Chromatogram characteristics

A chromatogram characteristic of di-ethylhexyl phthalate (DEHP), the most commonly used plasticizer for PVC. The mass spectrum on the right-hand side is a molecular fragmentation pattern which identifies this peak as DEHP. 

Fig. 14. Molecular weight characteristics of novolac resins. Shown is the size-exclusion chromatogram for a typical commercial novolac polymer. The unsymmetrical peak shape reflects the multimodal molecular weight distribution of the polymer.

Column design involves the application of a number of specific equations (most of which have been previously derived and/or discussed) to determine the column parameters and operating conditions that will provide the analytical specifications necessary to achieve a specific separation. The characteristics of the separation will be defined by the reduced chromatogram of the particular sample of interest. First, it is necessary to calculate the efficiency required to separate the critical pair of the reduced chromatogram of the sample. This requires a knowledge of the capacity ratio of the first eluted peak of the critical pair and their separation ratio. Employing the Purnell equation (chapter 6, equation (16)). 

The performance and basic characteristics of the packed column can be checked by running the test chromatograms as follows ... 

The 2D chromatograms reveal additional components of the natural mixtures. They also give a map of the essential oil, which is helpful in the identification of the components by the position and the characteristic colours of the derivatives on the plate. A further, considerable improvement in the separation performance can be obtained by using overpressured layer chromatography (OPLC). Harmala et al. (70) used 2D OPLC for the separation of coumarins from the genus Angelica. Figure 10.15 shows the one-dimensional (a) and two-dimensional (b) OPLC separations of 16 coumarins. 

The various characteristics of the chromatogram that have so far been considered are shown in figure 10. It is important to be able to... 

Frdns has demonstrated this possibility with reserpine and rescinnamine hy irradiating at the start zone for two hours and obtaining a characteristic zone pattern (fingerprint) after TLC separation of the photochemically produced derivatives [20J. Huck and Dworcak exposed developed chromatograms with vanilmandelic acid and homovanillic acid zones to diffuse daylight and observed the formation of fluorescent metabolites that were suitable for direct quantitative analysis [21]. 

Heating the chromatograms after treatment is not generally to be recommended, but it leads to characteristic color changes in some cases [4]. 

Note The colors obtained are characteristic for the various substance classes (Table 1). Thus color tones obtained for phenothiazine derivatives are mainly reddish [1], If the j chromatogram zone contains more than 10 pg substance per spot there is only a colored 1 outer ring surrounding an uncolored center (Fig. 1) [1, 4]. The Forrest reagent does not interfere with subsequent detection with the Dragendorff reagent [1, 4], 1... 

For illustration consider SEC chromatograms obtained for two polymers on the same chromatographic system. One sample is a linear homopolymer while the other is a branched polymer with the same chemical composition. In the latter sample assume that the polymer components of different molecular weight have uniform branching characteristics so that all have similar molecular size/weight relationships. 

Compare molecular size/weight characteristics of branched and linear species eluting at V in each chromatogram. Under the universal calibration formalism branched and linear components have the same hydrodynamic volume at V ... 

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