Retention factor and retention index of homologous series compounds in microemulsion electrokinetic chromatography employing suppressed electroosmosis

Abstract

The retention factor (k) and retention index (I) of homologous series compounds such as alkylbenzenes (BZ), alkylaryl ketones, alkylbenzoates, and alkylparabens in microemulsion electrokinetic chromatography (MEEKC) with suppressed electroosmosis were investigated in a wide range of SDS concentrations ([SDS]), temperatures, and concentrations of organic cosolvents (phi). Using BZ as standards, the retention indices of other homologous series compounds were determined and they were found to be independent of [SDS] and temperature, while are dependent on the types and concentrations of organic cosolvents. The retention factor linearly increases with increasing [SDS], while linearly decreases with increasing temperature. The value of log k linearly decreases with increasing phi for methanol, ethanol, or ACN, while decreases by a second-degree polynomial with increasing phi for 2-propanol. Excellent agreement was found between the observed and predicted values of log k of analytes in MEEKC at given [SDS] and phi, where the predicted values were obtained from modified equations of the linear relationship of log k as functions of [SDS], the number of carbons, and phi. Therefore, both k and I can be used for peak identification of homologous series compounds.

Comparison of resolution in microemulsion EKC and MEKC employing suppressed electroosmosis: Application to bisphenol‐A‐diglycidyl ether and its derivatives

“…The rationale for the same definition of k is given for MEKC and MEEKC with and without EOF because a change in k caused by any parameter can be explained by using the same principle of MEKC and MEEKC, as reported in our previous work [6,29]. The additional reason is that this definition can be used for charged analytes in MEKC and MEEKC without EOF and with neutral surfactants, where both the pseudostationary and aqueous phases are immobilized.…”

“…The greater decrease in log a CH2 with increase in the concentration of 2-PrOH in MEEKC is possibly because 2-PrOH has the interaction with the charged oil droplets [1,29], while other organic solvents have little or no interaction with the pseudostationary phase. A slight decrease in the values of log a CH2 with increase in the concentration of ACN in MEKC as shown in Fig.…”

“…The hydrophobicity of pseudo stationary phase may be compared using the logarithm of methylene selectivity, log a CH2 , that is obtained from a slope of a linear plot between log k of the homologous series in MEEKC or MEKC versus the number of carbons [29,30]. The greater the value of log a CH2 , the higher the hydrophobicity of the pseudo stationary phase [30].…”

“…At a given concentration of each organic cosolvent, log a CH2 obtained from MEEKC is Figure 3. Methylene selectivity, log a CH2 , for MEEKC (solid lines) and MEKC (dash lines) at various types and concentrations of organic cosolvents, where log a CH2 was obtained from the slope of a linear plot between log k of alkylbenzene series versus the number of carbons as described in our previous work [29]. Symbols n, r, m, and d refer to ACN, MeOH, EtOH, and 2-PrOH, respectively.…”

Advances in the theory and application of MEEKC

“…Suppression of EOF has been employed in MEEKC to reduce the migration time of neutral compounds [31,[33][34][35][36]. Under normal MEEKC conditions, employing a negatively charged surfactant such as SDS and a strong EOF flow with a positive applied voltage, neutral compounds partition strongly into the ME droplet and migrate towards the anode.…”