Structure and Chromatographic Behaviour

The sequences discussed below are thus of limited validity, the more so because inversions can occur in certain solvent systems. As already known from column chromatography, certain adsorbents can also bring about inversions (cf. p. 57 in [129]). Even though in genuine partition chromatography, an equatorial (e) hydroxyl group is always more polar than an axial (a) in the same position, this does not necessarily always apply in adsorption. 

Thus the following sequence of diminishing polarity has been foimd for the isomeric 3-hydroxycholestanes and 3 j8-hydroxycholest-5-ene on silica gel and alumina (29, 32)  

It is evident from this, that 5a-H and J -compounds of the 3)8-series and the 3a, 5 and 3 j8,5 8-isomers are difficult to separate on the other hand, the separation of the equatorial from the axial pairs causes no difficulty in partition chromatography. A combination of both chromatographic types would more easily effect separation here. A further comparison of adsorption (A) and partition (P) leads to the following list [29]  

The situation is complicated, however, by the partial inversion of sequences which, as already mentioned, can be brought about by certain adsorption systems (ether, ethyl acetate). 

The following sequences of decreasing polarity have been established for the hydroxyl groups in other positions [171]  

---

L. S. Bark and J. T. Graham, Studies in the relation ship between molecnlar structure and chromatographic behaviour, J. Chromatogr. 23 417-442 (1966) J. Chro-matogr. 25 357-366 (1966). 

The relation between structure and chromatographic behaviour is very much the same here as in paper chromatography it would be outside the scope of this article to discuss here the extensive material, especially since it has recently been compiled in a monograph [129]. 

The statements in section IV about the relation between structure and chromatographic behaviour apply to the more weakly polar 17-ketosteroids, the polarity of which increases in the order ... 

In a partition system, the relationship between chemical structure and chromatographic behaviour can be rationalized to a large extent It has been shown that the logarithm of the partition coefficient can be split up into contributions due to various structural features the respective group contributions A log a are additive. If the position of a band is expressed in terms of an R, value, one can calculate an Rm value... 

Using a chiral column, coated with a definite modified cyclodextrin as the chiral stationary phase, the elution orders of furanoid and pyranoid linalool oxides are not comparable [11, 12]. Consistently, the chromatographic behaviour of diastereomers and/or enantiomers on modified cyclodextrins is not predictable (Fig. 17.1, Table 17.1). Even by changing the non-chiral polysiloxane part of the chiral stationary phase used, the order of elution may significantly be changed [13]. The reliable assignment of the elution order in enantio-cGC implies the coinjection of structurally well defined references [11-13]. 

A disadvantage of the availability of many different silicas is the limited reproducibility of the packing materials. Apart from the factors described above, the chromatographic behaviour of the silica can be affected by chemical factors such as the structure of the surface (affected by heat treatments and by washing the column with acidic or basic solutions), the history of the material (previous usage) and the presence of contaminants (e.g. metal ions). The water content is another major factor. Physically adsorbed water can be removed from or added to the surface, but water bound to the surface as silanol groups (chemisorption) cannot be introduced or removed once the silica is packed into the column. 

In figure 7.1 a relative importance is assigned to each stage of the process. Because it is (as yet) impossible to predict the chromatographic behaviour of solutes from structural information alone and, moreover, because the structure of all sample components is usually not known, we have to rely on chromatographic experiments for the optimization of the selectivity. Consequently, the first step is that an instrument should be built . 

De Beer et al. [501] reported an extensive comparative study of the chromatographic behaviour of methyl esters and pentafluorobenzyl esters of these substances. They correlated retention data (Kovats retention indices) on nine stationary phases with the structure of the derivatives and with the polarity of the stationary phases with the aim of utilizing these dependences for identification purposes. However, much more significant is the better separation obtained with pentafluorobenzyl esters and the possibility of increasing the sensitivity of the analysis. 

Bate-Smith, E.C. Westall, R.G. 1950. Chromatographic behaviour and chemical structure... 

Bate-Smith, E.C. and Westall, R.G. (1950) Chromatographic behaviour and chemical structure. I. Some naturally occurring phenolic substances. Biochim, Biophys. Acta, 4, 427-440. 

Models that describe the relationship between GCxGC retention and structural properties should not be restricted to a fixed set of columns and chromatographic conditions. In GCxGC, any alteration in the operation parameters of the first column will cause a change in the elution conditions (flow, temperature) in the second affecting the retention in both and columns. For these reasons, practical objectives require that retention GCxGC behaviour can be predicted for different chromatographic conditions, as described in the next section. 

The number of theoretical plates N, included in Equation (8), is a measure of column efficiency, which can be individually applied to the two columns of a GCxGC set. But in comprehensive GCxGC, the use of two columns having phases with different characteristics results in the redefinition of peak capacity, a 1 D GC efficiency concept. It also results in the introduction of two new concepts related to the separation behaviour, orthogonality and chromatographic structure, which are specific to GCxGC. These three concepts are discussed in Sections 4.2, 4.3, and 4.4, respectively. 

For KS, in common with many other carbohydrate polymers of the GAG family (e.g. the heparan sulphate/heparin systems) and elsewhere, the anticipated molecular structures are not unique. They are present as distributions, both in terms of molecular size and in the nature of substituents and their placements placements. Thus they differ greatly from the ordered world of peptides and proteins and are more akin to synthetic polymers in their nature, their chromatographic and spectroscopic behaviour and in the resultant analytical problems. It is apt to consider substituent placements along the repeat unit backbone as exhibiting microstructural patterns within diad, triad or larger groups of monomer units, for which statistical data can be elicited, e.g. in terms of sulfation and its distribution. The spectroscopic route to KS structure can also be similar to that used for synthetic macromolecules. 

---