Determination of the retention factor of ionizable compounds in microemulsion electrokinetic chromatography

Abstract

Determination of the retention factor of ionized compounds in microemulsion electrokinetic chromatography requires two mobility measurements at the same pH: one in the presence of the microemulsion and another in plain buffer. However, it has been observed that in some cases subtracting one mobility from another determined in a different medium leads to negative retention factors, which makes no sense from a chemical point of view. This indicates that there is some error in the process which has a direct impact when retention factors are used for further applications. Here, we evaluate how the components of the microemulsion confer different properties to the buffer medium, particularly varying the viscosity parameter (which is inversely related to mobility). Whereas sodium dodecyl sulfate, the surfactant used in the microemulsion, has little effect on the medium viscosity (only an increase of 5%-6%), the presence of 1-butanol, used as a stabilizer, increases it by around 30%. Meanwhile, heptane, which is used as an oil, provokes a slight decrease. Consequently, the mobilities obtained in the microemulsion system are shifted to higher values (less negative mobilities) compared to mobilities obtained in the aqueous buffer, and so one cannot be directly subtracted from the other. Since the microemulsion-buffer medium cannot be directly reproduced, we propose a correction that takes into account the variation of viscosities. This is determined from the electrophoretic mobility of the benzoate ion. As this ion does not interact with the microemulsion, the ratio of its mobilities (measured in plain buffer and microemulsion) is equivalent to the ratio of viscosities, and can be used as the correction factor for other measurements. Thus, mobilities in buffer and microemulsion media are placed on the same scale, overcoming the errors in retention factor determination.