Influence of mobile phase acid-base equilibria on the chromatographic behaviour of protolytic compounds

Effect of temperature on the chromatographic retention of ionizable compounds

The retention mechanism of acids and bases in reversed-phase liquid chromatography (RPLC) has been experimentally studied by examining the temperature dependence of retention, with emphasis on the role of the buffer ionization equilibria in the retention and selectivity. Retention factors of several ionizable compounds in a typical octadecylsilica column and using buffers dissolved in 50% (w/w) methanol as eluents at three temperatures in the range of 25-50 degrees C were measured. Two pairs of buffer solutions were prepared by a close adjusting of their pH at 25 degrees C; differences in their ionization enthalpies determined a different degree of ionization when temperature was raised and, as a consequence, a different shift in the eluent pH. Predictive equations of retention that take into account the temperature effect on both the transfer and the ionization processes are proposed. This study demonstrates the significant role that the selected buffer would have in retention and selectivity in RPLC at temperatures higher than 25 degrees C, particularly for co-eluted solutes.

Determination of pKa by pH Gradient Reversed-Phase HPLC

pH gradient reversed-phase HPLC consists of a programmed increase during the chromatographic run of the eluting power of the mobile phase with regard to ionizable analytes. On the analogy of the conventional organic modifier gradient RP HPLC, in the pH gradient mode, the eluting strength of the mobile phase increases due to its increasing (with acid analytes) or decreasing (with basic analytes) pH, whereas the content of organic modifier is kept constant. We have shown previously that the pH gradient separations are technically possible using standard chromatographic equipment. Here we demonstrate that the method is uniquely suitable to determine pK(a) values of analytes. A strict theoretical model is proposed to determine pK(a) values based on the retention data from a pH gradient RP HPLC run. The pK(a) data so obtained are discussed in relation to the concentration of methanol in the mobile phase, the type of stationary phase, and the duration of the gradient. The pK(a) values determined by the pH gradient method are related to the respective data obtained conventionally in a series of isocratic experiments. A close similarity of the two types of chromatographically determined pK(a) data is demonstrated. The HPLC-derived pK(a) parameters correlate to the literature pK(a) values determined by titrations in water. The chromatographically derived and the reference pK(a) values are not identical, however. That is probably due to the effects on the chromatographic pK(a) of the specific sites of interactions with analytes on the surfaces of the HPLC stationary phases. Nonetheless, the proposed pH gradient HPLC method may supply in a fast and convenient manner comparable acidity parameters for larger series of drug candidates, including those available in only minute amounts, without need of their purification, and also when the compounds are provided as complex mixtures, like those produced by combinatorial chemistry.

Determination of the pH of binary mobile phases for reversed-phase liquid chromatography

The measurement of pH in chromatographic mobile phases has been a constant subject of discussion during many years. The pH of the mobile phase is an important parameter that determines the chromatographic retention of many analytes with acid-base properties. In many instances a proper pH measurement is needed to assure the accuracy of retention-pH relationships or the reproducibility of chromatographic procedures. Three different methods are common in pH measurement of mobile phases: measurement of pH in the aqueous buffer before addition of the organic modifier, measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with standard solutions prepared in the same mobile phase solvent, and measurement of pH in the mobile phase prepared by mixing aqueous buffer and organic modifier after pH calibration with aqueous standard solutions. This review discusses the different pH measurement and calibration procedures in terms of the theoretical and operational definitions of the different pH scales that can be applied to water-organic solvent mixtures. The advantages and disadvantages of each procedure are also presented through chromatographic examples. Finally, practical recommendations to select the most appropriate pH measurement procedure for particular chromatographic problems are given.

Differences in preparative loadability between the charged and uncharged forms of ionizable compounds

Ionizable compounds experience a drastic difference in preparative loadability as a function of pH. It can be shown that the preparative loadability of a compound in the ionic form is by a factor of 20 or more inferior to the loadability of the same compound in the unionized form. In this paper, we demonstrate the reason for this behavior, and show practical applications of the principle.

Analysis of basic compounds at high pH values by reversed-phase liquid chromatography

Reversed phase high performance liquid chromatography (RPLC) is currently the method of choice for the analysis of basic compounds. However, with traditional silica materials, secondary interactions between the analyte and residual silanols produce peak tailing which can negatively affect resolution, sensitivity, and reproducibility. In order to reduce these secondary interactions, which comprise ion exchange, hydrogen bonding, and London forces interactions, chromatographic analyses can be carried out at low or high pH values where silanol groups and basic compounds are mostly uncharged. The chromatographic behaviour of a particular bidentate stationary phase, Zorbax Extend C18, was studied with a set of basic and neutral compounds. Thanks to a higher chemical stability than traditional silica based supports, analyses were carried out with a high pH mobile phase, which represents a good alternative to the acidic mobile phases generally used to reduce ion exchange interactions. The performance of this bidentate stationary phase was also compared with that of other supports and it was proved that it is advantageous to work with high pH mobile phases when analyzing basic compounds.

Determination of ionisation constants of organic bases in aqueous methanol solutions using capillary electrophoresis

The pKa of eight organic bases was determined in aqueous and aqueous methanol solutions of 0-70% (v/v) methanol using capillary electrophoresis. The bases investigated include compounds commonly used to test the activity of RP columns in HPLC. The variation of pKa with temperature in aqueous methanol solutions was also investigated and found to closely resemble temperature coefficients reported for bases in purely aqueous solutions. pKa values determined by CE were compared to those reported using NMR spectroscopy. The good agreement of the results is evidence that either technique is suitable to perform pKa measurements.

Hydrophobic and cation exchange mechanisms in the retention of basic compounds in a polymeric column

A cation exchange retention mechanism concomitant with the well-known hydrophobic partition mechanism in a polymeric column has been observed and investigated. This exchange process is attributed to ionization of some acidic sites present in the polymer column at basic mobile phase pH values. Several drugs of different basicity have been chromatographed on a polymeric PLRP-S column with methanol-water and acetonitrile-water mobile phases. The cation exchange between the protonated basic drug and the buffer cations (Na+, K+ and BuNH4+) is observed at the pH range where the protonated drug and the ionized sites of the column coexist. This process produces a shift of the retention versus pH plot of the base to pH values lower than those expected from the pKa of the base as well as a maximum in the plot at basic pH values. These effects are more pronounced for acetonitrile-water mobile phases.

The effect of eluent pH and compound acid–base character on the design of generic-gradient reversed-phase high-performance liquid chromatography (RP-HPLC) methods for use in drug discovery

The design of generic reversed-phase high-performance liquid chromatography (RP-HPLC) gradient methods for the analysis of compound mixtures or 'cocktails' has been investigated with particular reference to the eluent pH and the type of compound (acid, base or neutral) analysed. The use of eluents with an acidic eluent pH, an approach which is widely employed, can lead to non-retention of polar bases resulting in 'failure' of the method. This problem is aggravated where the majority of compounds submitted for analysis are bases, which is typical of many drug discovery programs. The problem can be ameliorated through the use of eluents with near neutral pH. Although these neutral pH eluents can lead to co-elution when cocktails are analysed and possibly ion-suppression where mass spectrometry (MS) is the detection method, this can be avoided through optimisation of the gradient shape.

Effects of pH and the presence of micelles on the resolution of diuretics by reversed-phase liquid chromatography

A comparative study on the performance of two RPLC modes on the separation of 18 diuretics with diverse acid-base behaviour (acetazolamide, althiazide, amiloride, bendroflumethiazide, benzthiazide, bumetanide, canrenoic acid, chlorothiazide, chlorthalidone, ethacrynic acid, furosemide, hydrochlorothiazide, piretanide, probenecid, spironolactone, triamterene, trichloromethiazide and xipamide) was carried out. A conventional octadecylsilane column and acidic acetonitrile-water mobile phases, in the absence and presence of micelles of the anionic surfactant sodium dodecyl sulphate (SDS), were used. The effects of pH and the modifiers acetonitrile and SDS on peak asymmetry, efficiency, selectivity, resolution and analysis time, were examined. The comparison of both RPLC modes (aqueous- and micellar-organics) was done using the same processing tools, applying several polynomial and mechanistic equations to describe the retention. The best separations were obtained by maximising the product of peak purities, considering a wide range of experimental conditions. The study illustrates that, despite the theoretical and practical complexity of the problem, the predicted optimal chromatograms can be reproduced experimentally with great accuracy. None of the examined RPLC modes was able to yield baseline separation of the 18 diuretics. However, their selectivity was complementary, being appropriate for different combinations of a smaller number of the assayed diuretics.

pH Gradient Reversed-Phase HPLC

pH gradient HPLC is reported, which is a new original mode of reversed-phase high-performance liquid chromatography applicable to ionogenic analytes. The method consists of programmed increase during the chromatographic run of the eluting strength of the mobile phase with respect to the acid/base analytes separated. Unlike the well-established conventional gradient HPLC, where the eluting power of the mobile phase is increased with time due to the increasing content of organic modifier, in the pH gradient HPLC that is realized by linearly increasing (in the case of acids) or decreasing (in the case of bases) the pH of the eluent of a fixed organic modifier content, thus providing functional increase in the degree of analyte dissociation and, hence, a decrease in its retention. The pH gradient mode has typical features of gradient HPLC, such as reduced peak width and minimized peak-tailing due to peak compression, which is especially advantageous in the case of organic base analytes. It may be of special value for separation of those analytes which are susceptible to the higher concentrations of organic solvents, as many bioanalytes are. A theory of the pH gradient HPLC has been elaborated, and its full mathematical formalistic is presented step by step in a comprehensive manner. Although fundamental relationships at the basis of pH gradient HPLC are more complex than in the case of the organic gradient variant, the resulting mathematical model is easily manageable. Its applicability to predict changes in retention and separation of test mixtures of analytes accompanying the changes in chromatographic conditions has been demonstrated experimentally in both gradient and isocratic HPLC. The proposed model supplies a rational basis for modifications of eluent pH aimed at optimization of separations and for convenient assessment of chromatographically relevant physicochemical parameters of analytes, such as pK(a).

Determination of pKa values of organic bases in aqueous acetonitrile solutions using capillary electrophoresis

Capillary electrophoresis (CE) was used for the determination of ionisation constants (pKa) of a variety of organic bases in aqueous acetonitrile solutions over the range 0-60% (v/v) acetonitrile. These bases are used as test compounds in HPLC column evaluation, thus knowledge of their pKa in hydro-organic solutions is useful. The base pKa decreased with acetonitrile concentration and significant shifts from the aqueous pKa (up to -0.8) were found using 60% acetonitrile. The CE application was confirmed to be very suitable for fast and accurate pKa measurement in aqueous organic solutions.

Effect of temperature on pH measurements and acid-base equilibria in methanol-water mixtures

The knowledge of the acid-base equilibria in water-solvent mixtures of both common buffers and analytes is necessary in order to predict their retention as function of pH, solvent composition and temperature. This paper describes the effect of temperature on acid-base equilibria in methanol-water solvent mixtures commonly used as HPLC mobile phases. We measured the delta-correction parameter (delta = sw pH - ss pH = Ej - log sw(gamma)oh) between two pH scales: pH measured in the solvent concerned and referred to the same standard state, ss pH, and the pH measured in that solvent mixture but referred to water as standard state, sw pH, for several methanol compositions in the temperature range of 20-50 degrees C. These determinations suggest that the delta-term depends only on composition of the mixture and on temperature. In water-rich mixtures, for which methanol is below 40% (w/w), delta-term seems to be independent of temperature, within the experimental uncertainties, whereas for methanol content larger than 50% (w/w) the delta-correction decreases as temperature increases. We have attributed this decrease to a large increase in the medium effect when mixtures have more than 50% methanol. The pKa of five weak electrolytes of different chemical nature in 50% methanol-water at 20-50 degrees C are presented: the effect of temperature on pKa was large for amines, pyridine and phenol, but almost no dependence was found for benzoic acid. This indicates that buffers can play a critical role in affecting retention and selectivity in HPLC at temperatures far from 25 degrees C, particularlyfor co-eluted solutes.