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Applied Spectroscopy November 2000 Abstracts
(* denotes corresponding author)
Volume 54 (11)
Focal Point
Submitted Papers
Correlative X-ray photoelectron spectroscopy (XPS) and FT-IR studies of the complex heterogeneous structure of 50/50 PVC/PMMA polymer blends are presented. The comparable lateral resolution and parallel imaging capabilities of both techniques allow for a direct comparison of surface (XPS) and bulk (FT-IR) measurements of polymer blends. To eliminate substrate influence and film-to-film differences, the same areas on the polymer films are analyzed by both methods. The effect of PMMA molecular weight on surface separation and surface segregation is evaluated by using six blends with a constant PVC molecular weight and a PMMA molecular weight varying from 75 kDa to 2,132 kDa. Imaging capabilities of both methods were used for a qualitative comparison of the heterogeneous structure of the blends, while a quantitative comparison of the bulk and surface compositions of the same areas of the samples used small area spectroscopy from XPS and FT-IR. Based on the quantitative analysis, it is concluded that surface segregation of PMMA increases with increasing molecular weight. The determination of both surface and bulk properties of complex heterogeneous samples is important for a more complete understanding of structure of complex films.
Raman microimaging was used to estimate the effect of the silica filler on phase separation in binary polymer blends composed of BIMS and BR. The domain sizes, relative concentration of polymer components within domains, and distribution of particulate silica filler and zinc stearate curative were characterized for blends of different compositions and history of aging treatments. The presence of increased concentrations of precipitated silica results in better polymer morphology since domain sizes are reduced. Increased temperature treatment also decreases domain sizes up to about 150 °C, but treatment at 200 °C appears to induce separation of the elastomer components. Silica is usually found near the centers of the BIMS domains. There is little difference in silica distribution before and after curing.
The characterization of ink jet dyes and inks printed onto paper surfaces is of importance for the development of ink jet ink formulations and for use in forensic science. Raman spectroscopy is an effective and informative probe for this purpose if problems associated with fluorescence are overcome. A comparison is made here between two effective techniques, surface-enhanced resonance Raman scattering (SERRS) and near-infrared Fourier transform Raman scattering (NIR-FTR). SERRS provides a good method for obtaining in situ measurements by using a visible laser system. It is fast (1-10 s accumulations) and requires very low laser powers (<l mW). However, it requires the addition of a small dot of silver colloid to the paper surface in order to produce the enhancement. NIR-FTR is non-invasive and simpler to use. However much higher laser powers are required (>200mW), and the spectra accumulation time is significantly longer. Both methods overcome fluorescence effectively in most samples studied however each is more effective than the other with selected inks and paper substrates. SERRS involves resonant enhancement of the chromophore of the dye in contact with the silver surface and consequently it is the dye chromophore that is uniquely identified. With NIR-FTR, signals from the paper and the filler are also observed. Comparable spectral patterns with clear explicable differences are obtained from each method indicating in particular that SERRS spectra can be interpreted without recourse to specific surface selection rules. The combination of the two techniques provides some information on the electronic as well as the vibrational properties of the dyes in situ.
Rough silver surfaces enhance the Raman scattering of adsorbed molecules by many orders of magnitude. The size, shape, and surroundings of the metal particles greatly influence the resulting Raman intensities. With the controlled formation of well-separated silver islands direct investigation of the local origin of the Raman enhancement is greatly facilitated. We summarize the effects of experimental parameters during and after vapor-deposition of silver on the resulting surface morphology. By choosing suitable experimental conditions, particle sizes and shapes, as well as the interparticle distances can be controlled such that well-separated silver islands are formed that still show high Raman enhancement. The surfaces were characterized morphologically by means of Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Optical characterization was carried out by using UV/Vis and Raman spectroscopy.
Raman spectroscopy has been implemented as an extremely powerful tool for the vibrational analysis of solid phase synthesis throughout the development of combinatorial library scaffolds. In this report, the application of both on-line and batch mode Raman spectroscopic analysis will be presented for the comprehensive characterization of a multi-step small molecule synthesis. Throughout this application, near-infrared Raman spectroscopy also provides unprecedented utility for on-bead qualitative and quantitative investigations. Finally, the on-line experimental procedure presents the first opportunity for open loop feedback control of synthetic parameters through the real-time presentation of reaction progression by using Raman spectroscopy.
Two types of nanosecond temperature jump (T-jump) apparatuses applicable to time-resolved Raman measurements were constructed. T-jump was achieved by direct heating of water by using near-infrared (NIR) pulses at 1.89 mm in one type and at 1.56 m m in the other. The two NIR pulses were generated through stimulated Raman scattering (SRS) of H2 or D2 excited by the fundamental line of a Q-switched Nd:YAG laser, in which a single pass configuration with H2 was sufficient for 1.89 m m pulses but a seeding-amplification configuration with D2 was necessary for 1.56 m m pulses. The seeding-amplification configuration yielded significant improvements in conversion efficiency, pulse-to-pulse stability, and beam quality. These apparatuses were applied to transient Raman measurements of MoO42- solution and transient temperatures of the heated volume were determined from ratios of anti-Stokes to Stokes Raman intensities. Temporal behaviors of the temperature of heated volume upon illumination of nanosecond heat pulses at 1.89 m m or 1.56 m m were explored and its applicability to studies on the primary process of thermal reactions was examined. It became clear that the continuation time of raised temperature is determined only by replacement of sample in the case of thick cell and by both thermal transfer and sample replacement in a case of thin cell, while thermal diffusion is not effective for both cells.
A non-destructive analytical method based on the use of Fourier-Transform Raman Spectroscopy (FT-RS) for identification and quantitative analysis of hydroxyapatite (HAP) and collagen (COL) in bones was developed. Calibration curve based on the Raman spectra of known powder mixtures of HAP-COL was constructed by using the Raman bands at 960 cm-1 for HAP and 2937 cm-1 for COL. It was found that : I960/I2937 = 0.46/WC - 0.36 where I960 and I2937 are the relative intensities for the 960 and 2937 cm-1 Raman bands, respectively, and WC represents the weight fraction of collagen. The detection limit for HAP and COL was found to be 2.8 and 0.6 wt%, respectively. The potential of using RS for an in vitro point-by-point analysis ("mapping") of bones was demonstrated successfully on an artificially de-mineralized ox femur sample.
We present a new approach for quantitative chemical analysis by Raman spectroscopy on the example of phosphate analysis in highly concentrated industrial raw phosphoric acid. The proposed method is based on the direct measurement of the Raman shift of the n s(P-OH) band of phosphoric acid that suffers a blue shift upon increasing concentration. Correlation of the exact band position rather than band intensity with the concentration of the analyte eliminated the need to reference the measured Raman signal to an internal or external standard that is usually needed to account for changes in laser power or optical throughput. A calibration curve ranging from 100 – 1400 gL-1 phosphoric acid was established (coefficient of variation of the method, Vxo: 0.99 %, analysis time: 7 min per sample). The applicability of the developed method to industrial samples was proven by an interference study including sulfate, magnesium, aluminum, and iron(III) as well as by comparing the results to those of gravimetry. The observed peak shift due to changes in phosphate concentration was investigated by 2D correlation analysis that provided valuable insight in the inter- and intra-molecular interactions in highly concentrated phosphoric acid.
A method is described for determining petroleum fuel contamination in water based on solid-phase microextraction and Raman spectroscopy (SPME/Raman). In this method, contaminants are extracted from aqueous solutions into a solid phase and then detected directly in it by using spontaneous Raman spectroscopy. The solid phase consists of a small volume (~20 µL) of poly(dimethylsiloxane) that has an optical window in the 830-1600 cm-1 Raman shift region. This region is suitable for determining both aromatic and aliphatic fuel components. Four "real world" fuels - aviation gasoline, unleaded gasoline, jet fuel A, and #l fuel oil - were used to test this new method. As shown, the SPME/Raman determinations performed in the visible spectral region were not adversely affected from the fluorescence of the marker dyes in these fuels. The linear dynamic ranges for the determinations exceeded the water solubility limits and spanned 1-3 orders of magnitude. The limits of detection using the most sensitive Raman bands in each fuel were in the 8-12 ppm range with the exception of #1 fuel oil (25 ppm), and relative standard deviations typically were 2-10%. Finally, the SPME/Raman method was applied to the determination of total petroleum hydrocarbons in natural water matrices. Using hexane and undecane as standards, the results of the determinations compared favorably to the standard methods of purge-and-trap gas chromatography and liquid-liquid extraction followed by infrared detection,
Tricarbonyl(cyclohexadiene)iron(0) reporting groups have been derivatized by attachment of an aminopropylsilyl link and attached covalently to silica. The IR properties of the organometalcarbonyl moiety have been investigated by Fourier Transform infrared spectroscopy (FT-IR) and have been shown to retain their capacity to function as a key element of a molecular sensor assembly. The ?(CO) vibrational frequencies of the immobilized complexes shift with changes in the solvent environment when exposed to binary and ternary solvent systems. Principal components regression (PCR) was used to correlate the ?(CO) shifts of the probe group quantitatively with solvent composition with a RMSEP error of 0.9% for a binary solvent system.
In this work an innovative etching technique for tapering silver halide fibers is introduced. As silver halides form soluble complexes with thiosulfate in aqueous solution, the fiber can be chemically tapered by an etching process, which also warrants a high quality of the fiber surface. The evanescent field sensitivity of thus obtained tapered fibers was raised by more than one order of magnitude, demonstrated by calibration curves of tetrachloroethylene in hexane recorded with a tapered sensor fiber coupled to a Fourier transform infrared (FT-IR) spectrometer.
A real-time, spectral-multiplex method for the complete separation of circular dichroism (CD) spectra from linear birefringence (LB) spectra is presented. The method, called dual polarization modulation (DPM), involves the introduction of a second source of polarization modulation after the CD sample. The first source of polarization modulation, as in conventional CD spectrometers, is located before the sample. Intensity signals at the detector in-phase with each of the two polarization modulation frequencies are demodulated simultaneously in parallel and combined electronically in opposition to eliminate the LB spectrum by real-time cancellation. The accuracy of the cancellation can be adjusted electronically without the need to change the optical alignment of the instrument. The DPM method permits baseline-corrected CD spectra to be measured without the need of a subsequent CD background measurement.
A calibration procedure is presented to relate Fourier transform infrared reflection-absorption spectroscopy (IRRAS) spectra obtained by polarization modulation (PM) and conventional techniques. This procedure concerns only PM-IRRAS measurements obtained by using a lock-in amplifier. At the first stage, it consists in measuring two calibrated spectra, by adding a linear polarizer after the sample. The corresponding corrected PM-IRRAS spectra display band intensities close to those observed on IRRAS spectra (at about 6%) but the value of the baseline is lower. The calibration procedure can be performed at a second stage, by normalizing the corrected PM-IRRAS spectra of the sample and the bare substrate. In this case, the band intensities and the baseline of the normalized corrected PM-IRRAS and IRRAS spectra are similar.
This paper presents IR spectra of ß-D-glucose, cellobiose, and cellulose in the 1500-850 cm-1 range as well as the results of their deconvolution. Comparison of the results of deconvolution with the IR spectra of the investigated compounds obtained at room and helium temperatures of the sample as well as with the theoretical data has been made. It is shown that the use of the deconvolution method provides a better, compared to the low temperature spectra, resolution of individual absorption band components for all the compounds being analyzed, especially for the polysaccharide cellulose. The results of deconvolution of IR spectra of monosaccharide ß-D-glucose and disaccharide cellobiose are in good agreement with the data of normal coordinate analysis. It has been found that the minimum values of band halfwidths in the room temperature IR spectra of ß-D-glucose and cellobiose approximately coincide while for cellulose they increase by a factor of about 1.5.
2D FT-IR spectroscopy was applied to study the slower states of the membrane protein bacterioRhodopsin (bR) photocycle, with bR adsorbed on a ZnSe ATR-crystal. The M and the N state of the bR photocycle could be discriminated and 2D-spectra revealed a N state absorption band at 1536 cm-1 that is normally covered by the broader and bigger 1527 -1 M state absorption band. 2D FT-IR absorbance changes are presented in a more convenient way for analysis than in traditional 2D FT-IR spectroscopy by using 2D-phase spectra and phase/amplitude calculations.
A method has been developed for measuring formaldehyde in single puffs of mainstream (MS) and per cigarette in sidestream (SS) smoke by using tunable diode laser (TDL) infrared spectroscopy. Thirty to fifty percent of the total MS formaldehyde delivery per cigarette is generated in the lighting puff. This phenomenon is unique to formaldehyde compared to most other gaseous smoke components. The effect of the lighting technique, packing density of the tobacco in the cigarette rod, flow rate of air (heating rate) through the cigarette coal (burning zone), and tobacco type on the formaldehyde levels were studied. Calibration was based on measuring the second derivative spectral response of formaldehyde generated from certified paraformaldehyde permeation tubes. The accuracy of the measured value of this standard was determined to be within ± 0.7% of the certified formaldehyde delivery by performing periodic gravimetric measurements. In addition, the formaldehyde standard was collected in a water trap and analyzed using a colorimetric analytical technique, providing an agreement of ± 2 % with the TDL measurement. The TDL method precision was Ł 1% using the permeation standard. The MS formaldehyde deliveries for a Philip Morris monitor and Kentucky Reference 1R4F cigarette were 37 ± 5 and 27 ± 5 m g/cigt., respectively. The 1R4F value was in close agreement to results using an HPLC method reported in the literature. The MS sampling system can be quickly modified to determine formaldehyde levels in sidestream smoke. The SS formaldehyde delivery for the Philip Morris monitor cigarette was 2.1 ± 0.1 mg/cigt., (i.e., 0.27 ± 0.01 mg/min). The range of SS deliveries for reference, experimental, and commercial brands was much less than that observed for the MS smoke deliveries of similar cigarettes.
Derivative variable-angle synchronous spectrometry is developed to improve the selectivity of fluorescence measurements without loss of sensitivity. The first derivative variable-angle synchronous scanning permits the rapid simultaneous determination of tetracaine and procaine in a mixture from a single spectrum. The analyses were performed in 90% ethanol medium and 8.10-3 M NaOH. These local anesthetics can be determined in the presence of large amounts of cocaine. Talc (hydrous magnesium silicate) and kaolin (aluminum silicate hydroxide) are the most common compounds that can be present in cocaine samples but they can be eliminated by filtration, because they are insoluble in ethanol. Possible interference by other local anesthetics likely to be present along with the compounds to be determined have been investigated. A complete and exhaustive statistical analysis of the experimental data was realized to demonstrate the validity of the method. The detection limit according to Long and Winefordner was 2.6 µg L-1 for procaine and 1.20 µg L-1 for tetracaine and the detection limit proposed by Clayton was 3.00 µg L-1 for procaine and 1.38 µg L-1 for tetracaine.
Fluorescence from sugar crystal samples has previously been used to obtain information about factory imprint and sugar quality. Solid-phase fluorescence has potential as a fast screening method but the spectra are highly influenced by the measurement geometry and sugar crystal sample. The aim of the present study was to examine how the fluorescence measurements are related to the sugar crystals for a better understanding of both. Initially, five sugar samples of varied composition were sieved into five crystal size fractions. Fluorescence excitation-emission landscapes of the fractions were measured with solid transmission and reflection techniques and in solution. The transmission fluorescence was quenched at ultraviolet wavelengths and light scatter highly influenced the reflection fluorescence. Principal component analysis (PCA) showed that large crystals favored the transmission fluorescence whereas smaller crystals improved the reflection fluorescence measurements. The multi-way method PARAFAC was used to resolve spectra of individual components from the fluorescence landscapes. Transmission and solution components had similar spectral profiles at higher wavelengths characterizing a colorant and a colorant intermediate. The resolved components of the reflection data were very influenced by scatter. Color predictions based on a few significant wavelength variables equaled the model results of full-spectrum models by using partial least-squares regression (PLS). The variables corresponded to wavelength maxima of the resolved colorants and ultraviolet wavelengths characterizing colorant precursors.
Spectroscopic Techniques
FT-IR imaging yields large volumes of spatially specific spectral data in a short time. While chemical information about a specific sample area is a goal, complete morphological visualization similar to that obtained by optical microscopy is desirable. To obtain morphological images from this large data volume with the highest possible fidelity, specific techniques are required. However, in general, fast FT-IR imaging (3-5 min of collection time) leads to noisy morphological images due to hardware limitations. The data can be transformed by using transformations to eliminate noise. Image display techniques and filters can then be used to further refine display and carry out quantitative morphological analysis. This paper classifies various techniques and demonstrates their applicability to FT-IR absorbance image analysis.
A new sample preparation procedure for the analysis of molybdenum powder by direct -current/arc optical emission spectrometer for chromium, iron, and nickel is introduced. This procedure requires treatment of an oxidized molybdenum sample with nitric acid (HNO3) in order to insure homogeneity of the analyzed subsample. In this work, it has been shown that the Mo-Cr-Fe-Ni scales formed on the surface of the reduction tubes and boats during the hydrogen reduction process of molybdenum oxide are the main source of Cr-Fe-Ni contamination in the molybdenum powder. The formation mechanism of those scales is out of the scope of the present work. In this study, it was assumed that the chromium in Mo-Cr-Fe-Ni scales exists partly as a metal and partly as an oxide, which is why it is impossible to dissolve a contaminated sample of molybdenum powder completely in any commonly used mineral acid. The use of perchloric acid (HClO4) was not taken into consideration, since it has explosive and hazardous properties that make it difficult to use for routine sample preparation. For these reasons, the application of techniques that require a complete dissolution of the sample prior to analysis (AA, ICP-AES, ICP-MS, for example) can lead to low chromium results. It has been shown that in using a bigger subsample weight and an acid treatment procedure can dramatically improve the precision of chromium, iron, and nickel determination in the reduced molybdenum powder by using direct-current/arc optical emission spectrometry.
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