Adsorption-elution problem

To illustrate the general elution problem and its solution, let us consider the following situation. Consider a 20-component mixture with capacity factors k of the components forming a geometrical progression and exponentially dependent on the modifier concentration (molar or volume fraction c), in accordance with the Snyder-Soczewinski model of adsorption [2]. The log k versus log c plots of the 20 solutes are given in Fig. 1, which has a parallel Rf axis subordinated to the right-hand-side log k axis. It can be seen that no isocratic eluent can separate all the components. A pure modifier [c = 1.0 (100%)] separates well solutes 1-7, and the less polar solutes are accumulated near the solvent front for c = 0.1 (10%), solutes 8-14 are... 

The elution problem does not occur in size-exclusion chromatography, which is one of its major advantages. Instead, elution of all components is guaranteed, the final ones coinciding with the void volume at the very latest. The only exception to this is in the case of adsorption effects, peaks being eluted later than Vq, but this can be prevented by a careful choice of stationary and mobile phases. 

It has already been pointed out that the forms of solution to the adsorption wave problem are similar to those representing the elution of tracer in a residence-time distribution experiment. Closest to what we considered in Chapter 5 are solutions of Levenspiel and Bischolf [O. Levenspiel and K.B. Bischolf, Adv. Chem. Eng., 4, 95 (1963)] and Lapidus and Amundson [L. Lapidus and N.R. Amundson, J. Phys. Chem., 56, 984 (1952)]. With the rate equation written as in equation (9-8a), these are... 

It is evident from these results that the interactive properties of the investigated SEC PS/DVB or DVB gels are very different. Because polar electroneutral macromolecules of PMMA were more retained from a nonpolar solvent (toluene) than from polar ones (THF, chloroform), we conclude that the dipol-dipol interactions were operative. Columns No. 1 and No. 2 were very interactive and can be applied successfully to LC techniques that combine exclusion and interaction (adsorption) mechanisms. These emerging techniques are LC at the critical adsorption point (18), the already mentioned LC under limiting conditions of adsorption (15,18), and LC under limiting conditions of desorption (16). In these cases, the adsorptivity of the SEC columns may even be advantageous. In most conventional SEC applications, however, the interactive properties of columns may cause important problems. In any case, interactive properties of SEC columns should be considered when applying the universal calibration, especially for medium polar and polar polymers. It is therefore advisable to check the elution properties of SEC columns before use with the... 

Some GPC analysts use totally excluded, rather than totally permeated, flow markers to make flow rate corrections. Most of the previously mentioned requirements for totally permeated flow marker selection still are requirements for a totally excluded flow marker. Coelution effects can often be avoided in this approach. It must be pointed out that species eluting at the excluded volume of a column set are not immune to adsorption problems and may even have variability issues arising from viscosity effects of these necessarily higher molecular weight species from the column. 

Surface Adsorption Hater sample is passed through a column of the adsorbent and the adsorbed organic constituents subsequently eluted with a smaller volume of organic solvent. All sample types Adsorbents Include charcoal, macroretlcular resins, polyurethane foams, bonded phases and ion-exchangers. Generally have high capacity but sample discrimination may be a -problem. Sample modification and Incomplete recovery are further possible problems. 

When retention times of mixture components decrease, there may be problems with either the mobile or stationary phase. It may be that the mobile phase composition was not restored after a gradient elution, or it may be that the stationary phase was altered due to irreversed adsorption of mixture components, or simply chemical decomposition. Use of guard columns may avoid stationary phase problems. 

Membrane absorbers are continuous chromatographic supports, which circumvent some of the above-mentioned problems of particulate stationary phases. They were originally derived from membrane (filtration) technology. The immobilization of interactive (ionic, hydrophobic, or biospecific) groups on the surface of microfiltration membranes was found to increase the selectivity of certain separation procedure. Ideally such activated membranes, or membrane adsorbers, allow the selective adsorption of certain substances and substance classes, which may subsequently be eluted by means of a stepwise change of the mobile phase (elution buffer). More complete information on the various types of modern membrane technology can be found in some recent reviews [e.g., 31-33]. 

Because of the problems encountered with the water system, the use of aliphatic alcohols, ie.g., methanol, ethanol, and isopropanol, as modifiers of the adsorption strength has been recommended (44. 45. 50. 51). Usually, between 0.01 and 0.5% (v/v) alcohol is added to the eluent. As an example, the k values for the benzyl alcohols on a silica column are in the same range when eluted with dichloromethane containing either 0.1% water (50% water-saturated) or 0.15% methanol or 0.3% isopropanol (45). The preparation and preservation of these alcohol-eluent mixtures is accompanied by problems similar to those discussed with water-modified eluents. Also, column equilibration is slow (44). The efficiency of columns operated with alcohol-modified eluents is generally lower than that of water-modulated eluent system. At some alcohol concentrations, distorted peaks with tailing or frontal asymmetry have been observed 44), but olhei workers using another silica could not verify this observa tion (61). 

It is also possible to determine simultaneously by GPC the micelle molecular weight and aggregation number. It is important to ensure that the integrity of polymeric micelles during their elution through the size exclusion column is maintained. Adsorption of the polymer on the column may prove to be a problem (Yokoyama et al., 1993), especially at concentrations close to the CMC, where micelles consist of large loose aggregates (Jones and Leroux, 1999). 

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