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At least 469 records · Page 26Linked to original sources
PubMed2004

Tip-enhanced optical spectroscopy.

Spectroscopic methods with high spatial resolution are essential for understanding the physical and chemical properties of nanoscale materials including biological proteins, quantum structures and nanocomposite materials. In this paper, we describe microscopic techniques which rely on the enhanced electric field near a sharp, laser-irradiated metal tip. This confined light-source can be used for the excitation of various optical interactions such as two-photon excited fluorescence or Raman scattering. We study the properties of the enhanced fields and demonstrate fluorescence and Raman imaging with sub-20 nm resolution.

Lasers↗
PubMed2004

Resonant Raman spectroscopy of nanotubes.

Single and double resonances in Raman scattering are introduced and six criteria for the observation and identification of double resonances stated. The experimental situation in carbon nanotubes is reviewed in view of these criteria. The evidence for the D mode and the high-energy mode is found to be overwhelming for a double-resonance process to take place, whereas the nature of the radial breathing-mode Raman process remains undecided at this point. Consequences for the application of Raman scattering to the characterization of nanotubes are discussed.

Carbon↗
PubMed2004

The potential use of Raman mapping to investigate in vitro deposition of combination pressurized metered-dose inhalers.

Scanning near-infrared Raman microscopy has been used to map aerosol particulate deposits produced from pressurized metered-dose inhalers (pMDI). A commercially available combination asthma therapy pMDI (Ventide, Allen and Hanbury, UK), containing salbutamol and beclometasone dipropionate, was analyzed by conventional in vitro quantitative analysis and scanning Raman microscopy. Raman maps, taken from Andersen cascade impactor plate stages 3 and 5 (over 100 x 100 microm areas) suggested good correlation with chemical analysis of the respective stages. Scanning Raman microscopy allows visual differentiation between formulation components (not possible using conventional imaging techniques), while potentially allowing chemical quantification.

Aerosols↗
PubMed2006

Comparison of derivative preprocessing and automated polynomial baseline correction method for classification and quantification of narcotics in solid mixtures.

This work offers a real-world comparison of derivative preprocessing and a new polynomial method described by Lieber and Mahadevan-Jansen (LMJ) for baseline correction of Raman spectra with widely varying backgrounds. This comparison is based on their outcomes in factor analysis, analyte discrimination, and quantification. Both correction methods are applied to a Raman spectra data set taken from 85 solid samples of illegal narcotics diluted with various materials. It is found that neither approach outperforms the other, as they give similar principal component analysis (PCA) models and quantification errors: cocaine and heroin show cross-validation errors of approximately 8%, while MDMA is quantified to a cross-validation error of approximately 3-4%. The LMJ method does offer several other advantages, the most significant being the retention of original peak shapes after the correction, which simplifies the interpretation of the preprocessed spectra. The LMJ method is therefore recommended for use as a baseline correction method in future research with Raman spectroscopy.

Algorithms↗
PubMed1980

Resonance Raman studies of the primary photochemical event in visual pigments.

Resonance Raman multicomponent spectra of bovine rhodopsin, isorhodopsin, and bathorhodopsin have been obtained at low temperature. Application of the double beam "pump-probe" technique allows us to extract a complete bathorhodopsin spectrum from the mixture in both protonated and deuterated media. Our results show that the Schiff base of bathorhodopsin is fully protonated and that the extent of protonation is unaffected by its photochemical formation from either rhodopsin or isorhodopsin. The Raman spectrum of bathorhodopsin is significantly different than that of either parent pigment, thus supporting the notion that a geometric change in the chromophore is an important component of the primary photochemical event in vision. A normal mode analysis is carried out with particular attention devoted to the factors that determine the frequency of the C=N stretching vibration. We find that the increased frequency of this mode in protonated relative to unprotonated Schiff bases is due to coupling between C=N stretching and C=N-H bending motions, and the shift observed upon deuteration of the Schiff base can also be understood in these terms. Various models for the primary event are discussed in light of our experimental and theoretical results.

Animals↗
PubMedDate not supplied

Direct detection of 8-oxo-deoxyguanosine using UV resonance Raman spectroscopy.

8-oxo-deoxyguanosine (8-oxo-dG) is a major oxidative lesion in DNA and is responsible for mutation and cancer. Current techniques for detecting 8-oxo-dG are indirect methods. Thus, development of new methodologies is needed to directly detect such oxidative lesions. In this article, we have used ultraviolet resonance Raman (UVRR) spectroscopy as a novel analytical technique for the detection of 8-oxo-dG. Here, the UVRR spectrum of 8-oxo-dG was acquired and compared to that of deoxyguanosine (dG) and deoxyadenosine (dA). Data analysis shows a distinct UVRR spectrum of 8-oxo-dG with characteristic peaks. Detection of 8-oxo-dG was easily achieved from a mixture with dG. These results reveal that UVRR spectroscopy shows promise as a direct method for detecting 8-oxo-dG.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed2005

Quantitative Raman reaction monitoring using the solvent as internal standard.

Despite its potential, the use of Raman spectroscopy for real-time quantitative reaction monitoring is still rather limited. The problems of fluorescence, laser instability, low intensities, and the inner filter effect often outscore the advantages as narrow bands, the use of glass fibers, and low scattering of water and glass. In this paper, we present real-time quantitative monitoring of the catalyzed Heck reaction by using the solvent as internal standard. In this way, all multiplicative distortions, e.g., laser intensity variations or absorbance of the laser light, can be corrected for. We also show that a limited amount of fluorescence does not hamper the analysis. Finally, we present a new method to correct for the inner filter effect, i.e., the absorbance of Raman scattered light by the reaction medium. Simultaneous absorption measurements of the reaction mixture enable accurate correction of Raman signals for the inner filter effect. Thus, for reaction monitoring applications, a Raman spectrometer should be equipped with an absorbance measurement device.

Acrylates↗
PubMed2004

Coherent anti-stokes raman scattering spectral interferometry: determination of the real and imaginary components of nonlinear susceptibility chi(3) for vibrational microscopy.

We demonstrate coherent anti-Stokes Raman scattering (CARS) heterodyne spectral interferometry for retrieval of the real and imaginary components of the third-order nonlinear susceptibility (chi(3)) of molecular vibrations. Extraction of the imaginary component of chi(3) allows a straightforward reconstruction of the vibrationally resonant signal that is completely free of the electronic nonresonant background and resembles the spontaneous Raman spectrum. Heterodyne detection offers potential for signal amplification and enhanced sensitivity for CARS microscopy.

Algorithms↗
PubMed1986

Dedicated monitoring of anesthetic and respiratory gases by Raman scattering.

The monitoring of respiratory and anesthetic gases in the operating room is important for patient safety. This study measured the accuracy and response time of a multiple-gas monitoring instrument that uses Raman light scattering. Measurements of oxygen, carbon dioxide, nitrogen, nitrous oxide, halothane, enflurane, and isoflurane concentrations were compared with a gas mixer standard and with measurements made with an infrared anesthetic agent analyzer. Correlation coefficients were all greater than 0.999, and probable errors were less than 0.43 vol% for the gases and less than 0.03 vol% for the volatile anesthetics. Response time was 67 ms with a sample flow rate of 150 ml/min. There was some signal overlap between nitrogen and nitrous oxide and between the volatile anesthetic agents. Such overlap can be compensated for by linear matrix analysis. The Raman instrument promises a monitoring capability equivalent to the mass spectrometer and should prove attractive for the monitoring of respiratory and anesthetic gases in the operating room.

Air↗
PubMed2000

Ab initio inter-radical potential for p-phenylenediamine radical cation dimer.

Ab initio molecular orbital calculations were carried out to investigate the inter-radical interaction of the paired p-phenylenediamine radical cations in the singlet state. After initial optimization of the dimer in the parallel sandwich (D2h) and parallel displaced (Cs) configurations at the B3LYP/6-31G* theoretical level, the MP2/6-31G* and B3LYP/6-31G* single energies of the dimer were calculated as a function of the inter-radical distance R. The depths of the potential minima near R = 3.2 A were estimated to be in the order of the hydrogen bonding energy, assuming that the electrostatic contribution between the cations is canceled out by the attractive contributions due to the counter anions on the aspect of a simple electrostatic model. This can be related to the indications of the cation dimer formation in solution in the presence of counter anions at a low temperature reported previously in the literature by resonance Raman and electronic absorption spectra. The inter-radical (Raman active) frequencies of the dimer were calculated, one of which corresponds to the reported value at 161 cm(-1) observed in the resonance Raman spectrum in ethanol at 200 K by Yokoyama and Maeda (Chem. Phys. Lett. 48 (1977) 59).

Cations↗
PubMed2004

Normal coordinate analyses and barrier to internal rotation of nitroso- and nitroazides.

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.

Azides↗
PubMed2002

Near-infrared FT-Raman spectroscopy as a rapid analytical tool for the determination of diltiazem hydrochloride in tablets.

This study was performed to develop a fast and reliable analytical method for the quantitative determination of diltiazem hydrochloride in tablets. HPLC is currently the preferred method, but is time consuming due to extensive sample preparation. FT-Raman spectroscopy was used to quantitatively analyse diltiazem hydrochloride in commercially available tablets (Tildiem) and in experimental tablets prepared at lab-scale. The percentage of diltiazem hydrochloride in each tablet was determined by calculating-after vector normalisation-the total peak area of the spectral band between 1625 and 1560 cm(-1). No spectral interference from tablet excipients was seen at this spectral band. After FT-Raman spectroscopy the same samples were analyzed based on the HPLC method described in the USP XXIV. The drug dosage per tablet obtained from the vibrational spectroscopy method correlated well with the results obtained using HPLC analysis for both the commercial tablets (HPLC: 63.57+/-0.13 mg; Raman: 63.28+/-0.26 mg (n=50)) and the experimental tablets (HPLC: 181.02+/-0.25 mg; Raman: 181.22+/-0.35 mg (n=50)). FT-Raman is a reliable alternative for the HPLC method to quantify the amount of diltiazem hydrochloride in tablets. The spectroscopic method is faster because it eliminates sample pre-treatment. Furthermore the FT-Raman method has an additional advantage of not requiring solvents.

Chromatography, High Pressure Liquid↗
PubMed2005

Flow-through microdispenser for interfacing micro-HPLC to Raman and mid-IR spectroscopic detection.

A flow-through microdispenser has been coupled to a micro HPLC separation system and used as a solvent elimination interface for Fourier transform infrared (FTIR) and Raman spectroscopic detection of the separated compounds. Using the microdispenser picoliter sized droplets can be generated and deposited on an appropriate target placed on a computerized x, y-stage. Evaporation of volatile solvent and buffer is rapid and allows analysis of the obtained dry deposits by various techniques. Due to the destruction free character of Raman and FTIR spectroscopy they can be applied sequentially to interrogate the same deposit. In the reported application five phenolic acids typically present in wine have been separated on a C-18 column technique using a mixture of water, methanol and acetic acid as mobile phase. For spectrum acquisition infrared and Raman microscopes have been used. The spectra recorded from the dried deposits of the separated compounds agreed well with the reference spectra of corresponding components.

Chromatography, High Pressure Liquid↗
PubMed2005

DFT calculations, structural and spectroscopic studies on the products formed between IBr and N,N'-dimethylbenzoimidazole-2(3H)-thione and -2(3H)-selone.

The NBO charge distribution calculated at DFT level on the [LEX](+) species [LE=N,N'-dimethylbenzoimidazole-2(3H)-thione (3) and -2(3H)-selone (4)(Scheme 1); X=I, Br] suggests that the most likely products from the reaction 3 of 4 and with IBr are the 10-X-2 charge-transfer (CT) adduct and the 10-Se-3 "T-shaped" hypervalent adduct featuring a linear Br--Se--I system, respectively. This prediction is confirmed by the synthesis, and X-ray diffraction analysis of 3.IBr (I) and 4.I(0.72)Br(1.28)(II). In particular II, is a 10-Se-3 "T-shaped" hypervalent adduct containing an almost linear X--Se--X system [X--Se--X 179.07(3) degrees, X=I(0.36)/Br(0.64)], which is roughly perpendicular to the average plane of the benzoimidazole moiety. The FT-Raman spectra of I and II agree very well with their structural features. In particular, the complexity of the FT-Raman spectrum of II reflects the disorder in the X-ray crystal structure of this compound.

Algorithms↗