolet mirrors for synchrotron radiation

Reflectance spectra have been measured for many materials in the wavelength range from 90 Å to 400 Å. The angle of incidence ranged from 20° to 85°. The samples were chemical-vapor-deposited silicon carbide (CVD-SiC), a single crystal of silicon with (100) surface, fused quartz, zerodur, pyrex, gold, platinum, copper and two kinds of steel. To obtain reflectances for surfaces for practical use, measurements were made on surfaces sufficiently exposed to air. Reflectances for s-polarized light (R_s) were measured for all samples. On CVD-SiC, gold and platinum, reflectances for p-polarized light (R_P) were also observed. At short wavelengths and large angles of incidence, the difference between R_s and R_P is small, and R_P happened to exceed R_s in contrast to the result from the Fresnel equation. This may be due to the deviation of the present surfaces of mirrors from an ideal plane. Reflectance spectra of surfaces prepared by some feasible techniques are also presented.     

Micrograting design to retrieve IR spectra in the long wavelength region

To design microgratings, which can be used for correlation spectroscopy in the 725–1450 cm^?1 range, a gradient searching Davidon–Fletcher–Powell (DFP) algorithm was successfully applied to retrieve target spectra of SF₆, ethylene, SO₂, and NH₃. Owing to a unique fit parameter treatment, the DFP algorithm produced stable convergence for the minimization and provided superior matches between the diffraction spectrum and the target spectrum to those provided by the phase retrieval algorithm (PRA) or the extended PRA. In addition, the diffraction efficiencies for the micrograting line profiles obtained from the DFP algorithm were at least an order of magnitude greater than those from the PRA or the extended PRA. We also evaluated the effects of number of line element reduction or the dimensional errors in the line elements on the spectrum shape. The line depth error would not change the main spectral features but increase noise features. However, the line width error affects more sensitively the spectrum shape and the diffraction efficiency.     

Characterization of Raman spectra measured in vivo for the detection of cervical dysplasia

Raman spectroscopy has been shown to have the potential for providing differential diagnosis in the cervix with high sensitivity and specificity in previous studies. The research presented here further evaluates the potential of near-infrared Raman spectroscopy to detect cervical dysplasia in a clinical setting. Using a portable system, Raman spectra were collected from the cervix of 79 patients using clinically feasible integration times (5 seconds on most patients). Multiple Raman measurements were taken from colposcopically normal and abnormal areas prior to the excision of tissue. Data were processed to extract Raman spectra from measured signal, which includes fluorescence and noise. The resulting spectra were correlated with the corresponding histopathologic diagnosis to determine empirical differences between different diagnostic categories. Using histology as the gold standard, logistic regression discrimination algorithms were developed to distinguish between normal ectocervix, squamous metaplasia, and high-grade dysplasia using independent training and validation sets of data. An unbiased estimate of the accuracy of the model indicates that Raman spectroscopy can distinguish between high-grade dysplasia and benign tissue with sensitivity of 89% and specificity of 81%, while colposcopy in expert hands was able to discriminate with a sensitivity and specificity of 87% and 72%.     

New indicator for optimal preprocessing and wavelength selection of near-infrared spectra

Preprocessing of near-infrared spectra to remove unwanted, i.e., non-related spectral variation and selection of informative wavelengths is considered to be a crucial step prior to the construction of a quantitative calibration model. The standard methodology when comparing various preprocessing techniques and selecting different wavelengths is to compare prediction statistics computed with an independent set of data not used to make the actual calibration model. When the errors of reference value are large, no such values are available at all, or only a limited number of samples are available, other methods exist to evaluate the preprocessing method and wavelength selection. In this work we present a new indicator (SE) that only requires blank sample spectra, i.e., spectra of samples that are mixtures of the interfering constituents (everything except the analyte), a pure analyte spectrum, or alternatively, a sample spectrum where the analyte is present. The indicator is based on computing the net analyte signal of the analyte and the total error, i.e., instrumental noise and bias. By comparing the indicator values when different preprocessing techniques and wavelength selections are applied to the spectra, the optimal preprocessing technique and the optimal wavelength selection can be determined without knowledge of reference values, i.e., it minimizes the non-related spectral variation. The SE indicator is compared to two other indicators that also use net analyte signal computations. To demonstrate the feasibility of the SE indicator, two near-infrared spectral data sets from the pharmaceutical industry were used, i.e., diffuse reflectance spectra of powder samples and transmission spectra of tablets. Especially in pharmaceutical spectroscopic applications, it is expected beforehand that the non-related spectral variation is rather large and it is important to remove it. The indicator gave excellent results with respect to wavelength selection and optimal preprocessing. The SE indicator performs better than the two other indicators, and it is also applicable to other situations where the Beer-Lambert law is valid.     

Appearance of molecular hydrogen in the ultraviolet spectra of diamond-like carbon

Ultraviolet Raman scattering is used to study the state of hydrogen in diamond-like α-C:H. Scattering lines are observed corresponding to transitions between the rotational states of the hydrogen molecule. The interaction with the α-C:H matrix increases the internuclear distance and perturbs the electronic-vibrational terms of the H₂ molecule, so that resonance Raman scattering can be observed, excited by 10.04 eV photons. Pis’ma Zh. Tekh. Fiz. 23, 19–24 (June 26, 1997)     

Hidden isosbestic point(s) in ultraviolet spectra

The existence of isosbestic point(s) in a set of UV-visible spectra is rarely exploited for quantitative or qualitative information despite its interest. Indeed, the presence of isosbestic points means a quality and quantity conservation in the global composition of samples with a given relation between the concentration of absorbing compounds or mixtures of compounds. However, in some cases, for example, when a dilution occurs, no isosbestic point appears. This work shows that a simple operation, called normalization, can reveal the existence of hidden isosbestic point(s) (IP*). This paper first summarizes the required conditions for the presence of isosbestic points. Second, it demonstrates the significance of IP* revealed by normalization. An example of the exploitation of direct or hidden isosbestic points in the environmental field illustrates the interest in and application of the technique.     

Study of the peak shape in alpha spectra measured by liquid scintillation

Liquid-scintillation counting allows the measurement of alpha and beta activities jointly or only of the alpha-emitting nuclides in a sample. Although the resolution of the alpha spectra is poorer than that attained with semiconductor detectors, it is still an attractive alternative. We describe here attempts to fit a peak shape to experimental liquid-scintillation alpha spectra and discuss the parameters affecting this shape, such as the PSA (pulse-shape analyser) level, vial type, shaking the sample, etc. Spectral analysis has been applied for complex alpha spectra.     

Multi-functional apparatus for the measurement of wavelength and the spectra of light sources over a broad spectral range

A multifunctional apparatus used for the study and certification of the wavelength of light sources is described. The wavelength range of interest is from O.5– 4.0 pm. The instrument is capable of characterizing the radiation from spectral lamps, as well as gas and semiconductor lasers. An error analysis is presented for the measurement of wavelength using a Fabry-Perot interferometer.Translated from Izmeritel'naya Tekhnika, No. 3, pp. 29–31, March, 1995.     

Diffraction efficiency of 200-nm-period critical-angle transmission gratings in the soft x-ray and extreme ultraviolet wavelength bands

We report on measurements of the diffraction efficiency of 200-nm-period freestanding blazed transmission gratings for wavelengths in the 0.96 to 19.4 nm range. These critical-angle transmission (CAT) gratings achieve highly efficient blazing over a broad band via total external reflection off the sidewalls of smooth, tens of nanometer thin ultrahigh aspect-ratio silicon grating bars and thus combine the advantages of blazed x-ray reflection gratings with those of more conventional x-ray transmission gratings. Prototype gratings with maximum depths of 3.2 and 6 μm were investigated at two different blaze angles. In these initial CAT gratings the grating bars are monolithically connected to a cross support mesh that only leaves less than half of the grating area unobstructed. Because of our initial fabrication approach, the support mesh bars feature a strongly trapezoidal cross section that leads to varying CAT grating depths and partial absorption of diffracted orders. While theory predicts broadband absolute diffraction efficiencies as high as 60% for ideal CAT gratings without a support mesh, experimental results show efficiencies in the range of ～50-100% of theoretical predictions when taking the effects of the support mesh into account. Future minimization of the support mesh therefore promises broadband CAT grating absolute diffraction efficiencies of 50% or higher.     

Influence of binding layer on the reflective performance of a Au film in vacuum ultraviolet wavelength region

We investigate the influence of the binding layer on the reflectance of a Au film in vacuum ultraviolet (VUV) wavelength region theoretically and experimentally. The reflectance of Au films on quartz glass substrates with an approximately 2 nm binding layer of Ti, Cr, and Ir are estimated and fabricated. Their reflectance in the 115-140 nm wavelength region are measured continuously by the reflectometer located in the National Synchroton Radiation Laboratory. The testing results show that the addition of the binding layer indeed greatly enhances the interfacial adhesion of the Au layer to the quartz glass substrate, but it also exerts a considerably adverse impact on the reflectance of the Au layer in VUV wavelength region. In near normal incidence, the reflectance of the Au layer with a 2 nm thick binding layer is less than 20%, approximately 5% lower than those without the binding layer. The material used for the binding layer has little impact on the reflectance if this layer is thin enough.     

Method to determine snow albedo values in the ultraviolet for radiative transfer modeling

For many cases modeled and measured UV global irradiances agree to within +/-5% for cloudless conditions, provided that all relevant parameters for describing the atmosphere and the surface are well known. However, for conditions with snow-covered surfaces this agreement is usually not achievable, because on the one hand the regional albedo, which has to be used in a model, is only rarely available and on the other hand UV irradiance alters with different snow cover of the surface by as much as 50%. Therefore a method is given to determine the regional albedo values for conditions with snow cover by use of a parameterization on the basis of snow depth and snow age, routinely monitored by the weather services. An algorithm is evolved by multiple linear regression between the snow data and snow-albedo values in the UV, which are determined from a best fit of modeled and measured UV irradiances for an alpine site in Europe. The resulting regional albedo values in the case of snow are in the 0.18-0.5 range. Since the constants of the regression depend on the area conditions, they have to be adapted if the method is applied for other sites. Using the algorithm for actual cases with different snow conditions improves the accuracy of modeled UV irradiances considerably. Compared with the use of an average, constant snow albedo, the use of actual albedo values, provided by the algorithm, halves the average deviations between measured and modeled UV global irradiances.     

Method for integrating the absorption cross sections of spheres over wavelength or diameter

The absorption cross sections of spherical particles and droplets must be integrated over frequency or droplet size or both for various applications. Morphology-dependent resonances (MDRs) of the spheres can make evaluation of such integrals difficult because the MDRs can contribute significantly to the integrals even when their linewidths are extremely narrow, especially when the absorption is weak. A method of evaluating these integrals by use of Lorentzian approximations near MDRs is described. Calculated frequency-integrated absorption cross sections illustrate how the method obtains accurate cross sections with far fewer integration points than a method that uses equally spaced points. The method reported here suggests a way to integrate over frequency in more-complicated scattering and emission problems and should also be useful for integrating scattering and absorption by other shapes, e.g., spheroids and cylinders, for which the MDR positions and linewidths can be calculated.     

Vacuum ultraviolet photoionization mass spectra and cross-sections for volatile organic compounds at 10.5 eV

Vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) has been applied to the detection of volatile organic compounds (VOCs), including aromatic, chlorinated, and oxygenated compounds. Photoionization mass spectra of 23 VOCs were measured using SPI-TOFMS at 10.5 eV (118 nm). The limits of detection of VOCs using SPI-TOFMS at 10.5 eV were estimated to be a few ppbv. The mass spectra of 20 VOCs exhibit only the parent ion and its isotopes' signals. The ionization processes of the VOCs were discussed on the basis of the reaction enthalpies predicted by the quantum chemical calculations. Absolute photoionization cross-sections for 23 VOCs, including 12 newly measured VOCs, at 10.5 eV were determined in comparison to the reported absolute photoionization cross-section of NO.