Library of FT-Raman spectra of pigments, minerals, pigment media and varnishes, and supplement to existing library of Raman spectra of pigments with visible excitation.

Sixty pigments, minerals and media have been analysed by Fourier-transform Raman (FT-Raman) microscopy in order to assemble a database of reference FT-Raman spectra for scientists working at the Arts-Science interface. An earlier library of Raman spectra compiled using visible excitation has been extended by the addition of 22 further reference spectra obtained with 780.0, 647.1, 632.8 and/or 514.5 nm excitation. The relative merits of 1064 nm and visible excitation are discussed.     

Infrared and Raman microspectroscopy of foreign materials in tissue specimens

Infrared and Raman spectra of materials found in tissue specimens submitted for histopathologic diagnosis have been recorded. These foreign materials range in size from approximately 5 to 50 microm, and the vibrational spectra have been used to identify them. Examples include cholesterol and polytetrafluoroethylene (PTFE) in an implant case, polyethylene terephthalate (PET) and polyacrylonitrile (PAN) in a pilonidal cyst, and carbenicillin in a skin biopsy. In some instances, either the infrared or Raman spectra were sufficient to make a definitive identification, while in other cases both were necessary. Because some of the samples fluoresced with visible excitation at 532 nm, FT-Raman spectra with 1064-nm excitation were also recorded. The flexibility of sampling for vibrational microspectroscopy and the value of the recorded data in assisting pathologists render medical diagnoses in the examples cited and other cases are discussed.     

Fourier transform Raman spectroscopy in the visible region

Low-resolution (ca. 4cm^–1) Raman spectra of various liquids and solids, excited by an argon laser at 448 and 514.5 nm, were recorded using a BOMEM DA3.02 commercial interferometer. These spectra were compared with those obtained in the same experimental conditions on a conventional dispersive spectrometer. It is concluded that for low-resolution Raman studies with excitation in the visible region, the dispersive method is superior to the interferometric technique.     

Fluorescence rejection in Raman spectra of Syncrude Sweet Blend distillation fractions

Four techniques for the reduction or elimination of fluorescence from Raman spectra of Syncrude process samples were examined in this study. These methods are based on the retrieval of Raman bands from differential, or derivative spectra. Differential data were generated by subtracting similar spectra of a given sample obtained in three ways: (a) shifted detection utilizing an array detector and two successive spectrometer settings; (b) shifted excitation (dispersive Raman) where the two spectra are recorded using neighbouring laser lines and ordinary photon counting; (c) shifted excitation (FT-Raman) in which the laser frequency is changed in software before acquisition of the second spectrum. In addition to these differential techniques, derivative spectra were acquired directly with a dispersive Raman system by modulating the wavelength during scanning. These fluorescence rejection methods were applied to two groups of Syncrude Sweet Blend distillation fractions. For light gas oils (boiling range, 195-343 degrees C) the ratio of monocyclic and bicyclic aromatic species was determined and bands due to aliphatic CH(n) groups were characterized. Heavy gas oils (343-524 degrees C) yielded bands that allowed quantitation of monocyclic, bicyclic and total aromatic groups. Bands due to aliphatics were also identified for the heavy gas oils. These results constitute a significant advance compared to the information obtainable using conventional dispersive and FT-Raman spectroscopy for the analysis of hydrocarbon distillation fractions.     

Non-destructive analysis of pigments and other organic compounds in lichens using Fourier-transform Raman spectroscopy: a study of Antarctic epilithic lichens

Lichens in Antarctic habitats are subjected to environmental extremes, including UVB radiation, desiccation and low temperatures, as well as to rapid fluctuations in these. Lichens synthesise a variety of chemical compounds in response to their environmental conditions which contribute towards their colour, and which act as protectants against physiological stresses. The fluorescence generated by the lichens at 532 nm can be used in epifluorescence microscopy to identify their presence on substrata but this can severely affect the Raman spectra using visible excitation. The advantage of the near infrared excitation used in FT-Raman spectroscopy in minimising fluorescence emission facilitates the molecular characterisation of lichen encrustations without having to remove the thallus from its substrate or remove or otherwise damage any part of the thallus. Spectroscopic biomarkers are proposed which allow the lichens to be characterised by the identification of characteristic lichen substances; the use of these biomarkers for the preliminary taxonomic identification of Antarctic lichens is examined and some potential pitfalls are described.     

On-line FT-Raman and dispersive Raman spectra database of artists’ materials (e-VISART database)

Raman spectroscopy has been widely applied in the analysis of different types of artwork. This technique is sensitive, reliable, non-destructive and can be used in situ. However, there are few references in the literature regarding specific Raman spectra libraries for the field of artwork analysis. In this paper, the development of two on-line databases with Fourier transform Raman (FT-Raman; 1064 nm) and dispersive Raman (785 nm) spectra of materials used in fine art is presented; both are implemented in the e-vibrational spectroscopic databases of artists materials database (e-VISART). The database provides not only spectra, but also information about each pigment. It must be highlighted that for each pigment or material several spectra are available from different dealers. Some of the FT-Raman spectra available in the e-VISART database have not been published until now. Some examples in which the e-VISART database has been successfully used are presented.     

Non-destructive Raman analyses--polyacetylenes in plants

Ferdinand Bohlmann has described the isolation, the identification and the structure elucidation of acetylene compounds in many plants, and confirmed it by its synthesis. We have recorded the Raman spectra of most of these plants non-destructively by FT-Raman spectroscopy using radiation at 1064 nm. We could not observe any interfering fluorescence. We found acetylene compounds in some plants, even distinct compounds with different concentration in various parts of it. The distribution of the different compounds over the plant can be observed and their changes during the ontogenesis can be followed by a FT-Raman mapping technique. Of special help is a library of Raman and IR spectra and the structure of the compounds, synthesized by Bohlmann. Thus, the Raman technique allows analyses in a very short time replacing the usual time-consuming separation procedures and avoiding artefacts during clean-up procedures.     

Raman spectrometry and neural networks for the classification of wood types—1

In this work the coupling of near infrared (NIR) Fourier-transform (FT) Raman spectroscopy and neural computing for spectral feature extraction and classification of woods is reported. A NIR FT-Raman spectrometer operating at 1064 nm was used for all measurements; particular attention was paid to the effects of sample fluorescence and heating. It was demonstrated that fluorescence rejection is accomplished only for the lighter colored woods and that fluorescence was found to be severe for 10 of the 71 woods studied in this work even using excitation at 1064 nm. It was further found that hardwoods were no more or less susceptible to sample heating than softwoods. Feed-forward neural networks were used to extract the principal features of wood spectra at resolutions of 4, 8 and 16 cm⁻¹ and to classify spectra as either temperate hardwoods or temperate softwoods. Neural networks were constructed using zero and two processing elements in the hidden layer. It was shown that neural networks with two hidden processing elements perform near optimally, since each hidden layer processing element may function as either a hardwood or softwood feature detector. This work represents the first time that FT-Raman spectroscopy and neural network technology have been coupled for spectral feature extraction and classification.     

Anti-Stokes Yb3+ emission--valuable structure information in spectra of rare earth compounds measured with FT-Raman spectrometers

Raman spectroscopy is a powerful and simple method which proved to be very useful in studies of solids. The most widely used Raman spectrometers are FT-Raman instruments with YAG:Nd(3+) laser as an excitation source. However, in the case of samples containing rare earth elements, the quality of FT-Raman spectra is often low due to strong fluorescence effects. We show that, in such cases, anti-Stokes part of the Raman spectra often contains strong, well resolved bands identified as multiphonon-assisted emission bands of Yb(3+) present as an impurity. We show on several examples that analysis of these bands may provide useful structure information, similar to that obtained by "Eu structure probe" method in optical spectroscopy. The Yb(3+) emission can be also measured using standard luminescence detection systems. However, the application of FT-Raman system allows one to obtain good quality spectra in a much cheaper, easier and faster way (in times as short as a few seconds). Moreover, high-sensitivity of FT-Raman spectrometers allows to detect even very small amounts of Yb(3+) impurity.     

Normal Raman and surface enhanced Raman spectroscopic experiments with thin layer chromatography spots of essential amino acids using different laser excitation sources

A comparative study of the feasibility and efficiency of Raman spectroscopic detection of thin layer chromatography (TLC) spots of some weak Raman scatterers (essential amino acids, namely, glycine and L-forms of alanine, serine, valine, proline, hydroxyproline, and phenylalanine) was carried out using four different visible and near-infrared (NIR) laser radiations with wavelengths of 532, 633, 785, and 1064 nm. Three types of commercial TLC plates were tested and the possibility of inducing surface enhanced Raman scattering (SERS) by means of Ag-sol was also investigated. The spectra obtained from spotted analytes adsorbed on TLC plates were of very different quality strongly depending on the excitation wavelength, the wetness of the samples, and the compounds examined. The best results were obtained with the simple silica TLC plate, and it has been established that the longest wavelength (lowest energy) NIR excitation of a Nd:YAG laser is definitely more suitable for generating normal Raman scattering of analyte spots than any of the visible radiations. Concerning SERS with application of Ag-sol to the TLC spots, 1-3 orders of magnitude enhancement was observed with wet samples, the greatest with the 532 nm radiation and gradually smaller with the longer wavelength excitations. It is shown, however, that due to severe adsorption-induced spectral distortions and increased sensitivity to microscopic inhomogeneity of the sample, none of the SERS spectra obtained with the dispersive Raman microscope operating in the visible region were superior to the best NIR normal FT-Raman spectra, as far as sample identification is concerned.     

Experimental aspects of Fourier transform Raman spectroscopy

Using a commercial Fourier transform infrared spectrometer and the 1.064m line of a CW NdYAG laser, we have measured the Raman spectra of a wide variety of materials. The Raman scattered light, Stokes shifted toward the mid-infrared, is collected, using a 90° lens geometry, and focused through the emission port of the spectrometer. After passing through the Michelson interferometer, the light is detected by a thermoelectrically-cooled high-sensitivity germanium detector. The Fourier transform of the resulting interferogram gives the Raman spectrum. This new technique allows spectra to be obtained of samples which were previously completely masked by competing fluorescence. In addition, FT-Raman also allows moieties, such as hydrocarbon chains, which are not present in resonance enhanced spectra, to be investigated. We will discuss our approach toward FT-Raman, which is compatible with traditional Raman spectroscopy, present representative spectra of liquids and solids, and draw some comparisons and contrasts between dispersive and FT measurements.     

Fourier transform Raman spectrometry

Conclusions FT-Raman spectrometry has advanced significantly through the combination of CW Nd: YAG lasers, high sensitivity near-infrared detectors, and state-of-the-art filter technology. Obtaining fluorescence-free spectra through nearinfrared excitation has now become practical.Raman spectrometry provides data which is complementary in nature to infrared spectra and often helps provide unambiguous structural determination. Sample preparation, if necessary, is often easier than for the mid-infrared analysis. Sample categories for which Raman spectrometry can provide significant data include: aqueous soultions, catalysts [9], thin films, minerals and inorganics [10], and biological [9, 11] compounds, Typical scan times are 1 min or less. FT-Raman has truly become a powerful, general purpose analytical tool.     

In-situ surface-enhanced Raman scattering and FT-Raman spectroscopy of black prints

The black inkjet and laser prints were analysed with regard to application in forensic analysis of questioned documents. The purpose of this work was to study spectral properties and compare the suitability of surface-enhanced Raman scattering (SERS) with Fourier transform Raman spectra of prints. This work aimed to find optimal surface-enhanced Raman spectroscopic approach for the future analysis of documents using statistical methods. In this work, we analysed eight prints of four laser and four inkjet devices. The samples were measured using two dispersive Raman devices; (DXR Raman microscope with excitation line 532 nm, Foram 685-2 spectrometer − 685 nm) and FT-Raman device (Bruker Spectrometer MultiRAM with excitation line 1064 nm). The silver nanoparticles (AgNPs) colloid for SERS experiment were synthesised and checked by UV–vis spectroscopy and scanning electron microscopy (SEM). The remarkable differences caused by centrifugation of silver colloid were observed just in the SEM images. The main contribution of this paper is to propose the novel approach achieving sufficient SERS signal intensity of black prints using the both, laser and inkjet printers. Moreover, this method is based on just a single metal colloid, and the analysis can be performed in-situ, i.e. directly on the printed sample surface. We consider the SERS could by highly promising and universal for applications in the forensic analysis of printed documents with the combination of statistical method when conventional methods are not effective.     

UV resonance raman investigation of electronic transitions in alpha-helical and polyproline II-like conformations

UV resonance Raman (UVRR) excitation profiles and Raman depolarization ratios were measured for a 21-residue predominantly alanine peptide, AAAAA(AAARA) 3A (AP), excited between 194 and 218 nm. Excitation within the pi-->pi* electronic transitions of the amide group results in UVRR spectra dominated by amide vibrations. The Raman cross sections and excitation profiles provide information about the nature of the electronic transitions of the alpha-helix and polyproline II (PPII)-like peptide conformations. AP is known to be predominantly alpha-helical at low temperatures and to take on a PPII helix-like conformation at high temperatures. The PPII-like and alpha-helix conformations show distinctly different Raman excitation profiles. The PPII-like conformation cross sections are approximately twice those of the alpha-helix. This is due to hypochromism that results from excitonic interactions between the NV 1 transition of one amide group with higher energy electronic transitions of other amide groups, which decreases the alpha-helical NV 1 (pi-->pi*) oscillator strengths. Excitation profiles of the alpha-helix and PPII-like conformations indicate that the highest signal-to-noise Raman spectra of alpha-helix and PPII-like conformations are obtained at excitation wavelengths of 194 and 198 nm, respectively. We also see evidence of at least two electronic transitions underlying the Raman excitation profiles of both the alpha-helical and the PPII-like conformations. In addition to the well-known approximately 190 nm pi-->pi* transitions, the Raman excitation profiles and Raman depolarization ratio measurements show features between 205-207 nm, which in the alpha-helix likely results from the parallel excitonic component. The PPII-like helix appears to also undergo excitonic splitting of its pi-->pi* transition which leads to a 207 nm feature.     

Surface-enhanced Raman scattering of trans-1,2-bis (4-pyridyl)-ethylene on silver by theory calculations

Surface-enhanced Raman spectra of trans-1,2-bis (4-pyridyl)-ethylene (t-BPE) on silver foil were detected at laser line of 514.5, 633, 785 and 1064 nm, respectively. The structure of Ag-t-BPE, Ag4-t-BPE, Ag6-t-BPE, Ag10-t-BPE and Ag20-t-BPE complexes has been calculated using a local version of the Amsterdam density functional program package. The Raman spectra and electronic polarizability of t-BPE-Ag at 514.5, 633, 785 and 1064 nm excitation lines were calculated. The Raman bands of t-BPE were assigned according to the calculation of potential energy distribution. The experimental and calculated Raman spectra of t-BPE-Ag at 514.5, 633, 785 and 1064 nm were compared. The relative Raman intensities change at different excitation lines were discussed based on the Raman enhanced mechanism and surface selection rules.     

Excitation wavelength dependent surface-enhanced Raman spectra of a dipping film of azobenzene-containing long-chain fatty acid on a silver mirror

Surface-enhanced Raman scattering (SERS) of dipping films of azobenzene-containing long-chain fatty acids, nAmH (n=8, 12, m=3, 5), on silver mirrors measured with a wide range of excitation wavelengths in the 457.9-1064 nm region is reported. The obtained Raman spectra show great SERS effect even with the 1064 nm excitation, and the excitation with 457.9, 476.5, and 488.0 nm gives surface-enhanced resonance Raman scattering (SERRS) due to the resonance effect of the symmetry-forbidden n-pi* transition of the azo group. Of particular note in the present study is that the SERS spectra with the excitation in the 532-1064 nm region yield Raman bands whose frequencies are almost identical to those bands in Raman spectra of nAmH in solid state while the SERRS spectra with the excitation in the 457.9-514.5 nm region show not only a set of bands which correspond to those of nAmH in the solid state but also a set of bands whose frequencies show a significant shift from those of the bands of nAmH in the solid state. These observations lead us to conclude that there are two kinds of molecular aggregates in the dipping films of azobenzene-containing long-chain fatty acid in which azobenzene moieties are condensed to form small bundles.     

1064-nanometer-excited Fourier transform Raman spectroscopy of conducting polymers

Application of 1064-nm-excited Fourier transform (FT)-Raman spectroscopy to the characterization of conducting polymers is described. 1064-nm-excited FT-Raman spectra with high signal-to-noise ratios are obtained from polyacetylene (PA), poly(1,4-phenylene) (PPP), poly(1,4-phenylene vinylene) (PPV) and poly(2,5-thienylene vinylene) (PTV) in their neutral (insulating) state. The resonant Raman spectra of acceptor- or donor-doped (conducting) PA and PPV are also obtained wih 1064-nm excitation. The resonant Raman spectra of Na-doped PA change in two stages with increasing dopant concentration, the first change corresponding to the increase in electrical conductivity and the second to the appearance of a Pauli susceptibility. The 1064-nm-excited FT-Raman spectrum of Na-doped PPV indicates existence of negative bipolarons which are equivalent to divalent anions extending over a few repeating units in the polymer chains.     

活性碳纤维阴极电芬顿反应降解微囊藻毒素研究

以具有高比表面积的活性碳纤维作为阴极,通过电芬顿反应降解水中微囊藻毒素(MCRR,MCLR)的电化学方法系统考察了电流密度、pH值和Fe2+浓度等因素对微囊藻毒素降解效果的影响.实验结果表明,在Fe2+浓度为1.0mmol/L和电流密度为6.6mA/cm2条件下,电化学处理60min,MCRR(8.81mg/L)去除率为75%,MCLR(6.36mg/L)去除率为94%.证明过氧化氢可以通过电化学还原在活性碳纤维阴极表面高效产生,微囊藻毒素可被高效降解去除.     

Raman spectroscopic study of the tautomeric composition of adenine in water

An experimental and theoretical study of the tautomeric composition of adenine (Ade) in water using Raman spectroscopy is reported. Experimental resonance Raman spectra of adenine at excitation wavelengths of 200, 218, and 266 nm were compared with quantum-mechanical calculations of N(9)H- and N(7)H-adenine tautomers and their cations. Both theoretical and experimental studies of nonresonance Raman spectra (457 nm excitation) of adenine were also performed for comparison. A satisfactory agreement of the calculated results with the experimental data was obtained. The Raman spectra are interpreted, and the basic regularities of the Raman intensity distribution are explained. On the basis of the analysis performed, the tautomeric composition of adenine in water is revealed. It is shown that the Ade-N(9),N(1)H(+) cation is the predominant form and that some neutral forms of Ade-N(9)H and Ade-N(7)H tautomers exist in water at pH 3.     

Mode-focusing in molecules by feedback-controlled shaping of femtosecond laser pulses

The feasibility of mode-selective excitation with broadband femtosecond laser pulses is demonstrated for toluene in liquid phase. A learning-loop optimal control scheme was applied to a stimulated Raman excitation process. Modifications of the phase shape of one of the exciting pulses resulted in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. An evolutionary algorithm guided the coherent excitation process to a selective enhancement or suppression of one or more vibrational modes over the complete coherence lifetime spanning several picoseconds. New ways of spectral filtering as well as exciting possibilities of mode-selective studying of chemical reaction dynamics are indicated.