Eluotropic strength

Binders (TbC) 671 Bipolar pulse conductivity detector (LC) 588 Bonded phases (GC) 125 crosslinked 126 estersils 125 nonextractable 126 siloxane 125 Bonded phases (LC) 324 carbon loading 335 cleavage of ligands 336 eluotropic strength (LSC) 382 endcapping 326 hydrophobicity 364 metal impurities 369 models for surface 337 physical characteristics 333, 366... 

In principle, enhanced sensitivity can be achieved when the column focusing of the analyte is possible. The sample is preferably dissolved in a solvent with lower eluotropic strength compared to that of the mobile phase at the start of a chromatographic run. A solvent with... 

The concept of eluotropic strength has been invoked here without a rigorous definition. Snyder (130) developed a series of eluotropic values for solvents by using retention values measured on alumina columns. Colin and Guiochon (86) used a definition similar to that of Snyder to evaluate eluotropic strengths of methanol-water mixtures on various column surfaces. The eluotropic strength, e, was calculated by using the equation... 

Pio. 22. Eluotropic strength of water-acetonitrile on pyrocaibon. The eluites used are ( ) 2,3.4.S-tetramethylbenzene. (O) 1.3.4-trimethylbenzene, and ( ) 3,4,S-trimethylphenol. Reprinted with permission Horn Colin et at. (Ml). 

Common solvent systems are shown in Table 2. Although there is a direct correlation between increasing Rf values and increasing the eluotropic strength of the organic solvent, Wilson pointed out that this correlation exists over a very wide range, contrary to what is seen on RP-HPLC. Additionally, Revalues are not particularly sensitive to pH, which makes RP-TLC a particularly robust and simple technique to use. 

One single eluent is often not convenient to achieve the desired separation. Intermediate values of e° are required. For that purpose the simplest thing is to use binary mixtures of eluents an apolar diluent A and a modifier B. Calculation of eluotropic strength is not straightforward. It can be achieved through the formula derived by Snyder. The easiest way is to draw plots of e° versus volume of the strongest eluent B (Fig. 6). 

If, in this example, the best recovery were observed for the monomethyl phthalate and the least recovery observed for the mono-n-octyl phthalate (i.e., the order in recovery at pH 2 were reversed), an inadequate volume or eluotropic strength of the elution solvent might be the cause of reduced recovery for the more hydrophobic analytes. 

Relative elution solvent strength (or eluotropic strength) is depicted in solvent polarity charts (Figure 2.39). The relative elution strength for a solvent on a polar, normal-phase sorbent such as silica or alumina increases in reverse order to that measured on a nonpolar, reversed-phase sorbent. Ac-... 

Figure 2.40. Dependence of SPE desorption on elution solvent eluotropic strength. Graphic based on selected data from Ref. 111.

A liquid-liquid system can be created by coating a particulate matter with a thin layer of a liquid phase, similar to the way packed columns are used in GLC. To maintain such an LLC column, the stationary phase should be insoluble in the mobile phase, just as GLC phases need to be involatile at the temperature of operation. Unfortunately, insolubility is an absolute demand that can at best be approximated in practice. The solubility of the stationary phase in the mobile phase becomes even more critical once some flexibility is desired with regard to the choice of the mobile phase. For example, mixtures of several pure solvents are usually required in order to adapt the eluotropic strength (polarity) of the mobile phase such that the capacity factors fall in the optimum range. 

To a first approximation (eqn.3.29) we may expect mixtures of the same polarity to yield the same capacity factors. In other words, mixtures with the same solubility parameters are expected to have the same eluotropic strength, and therefore they might be called iso-eluotropic mixtures. If we use THF (T) instead of methanol in a binary mixture with water, the following equation relates two iso-eluotropic mixtures... 

Eqns.(3.54) and (3.55) are very convenient for the definition of iso-eluotropic mixtures and for the calculation of the eluotropic strength of multicomponent mobile phases, in terms of a corresponding binary methanol/water mixture. 

The solubility parameter model appears to work very well for the prediction of iso-eluotropic mixtures in LLC and RPLC. However, in LSC the retention mechanism is very different from the one that was assumed at the outset of this section, and hence a different model should be applied to allow the description and possibly prediction of the eluotropic strength in LSC. This model will be described in section 3.2.3. 

In this equation 5° is the adsorption energy of the solute on a standard adsorbent. At is the adsorption area of the solute molecule. e° is the adsorption energy of the solvent per unit area on the same standard adsorbent, usually referred to as the solvent strength or eluotropic strength, a is the activity of the adsorbent and Va is the volume of the adsorbed solvent per gram of stationary phase. Hence, Va can be seen as a compensation factor for the dimensions of Ka i. Ka i/ Va is a dimensionless quantity. 

Eluotropic strength (e°, eqn.3.72) of some common solvents for LSC. Data taken from refs. [349] en [351],... 

Figure 3.18 Variation of the eluotropic strength ( %) with the composition of the mobile phase in LSC (eqn.3.73). Weak solvent is n-pentane Strong solvents (from bottom) carbon tetrachloride, n-propylchloride, methylene chloride, acetone, pyridine. Figure taken from ref. (349]. Reprinted with permission.

Eqn.(3.73) suggests that any mixture of two solvents with the same ° value (iso-eluotro-pic solvents) will also have the same eluotropic strength. This would allow the application of a similar strategy for the definition of iso-eluotropic multicomponent mobile phase mixtures as was used for RPLC in section 3.2.2.1. In practice, the situation in LSC has proved to be more complicated, because an effect described as solvent localization limits the validity of eqns.(3.72) and (3.73) if polar components (such as acetonitrile or methyl t-butyl ether) are present in the mobile phase. This makes it difficult to calculate the composition of iso-eluotropic mixtures for LSC with sufficient accuracy for optimization purposes [360-363]. 

The concentration of the counterion can be used to control the retention in IEC. It plays a role similar to that of the eluotropic strength of the eluent in RPLC or LSC, in that it affects retention much more than it does selectivity. The capacity factor can be related to the distribution coefficient of the solute (Dx) ... 

The type of counterion used may affect the retention considerably. The eluotropic strength of the different counterions is usually expressed as an eluotropic series. An example of this is shown in table 3.6. 

Table 3.10d lists the parameters for LSC. Again, most separations may be optimized by optimizing the eluotropic strength (primary parameter) and the nature (secondary parameter) of the mobile phase. The latter parameter involves the preparation of different iso-eluotropic mixtures containing different solvents, or small quantities of very polar components ( modulators ). As in the case of RPLC, there are several additional parameters that are not frequently exploited. 

For the important case of optimizing the solvent eluotropic strength in RPLC, a more elegant alternative is available. We have seen in chapter 3 (section 3.2.2) that eqn.(3.45) is a good approximation for the retention behaviour of solutes in RPLC in the range of optimum capacity factors (1 < k< 10). In chapter 3 we also discussed the validity of the empirical equation... 

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