Library Raman

Plenary 15. B Scluader et al, e-mail address beriilrard.scluader uni-essen.de (NIR-FTRS). A review of the use of Raman spectroscopy in medical diagnostics. Its possibilities, limitations and expectations. Emphasizes the need for a library of reference spectra and the applications of advanced analysis (chemometry) for comparing patient/library spectra. 

Woodruff and co-workers introduced the expert system PAIRS [67], a program that is able to analyze IR spectra in the same manner as a spectroscopist would. Chalmers and co-workers [68] used an approach for automated interpretation of Fourier Transform Raman spectra of complex polymers. Andreev and Argirov developed the expert system EXPIRS [69] for the interpretation of IR spectra. EXPIRS provides a hierarchical organization of the characteristic groups that are recognized by peak detection in discrete ames. Penchev et al. [70] recently introduced a computer system that performs searches in spectral libraries and systematic analysis of mixture spectra. It is able to classify IR spectra with the aid of linear discriminant analysis, artificial neural networks, and the method of fe-nearest neighbors. 

The Aldrich Library of Infrared Spectra , 3rd ed., Aldrich chemical Company, Milwaukee, WI, 1981, by Pouchert contains more than 12,000 IR spectra so arranged that the user can readily see the change that takes place in a given spectrum when a slight change is made in the structure of a molecule. The same company also publishes the Aldrich Library of FT-IR Spectra and the Aldrich Library of NMR Spectra , both also by Pouchert. A similar volume, which has ir and Raman spectra of about 1000 compounds, is Raman/Infrared Atlas of Organic Compounds , 2nd ed., VCH, New York, 1989, by Schrader. 

Decades of combined spectral and chemistry expertise have led to vast collections of searchable user databases containing over 300 000 UV, IR, Raman and NMR spectra, covering pure compounds, a broad range of commercial products and special libraries for applications in polymer chemistry (cf. Section 1.4.3). Spectral libraries are now on the hard disks of computers. Interpretation of spectra is frequently made only by computer-aided search for the nearest match in a digitised library. The spectroscopic literature has been used to establish computer-driven assignment programs (artificial intelligence). 

An important tool for the fast characterization of intermediates and products in solution-phase synthesis are vibrational spectroscopic techniques such as Fourier transform infrared (FTIR) or Raman spectroscopy. These concepts have also been successfully applied to solid-phase organic chemistry. A single bead often suffices to acquire vibrational spectra that allow for qualitative and quantitative analysis of reaction products,3 reaction kinetics,4 or for decoding combinatorial libraries.5... 

The bottleneck in utilizing Raman shifted rapidly from data acquisition to data interpretation. Visual differentiation works well when polymorph spectra are dramatically different or when reference samples are available for comparison, but is poorly suited for automation, for spectrally similar polymorphs, or when the form was previously unknown [231]. Spectral match techniques, such as are used in spectral libraries, help with automation, but can have trouble when the reference library is too small. Easily automated clustering techniques, such as hierarchical cluster analysis (HCA) or PCA, group similar spectra and provide information on the degree of similarity within each group [223,230]. The techniques operate best on large data sets. As an alternative, researchers at Pfizer tested several different analysis of variance (ANOVA) techniques, along with descriptive statistics, to identify different polymorphs from measurements of Raman... 

The deconvolution of compound libraries prepared by the mix-and-split method can be greatly simplified by using polymeric supports that have been labelled with various dyes prior to library synthesis. In this case, the first monomer can be identified from the color or the UV spectrum of each bead. This type of labelling can, for instance, be achieved by partial derivatization of the support with different dyes [47], or by the use of polymers prepared from monomers showing characteristic IR or Raman spectra [48],... 

IR spectroscopy is a fast, simple, and cheap method for the qualitative detection of certain functional groups on insoluble supports [77-79]. Dried supports can be used directly to prepare KBr pellets for standard recording of IR spectra [54,80-82], Newer IR-based techniques, which require much less support material than required for a KBr pellet, include single-bead FT-IR spectroscopy [16,77,83-86], single-bead Raman spectroscopy [87], near-IR multispectral imaging [88], and the simultaneous analysis of several different beads by FT-IR microscopy for analysis of combinatorial libraries [89,90],... 

IR spectroscopy is not a very sensitive analytical tool and is, therefore, not well suited to the detection of small amounts of material. If, however, intermediates have intense and well-resolved IR absorptions, the progress of their chemical transformation can be followed by IR spectroscopy [83,88,91-93], Near-infrared spectroscopy, in combination with an acousto-optic tunable filter, can be sufficiently sensitive to enable the on-bead identification of polystyrene-bound di- and tripeptides, even if the peptides have very similar structures (e.g., Leu-Ala-Gly-PS and Val-Ala-Gly-PS) or differ only in their amino acid sequence (e.g., Leu-Val-Gly-PS and Val-Leu-Gly-PS) [94]. Special resins displaying an IR and Raman barcode have been developed, which may facilitate the deconvolution of combinatorial compound libraries prepared by the mix-and-split method [48]. 

This example can be classified as a forensic investigation. When Raman is used to identify unknown materials in formulations, the spectral analysis may be challenging when using Raman libraries as they do not have the compound diversity compared with spectral diversity of FT-IR libraries. In addition the... 

Though especially the cultural parameters of the studied cells have a huge impact on their signature variability it seems that - a decent Raman signature library provided - the majority of cultivation differences can be accommodated and thus no extreme standardization of culturing conditions is required for identification via this method. But for the sake of taxonomic resolution care has to be taken in finding appropriate culturing conditions to affect the implementation possibilities of Raman spectroscopy in microbial analysis positively [43]. 

Rahman SS, Busby DJ, Lee DC, Infrared and Raman spectra of a single resin bead for analysis of solid-phase reactions and use in encoding combinatorial libraries, J. Org. Chem., 63 6196-6199, 1998. 

Figure 5-10 Raman spectra of an unknown samples and three library spectra. On the right two columns are the Euclidean distances and dot products of the unknown spectrum with each of the three library spectra.

The majority of Raman spectra reported in the literature are uncorrected for instrument response, so one could argue that the most common response correction is none at all. Uncorrected spectra are still valuable for qualitative applications involving comparison of peak frequencies and for quantitative comparisons where the response function is unknown but constant. For example, a quantitative analysis of two components based on the relative heights (or areas) of two Raman bands can be calibrated with known solutions and applied to unknowns without determination of the response function. However, there are many situations in which response function calibration is important, including variations in relative intensity with different instruments, variations caused by instrumental drift or repair, and subtraction of library spectra (see Fig. 5.6). If a quantitative analysis is based on a calibration curve without response correction, a new curve must be collected if a change in response... 

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