Processes involved in sweeping under inhomogeneous electric field conditions as sample enrichment procedure in micellar electrokinetic chromatography.

Sweeping under inhomogeneous electric field conditions has been described as a process that includes stacking or destacking of the micelles when entering the sample zone, sweeping of analytes by the stacked or destacked micelles, and destacking or stacking of the swept analyte zone. However, there is ongoing debate that not only the retention factor of the analyte but also the electric conductivity of the sample solution or the concentration of an added salt can have an impact on the enrichment efficiency. Revisiting the equations describing sweeping, a factor θ (phase ratio shift factor) is defined to quantitatively describe the change of the retention factor between the sample and separation zones. The influence of the sample matrix composition on the experimentally obtained sweeping efficiency is studied with SDS as pseudostationary phase taking parabens, benzamide and anilines as model analytes. To this end, a robust and reliable method for the assessment of the sweeping efficiency is developed. The values obtained via this method are very precise and agree well with theoretically predicted ones. The results obtained for varied buffer concentration and varied concentration of NaCl in the sample solution show that under the conditions of our experimental study, the approximation of assuming θ to be equal to the reciprocal value of the field strength enhancement factor γ is valid. Accordingly, the sweeping efficiency for neutral analytes is independent of the electric conductivity of the sample matrix. It is also shown that under specific conditions unexpectedly high enrichment factors are obtained which are ascribed to the focusing of neutral analytes by micellar transient isotachophoresis (mtITP). The results obtained in this study can be used as a guide for better understanding of the sweeping process and the factors affecting the sweeping efficiency in micellar electrokinetic chromatography (MEKC).

El-Awady M ，Huhn C ，Pyell U 《-》

Processes involved in sweeping as sample enrichment method in cyclodextrin-modified micellar electrokinetic chromatography of hydrophobic basic analytes.

Sweeping is an enrichment method in MEKC, which includes following steps: stacking/destacking of the micelles, sweeping of analyte by the stacked/destacked micelles, destacking/stacking of the swept analyte zone and additional focusing/defocusing due to the retention factor gradient effect (RFGE). In this study, we investigate additional processes, regarding online focusing in cyclodextrin-modified MEKC (CD-MEKC) of hydrophobic basic analytes: dynamic pH junction (sample with pH different from that of BGE) and adsorption of analyte onto the capillary wall within the sample zone. It is demonstrated that the developed method for the assessment of the sweeping efficiency is also applicable to CD-MEKC taking ethylparaben as an example of acidic analytes and desloratadine as an example of basic analytes using different types of β-cyclodextrin. Our previous results regarding RFGE as an additional focusing/defocusing effect in sweeping-MEKC are confirmed for the case that the apparent distribution coefficient differs for the sample and the BGE due to a different content of the complex-forming agent cyclodextrin and due to a pH difference between the sample and the BGE. Despite being significantly more hydrophobic than ethylparaben, desloratadine shows an unexpectedly low enrichment factor. This enrichment factor is nearly unaffected by the addition of CD to the BGE. This unexpected behavior is attributed to wall adsorption of the protonated hydrophobic basic analyte within the sample zone, which significantly counteracts the sweeping process. This assumption is corroborated by an improvement in the enrichment factor achieved via addition of a dynamic coating agent (triethylamine) to the sample solution.

El-Awady M ，Pyell U 《-》

Sweeping as a multistep enrichment process in micellar electrokinetic chromatography: the retention factor gradient effect.

The application of a new method developed for the assessment of sweeping efficiency in MEKC under homogeneous and inhomogeneous electric field conditions is extended to the general case, in which the distribution coefficient and the electric conductivity of the analyte in the sample zone and in the separation compartment are varied. As test analytes p-hydroxybenzoates (parabens), benzamide and some aromatic amines are studied under MEKC conditions with SDS as anionic surfactant. We show that in the general case - in contrast to the classical description - the obtainable enrichment factor is not only dependent on the retention factor of the analyte in the sample zone but also dependent on the retention factor in the background electrolyte (BGE). It is shown that in the general case sweeping is inherently a multistep focusing process. We describe an additional focusing/defocusing step (the retention factor gradient effect, RFGE) quantitatively by extending the classical equation employed for the description of the sweeping process with an additional focusing/defocusing factor. The validity of this equation is demonstrated experimentally (and theoretically) under variation of the organic solvent content (in the sample and/or the BGE), the type of organic solvent (in the sample and/or the BGE), the electric conductivity (in the sample), the pH (in the sample), and the concentration of surfactant (in the BGE). It is shown that very high enrichment factors can be obtained, if the pH in the sample zone makes possible to convert the analyte into a charged species that has a high distribution coefficient with respect to an oppositely charged micellar phase, while the pH in the BGE enables separation of the neutral species under moderate retention factor conditions.

El-Awady M ，Pyell U 《-》

Sweeping of neutral analytes in electrokinetic chromatography with high-salt-containing matrixes.

Quirino JP ，Terabe S ，Bocek P 《-》

Sweeping with electrokinetic injection and analyte focusing by micelle collapse in two-dimensional separation via integration of micellar electrokinetic chromatography with capillary zone electrophoresis.

Zhang Z ，Du X ，Li X 《-》