Resolution eluting bands

Figure 19.7. Cyclic batch elution chromatography obtaining high product purity and high throughput by using incomplete resolution (overlapping bands) and recycling the mixed fraction (mf) to the feedstock (a) Control of band separation and cut points determines fractional impurities t mij mr and Ami /mr2-<4l)> (b) Chromatogram for separation of pure ds- and trans- 1,3-pentadiene. Components 1, isoprene 2, trans- 1,3-pentadiene 3, cis-l,3-pentadiene 4, cyclo-pentadiene. Component 1 is eluted at almost the same time as component 4 of the...

Chromatograms of the optical resolution of (I) and (II) Eire shown in Figure. 1. Compound (II) is completely resolved into its enantiomers chromatographically. The g-factor remains at its constant optimum value throughout the elution band of each enantiomer. Compound (I) in contrast is incompletely resolved, giving a single absorbance elution band, but a double bisignate difference absorbance band. The fractions of (I) eluted in the volume between the vertical dashed lines are incompletely resolved. Values for the g-max factors have been determined from the CD and absorption spectra of (I) and (II). 

Material for analysis can be recovered or examined by determination of either the distribution of a solute in a series of tubes or its distribution as it is eluted from such a series. To indicate quantitatively the effectiveness of the selective separation of one substance from another, the term resolution may be used. Resolution of bands may be defined as the ratio of band separation to average band width, and similarly, resolution of peaks as the ratio of peak separation to average peak width (Section 24-6). 

The right combination of the two immiscible liquid phases, mobile and stationary, can lead to highly selective separations at ordinary temperatures for both volatile and nonvolatile solutes. An important characteristic is that th partition ratio ideally is independent of concentration the elution bands are therefore more symmetrical and less subject to tailing than those observed in adsorption chromatography. Consequently, better resolution is usually possible. The principal problem is that of stabilizing the stationary phase the stationary and mobile phases are not completely insoluble in each other, even if one is aqueous and the other a hydrocarbon. 

If the resolution of early eluted bands is unsatisfactory and the separation time is long, the sample separation is usually improved by temperature or solvent gradients. 

Finally, we note that the fact that the displacement chromatogram is carried out at the watershed point of the first component guarantees that the profile of its band will be that of an overloaded elution band, just resolved from tine isotachic band of the second component when the isotachic train is formed. As illustrated in Figure 12.4b, changing the amount of the other components in the sample, hence the width of their zones, does not affect the resolution of this overloaded elution profile with the band of the second component. Similarly, increasing the column length or the displacer concentration would not affect the resolution of the bands rmder isotachic conditions. The only way to affect the heights of the zone and, to some extent, the width of the mixed zones is to increase the concentration of the displacer. 

There is no competition between the two components. Accordingly, the elution band profiles of the two components are the same, whether a given amoimt of each component is injected pure or as a mixture. They recognized that this assumption is inexact when the resolution between the bands is moderate (i.e., lower than 2). Due to the large influences of the displacement and the tag along effects on the band profiles, this assumption is unrealistic for bands having a resolution of the order of 1. 

A high specific surface area of the stationary phase is desirable as this allows stronger interaction of the analytes with the stationary phase, often leading to higher resolution of difficult-to-separate analytes and also counteracting problems with loadability, which manifest themselves in asymmetric and broad elution bands. 

The resolution Rs of two closely eluting bands is defined as the difference between the band centers divided by the average bandwidth, measured in the same... 

The general elution problem in chromatography. Improving the resolution of the overlapping bands in chromatogram (a) results in a longer analysis time for chromatogram (b). 

When only a few solutes are separated, they may occupy only a small portion of the total column volume at any given instant. In such cases, the productivity is improved by cyclic feed injections, timed so that the most strongly retained component from an injection elutes just before the least strongly retained component from the following injection (see Fig. 16-57). For a mixture of two components with k > 1, when the same resolution is maintained between bands of the same injections and bands of successive injections, the cycle time tc and the plate number requirement are ... 

Recalling that a separation is achieved by moving the solute bands apart in the column and, at the same time, constraining their dispersion so that they are eluted discretely, it follows that the resolution of a pair of solutes is not successfully accomplished by merely selective retention. In addition, the column must be carefully designed to minimize solute band dispersion. Selective retention will be determined by the interactive nature of the two phases, but band dispersion is determined by the physical properties of the column and the manner in which it is constructed. It is, therefore, necessary to identify those properties that influence peak width and how they are related to other properties of the chromatographic system. This aspect of chromatography theory will be discussed in detail in Part 2 of this book. At this time, the theoretical development will be limited to obtaining a measure of the peak width, so that eventually the width can then be related both theoretically and experimentally to the pertinent column parameters. 

At this point, it is important to stress the difference between separation and resolution. Although a pair of solutes may be separated they will only be resolved if the peaks are kept sufficiently narrow so that, having been moved apart (that is, separated), they are eluted discretely. Practically, this means that firstly there must be sufficient stationary phase in the column to move the peaks apart, and secondly, the column must be constructed so that the individual bands do not spread (disperse) to a greater extent than the phase system has separated them. It follows that the factors that determine peak dispersion must be identified and this requires an introduction to the Rate Theory. The Rate Theory will not be considered in detail as this subject has been treated extensively elsewhere (1), but the basic processes of band dispersion will be examined in order to understand... 

In the chromatography of plant extracts on an enlarged scale, there are a few main problems general elution because of the differentiated polarity of complex mixture components being separated the structural and chemical analogy of compounds and resolution decrease due to band broadening. 

The plate number in equation (4.56) corresponds to the value when the effective value of the capacity factor (equal to k when the band is at the column midpoint) is equal to the capacity factor in isocratic elution for the same column. The effective value of the capacity factor, k, is simply 1/1.15b. In most cases k, will be large and equation (4.57) is simplified by equating l/k, to zero. The resolution between two adjacent bands in a gradient program, again analogous to isocratic elution, is e q>ressed by equation (4.58)... 

Jandera, P. and Churacek, J., Gradient elution in liquid chromatography. I. The influence of the composition of the mobile phase on the capacity ratio (retention volume, band width, and resolution) in isocratic elution — theoretical considerations, /. Chromatogr., 91, 207, 1974. 

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