Stefan Keller

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

Abstract 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 Plancks and Kirchhoffs 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.

NIR Raman spectroscopy in medicine and biology: results and aspects

Abstract Analyses of biomaterial by ‘classical’ Raman spectroscopy with excitation in the visible range has not been possible since the fluorescence of many essential constituents of all animal and plant cells and tissues overlays the Raman spectra completely. Fluorescence, however, is virtually avoided, when Raman spectra are excited with the Ndxa0:xa0YAG laser line at 1064xa0nm. Within seven dissertations we explored different fields of potential applications to medical diagnostics. Identification and qualification of tissues and cells is possible. Tumors show small but significant differences to normal tissues; in order to develop a reliable tool for tumor diagnostics more research is necessary, especially a collection of reference spectra in a data bank is needed. Raman spectra of biomineralization structures in teeth and bones show pathological tissues as well as the development of new mineralized structures. NIR Raman spectra of flowers, leaves, and fruit show, without special preparation, their constituents: alkaloids, the essential oils, natural dyes, flavors, spices and drugs. They allow application to taxonomy, optimizing plant breeding and control of food.

NIR FT Raman spectroscopy—a new tool in medical diagnostics

Abstract 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.

Quality control of food with near-infrared-excited Raman spectroscopy

SummaryThe former considerable handicap of Raman spectroscopy in the visible range, the disturbing fluorescence of impurities, has now been eliminated: Raman spectra are excited by light quanta of the near-infrared range; their energy, however, is too small to excite fluorescence spectra. Now Raman spectroscopy can be applied to many ‘real world samples’, to quality control of raw material, to production and product control. This paper gives examples of the application to food analysis. Many problems can be solved without a special sample preparation, even by using fibre bundles on-line at the sample site, in containers and in real time for production control. The configuration and amount of C=C bonds in lipids can be determined directly, also the nature and amount of proteins and carbohydrates, and the composition of food products. Natural and synthetic colours, flavours and vitamins can be detected on TLC plates, especially if Raman scattering is enhanced by resonance. Traces of distinct compounds can be detected by the SERS technique (the surface-enhanced Raman spectroscopy). The powerful new tool of analytical chemistry promises many useful applications and the replacement of time-consuming traditional methods.

Monitoring of the polymerization of vinylacetate by near IR FT Raman spectroscopy

Abstract Emulsion polymerization of vinylacetate was studied to monitor the reaction on-line by the near infrared FT-Raman technique. The experimental conditions for monitoring the reaction by Raman spectroscopy were determined and discussed with regard to their applicability to industrial production of the polymer. The reaction goes to approximately 90% completion in 50 min and the polymerization can be succesfully monitored by taking full-scan measurements between 20–3500 cm −1 at every 8 min.

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.

Medical Diagnostics with NIR-FT-Raman Spectroscopy

The fluorescence of natural constituents of bio-material may conceal its Raman spectra. This fluorescence is reduced by shifting the existing radiation to longer wavelengths. For several reasons the optimum is the excitation with 1064 nm radiation, produced by the Nd:YAG laser. In order to explore the applicability to medical diagnostics we installed an NIR-FT-Raman spectrometer in the Universitaetsklinikum Essen. The results complied within 5 dissertations show that there are some useful potential applications in this field. However, much more work and, especially, international cooperation, is needed to develop this tool further for routine application.

Raman: FT or dispersive--is that the question?

The typical properties of the Raman spectrometers for the near-infrared range are discussed. Multiple monochromators, polychromators, and interferometers have specific advantages and disadvantages. They complement each other nicely.