Spectro-chromatograms

Figure 5.37 An example of a spectro-chromatogram recorded with a multichannel UV absorbance detector in LC. The sample contains a series of dipeptides, (a) (top) pseudo-isomeric three-dimensional plot dimensions are time, wavelength and absorption, (b) (bottom) contour plot with constant absorption lines. Figure taken from ref. [588]. Reprinted with permission.

The resulting spectro-chromatograms (SCG) are 3D-representations of the tar matrices with the UV-absorbances as function of the retention time in the gel column and the wavelength of absorption, respectively fig. 3). Sections of the SCG parallel to the retention time axis at 215 nm UV-absorption ("tar profiles" in the following) enable quick qualitative tar characterization. For the quantitative evaluation of the SCG, chemometric methods such as factor analysis and the classical least squares method are applied. This requires the set-up of a spectral library which contains the SCG of the quantitative important tar compounds. 

Fig. 3 Spectro-Chromatogram of a "primary" tar sample from the wood pyrolysis unit...

GPC analysis is conducted with the tar samples from the experimental series on homogeneous tar conversion with short gas residence times. Up to now, qualitative information about tar composition can be extracted from the comparison of the tar profiles. As outlined before, the quantitative evaluation of the spectro-chromatograms is performed by means of chemometric methods. 

Fluorescence spectroscopy is characterized by a greater selectivity when compared with other spectro-photometric techniques because there is an excitation and an emission spectra, with maxima usually quite characteristic of a particular compound. It is also selective because of the limited number of organic compounds that fluoresce. It has greater sensitivity than spectrophotometric methods in solution 10 9-10 12 M can usually be measured while on a thin-layer chromatogram some naturally fluorescent compounds may be detected instrumentally in sub-nanogram amounts. 

Selenomethionine is the predominant selenium moiety in plants whereas selenocysteine is probably formed from glycine and selenite in mammalian tissues. Analytical approaches are typically based on degradation of the original matrix to these and other amino acids followed by their determination. For this purpose, GC with element-selective or mass spectro-metric detection provides a valuable alternative to HPLC due to improved sensitivity. Selenoamino acids are derivatized with isopropylchloroformate and bis(p-methoxyphenyl) selenoxide, and with ethyl chloroformate. Figure 3 shows comparative selenium-specific GC-AED chromatograms for ethylated extracted enzymatic yeast hydrolyzates of archived selenized yeast employed in the Clark trial , a reference fresh selenized yeast and reference selenomethionine. 

Finally, the most powerful option available for volatiles analysis is the introduction of a third mass spectro-metric dimension in a comprehensive GC system. " MS techniques improve component identification and sensitivity, especially for the limited spectral fiagmenta-tion produced by soft ionization methods, such as chemical ionization (Cl) and field ionization (FI). The use of MS to provide a imique identity for overlapping components in the chromatogram makes identification much easier. However, quadrupole conventional MS is unable to reach the resolution levels required for such separations. Only TOF-MS possesses the necessary speed of spectral acquisition to give more than 50 spectra per second. 

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