Showing papers in "Applied Spectroscopy in 1996"
TL;DR: In this paper, a new imaginary refractive index spectrum of water has been determined between 4000 and 700 cm-1 as the average of spectra calculated from ATR spectra recorded by four workers in their laboratory over the past seven years.
Abstract: The previously reported nonreproducibility of the intensity of the OH stretching band of liquid water has been explored. It was found that it can be eliminated in measurements with the Circle® multiple ATR cell by ensuring that the ATR rod is coaxial with the glass liquid holder. It was also found that normal laboratory temperature variations of a few degrees change the intensity by ≤~1% of the peak height. A new imaginary refractive index spectrum of water has been determined between 4000 and 700 cm-1 as the average of spectra calculated from ATR spectra recorded by four workers in our laboratory over the past seven years. It was obtained under experimental and computational conditions superior to those used previously, but is only marginally different from the spectra reported in 1989. In particular, the integrated intensities of the fundamentals are not changed significantly from those reported in 1989. The available imaginary refractive index, k, values between 15,000 and 1 cm-1 have been compared. The values that are judged to be the most reliable have been combined into a recommended k spectrum of H2O(l) at 25°C between 15,000 and 1 cm-1, from which the real refractive index spectrum has been calculated by Kramers-Kronig transformation. The recommended values of the real and imaginary refractive indices and molar absorption coefficients of liquid water at 25 ± 1 °C are presented in graphs and tables. The real and imaginary dielectric constants and the real and imaginary molar polarizabilities in this wavenumber range can be calculated from the tables. Conservatively estimated probable errors of the recommended k values are given. The precision with which the values can be measured in one laboratory and the relative errors between regions are, of course, far smaller than these probable errors. The recommended k values should be of considerable value as interim standard intensities of liquid water, which will facilitate the transfer of intensities between laboratories.
588 citations
TL;DR: In this article, a method for the quantitative analysis of gas-phase infrared spectra is described in which calibration spectra are calculated from a database of absorption line parameters rather than measured in a real spectrometer.
Abstract: A method for the quantitative analysis of gas-phase infrared spectra is described in which calibration spectra are calculated from a database of absorption line parameters rather than measured in a real spectrometer. The synthetic calibration spectra are computed with the use of the program MALT (Multiple Atmospheric Layer Transmission), including environmental (pressure, temperature, pathlength, etc.) and instrumental (resolution, line shape, wavenumber shift) effects in the calculation, so that the calculated spectra closely approximate real measured spectra. The synthetic calibration spectra are then used in quantitative analysis as if they were real spectra. In conventional laboratory studies, the method circumvents the need for time-consuming collection of large sets of laboratory calibration spectra often required when many absorbing gases must be analyzed. It is particularly useful in long open-path and solar FT-IR spectroscopy when no sample cell is available for recording calibration spectra. Examples are presented from conventional laboratory spectra in a closed-cell, open-path FT-IR spectra used to determine trace gas fluxes in an open field, and solar absorption spectroscopy using ground-based FT-IR spectrometers.
336 citations
TL;DR: A portable instrument based on laser-induced breakdown spectroscopy (LIBS) has been developed for the detection of metal contaminants on surfaces as mentioned in this paper, which consists of a sampling probe connected to the main analysis unit by electrical and optical cabling.
Abstract: A portable instrument, based on laser-induced breakdown spectroscopy (LIBS), has been developed for the detection of metal contaminants on surfaces. The instrument has a weight of 14.6 kg, fits completely into a small suitcase (46 x 33 x 24 em), and operates from 115 V ac. The instrument consists of a sampling probe connected to the main analysis unit by electrical and optical cabling. The hand-held probe contains a small laser to generate laser sparks on a surface and a fiber-optic cable to collect the spark light. The collected light is spectrally resolved and detected with the use of a compact spectrograph/CCD detector system. The instrument has been evaluated for the analysis of metals in the environment: Ba, Be, Pb, and Sr in soils; Pb in paint; and Be and Pb particles collected on filters. Detection limits in ppm for metals in soils were 265 (Ba), 9.3 (Be), 298 (Pb), and 42 (Sr). The detection limit for Pb in paint was 0.8% (8000 ppm), corresponding to 0.052 mg/cm2. The higher limit obtained for Pb in paint is attributed to the use of the 220.35-nm Pb(II) line instead of the stronger 405.78-nm Pb(I) line used for soils. Spectral interferences prevented use of the 405.78-nm line to determine Pb in paint. The surface detection limit for Be particles on filters was dependent on particle size and ranged from 21 to 63 ng/cm2. The detection limit for Pb particles on filters was 5.6 μg/cm2.
322 citations
TL;DR: In this paper, the effects of chemical speciation and matrix composition on Pb and Ba measurements have been investigated by using sand and soil matrices, and a cylindrical lens was used to focus the laser pulses on the samples because it yielded higher measurement precision than a spherical lens.
Abstract: With the use of laser-induced breakdown spectroscopy (LIBS), the effects of chemical speciation and matrix composition on Pb and Ba measurements have been investigated by using sand and soil matrices. A cylindrical lens was used to focus the laser pulses on the samples because it yielded higher measurement precision than a spherical lens for the experimental conditions used here. The detection limits for Pb and Ba spiked in a sand matrix were 17 and 76 ppm (w/w), respectively. In spiked soil, the detection limits were 57 and 42 ppm (w/w) for Pb and Ba, respectively. Measurement precision for five replicate measurements was typically 10% RSD or less. Two factors were found to influence emissions from Pb and Ba present in sand and soil matrices as crystalline compounds: (1) compound speciation, where Ba emission intensities varied in the order carbonate > oxide > sulfate > chloride > nitrate, and where Pb emission intensities varied in the order oxide > carbonate > chloride > sulfate > nitrate; and (2) the composition of the bulk sample matrix. Emissions from Ba(II) correlated inversely with the plasma electron density, which in turn was dependent upon the percent sand in a sand/soil mixture. The analytical results obtained here show that a field-screening instrument based on LIBS would be useful for the initial screening of soils contaminated with Pb and Ba.
266 citations
TL;DR: In this paper, the authors describe an approach to single-molecule detection and explore assays that can be done at the single-species level that would be difficult or impossible with bulk measurements.
Abstract: Over the past five years, several groups have developed the capability to detect and identify single fluorescent molecules in solution as the molecules flow through a focused laser beam. The history of the approach to single-molecule detection in fluid solution is shown in Fig. 1. Approximately one dozen molecular species have been detected at this level of sensitivity. Fluorescence-based, single-molecule detection techniques are expected to have a significant impact in fields where fluorescence detection and quantification are broadly applied, e.g., analytical chemistry, biology, and medicine. Single-molecule detection is a new way of doing analytical chemistry, and new applications will arise. In this article, we describe our approach to single-molecule detection and explore assays that can be done at the single-species level that would be difficult or impossible with bulk measurements.
254 citations
TL;DR: In this paper, the effect of sampling geometry on laser-induced breakdown spectroscopy (LIBS) measurements was investigated, including the type of lens (cylindrical or spherical) used to focus the laser pulse onto the sample, the focal length of the lens (75 or 150 mm), the lens-to-sample distance (LTSD), the angle-of-incidence of the laser pulses onto a sample, and the method used to collect the plasma light (lens or fiber-optic bundle).
Abstract: In laser-induced breakdown spectroscopy (LIBS), a focused laser pulse is used to ablate material from a surface and form a laser plasma that excites the vaporized material. Geometric factors, such as the distance between the sample and the focusing lens and the method of collecting the plasma light, can greatly influence the analytical results. To obtain the best quantitative results, one must consider this geometry. Here we report the results of an investigation of the effect of sampling geometry on LIBS measurements. Diagnostics include time-resolved spectroscopy and temporally and spectrally resolved imaging using an acousto-optic tunable filter (AOTF). Parameters investigated include the type of lens (cylindrical or spherical) used to focus the laser pulse onto the sample, the focal length of the lens (75 or 150 mm), the lens-to-sample distance (LTSD), the angle-of-incidence of the laser pulse onto the sample, and the method used to collect the plasma light (lens or fiber-optic bundle). From these studies, it was found that atomic emission intensities, plasma temperature, and mass of ablated material depend strongly on the LTSD for both types of lenses. For laser pulse energies above the breakdown threshold for air, these quantities exhibit symmetric behavior about an LTSD approximately equal to the back focal length for cylindrical lenses and asymmetric behavior for spherical lenses. For pulse energies below the air breakdown threshold, results obtained for both lenses display symmetric behavior. Detection limits and measurement precision for the elements Be, Cr, Cu, Mn, Pb, and Sr, determined with the use of 14 certified reference soils and stream sediments, were found to be independent of the lens used. Time-resolved images of the laser plasma show that at times >5 μs after plasma formation a cloud of emitting atoms extends significantly beyond the centrally located, visibly white, intense plasma core present at early times (<0.3 μs). It was determined that, by collecting light from the edges of the emitting cloud, one can record spectra using an ungated detector (no time resolution) that resemble closely the spectra obtained from a gated detector providing time-resolved detection. This result has implications in the development of less expensive LIBS detection systems.
216 citations
TL;DR: A Lyot-type liquid crystal tunable filter (LCTF) suitable for high-definition Raman chemical imaging has been developed in this paper, which provides diffraction-limited spatial resolution and 7.6-cm-1 spectral bandpass across the full free spectral range of the imaging spectrometer.
Abstract: A Lyot-type liquid crystal tunable filter (LCTF) suitable for high-definition Raman chemical imaging has been developed. The LCTF has been incorporated into an efficient Raman imaging system that provides significant performance advantages relative to any previous approach to Raman microscopy. The LCTF and associated optical path is physically compact, which accommodates integration of the LCTF within an infinity-corrected optical microscope. The LCTF simultaneously provides diffraction-limited spatial resolution and 7.6-cm-1 spectral bandpass across the full free spectral range of the imaging spectrometer. The LCTF Raman microscope successfully integrates, in a facile manner, the utility of optical microscopy and the analytical capabilities of Raman spectroscopy. In this paper the LCTF Raman imaging system is described in detail, as well as results of initial studies of polymer and corrosion product model systems.
141 citations
TL;DR: In this article, a multivariate calibration model for five different components in synthetic mixtures prepared in aqueous solutions is presented, based on partial least-squares regression of raw and Fourier filtered absorbance spectra.
Abstract: Calibration models are generated and evaluated for the measurement of five different components in synthetic mixtures prepared in aqueous solutions. Mixtures of glucose, glutamine, ammonia, lactate, and glutamate were prepared to simulate concentration levels expected during routine bioreactor fermentation processes. Near-IR spectra were collected from these solutions over the spectral range from 5000 to 4000 cm-1. This spectral information was used to build individual multivariate calibration models for each analyte. Models were constructed on the basis of partial least-squares regression of raw and Fourier filtered absorbance spectra. Each analyte could be detected selectively with mean percent errors of prediction ranging from 4 to 8%.
131 citations
TL;DR: Raman spectrometry has traditionally been limited to the study of pure, nonfluorescent samples as discussed by the authors, and the ability to obtain Raman spectra in the deep red or near-infrared (NIR) could not be generally realized in reasonable measurement times, primarily because of the insensitivity of the available detectors.
Abstract: Raman spectrometry has historically been limited to the study of pure, nonfluorescent samples. During the period 1966-1986, several workers demonstrated that the use of deep-red or near-infrared (NIR) lasers would allow the observation of Raman spectra without exciting fluorescence. The ability to obtain Raman spectra in the deep red or NIR could not be generally realized in reasonable measurement times, primarily because of the insensitivity of the available detectors (photomultiplier tubes or diode arrays). In addition, the krypton or solid-state ion-pumped lasers required for these measurements were very expensive.
122 citations
TL;DR: Vibrational optical activity (VOA) is no longer a curious novelty in the field of molecular spectroscopy as mentioned in this paper, and it has matured to a point where the phenomenon is well understood theoretically, can be measured and calculated routinely, and is being used to uncover exciting new information about the structure of optically active molecules.
Abstract: Vibrational optical activity (VOA) is no longer a curious novelty in the field of molecular spectroscopy. After recently celebrating twenty years of development since its early years of discovery, VOA has matured to a point where the phenomenon is well understood theoretically, can be measured and calculated routinely, and is being used to uncover exciting new information about the structure of optically active molecules. Beyond this, VOA has been shown to be a sensitive, noninvasive diagnostic probe of chiral purity or enantiomefic separation with potential use in the synthesis and manufacture of chiral drugs and pharmaceutical products.
119 citations
TL;DR: Fluorescence quantum yield measurements are reported for visible and UV excitation for neat and dilute crude oil solutions, extending earlier work with excitation in the long wavelength visible and the NIR as discussed by the authors.
Abstract: Fluorescence quantum yield measurements are reported for visible and UV excitation for neat and dilute crude oil solutions, extending earlier work with excitation in the long wavelength visible and the NIR. Large and monotonically increasing quantum yields are found with shorter wavelength excitation (to 325 nm), and all crude oils are shown to have nearly the same relative dependence of quantum yield on excitation wavelength. These observations are explained by the energy dependence of internal conversion. Dilute solutions of light crude oils exhibit higher quantum yields than those of heavy crude oils because of their lack of large chromophores. The fraction of fluorescence emission resulting from electronic energy transfer (with subsequent fluorescence emission) for neat crude oils was previously shown to vary from ~100% for ultraviolet excitation to ~0% for near-infrared excitation; this large variation correlates well with and is explained by the very large variation in quantum yields with excitation wavelength. Comparison of quantum yields from neat and dilute solutions shows that quenching is the other major process which occurs with chromophore interactions. The quantum yields of a maltene and resin are large and similar, while the asphaltene exhibits much smaller quantum yields because of its lack of small chromophores.
TL;DR: In this paper, the applicability of laser-induced fluorescence (LIF) spectroscopy as a nondestructive analytical technique for artwork diagnostics is investigated, which is shown to be suitable for differentiating among the various cadmium pigments used in this study and furthermore, to be capable of identifying individual components in mixtures of these pigments on the basis of their characteristic fluorescence emission.
Abstract: The applicability of laser-induced fluorescence (LIF) spectroscopy as a nondestructive analytical technique for artwork diagnostics is investigated. In this work, LIF is employed in the examination of a set of cadmium sulfide- and cadmium selenide sulfide-based pigments in a series of oil painting test samples. Fluorescence spectra of the oil colors are recorded upon pulsed laser excitation at 532, 355 (Nd:YAG), and 248 nm (KrF excimer). The technique is shown to be suitable for differentiating among the various cadmium pigments used in this study and, furthermore, to be capable of identifying individual components in mixtures of these pigments on the basis of their characteristic fluorescence emission. Future prospects and the potential for the extension of LIF from a research laboratory technique into a conservator's tool for artwork diagnostics are discussed.
TL;DR: By use of near-infrared (NIR) spectroscopy, simultaneous, multiple-constituent estimation of important bioprocess parameters can be obtained in a time frame (<1 min assay) that was previously unattainable.
Abstract: By use of near-infrared (NIR) spectroscopy, simultaneous, multiple-con-stituent estimation of important bioprocess parameters can be obtained in a time frame (< 1 min assay) that was previously unattainable. Therefore, with NIR spectroscopy the opportunity exists to incorporate real-time chemical information into bioprocess monitoring or control strategies which will lead to significant bioproeess improvements. The NIR spectroscopic analysis of unmodified whole broth samples for acetate, ammonium, biomass, and glycerol is described for an industrial Escherichia coil fed-batch fermentation bioprocess. For acetate and glycerol, suitable results were obtained from multiple linear least-squares regression (MLR) analysis. A more sophisticated partial least-squares (PLS) regression analysis was necessary to adequately model ammonium and biomass. The respective prediction errors (1σ) of 0.7 g/L, 1.4 g/L, 0.7 g/L, and 7 mmol/L for acetate, biomass, glycerol, and ammonium compare well with the error of the wet chemical reference methods used to derive the calibration algorithms.
TL;DR: In this article, an indium antimonide (InSb) focal-plane array detector and a commercially available step-scan Fourier transform infrared spectrometer (FTIR) were used to generate image contrast in sections of monkey brain tissue and relate these data to specific lipid and protein fractions.
Abstract: We demonstrate a new mid-infrared and near-infrared imaging approach which is ideally suited to microscopic applications. The method employs an indium antimonide (InSb) focal-plane array detector and a commercially available step-scan Fourier transform infrared spectrometer (FTIR). With either a KBr or a CaF2 beamsplitter, images from 1 to 5.5 μm (10,000–1818 cm-1) can be rapidly acquired with the use of all the available pixels on the detector. The spectral resolution for each image is easily varied by changing the number of acquired images during the interferometer scan. We apply this technique to noninvasively generate image contrast in sections of monkey brain tissue and to relate these data to specific lipid and protein fractions. In addition, we describe several computational methods to highlight the spatial distributions of components within a sample.
TL;DR: Fourier transform infrared spectroscopy (FT-IR), surface-enhanced Raman spectroscopic (SERS), and nuclear magnetic resonance (NMR) (1H, 13C, and 31P) have been applied to the study of fractions of humic acids in Irish peat as discussed by the authors.
Abstract: Fourier transform infrared spectroscopy (FT-IR), surface-enhanced Raman spectroscopy (SERS), and nuclear magnetic resonance (NMR) (1H, 13C, and 31P) have been applied to the study of fractions of humic acids in Irish peat. Owing to the high fluorescence of these materials, no Raman spectra have been obtained, up to now, for the characterization of humic acids. The SERS technique was employed for the first time in the study of these complex substances, demonstrating that very valuable information about the aromatic groups and the spacial conformation of these macromolecules in water solution can be obtained. The combined use of these techniques has evidenced an increase of oxygenated groups in those fractions containing humic acids with low molecular weight. The presence of a greater number of carboxylate and phenolic groups can explain the high concentration of metals and phosphate ions found in these fractions.
TL;DR: Together, instrumentation, optics, and computing combine to poise biomedical near-IR spectrometry for great advances as it enters the next century.
Abstract: Near-infrared spectrometry is being applied in the solution of problems in many areas of biomedical and pharmaceutical research, including cardiovascular radiology, brain imaging, formulation, quality/process control, and even clinical trials The technique can also play a role in the biotechnology industry in the nondestructive analysis of small quantities of expensive materials This report first defines near-IR spectrometry and imaging and then describes its application to atherosclerosis and stroke research New developments in near-IR optics and instrumentation that make effective biomedical near-IR spectrometry possible are related, and, finally, new computational research results in parallel supercomputing for near-IR imaging are described Together, instrumentation, optics, and computing combine to poise biomedical near-IR spectrometry for great advances as it enters the next century
TL;DR: In this article, the sensitivity and sampling volume of various remote fiber-optic Raman probes were investigated. And the results showed that beveled probes can collect light from a restricted volume closer to the probe end than flat-tip probes.
Abstract: We have developed models describing the sensitivity and sampling volume of various remote fiber-optic Raman probes—single-fiber, lensed, dual-fiber beveled-tip, dual-fiber flat-tipped, and multi-fiber flat-tipped. The models assume clear samples and incorporate radii, separation, bevel angle, and numerical aperture of the fibers; overlap geometry of illumination and excitation light cones; and refractive index of immersion medium. For the Raman spectra of solid samples in air, single-fiber and lensed probes are predicted to yield the highest Raman signal. Beveled probes should provide greater Raman signal strength than do flat-tipped probes because beveled probes can collect light from a restricted volume closer to the probe end. Although multiple collection fibers improve Raman signal strength, progressively distant concentric fiber rings contribute less and sample material further from the probe.
TL;DR: In this article, three FT-IR sampling techniques were used to study tricalcium silicate (C3S), hydrated C3S, calcium hydroxide, and calcium silicate hydrate (C-S-H).
Abstract: The influence of FT-IR sampling techniques on the characterization of cement systems was investigated. Three FT-IR techniques were used to study tricalcium silicate (C3S), hydrated C3S, calcium hydroxide, and calcium silicate hydrate (C-S-H). They include transmission spectroscopy (TS), photoacoustic spectroscopy (PAS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The TS technique (using KBr pellets) was the most labor-intensive but was found to give the simplest spectra with well-defined bands. The PAS technique was found to be the simplest technique but yielded bands at lower wavenumber than TS. DRIFTS was determined to be a good alternative for cement powders since it provided spectra similar to those for the TS technique. DRIFTS required more sample preparation than PAS but less sample preparation than the KBr pellet technique.
TL;DR: In this paper, Fourier transform infrared (FT-IR) microspectroscopy was applied to three distinct types of plant tissue to study the changes in cell wall structure and composition of flax epidermal cells during development.
Abstract: Fourier transform infrared (FT-IR) microspectroscopy was applied to three distinct types of plant tissue. Reflectance microspectroscopy of nutshells highlighted the differences between the chemistries of the inner and outer surfaces and the tissue as a whole. The outer surfaces were suberized, while the inner surfaces contained absorbances indicative of lignin or tannins or both. Transmission microspectroscopy was used to follow the changes in cell wall structure and composition of flax epidermal cells during development and showed that initial development was accompanied by suberin and lignin deposition, which was followed by polysaccharide deposition characteristic of secondary cell wall formation. These results were compared with those obtained from bamboo. Transmission microspectroscopy was also used to study the infection of potato tubers by Erwinia carotovora ssp. carotovora in aerobic and anaerobic conditions. The spectra suggested that both infective conditions produced cell wall degradation, whereas anaerobic infection was accompanied by extensive breakdown of starch and plasma membrane.
TL;DR: In this paper, a tungsten coil atomizer, a miniature spectrometer/charge-coupled device combination mounted on a PC card, and a near-line background-correction method was used for lead detection.
Abstract: A compact, inexpensive atomic absorption spectrometer has been designed, constructed, and evaluated for the determination of lead at the μg/L level. The new device is made feasible by the combination of a reliable tungsten coil atomizer, a miniature spectrometer/charge-coupled device combination mounted on a PC card, and a near-line background-correction method. The finished spectrometer can be powered by a normal 12-V car battery, controlled with a laptop computer, and transported in any automobile. The overall dimensions of the original prototype system are 19 in. x 8 in. x 3 in. (excluding the computer), and it has no moving parts. The total estimated cost of the system including the computer is less than d6000. The limit of detection for Pb is 20 pg (20-μL sample volume), the linear dynamic range is two orders of magnitude, and the precision for the technique is 5% RSD at concentrations ten times greater than the detection limit. The accuracy of the technique was determined with the use of NIST SRM #1579a "Powdered Lead-Based Paint" containing 11.995 wt % Pb and NIST SRM 955a "Lead in Blood" containing 54.43 μg/dL Pb. The accuracy for the paint sample was 95.1% (11.41 wt % found) with the use of the calibration curve method (aqueous standards) and 97.2% (11.66 wt % found) with the method of standard additions. The accuracy for the blood sample was 93.5% (50.9 μg/dL found) with the calibration curve method and 96.6% (56.3 μg/dL found) with the method of standard additions. The limiting source of noise for the instrument is detector noise, so that the performance of the device can be improved by increasing the optical throughput of the system.
TL;DR: In this paper, the authors compared the performance of flat-tip, beveled and single-lensed focused fiber-optic Raman probes and evaluated the utility of optical filters for reducing fiber background.
Abstract: We compare relative performances of flat-tipped, beveled (two-fiber and six-around-one), and single-lensed focused fiber-optic Raman probes and, where feasible, evaluate the utility of optical filters for reducing fiber background. The sensitivity profile of each probe is determined by measuring the relative intensity of light backscattered off a flat surface as a function of distance from the probe tip. The experimental results are compared with a simple light-cone-overlap model incorporating fiber numerical aperture, fiber and immersion medium refractive indices, separation between excitation and collection fibers, number of fibers, and fiber bevel angle and/or lens focal length. The model and sensitivity profiles are used to interpret the sampling regions for Raman spectra obtained by using each of the probes with a clear, transparent sample (single-crystal sparry calcite), a white, partially transparent sample (acetaminophen tablet), and a set of organic liquids of varying refractive index. The sensitivity of the tested commercial lensed probe drops off symmetrically about the focal point. For both solid samples, the intensity of fiber background follows a profile determined primarily by laser backscattering off the surface, whereas the sample Raman signal follows a profile dependent upon sampling depth.
TL;DR: In this article, Raman optical activity (ROA) spectra of proteins, proteins, carbohydrates, glycoproteins, and nucleic acids are compared in aqueous solution.
Abstract: Recent advances in Raman optical activity (ROA) instrumentation are outlined which have enhanced significantly the quality of vibrational ROA spectra of biopolymers in aqueous solution. Peptides, proteins, carbohydrates, glycoproteins, and nucleic acids now provide excellent ROA spectra which contain detailed information about solution structure. ROA spectra can be measured just as easily in D2O as in H2O solution, and, as illustrated for bovine serum albumin and concanavalin A, a comparison of the two can be highly informative. In addition to signatures of extended secondary structure, protein ROA spectra also contain signatures related to loops and turns which are valuable for studying tertiary structure and dynamics, exemplified here by a comparison of the ROA spectra of reduced lysozyme and unordered poly-L-lysine, by the ROA spectra of acid molten globule α-lactalbumin at different temperatures, which reveal a native-like tertiary fold, and by changes in the ROA spectrum of native lysozyme on binding to a saccharide inhibitor. Carbohydrate ROA spectra contain signatures of all the central features of their stereochemistry and, as shown by a comparison of laminaribiose with laminarin, can also probe extended secondary structure in polysaccharides. Results on a glycoprotein, orosomucoid, suggest that ROA can provide information about both the protein and the carbohydrate components. Preliminary results on nucleic acids are outlined with the ROA spectra of Poly(rA).Poly(rU) and Poly(rI).Poly(rC) shown as examples.
TL;DR: In this paper, a time-resolved laser-induced breakdown spectroscopy was applied for quantitative measurement of lead content in concrete at levels down to 10 ppm using a Q-switched ND: YAG laser operating at a 106-μm wavelength and a repetition rate of 10 Hz.
Abstract: Time-resolved laser-induced breakdown spectroscopy was applied for quantitative measurement of lead content in concrete at levels down to 10 ppm The breakdown was formed at the sample surface by a Q-switched ND: YAG laser operating at a 106-μm wavelength and a repetition rate of 10 Hz Contamination levels were inferred from the ratio of the integrated emission line of lead to a known reference line of the matrix The lead contamination can be determined on an absolute scale down to 10 ppm at an optimum delay time of 30 μs These results were derived from analysis of the temporal evolution of the calibration function within a 01- to 190-μs time range The calibration function exhibits no dependence on the incident laser pulse energy, which was varied from 250 to 400 mJ
TL;DR: In this paper, the effects of optical saturation on fluorescence and thermal lens measurements for rhodamine 6G in various media and over a wide energy range were investigated and the photothermal method gave accurate absolute values of Pf ranging from 0.93 to 0.95, depending on the solvent.
Abstract: The high incident irradiances available with pulsed lasers can lead to a significant depletion of the ground-state population of the chromophore and to optical saturation effects. As a result, the optical absorption coefficient decreases as a function of the excitation energy and, because the amount of energy released by radiative and nonradiative relaxation processes depends on the amount of energy absorbed, nonlinear energy-dependent signals are obtained. Therefore, large errors can be introduced when fluorescence and photothermal data are used to determine fluorescence quantum yields. This work provides experimental results describing the effects of optical saturation on fluorescence and thermal lens measurements for rhodamine 6G in various media and over a wide energy range. It is shown that, when optical saturation is avoided, the photothermal method gives accurate absolute values of Pf ranging from 0.93 to 0.95, depending on the solvent. On the contrary, fluorescence measurements seem to be sensitive to additional experimental artifacts that are more difficult to characterize and to eliminate.
TL;DR: In this article, a molecular depth profiling study of a PMMA/PVOH laminate on quartz using confocal Raman microspectroscopy was performed, and it was demonstrated that this technique can be successfully employed to study the hydrogen-bonding interaction between the ester and alcohol groups near the interfacial region.
Abstract: We report a molecular depth profiling study of a PMMA/PVOH laminate on quartz using confocal Raman microspectroscopy. It is demonstrated that this technique can be successfully employed to study the hydrogen-bonding interaction between the ester and alcohol groups near the interfacial region. The carbonyl, v(C=O), band of PMMA shows significant broadening in the interfacial region. Various PMMA/PVOH laminates with different PMMA molecular weights have been studied, and it is demonstrated that the PMMA layers with lower molecular weight show a greater degree of interpenetration for a given annealing time.
TL;DR: In this article, infrared spectroscopy has been coupled with the polarization modulation technique in order to characterize the molecular orientation in films of azopolymers irradiated in situ with polarized visible light.
Abstract: Infrared spectroscopy has been coupled with the polarization modulation technique in order to characterize the molecular orientation in films of azopolymers irradiated in situ with polarized visible light. The results obtained on disperse red 1-containing amorphous azopolymer (pDR1A) demonstrate the high efficiency of polarization modulation infrared linear dichroism to determine quantitatively the time dependence of the orientation function of several chemical groups during the orientation (laser on) and relaxation (laser off) processes. The difference dichroic spectra show that polarized visible light induces a preferential orientation of the azobenzene groups perpendicular to the direction of the polarization of the writing laser. This orientation of the side chains also results in a slight cooperative orientation of the C=O ester groups of the main chain of the polymer. The time dependence of the orientation function shows that the orientation and relaxation processes could be described by a biexponential function involving
TL;DR: In this paper, the performance characteristics of a laser-induced plasma for real-time determination of various gas-phase metal hydrides, specifically Sn and As, were evaluated in He and N2 atmospheres at 300 and 760 Torr.
Abstract: A study was performed to evaluate the performance characteristics of a laser-induced plasma for real-time determination of various gas-phase metal hydrides, specifically Sn and As. The choice of carrier gas composition and the effect of the pressure on the temporal emission behavior of neutral atoms excited by the laser-induced plasma were investigated. Metal hydrides were generated by using a NaBH4-based hydride generation system. The hydrides were equilibrated into an evacuated cell and isolated from the generator prior to measurement. Laser-induced breakdown spectroscopy (LIBS) spectra of Sn and As were recorded in He and N2 atmospheres at 300 and 760 Torr. The temporal behavior of the LIBS signal was most affected by gas composition, gas pressure, and intensity of the laser beam. The Sn neutral atom emission (284.0 nm) in a N2 atmosphere decreased exponentially with time. In contrast, with a He atmosphere and identical experimental conditions, the Sn signal increased logarithmically with time over the first 100 s. Then the signal maintained a steady-state value until approximately 400 s, after which it decreased exponentially. The steady-state time depends on the concentration of metal hydride. The variation of the LIBS signal with time was mirrored for the As neutral atom emission in He and N2 atmospheres. Various experiments have been performed to find the possible reason for the signal variation with time. It was found that chemical reactions in the laser plasma that might deplete the metal from the gas volume were responsible for the decrease in the signal with time.
TL;DR: In this paper, the authors proposed a new approach to solve the phase retrieval problem in optical spectroscopy by using the maximum entropy model, which has been shown to work well in practice.
Abstract: The problem of phase retrieval appears in optical spectroscopy when a power spectrum P(ω) = |f(ω)|2is measured while the entire complex function f(ω) = |f(ω)| exp iθ(ω) is needed for obtaining the desired material properties. Recently we proposed a new approach to solve this problem in optical spectroscopy by using the maximum entropy model. Here we give a short review of its theory and show how to improve the phase retrieval procedure to work well in practice. The usage and applicability of the procedure, especially in reflectance spectroscopy of solids, are demonstrated with practical examples. Its use in other applications is also discussed.
TL;DR: In this article, the anti-Stokes Raman scattering was collected by using chromatic aberration of a lens to obtain high-quality anti-stokesraman spectra from glucose in plasma and serum, which is a strongly fluorescent biological sample.
Abstract: This study demonstrates the potential of anti-Stokes Raman spectroscopy in investigating biological samples in a nondestructive manner; quantitative analysis of glucose in plasma and serum has been studied as an example. The efficient collection of anti-Stokes Raman scattering by use of chromatic aberration of a lens has allowed us to obtain high-quality anti-Stokes Raman spectra from glucose in plasma and serum, which is a strongly fluorescent biological sample. The concentration of glucose in these materials can be estimated by the anti-Stokes Raman intensity of the band at 1130 cm-1 due to the C-O stretching mode. The correlation coefficient between the concentration and the intensity has been calculated to be 0.993 and 0.991 for glucose in plasma and serum, respectively. The detection limits for these materials have been found to be 45 mg/dL and 50 mg/dL, respectively.
TL;DR: In this paper, a single micrometer-sized monomer droplet was used to investigate polymerization reactions in a single micro-scale monomer particle and the results demonstrate that the combination of the optical levitation and Raman spectroscopy allows nondestructive in situ measurements of single particles.
Abstract: Raman spectroscopy was used to investigate polymerization reactions in a single micrometer-sized monomer droplet. An Ar+ laser levitated the microparticles and simultaneously excited the Raman scattering. The polymerization reaction was initiated by exposing the monomer droplets to the UV radiation of a mercury are excitation lamp. The Raman spectrum of the reacting particle was investigated on-line. The results demonstrate that the combination of the technique of optical levitation and Raman spectroscopy allows nondestructive in situ measurements of single particles and is therefore very useful for the study of fundamental processes.