Chaotropic counteranions

Y. V. Kazakevich, R. LoBrutto, and R. Vivilecchia, Reversed-phase high-performance liquid chromatography behavior of chaotropic counteranions, /. Chromatogr. A 1064 (2005), 9-18. 

Semiempirical expression was derived for the description of the retention of chaotropic counteranions in reversed-phase conditions [165]. Overall expression for the description of the retention dependencies of analyte ions versus eluent composition will have only four unknowns and allow numerical approximation of experimental retention data (shown as a function of the mole fraction of organic eluent component). 

Increasing the chaotropic counteranion concentration of perchlorate, hexa-fluorophosphate, and tetrafluoroborate in the mobile phase for basic compounds studied led to an increase in the apparent efficiency of the system until the maximum plate number for the column is achieved [153], In Figure 4-57A the efficiency for three basic ophthalmic drug compounds increases relatively fast when the concentration of counteranion BFf was increased from 1 mM... 

The analysis of Dorzolamide HCl at pH 2 with phosphoric acid shows early elution. The addition of hexafluorophosphate to the mobile phase leads to an enhancement of the retention. Figure 4-59 is an overlay of Dorzolamide HCl chromatograms at four increasing PFe concentrations. As the concentration increased, peak tailing decreased, and peak efficiency and analyte retention increased. Figure 4-60 shows the effect of different counteranions on basic analyte retention and peak efficiency. Depending upon the desired selectivity between a neutral component and a charged basic analyte, a particular chaotropic counteranion could be employed. 

The use of chaotropic counteranions for a chromatographic separation is beneficial as a method development strategy. These modiflers may replace the need for changing column type and/or addition of hydrophobic ion-pairing reagents for the more challenging separations. Further studies are needed to fully elucidate the detailed mechanism of chaotropic mobile-phase additives. 

Kazakevich YV, LoBmtto R, Vivileccliia R (2005) Reversed-phase higlr-perfoimance liquid cln omatography behavior of chaotropic counteranions. J Clri omatogr A 1064 9—18. 

When these counteranions are present in the eluent and interact with the protonated basic analyte they tend to disrupt the solvation of the basic analyte. These anions can be classified as chaotropic counteranions. ... 

Figure 5-15 shows how the retention of 4-ethylpyridine varies with pH. The greatest increase in retention occurred when trifluoroacetic acid and perchloric acid were employed. The phosphoric modifier did not increase the retention of the 4-ethylpyridine significantly at decreasing pH values. Therefore the trifluoroacetate and perchlorate had a greater influence than phosphate as chaotropic counteranions at any particular pH. It was shown that this chaotropic effect began to occur at pHs less than 3. At all pHs greater than 4 the retention factors of the basic compound were similar and were independent of what type of modifier was used in the mobile phase. 

These were all done with perchloric acid as the modifier. It is considered to be a strong chaotropic agent. Weak chaotropic counteranions will produce the same type of retention dependence, but the overall effect of weak chaotropes on the analyte retention is much less pronounced. 

At a certain pH when different acidic modifiers are employed the counteranion concentrations are not the same. How can one compare the effect of the chaotropic counteranion on the retention of basic analytes ... 

Yes, you can adjust the pH of the mobile phase by addition of an acid (preferably with low pA ) and then increase counteranion concentration by adding its salt. The retention increase will be solely due to the increase of the chaotropic counteranion. This approach may be needed to fine tune a method. For example, if a mixture of acids and bases is not optimally resolved at a certain pH then the addition of perchlorate anion will increase the retention of only the protonated basic compounds without affecting the retention of the acidic compound or other basic compounds that are not fully protonated at this pH. In order to calculate the total concentration of perchlorate anions present, the concentration of perchlorate anion from the addition of perchloric acid and sodium perchlorate must be known. 

The sodium perchlorate salt was added, but what other salts that contain chaotropic counteranions could I use ... 

The chaotropic effect is dependent on the concentration of the free counteranion and not the concentration of the protons in solution at pH < basic analyte Ka. This suggests that change in retention of the protonated basic analyte may be observed with the increase in concentration of the counteranion by the addition of a salt at a constant pH as shown in Figure 4-47 for a pharmaceutical compound containing an aromatic amine with a pKa of 5. 

Disruption of the basic analyte solvation shell should be possible with practically any counteranion employed, and the degree of this disruption will be dependent on the chaotropic nature of the anion. Chaotropic activity of counteranions has been established according to their ability to destabilize or bring disorder (bring chaos) to the structure of water [148,149]. 

As was shown above, the chaotropic effect is related to the influence of the counteranion of the acidic modifier on the analyte solvation and is independent on the mobile-phase pH, provided that complete protonation of the basic analyte is achieved. Analyte interaction with a counteranion causes a disruption of the analyte solvation shell, thus affecting its hydrophobicity. Increase of the analyte hydrophobicity results in a corresponding increase of retention. This process shows a saturation limit, when counteranion concentration is high enough to effectively disrupt the solvation of all analyte molecules. A further increase of counteranion concentration does not produce any noticeable effect on the analyte retention. 

Chaotropic Model. If the counteranion concentration is low, some analyte molecules have a disrupted solvation shell, and some do not due to the limited amount of counteranions present at any instant within the mobile phase. If we assume an existence of the equilibrium between solvated and desolvated analyte molecules and counteranions, this mechanism could be described mathematically [151]. 

Effect of Different Counteranions. The chaotropic theory was shown to be applicable in many cases where small inorganic ions were used for the alteration of the retention of basic pharmaceutical compounds [153-157]. Equation (4-39) essentially attributes the upper retention limit for completely desolvated analyte to the hydrophobic properties of the analyte alone. In other words, there may be a significantly different concentration needed when different counterions are employed in the eluent for complete desolvation of the analyte. Therefore, the resulting analyte hydrophobicity and thus retention characteristics of analyte in completely desolvated form should be essentially independent on the type of counteranion employed. Experimental results, on the other hand, show that the use of different counterions... 

In the chaotropic model, counteranions disrupt the analyte solvation shell, thus increasing its apparent hydrophobicity and retention. 

It has been shown that the PFe counteranion has had the greatest effect on the improvement of the peak asymmetry at low concentrations compared to other chaotropic additives. At the highest concentration of counteranions (PFe , CIO4, BF4), the number of plates for most of the basic compounds studied was similar to that of the neutral markers. In contrast, the neutral... 

The overall chaotropic effect within a given concentration range is not similar at different temperatures. A plot of k vs counteranion concentration is given in Fig. 5-26. 

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