Showing papers in "Applied Spectroscopy in 1997"

FT-Raman Spectroscopy of Wood: Identifying Contributions of Lignin and Carbohydrate Polymers in the Spectrum of Black Spruce (Picea Mariana):

Abstract: Good-quality Raman spectra of most wood species can now be obtained by using near-infrared Fourier transform Raman spectroscopy. To make effective use of such spectroscopic information, one needs to interpret the data in terms of contributions from various wood components and, for each component polymer, in terms of vibrational modes of its substructural units/groups. In the present work, Raman spectral features of black spruce (Picea mariana) wood were associated with lignin and/or carbohydrate polymers. Lignin's spectral contributions were recognized in several ways. In addition to spectra of milled-wood and enzyme lignins, a spectrum of native lignin was obtained by subtracting the spectrum of acid chlorite delignified black spruce from the spectrum of an untreated wood sample. A comparison of lignin spectra indicated that the Raman features of the three lignins are very similar. Raman contributions of carbohydrate polymers, namely, those of cellulose and hemicellulose, were identified by using authentic and/or isolated samples and, in the case of cellulose, by using previously published spectra. Such an analysis showed that the hemicellulose present in black spruce did not give rise to any new, unique features that were not already present due to cellulose. Therefore, it was concluded that the hemicellulose contribution is broad and is hidden under the Raman contribution of cellulose. Also, peak positions of lignin contributions did not overlap with those of cellulose, and there were spectral regions where either lignin or cellulose contributed.

Laser Diagnostics of Painted Artworks: Laser-Induced Breakdown Spectroscopy in Pigment Identification

Abstract: Laser-induced breakdown spectroscopy (LIBS) was employed for the in situ analysis of pigments used in painting. LIBS spectra were collected from a wide variety of pigments in powder form and in oil color test samples. Appropriate emission lines for the identification of the metallic elements in the pigments examined are proposed. Under optimal experimental parameters, the technique is minimally destructive; two pulses from a laser beam focused on the sample surface result in the formation of a small crater with typical diameter around 40 mu m and depth of no more than 10 mu m. Furthermore, recording LIBS spectra from successive laser pulses on the same spot of a model oil painting resulted in information regarding the pigment composition of several paint layers, showing the capability of the technique in performing depth profile analysis. Finally, a test case is presented in which an 18th century oil painting, subjected to partial restoration, was examined by LIBS, and the different pigments used in the original and in the restored part of the work were clearly identified. The results of our studies demonstrate the applicability of LIBS in the rapid, in situ, and practically nondestructive determination of pigments in painted artworks.

Trace Element Analysis in Water by the Laser-Induced Breakdown Spectroscopy Technique

Abstract: In this paper, results are presented showing the feasibility of the laser-induced breakdown spectroscopy (LIBS) technique as a fast and sensitive analytic tool for quantitative measurement of trace elements in water. Many ionic elements were detected; the system linearity was tested by analyzing water samples containing known concentration of Mg and Ca, whereas Cr-polluted samples were exploited to test the system sensitivity limit to impurities.

Discrete particle detection and metal emissions monitoring using laser-induced breakdown spectroscopy

Abstract: The unique conditions for the application of laser-induced breakdown spectroscopy (LIBS) as a metal emissions monitoring technology have been discussed. Because of the discrete, particulate nature of effluent metals, the utilization of LIBS is considered in part as a statistical sampling problem involving the finite laser-induced plasma volume, as well as the concentration and size distribution of the target metal species. Particle sampling rates are evaluated and Monte Carlo simulations are presented for relevant LIBS parameters and wastestream conditions. For low metal effluent levels and submicrometer-sized particles, a LIBS-based technique may become sample limited. An approach based on random LIBS sampling and the conditional analysis of the resulting data is proposed as a means to enhance the LIBS sensitivity in actual wastestreams. Monte Carlo simulations and experimental results from a pyrolytic waste processing facility are presented, which demonstrate that a significant enhancement of LIBS performance, greater than an order of magnitude, may be realized by taking advantage of the discrete particulate nature of metals.

Determination of the Relative Amount of Nucleic Acids and Proteins in Leukemic and Normal Lymphocytes by Means of Fourier Transform Infrared Microspectroscopy

Abstract: Previous Fourier transform infrared (FT-IR) spectroscopic studies on neoplastic and normal cells have shown different band profiles and intensity associated with absorptions of proteins and nucleic acids. In the present study, an interpretation of such differences has been attempted by comparing the spectra of DNA/RNA/protein mixtures with the spectra, particularly, obtained for lymphocytes from B-chronic lymphatic leukemia (B-CLL) patients and normal donors. FT-IR microspectroscopy analysis showed a good agreement between the intensity and the band profile of the spectra of leukemic lymphocytes and those of the binary mixture made up of 75% human serum albumin and 25% DNA. The addition of small amounts of RNA (1-5% ) modified the band shape, making it more similar to the spectrum of normal lymphocytes. An attempt was also made to estimate the relative amounts of DNA and RNA. The results demonstrated an increase in the DNA/RNA ratio value in neoplastic lymphocytes with respect to that reported in literature for normal ones.

Quantitative Comparison of Five SERS Substrates: Sensitivity and Limit of Detection

Abstract: Five surface-enhanced Raman scattering (SERS) substrates were quantitatively compared for ease of preparation, sensitivity, limit of detection (LOD), reproducibility, and stability. Specifically, vapordeposited Ag films, electrochemically roughened Ag electrodes, nitric acid-etched Ag foil, Tollens-produced Ag films, and photoreduced Ag films on TiO were examined. Of these substrates, post2 deposition-annealed Ag films exhibited the greatest sensitivity and lowest LOD, with 152 +/- 1 counts per femtomole and an LOD of 0.36 +/- 0.02 femtomoles of trans -1,2-bis(4-pyridyl)ethene (BPE). The substrate demonstrating the poorest sensitivity and highest LOD was Ag deposited from the Tollens reaction, with 0.38 +/- 0.01 counts per femtomole and an LOD of 270 +/- 20 femtomoles of BPE. The easiest substrate to prepare, nitric acid-etched Ag foils, exhibited a sensitivity of 0.485 +/- 0.008 counts per femtomole and an LOD of 200 +/- 10 femtomoles of BPE.

Near-Infrared Raman Spectrometer Systems for Human Tissue Studies:

Abstract: We have built two types of instruments for near-infrared Raman spectroscopy studies of human tissues, one for laboratory measurements and one for clinical use. The laboratory systems are designed to collect the highest quality spectra possible and allow different excitation/collection wavelengths to be studied. The clinical systems are designed to collect spectra via optical fibers within a few seconds and to be mobile and hospital-compatible. These systems are capable of detecting weak near-infrared Raman bands hidden in large background signals. Calibration and background subtraction procedures are described, and system performance is evaluated.

Interpretation of Raman Spectra of Nitro-Containing Explosive Materials. Part I: Group Frequency and Structural Class Membership

Abstract: Fourier transform (FT)-Raman spectroscopy has been used to obtain high-quality spectra of 32 explosive materials. The majority of the spectra of these explosives have not previously been reported. Twenty-eight of the explosives have been categorized into three classes (nitrates esters, nitro-aromatics, and nitramines) based on their chemical structure, the position of the antisymmetric and symmetric stretching vibrations of the nitro group, and the shapes of the band envelopes. The spectra of exceptional explosives are discussed in terms of their unique structures or compositions.

On-Line Monitoring of Laser Cleaning of Limestone by Laser-Induced Breakdown Spectroscopy and Laser-Induced Fluorescence

Abstract: The application of laser-induced breakdown spectroscopy (LIBS) to monitor the laser cleaning process of polluted limestone from a historic building is examined. The combination of a Q-switched Nd: YAG pulsed laser with on-line diagnostics by the LIBS technique is shown to be very useful for controlling and characterizing the cleaning process in order to avoid overcleaning. In addition, the coupling of this spectroscopic technique to the cleaning process provides important information about the optimal experimental conditions to be selected for achieving an adequate cleaning procedure. Furthermore, the spectroscopic study of the plasma emission can be used to determine the elemental composition of both the black crust and the underlying stone. The application of LIBS as a diagnostic technique to monitor and control the laser cleaning process of limestone is based on the different elemental composition of the black encrustations covering the stone surface and the underlying stone. On the other hand, a different experimental setup for probing the ablation products by laser-induced fluorescence (LIF), in order to achieve a signal amplification of some atomic emission lines with weak intensity in the LIBS spectrum, is described.

In Situ Spectroscopy of Polymers Subjected to Supercritical CO2: Plasticization and Dye Impregnation

Abstract: In situ FT-IR spectroscopy has been used to study poly(methyl methacrylate) films subjected to high-pressure and supercritical CO2. Spectral changes indicate increased molecular mobility of ester groups due to the plasticization effect of CO2 on PMMA. This increase in PMMA segmental mobility has been used to impregnate Disperse Red 1 dye (DR1) into polymer film from a supercritical fluid solution. The enhanced diffusion process was observed in situ via FT-IR and UV/vis spectroscopy.

Two-Dimensional FT-IR Spectroscopy: A Powerful Method to Study the Secondary Structure of Proteins Using H-D Exchange

Abstract: Two-dimensional infrared spectroscopy has been used for the first time to study the conformation of proteins by hydrogen-deuterium exchange. In order to generate the two-dimensional synchronous and asynchronous maps, hydrogen-deuterium exchange of the amide protons of proteins deposited on attenuated total reflection crystals has been used as an external perturbation. Owing to the fact that the amide protons associated with each conformation are not exchanged at the same rate, the different conformational contributions of the amide bands could be separated. The use of different sampling time domains turned out to be very helpful in order to separate more efficiently the fast kinetics from the slower ones. The results obtained on myoglobin show that this method is particularly useful to unravel the different components under the poorly resolved amide I, II, and II ' bands of proteins. The analysis of the synchronous and asynchronous maps of myoglobin demonstrates that the amide I band of this protein is composed of at least four components that could be assigned to alpha -helical, intermolecular beta -sheet, beta -turn, and random coil conformations.

Quantitative Analysis of Aluminum Impurities in Zinc Alloy by Laser-Induced Breakdown Spectroscopy:

Abstract: Laser-induced breakdown spectroscopy has been employed for the spectroscopic study of a laser-produced Zn alloy plasma for the analysis of Al impurities in Zn alloy. The laser-produced Zn alloy plasma was generated by a pulsed Nd:YAG laser (105 mJ, 3 ns). The physical characteristics of the laser-produced Zn alloy plasmas and an optimum condition for the composition analysis are reported. With the use of the relative intensities of spectral emission lines from standard samples, calibration curves for the analysis of aluminum in Zn samples were obtained in the case of air, vacuum, and Ar environments.

Infrared Spectroscopy in Aqueous Solution: Difficulties and Accuracy of Water Subtraction

Abstract: The long-term reproducibility of a set of water spectra in the infrared region with cell thicknesses of less than 10 mu m was tested. With the application of a computer routine for subtraction in the range from 2600 to 1100 cm-1 and the use of additional phase and nonlinearity corrections, an accuracy better than one milliabsorbance unit (mAU) can be achieved, if the temperature is kept constant within a tenth of a degree or if a temperature correction factor is implemented in the subtraction routine. The subtraction of water from an aqueous protein solution reduces the spectral range for a correction to 2300-1800 cm-1.In this region the protein absorption, as derived from dry protein films, is constant, and because of the association band at 2125 cm-1 water exhibits good subtraction properties. However, the reduction of the spectral range for the water subtraction leads to an increase in the residual error to 12 mAU at 1650 cm-1. This standard deviation can be reduced to 6-7 mAU by applying a second subtraction range from 4000 to 3650 cm-1. A further reduction of this error to 3-5 mAU is obtained by implementing a constant ratio of the integrated intensities of the amide I and amide II bands in the procedure of subtracting water from aqueous protein solutions. The remaining inaccuracy is mainly

Two-Dimensional Infrared and Near-Infrared Correlation Spectroscopy: Applications to Studies of Temperature-Dependent Spectral Variations of Self-Associated Molecules

Abstract: This paper demonstrates the potential of generalized two-dimensional (2D) Fourier transform (FT) infrared (IR) and near-infrared (NIR) correlation spectroscopy in the studies of temperature-dependent spectral variations of self-associated molecules. Three examples of the 2D correlation analysis are discussed in this paper. The first two are concerned with the temperature-dependent IR and NIR spectral changes of N -methylacetamide (NMA) in the pure liquid state. The 2D IR correlation approach revealed that almost all the peaks of NMA in the 3400-1100-cm-1 region consist of two to four separate bands. For example, the amide I band contains contributions from at least four distinct bands at 1685, 1665, 1650, and 1635 cm-1. The analysis of the asynchronous 2D IR spectrum in the amide I region showed that the sequence of spectral intensity change in the ascending order of temperature was given by 1635 < 1650 < 1665 < 1685 cm-1. These bands at 1635, 1650, 1665, and 1685 cm-1 were assigned to the amide I modes of chain oligomers of various sizes and dimer of NMA; the longer the chain, the lower the frequency. The closeup view of 2D NIR correlation spectra of NMA obtained at narrow spectral and temperature windows enabled us to propose not only band assignments in the 6800-6050-cm-1 region but also a detailed mechanistic picture of the thermally induced dissociation of NMA for each temperature range. We also applied the generalized 2D correlation approach to the analysis of a set of FT NIR spectra of oleyl alcohol under temperature variations. The 2D NIR analysis enhanced the spectral resolution and simplified the spectra with overlapped bands. For example, it was found that a band at 7090 cm-1 arising from the first overtone of an OH stretching mode of the monomeric alcohol consists of two bands due to the rotational isomerism of the free OH group. An intriguing possibility of correlating various overtone and fundamental bands to establish unambiguous assignments was also suggested from the 2D NIR study on oleyl alcohol.

Spectrochemical Analysis of Liquids Using Laser-Induced Plasma Emissions: Effects of Laser Wavelength on Plasma Properties:

Abstract: We spectroscopically determined the temperature and electron density of the plasma plumes produced by pulsed-laser ablation of aqueous solutions containing sodium, lithium, and rubidium. With the use of a Nd:YAG laser at 532 nm and fluence of 3 J/cm2, the plasma produced was hot (low eV range) and extensively ionized, with electron density in the 1018 cm -3 range. Analyte line signals were initially masked by intense plasma continuum emissions and would only emerge briefly above the background when the plume temperature dropped below 1 eV during the course of its very rapid cooling. Since ionization was thermally induced, the intense plasma flash was inevitable. In contrast, 193-nm laser ablation at similar fluence generated plasmas of much lower ( < 1 eV) temperature but comparable electron density. Plasma continuum emissions were relatively weak, and the signal-to-background ratio was a thousand times better. Consequently, this ''cold'' plasma was ideal for sampling biologically important elements such as sodium, potassium, and calcium.

Structural Characterization of β-Lactoglobulin in Solution Using Two-Dimensional FT Mid-Infrared and FT Near-Infrared Correlation Spectroscopy:

Abstract: Two-dimensional (2D) FT-IR correlation analysis was applied to both the mid-IR (MIR) and near-IR (NIR) regions to investigate changes in the secondary structures of beta -lactoglobulin in D2O (or H2O) solvent systems consisting of varying concentrations of bromoethanol. Mid-IR correlation spectra indicate that the amide I bands corresponding to different structures (i.e., alpha -helical structures at 1650 cm-1, aggregated beta -strands at 1620 cm-1, and beta -sheet at 1636 cm-1) exhibit apparently different spectral response towards varying concentrations of bromoethanol. We propose that the mechanism for the conversion of the beta -sheet into alpha -helix occurs in terms of two parallel pathways, i.e., (1) beta -sheets aggregated beta -strands alpha -helix, and (2) beta -sheets alpha -helix. Although the amide B/amide II combination bands give no spectral features relating to the secondary structure, changes were found in the C-H combination bands that suggest an interaction between the solvent and the protein.

External Reflection Infrared Spectroscopy of Anisotropic Adsorbate Layers on Dielectric Substrates

Abstract: Monolayers of octadecylsiloxane were formed on native silicon (Si/ SiO ) and glass surfaces by adsorption from dilute solutions of oc2 tadecyltrichlorosilane and were investigated by polarization- and angle-dependent external reflection infrared spectroscopy. In contrast to metal substrates, both the parallel and perpendicular vibrational components of the adsorbate can be detected on these dielectric surfaces. The monolayer reflection spectra show significant changes as a function of the light incidence angle and the polarization of the infrared radiation, which contain detailed information on the surface orientation of the film molecules. Spectral simulations based on classical electromagnetic theory yield an average 10 tilt angle of the hydrocarbon chains with respect to the surface normal on both silicon and glass surfaces. Despite this apparent structural identity of the monolayer films on silicon and glass, significant differences are observed in the monolayer reflection spectra resulting from purely optical effects of the substrate.

Application of Molecular Spectroscopy in the Mid-Infrared Region to the Determination of Glucose and Cholesterol in Whole Blood and in Blood Serum

Abstract: The feasibility of applying molecular spectroscopy in the mid-infrared region to the quantitation of glucose and cholesterol in whole blood and in blood serum was examined by measuring the samples in the dry state on a polyethylene carrier. The partial least-squares (PLS) algorithm was employed to set up the calibration models. Analysis was performed by normalizing the spectra to a standard area. Evidence that this approach is warranted was obtained through experiments where potassium thiocyanate served as the internal standard. The following average PRESS values (PRESS stands for prediction residual error sum of squares) emerged from the calibration: glucose, 22.3 mg/dL; cholesterol, 31.3 mg/dL. Such errors are comparable with those achieved in attenuated total reflectance (ATR) measurements. The calibration models set up for blood serum samples fit better than those for whole blood samples. If the procedure of sample application onto the carrier is improved, the method may work well for the determination of glucose and cholesterol in whole blood and in blood serum.

Spectrochemical analysis of liquids using laser-induced plasma emissions: effects of laser wavelength

Abstract: The plasma plume emissions produced by pulsed ( ~ 10 ns) laser ablation of liquid jets were monitored for spectrochemical analysis. Laser wavelengths at 532 and 193 nm were used, and sodium was the test analyte. As expected, the 532-nm laser pulse produced very intense plasma continuum emissions that masked the sodium signal for the first hundred nanoseconds, especially near the bright core of the vapor plume. Neither time-gating nor spatial masking could significantly improve the single-shot signal-to-noise ratio, since the transient nature of the emissions placed stringent demands on timing precision while the small size of the plume required accurate mask positioning-both antithetical to the inherent instability of jet ablation. In sharp contrast, the 193-nm laser pulse produced relatively dim plasma flash but intense sodium emissions, rendering it ideal for analytical applications.

Influence of Atmosphere and Irradiation Wavelength on Copper Plasma Emission Induced by Excimer and Q-Switched Nd:YAG Laser Ablation

Abstract: Laser-induced copper plasma emission was investigated by timeintegrated spatially resolved spectrometry. The comparative work on the plasma emission characteristics-specifically,self-absorption, line broadening, emission intensity, and metal ion formation-was carried out by the use of two different laser systems (XeCl excimer: lambda = 308 nm; Nd:YAG: lambda = 1064 and 532 nm). The characteristics of plasma emission produced by different wavelength radiation on copper atomic and ion lines under various atmospheric gases (argon, neon, and helium) and pressures (760-10 Torr) are presented. The differences in self-absorption and line broadening phenomena in the plasma location were explained by shock wave-driven excitation mechanism for atoms in the outer region in the plasma. The excitation temperatures of the plasma induced by 308- or 532-nm irradiation were also much higher that those induced by 1064-nm irradiation.

Quantitative Determination of the Scattering and Absorption Coefficients from Diffuse Reflectance and Transmittance Measurements: Application to Pharmaceutical Powders

Abstract: A measurement and data evaluation technique for the separate determination of the scattering and absorption properties of loose pharmaceutical powders is described. The equation of radiative transfer for diffuse reflectance and transmittance is analytically solved by using a three-flux approximation. Combined transmittance and reflectance measurements then allow one to derive both the scattering and the absorption coefficients. The scattering and absorption coefficients provide more information about particle size, degree of agglomeration, and chemical composition of the samples in comparison to the usual determination of only the ratio of the coefficients from diffuse reflectance measurements on optically thick samples. Furthermore, the theory of diffuse reflectance of optically thick samples according to Kubelka and Munk is compared to the three-flux approximation. The influence of the particle size on the scattering and absorption coefficient is investigated, and it is shown that the assumption of a wavelength-independent scattering coefficient, though often made in reflectance spectroscopy, is not generally valid.

Rapid Micro-Raman Imaging Using Fiber-Bundle Image Compression:

Abstract: A new technique for rapid Raman imaging and chemical analysis of micro-composites and biomaterials, with potential applications in real-time robotic vision, automated manufacturing, and medical imaging, is described and demonstrated. The key feature of this new instrument is a fiber-optic bundle used to compress two-dimensional images onto a one-dimensional fiber stack, which serves as the entrance slit of an imaging optical spectrograph. Thus a complete Raman spectrum is simultaneously collected from every point within a sample in a single scan of a charge-coupled-device (CCD) detector. The method is demonstrated by using Raman imaging of a microscopic mixed-salt sample. Its efficiency relative to alternative Raman imaging methods is quantitatively evaluated, and potential applications in other spectral imaging measurements are discussed.

Effect of data preprocessing methods in near-infrared diffuse reflectance spectroscopy for the determination of the active compound in a pharmaceutical preparation

Abstract: Near-infrared diffuse reflectance spectroscopy (NIRS) with a fiberoptic probe was used for the determination of the active compound in a commercial pharmaceutical preparation. In order to reduce the strong scatter in the spectra and prevent scatter-induced changes in measurements from prevailing over concentration-induced changes, several data preprocessing methods were tested: normalization, derivatives, multiplicative scatter correction, standard normal variate, and detrending. The effectiveness for reducing the scattering of each data preprocessing was assessed, and the best results were obtained with the use of the second derivative. The effect of the treatments on the quantitation of the active compound by partial least-squares regression (PLSR) was studied, similar results being obtained in all cases, with a relative standard error of prediction lower than 1.55%.

Elemental Fractionation of Glass Using Laser Ablation Inductively Coupled Plasma Mass Spectrometry

Abstract: Three laser wavelengths (1064, 532, and 266 nm) were employed for laser ablation at varied laser pulse energies to study the effect of irradiance and wavelength upon analytical results for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Two important results were observed and are reported here: (1) the intensity of the MS signal came to a local minimum when the laser focal point was on the sample surface, and (2) elemental fractionation based upon laser wavelength and laser pulse energy was observed. For the waste glass simulant studied, ablation with 1064-nm (IR) and 532-nm (green) radiation produced elemental fractionation that relates to the melting point of the elemental oxide, whereas with 266-nm (UV) ablation the response was independent of the elemental oxide melting point. At high laser powers, ablation at 266 nm produced an elemental bias based upon the mass of the elements. These observations suggest the use of ultraviolet radiation at low pulse energies to obtain improved analytical results.

Improved Prediction Error Estimates for Multivariate Calibration by Correcting for the Measurement Error in the Reference Values

Abstract: The validation of multivariate calibration models using measured reference values leads to a so-called apparent prediction error estimate, which is systematically larger than the true prediction error. The reason for this difference is clear: the measured reference values contain an irrelevant random component, the measurement error, which cannot be predicted by any model, not even the ''true'' one. However, the contribution of the measurement error in the reference values to the apparent prediction error estimate is interpreted as an inadequacy of the calibration model rather than an inadequacy of the reference values themselves. This phenomenon of confounding has been pointed out recently by several researchers, but no generally applicable solution was given. In this paper we propose a simple correction procedure that yields a more realistic estimate of the true prediction error. A large potential improvement over the conventional estimate is demonstrated for a variety of applications taken from the literature.

Comparison of Near-IR, Raman, and Mid-IR Spectroscopies for the Determination of BTEX in Petroleum Fuels

Abstract: We report for the first time a direct comparison of the three most common vibrational analysis techniques for the determination of individual BTEX components (benzene, toluene, ethylbenzene, ortho -xylene, meta -xylene, and para -xylene) in blended commercial gasolines. Partial least-squares (PLS) regression models were constructed for each BTEX component by using each of the three spectroscopic techniques. A minimum of 120 types of blended gasolines were used in the training set for each BTEX component. Leave-oneout validation of the training sets yields lower standard errors for Raman and mid-IR spectroscopies when compared to near-IR for all six BTEX components. In general, mid-IR has slightly lower standard errors than Raman. These trends are upheld when the models are tested by using independent test sets with a minimum of 40 types of blended gasolines (all of which differ in composition from the training set).

Simplified Calibration of Instrument Response Function for Raman Spectrometers Based on Luminescent Intensity Standards

Abstract: Published Raman spectra are rarely corrected for variations in spectrometer sensitivity across the Raman spectrum, which leads to often severe distortion of relative peak intensities that impede calibration transfer and library searching. A method was developed that uses the known luminescence of standards which fluoresce in response to laser irradiation. Since the standards are observed with the same sampling geometry as the Raman sample of interest, their spectra are subject to the same instrumental response function. After one-time calibration of the standards' fluorescence output against a known tungsten source, the unknown Raman spectrum may be corrected for instrumental response by a simple formula. In practice, the user need only run the standard under the same conditions as the Raman sample, then apply a short GRAMS algorithm. The approach is demonstrated for coumarin 540a and Kopp 2412 glass standards, with 514.5- and 785-nm laser light, respectively. Once the corrected spectrum is in hand, the absolute Raman cross section of a given Raman feature may be determined by comparison to known scatterers such as benzene.

Surface-Enhanced Vibrational Spectroscopy: A New Tool in Chemical IR Sensing?

Abstract: In order to further improve the sensitivity of chemical IR sensors, research into a significant change in the signal generation mechanism was carried out. This effort was realized by generating the surface-enhanced IR absorption and Raman-scattering effect at silver- and gold-island-covered Ge- and ZnSe ATR surfaces for surface-enhanced infrared absorption (SEIRA) and Ag-layer-covered brass plates for surface-enhanced Raman scattering (SERS), respectively, and linking the optimized tools to the IR sensor concept. By careful optimization of the generation of silver-island and goldisland films on Ge reflection elements, intensity gains by a factor of 50 were achieved. Chemical interactions between the silver-island layer and the various analytes investigated up to now ( p -nitro benzoic acid, as a model compound, and selected aromatic and chlorinated pesticides) are likely, as indicated by significant IR spectral changes; the substances are, however, bound to the surfaces in a reversible manner. This property of the novel SEIRA sensor can be favorably exploited in flow-through systems. In a similar way, by a specially optimized treatment of anodically oxidized brass plates in a silver bath, repeatably usable SERS targets for the trace analysis of pesticide mixtures in aqueous systems could be developed. A signal enhancement factor of 1.4 million was obtained with pyridine as the standard sample. The data shown in this work suggest that both SEIRA and SERS can be coupled to a flow injection system in a reversible way for the molecular-specific trace analysis of organic compounds in aqueous solutions.