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.     

Fourier transform Raman instrumentation

This paper gives an outline of the steps necessary to convert an FTIR interferometer, with NIR capability, into an NIR FT Raman spectrometer. The example of the conversion of a Perkin-Elmer 1720X FTIR instrument is used to bring out the important points. Where appropriate, explanations are given for the choices of particular components. Finally, two examples of possible accessories are given, i.e. the acquisition of depolarization ratios and remote Raman spectra.     

Fourier transform Raman spectroscopy

The basic experimental aspects of Fourier transform Raman Spectroscopy are reviewed with an emphasis on detector technology. The sensitivity is comparable to dispersive Raman Spectroscopy using visible lasers. The ease of spectral subtraction is demonstrated and examples are given showing the elimination of fluorescence.     

Some consequences of the Fourier transform in Fourier transform Raman spectroscopy

This paper reviews some of the differences between dispersive and Fourier transform (FT) Raman spectroscopy with the goal of highlighting some of the advantages and disadvantages of FT-Raman spectroscopy. In particular, the use of filters, Connes advantage, trading rules and the size of the multiplex and throughput advantages are discussed.     

Fourier transform Raman spectroscopy of kandite clays

The Raman spectra of the kandite clay minerals, kaolinite, halloysite, dickite and nacrite, have been measured in the 180–3000 cm⁻¹ region using Fourier transform near-IR Raman spectroscopy. These clays have a very small Raman cross-section and long data collection times were often required to obtain good spectra. Each clay has its own unique characteristic Raman spectrum which enables each kandite to be identified easily. In contrast, it is quite difficult to distinguish kandite clays by IR spectroscopy. Nacrite and dickite have relatively intense Raman peaks in the 1000–1100 cm⁻¹ region, whereas kaolinite is characterized by an intense peak at 685 cm⁻¹ and halloysite at 470 cm⁻¹.     

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.     

Tracking infrared signatures of drugs in cancer cells by Fourier transform microspectroscopy

Aimed at developing accurate, reliable and cost-saving analytical techniques for drugs screening we evaluated the potential of Fourier Transform (FT) InfraRed (IR) microspectroscopy (microFTIR) as a quantitative pre-diagnostic approach for the rapid identification of IR signatures of drugs targeting specific molecular pathways causing Chronic Myeloid Leukemia (CML). To obtain reproducible FTIR absorbance spectra at the necessary spatial resolution we optimized sample preparation and acquisition parameters on a single channel Mercury-Cadmium-Telluride (MCT) detector in the spectral interval of frequencies from 4000 to 800 cm(-1). Single K562 cells were illuminated by Synchrotron Radiation (SR) and a number of ~15 K562 cells spread in monolayer were illuminated by a conventional IR source (Globar), respectively. Combining IR spectral data with the results of complementary biochemical investigations carried out in samples by different analytical methods we identified and cross-validated IR signatures of drugs targeting the oncogenic protein BCR/ABL and its associated abnormal tyrosine kinase activity in K562 cell line. Unsupervised pattern recognition performed by Hierarchical Cluster Analysis (HCA) clustered the spectra of single K562 cells in two distinct groups roughly corresponding to living and to apoptotic cells, respectively. The corresponding IR spectral profiles were assumed to represent drug-resistant and drug-sensitive cells. Significant variations with increasing percentages of apoptotic cells were observed after the treatment of K562 cells with drugs that directly or indirectly target BCR/ABL. In conclusion, we suggest that microFTIR associated with multivariate data analysis may be useful to assess drug compounds in ex vivo cancer cell models and possibly peripheral blast cells from CML patients.     

Quantitative aspects of near-infrared Fourier transform Raman spectroscopy

Three fundamental behaviors of vibrational spectroscopy data manipulation routinely associated with Fourier transform infrared (FTIR) spectroscopy are evaluated for near-infrared (NIR) Fourier transform Raman spectroscopy. Spectral reproducibility, spectral subtraction and sensitivity are examined relative to the NIR FT-Raman experiment. Quantitative predictive ability is compared for identical sets of samples containing mixtures of the three xylene isomers. Partial least-squares analysis is used to compare predictive ability. IR performance is found to be better than Raman, though the potential for method development using NIR FT-Raman is shown to be quite promising.     

Performance enhancement of a Fourier transform Raman microprobe

Improvements to an existing Fourier transform Raman microprobe are described which include the optimization of the spectrometer for use in the near-IR region, the laser input optics in the microscope and optics employed to couple the microscope to the spectrometer. Several examples are presented which demonstrate the effect of these improvements on the microprobe's spatial resolution, Rayleigh rejection and ability to analyze weak Raman scattering materials with moderate excitation powers. Polarized data from a single crystal and data from the use of holographic notch filters are also presented. Future improvements to the system are also 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.     

Synthetic polyisoprenes studied by Fourier transform Raman spectroscopy

Fourier transform Raman spectra are presented for the cis-1,4 and trans-1,4 isomers of polyisoprenes Vibrational intensities are used to determine quantitatively the amounts of each isomer in the microstructure. Improvements over previous work are suggested for the quantitative assessment of 1,4 microstructure. Also, changes in the Raman spectrum due to oxidative degradation show that preferential oxidative degradation for the vinyl-3,4 units occurs. The α and β forms of trans-1,4 polyisoprene were studied the ν(CC) bands resolved were identified 4 cm⁻¹ apart. A study of the copolymerization of methyl methacrylate with isoprene showed that the 1,4 form is the most favoured form produced on copolymerization. Accurate cis-1,4 and trans-1,4 microstructural information could not, however, be determined.     

Analysis of high-explosive samples by Fourier transform Raman spectroscopy

The application of spectroscopic techniques to the detection and identification of explosive materials is of considerable importance in forensic investigations. However, forensic scientists do not routinely use conventional Raman spectroscopy in their analysis. This is due to the problems of high background scatter and time-consuming sample alignment. The development of Fourier transform Raman spectroscopy has overcome these problems. Using this technique three samples of different batches of Semtex labelled Semtex A, Semtex B and Semtex C were analysed. Semtex A was found to contain the explosive cyclotrimethylenetrinitramine (RDX), Semtex B contained pentaerythritol tetranitrate (PETN) and Semtex C contained a mixture of RDX and PETN.     

Fourier transform Raman and IR spectra of snake skin

The Fourier transform (FT) Raman and IR spectra of the shed dorsal skin of the snake Elaphe obsoleta (American black rat snake) are reported. Vibrational spectroscopic assignments are proposed for the first time. Although good quality Raman spectra were obtained from the hinge regions using an FT Raman microscope, the dorsal scale regions fluoresced even with 1064 nm IR excitation. This was ascribed to pigmentation markings on the scales.     

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.     

Fourier transform Raman and infrared and surface-enhanced Raman spectra for rhodamine 6G

The vibrational spectra of rhodamine 6G (R6G) are discussed on the basis of Fourier transform infrared and Fourier transform Raman spectra obtained far from resonance which are compared with resonance Raman and surface-enhanced resonance Raman spectra obtained with excitation at 457.9 nm. The behaviour of several bands is described and tentative assignments are proposed. Stong resonance Raman effects are observed for bands assignable to xanthene ring stretching modes and also xanthene ring deformation modes. Some of these are sensitive to the complexing of R6G with silver colloids.     

Near-infrared Fourier transform Raman spectroscopy: Facing absorption and background

Visible wavelength excitation enables Raman spectra to be recorded successfully from approximately 10% of the “real, live” samples encountered in routine analytics without recourse to purification procedures. Fluorescence from impurities present in the sample often masks the Raman spectrum. This is especially true of the industrial environment. The great advantage of the newly-developed technique of near-infrared Fourier transform Raman spectroscopy (NIR FTR) is that fluorescence arising from sample impurity is not excited. Now about 90% of all samples show Raman spectra. However, it is possible to increase both the number of samples open to study using NIR FTR and to improve the quality of the spectra by optimizing the sampling arrangement. This involves taking into consideration the optical properties of the sample, especially the absorption spectrum and thermal emission characteristics, according to Planck's and Kirchhoff's laws. Only a few samples continue to show continuous backgrounds; this is sometimes true even if no background is apparent with visible excitation. The sources of such backgrounds are described, as are means to reduce or eliminate most of them.     

Monitoring of enzyme catalysed reactions by Fourier transform Raman spectrometry

Esterification of mandelic acid catalysed by Candida antarctica lipase B was studied by Fourier transform (FT) Raman spectrometry in non-aqueous medium. It was found that there is a strict correlation between the intensity of the Raman scattering and the activity of the enzyme. FT-Raman spectrometry seems to be a fast and reliable method for selecting the appropriate enzyme and for determining the optimal enzyme water content. In addition, valuable information can be obtained from the spectra regarding the mechanism of enzyme–substrate bonding.     

Fourier transform infrared and Raman spectral investigations of 5-aminoindole

Fourier transform infrared (FT-IR) and Raman (FT-Raman) spectra of 5-aminoindole has been recorded and analysed. The FT-IR spectrum of the compound was recorded in a BrukerIFS 66 V spectrometer in the range 4000-400 cm(-1) and the FT-Raman spectrum was also recorded in the same instrument in the region 3500-100 cm(-1). Observed frequencies for normal modes are compared with those calculated form normal co-ordinate analysis. The shift in the frequencies of the fundamental modes with the substituent amino group and the mixing of different normal modes are discussed with the help of potential energy distribution (PED) calculated through normal co-ordinate analysis.