Retention of basic compounds

D. Skyora, E.Tesarova, and M. Popl, Interactions of basic compounds in reversed-phase high-performance liquid chromatography influence of sorbent character, mobile phase composition, and pH on retention of basic compounds, /. chromatogr. A 758 (1997), 37-51. 

The effect of organic modifiers. The effect of organic mobile-phase modifiers on retention and stereoselectivity of abscisic acid enantiomers has also been examined using methanol, ethanol, and 2-propanol (91). The results of this study indicate that hydrophobic interactions are involved in the retention, but not the stereochemical resolution of the solutes. Miwa and co-workers (91) have also postulated that hydrophobic and hydrogenbonding interactions are important in the retention of basic compounds. 

T. Hanai, Y. Masuda and H. Homma, Chromatography in silica retention of basic compounds on a carboxyl ion exchanger, /. Liq. Chromatogr. Relat Technal, 2005, 28, 3087-3097. 

Figure 7.7 The pH effect on k for the retention of basic compounds in ion-exchange liquid chromatography using a carbo>yl phase.

Competing amines such as triethylamine and di-rc-butylamine have been added to the mobile phase in reversed-phase separations of basic compounds. Acetic acid can serve a similar purpose for acidic compounds. These modifiers, by competing with the analyte for residual active sites, cause retention time and peak tailing to be reduced. Other examples are the addition of silver ions to separate geometric isomers and the inclusion of metal ions with chelating agents to separate racemic mixtures. 

As the pH of the mobile phase markedly influences the retention of ionizable compounds, it can be assumed that the separation capacity of RP-HPLC for ionizable analyses can be modified by changing the pH of the mobile phase. The theory of effect of pH gradient on the performance of RP-HPLC systems has been recently elaborated. The basic equation describing the dependence of the retention of the solute on the gradient of pH or organic modifier is ... 

In acidic eluents (Figure 3.9C and D), the retention of acidic compounds becomes stronger and that of basic compounds becomes weaker. In this system, uric acid (h) is not suitable as a void volume marker due to its longer retention time. In neutral and basic eluents, an ionized acid can be used as the marker because no other compounds are eluted more rapidly. Fructose (c) is a very... 

Changes in mobile-phase components such as pH, ionic strength, and water content have been systematically studied [3,310,316,317]. These studies indicate that retention of basic analytes is mediated primarily by the cation-exchange properties of the silica [2]. Interestingly, it has been suggested from retention data of various pharmaceuticals that the retention mechanisms of silica with aqueous eluents and reversed-phase systems are similar [317,318]. Due to the ion-exchange properties of silica, mobile-phase pH adjustments are useful in changing the retention of ionic compounds. 

FIGURE 7.3 Effect of l-ethyl-3-methylimidazolium tertafluoroborate concentrations on retention factors (k ) of basic compounds. Symbols = octopamine o = synephrine = tyramine. (From Tang, F. et al. J. Chromatogr. A. 2006,1125,182-188. With permission from Elsevier Science.)... 

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... 

Two basic approaches have been described to enhance the affinity of ionic compounds for the stationary phase in reversed-phase systems. The first approach, sometimes inappropriately referred to as ionic suppression involves the adjustment of the pH mobile phase so that the compound of interest is in the neutral or unionized form. The second approach to enhance the retention of ionic compounds on reversed-phase supports involves the application of ion-pair extraction techniques. 

In LC-ESI-MS, the role of the mobile phase pH is complicated. In practice, often a compromise must be strack between analyte retention and ionization. From the perspective of generating preformed ions in solution, the optimum conditions for the ESI analysis of basic compounds, e.g., amines, would be an acidic mobile phase with a pH at 2 units below the dissociation constant pIQ of the analytes, while for acidic compounds, e.g., carboxylic acid or aromatic phenols, a basic mobile phase with a pH two units above the pK, of the analytes is preferred [97]. These conditions are uirfavourable for an analyte retention in RPLC. The analytes elute virtually umetained. In RPLC, it is important to reduce protolysis of basic and acidic analytes, i.e., to assure that the compounds are... 

Satisfactory results are obtained with compounds of different nature. Of particular interest is the case of basic compounds, such as phenethylamines and 3-blockers, which experience large efficiency enhancements in SDS systems.This makes the use of special columns less necessary. The surfactant layer adsorbed on the column prevents the interaction of basic compounds with free silanol groups, which accounts for the low efficiencies observed with conventional columns in aqueous-organic RPLC. For acidic compounds such as sulfonamides, the efficiencies are comparable with both MLC and conventional RPLC, but for low polar compounds such as steroids, efficiencies are comparably poorer in MLC. However, in this technique, low polar steroids are eluted at sufficiently short retention times via a small amount of... 

Feng, Y.Q. Fu, H.J. Zhang, Q.H. Da, S.L. Zhang, Y.J. Retention behavior of basic compounds on alkylphosphonate-modified magnesia-zirconia composite stationary phase in RPHPLC. Chromatographia 2000, 52, 165 168. 

Goldberg compared several types of octadecyl columns for their retention of neutral, acidic and basic compounds. Vastly differing chromatographic properties were found. A more extended study of the diversity of octadecyl bonded phases was made by Engelhardt et al. 1. The suitability of the various materials for the analysis of basic compounds is also taken... 

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