NIR FT Research Articles

Spectral distortion and sample heating in 1064 nm FT-Raman spectra of aqueous dispersions of dipalmitoylphosphatidylcholine

To estimate the spectral distortion and the sample heating in the 1150-700 cm −1 NIR FT - Raman spectrum of NIR-absorptive scattering sample, we measured 1064-nm excited FT - Raman spectra of H 2O and D 2O dispersions of dipalmitoylphosphatidylcholine (DPPC). To evaluate the differential NIR Raman signal absorption by sample, which may result in the spectral distortion, and the absorptions of Rayleigh scattering and exciting laser, which may cause sample heating, we measured the NIR absorption spectra of DPPC - H 2O and DPPC - D 2O dispersions. Turbidity measurements at 1064, 1200 and 1540 nm as a function of temperature were performed for the H 2O and D 2O dispersions to evaluate turbidity changes in the phase transitions. The following observations were made: (1) The NIR Raman signal absorptions free from the sample heating effect were estimated to be about 20% and 4–8% in the H 2O and D 2O dispersions, respectively, for the CC skeletal optical modes (SOMs) and for the 874 and 889 cm −1 vibrations. (2) The Raman intensity attenuations caused by sample heating were about 10% for the CC SOMs, while an anomalous spectral artifact was observed in the 874 and 889 cm −1 features in comparison to those in the 514.5 nm Raman spectrum. It was interpreted in terms of the difference in the thermal behavior between the 874 and 889 cm −1 bands, that is, the thermal behavior of the 874 cm −1 band was similar to that of the CC SOMs, while the 889 cm −1 Raman intensity anomalously changed in comparison to that of the corresponding 514.5 nm Raman band, which is free from sample heating resulting from visible-region absorption by sample: about 35% decrease was estimated for both of the H 2O and D 2O dispersions. The local laser sample heating in the 1064 nm Raman measurement more significantly affected the 870–890 cm −1 spectral features than the CC SOM vibrations. (3) From the observed differential turbidity at 1200 nm in the two phases, about the 6 and 5% changes in the Raman intensities of the CC SOMs were estimated during the pre-transition for the H 2O and D 2O dispersions, respectively, and about 5% and 4% changes were estimated during the main transition. Since it may be estimated that the energy which had been absorbed from the NIR laser was near, but not beyond the Δ H for the pretransition of DPPC, it was concluded that usual discussions in model and natural biomembrane studies using the relative intensities of the CC SOMs will be still useful in the 1064 nm FT - Raman investigation of DPPC as long as the sample temperature is thermostatically set up so that the temperature of the sample spot irradiated by laser does not elevate over the pretransition temperature during Raman measurement.

NIR FT Raman spectroscopy—a new tool in medical diagnostics

Raman spectroscopy may aid the development of new tools to make medical diagnostics more objective. By shifting the exciting radiation of Raman spectrometers from the visible to the NIR region, the competing fluorescence of normal cell components can be reduced. The fluorescence of organic molecules is virtually eliminated when excitation by the Nd: YAG laser radiation at 1064 nm is employed. In combination with optimized interferometers and remote probes, it is now possible to record Raman spectra in vivo in acceptable times. Software programs are developed which interpret the spectra to help in the identification of suspicious tissues and micro-organisms, and for establishing medical diagnosis. In each case, the information distributed over the whole spectrum has to be evaluated.

Reduction of the multiplex disadvantage in NIR FT Raman spectroscopy by the use of interference filters

One of the positive features of interferometers is their multiplex advantage. However, under some very special conditions, such as for the detection of a weak band in a spectrum with high background or very strong solvent bands, there may be a multiplex disadvantage. We have recorded the Raman spectra of benzene in CCl 4, phenylalanine in water and as a solid, and oxidized glutathione in water. The results show that the multiplex disadvantage can be eliminated by suitable filters which transmit only the weak band.

Near-infrared Fourier transform Raman spectra of protonated and deuterated trans-azobenzene isotopomers

Near-infrared Fourier transform (NIR FT) Raman spectra of five trans-azobenzene isotopomers and their protonated and deuterated forms were recorded. Trifluoroacetic acid was found to be a good protonation agent for measuring Raman spectra of protonated azobenzene. An assignment of the most relevant vibrational modes (NN stretching and N H N 2 H in-plane bending) of the protonated and deuterated species is proposed. The NN stretching band in trans-azobenzene isotopomers decreases by about 40 cm −1 upon protonation, while it varies within ± 10 cm −1 upon deuteration with respect to the unprotonated (undeuterated) form. A comparison is made with the protonated form of the isoelectronic trans- N-benzylideneaniline.

NIR FT Raman examination of phthalocyanines at 1064 nm

NIR FT Raman examination of phthalocyanines at 1064 nm

NIR FT Raman examination of phthalocyanines at 1064 nm

NIR FT Raman examination of phthalocyanines at 1064 nm

Raman studies of superionic phase transition in (NH4)4H2(SeO4)3

Abstract NIR FT Raman spectroscopy was used to investigate molecular groups dynamics in the vicinity of superionic phase transition in (NH4)4H2(SeO4)3. It was found that the low temperature phase transition becomes unstable about 15 K below the superionic phase transition Ts. The observed changes refer mainly to the dynamics of SeO4 and HSeO4 entities. The Raman spectra of PEO+ +(NH4)4H2(SeO4)3 polymer are presented as well.

NIR FT Raman spectroscopic studies of η-Al 2O 3 and Mo/η-Al 2O 3 catalysts

NIR FT Raman spectra excited at 1064 nm have been obtained from samples of Mo/η-Al 2O 3 catalysts after treatment at 773 K in vacuum for 12 h. The samples were heated in quartz tubes which were sealed and transferred directly to the spectrometers without exposure to the ambient atmosphere. The broad background previously observed in the NIR FT Raman spectra of hydrated Mo/η-alumina samples disappeared after the dehydration. Upon exposure to ambient air the broad background signal reappeared suggesting that the broad background feature can be attributed to surface hydroxyl groups present in the hydrated samples. Furthermore, it was observed that upon treating the η-Al 2O 3 with water the broad background signal disappeared and the presence of bayerite (Al(OH) 3) or boehmite (AlO(OH)) was detected depending on the experimental details. The identification of the various phases is supported by XRPD analysis. The overall conclusion is that the broad NIR FT Raman background can be attributed to surface hydroxyl groups in hydrated η-Al 2O 3.

NIR FT Raman spectroscopy in medical diagnosis

NIR FT Raman spectroscopy in medical diagnosis

Application of near‐infrared‐Fourier transform Raman spectroscopy in medical research

AbstractUsing NIR–FT Raman spectroscopy we have obtained useful spectra of many kinds of biomaterials. These spectra contain—with only simple sample preparation—information that can be used both for component analysis and for tissue identification. In addition, pathological changes in the spectra of eye lenses and of brain, liver and colon tissues are observed, demonstrating the usefulness of the technique as a tool of medical diagnosis.

Micro and two-dimensional NIR FT raman spectroscopy

The former major problem in conventional Raman spectroscopy in the visible range, the disturbing fluorescence of impurities, has now been eliminated: Raman spectra can be excited by light quanta in the near-infrared range, the energy of which is too low to excite fluorescence spectra. An inherent disadvantage of this technique, the v4-dependence of the intensity of the Raman radiation, is compensated for by using interferometers, which are more powerful, by a factor of several hundred, than grating spectrometers. Raman spectroscopy can now be applied to analyses of ‘real world samples’ bio materials, food, paintings, micro electronics and ‘new materials’, as well as to quality control of raw materials, to production and product control without special sample preparation. By using fiber bundles, Raman spectra can be recorded on line at the sample site, in containers and in real time. For successful recording of NIR FT Raman spectra of small samples a compromise between large lateral resolution and a large signal/noise ratio has to be found. Its theoretical base and practical approach is discussed. Confocal microscopes allow recording of NIR FT Raman spectra of small particles or inclusions. They can be coupled to the spectrometer by fiber optics, so that they may be placed at some distance from the spectrometer. By using computer-driven x-y stages, systematic mapping of the distribution of specific compounds on the surface of different samples is possible with the FT Raman microscope, as well as with the ordinary sample arrangement.

Time-Resolved and Two-Dimensional NIR FT-Raman Spectroscopy

The near-infrared Fourier transform (NIR FT) Raman technique permits the measurement of Raman spectra without interference by fluorescence. Absorption by molecules containing X-H bonds in the NIR range requires a 180° scattering geometry. In this way, Raman spectroscopy of samples on surfaces is possible, both the detecting of small spots and the mapping of the sample distribution over larger areas. The spatial resolution extends into the micrometer range. Mapping of the inorganic pigment distribution of an initial letter of a mediaeval manuscript is demonstrated. For time-resolved measurements, the step-scan technique, previously developed for infrared spectroscopy, may be used in NIR FT-Raman spectroscopy as well. It allows the study of photochemical and photophysical processes, the application of modulation techniques, and the investigation of “noisy” samples. Photo-isomerization of the dye merocyanine 540 has been observed with the step-scan technique upon periodic excitation with a flash lamp.

Can a Raman renaissance be expected via the near-infrared Fourier transform technique?

The near-infrared Fourier transform (NIR FT) Raman technique excludes virtually all fluorescence of the sample or of impurities. Therefore, it can solve many analytical problems that could not be handled with conventional Raman spectrometry in the visible range. The interferometers with the Fourier transform technique permit shorter recording times at a comparable signal-to-noise ratio compared with conventional techniques. An inherent limitation is that Raman lines very near the exciting line cannot be observed as FT Raman instruments must have a “secondary filter” which eliminates radiation at the frequency of the exciting line and the Rayleigh wings. An important advantage is the ability to investigate the spectra of light-sensitive material in the electronic ground state and, during illumination in the ultraviolet or visible range, the structural changes due to this illumination. The NIR FT technique closes a gap: together with the well established Raman techniques with dispersive single- and multi-channel detectors and the non-linear Raman effects, all techniques serve to apply Raman spectrometry to every problem where it could be appropriate: spectroscopists now have all the tools required to bring about the Raman renaissance.

An observation of resonance Raman in the near‐infrared

AbstractThe NIR FT Raman spectrum of bacteriorhodopsin is reported. By comparison with previously published spectra it appears that there is some degree of resonance enhancement. Also reported is the spectrum of a sample of blue dyed cotton. Resonance enhancement is proposed to explain strong features in the spectrum not present in either undyed or red dyed material.

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.