Investigation of sweeping as a sample enrichment method in micellar electrokinetic chromatography in the analysis of pharmaceutical preparations and biological fluids
The present thesis deals with the study of sweeping as one of the most important sample preconcentration techniques in micellar electrokinetic chromatography (MEKC). The work includes the study of the fundamentals of sweeping as well as the application in the pharmaceutical field. The thesis is divi...
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The present thesis deals with the study of sweeping as one of the most important sample preconcentration techniques in micellar electrokinetic chromatography (MEKC). The work includes the study of the fundamentals of sweeping as well as the application in the pharmaceutical field. The thesis is divided into four main parts.
In the first part of the thesis, the processes involved in sweeping under homogeneous and under inhomogeneous electric field conditions are theoretically discussed. These processes include stacking or destacking of micelles when entering the sample zone, sweeping of analytes by the stacked or destacked micelles, and destacking or stacking of the swept analyte zone. A new robust and reliable method for the assessment of the sweeping efficiency is developed based on recording the peak height dependent on the injected sample volume. The values obtained via this method agree well with theoretically predicted ones. The results obtained show that 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 attributed to the focusing of neutral analytes by micellar transient isotachophoresis.
In the second part of the thesis, we extend our developed method for the assessment of sweeping efficiency to the general case, in which the distribution coefficient and the electric conductivity is varied in the sample and BGE compartments. It is shown that in the general case – in contrast to the classical description of sweeping – 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 BGE. An additional focusing/defocusing step is confirmed and the term “Retention factor gradient effect (RFGE)” is introduced. The validity of the derived equation is confirmed 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).
In the third part of the thesis, the processes involved in sweeping in cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) are theoretically discussed with a special focus on dynamic pH junction and adsorption of the analyte onto the capillary wall. Ethylparaben (pharmaceutical preservative) as an example of acidic analytes and desloratadine (antihistaminic drug) as an example of basic analytes are investigated using different types of ß-cyclodextrins. The presence of RFGE as an additional focusing/defocusing effect in sweeping-CD-MEKC is confirmed under the conditions of different content of cyclodextrin and different pH between the sample and the BGE. Desloratadine shows an unexpectedly low enrichment factor compared to the less hydrophobic ethylparaben. This unexpected behavior is ascribed to the strong adsorption of the protonated species of this drug onto the inner capillary wall in the sample zone that significantly counteracts the sweeping process. This effect is confirmed by the improvement in the enrichment factor achieved by the addition of a dynamic coating agent to the sample solution.
In the fourth part of the thesis, a CD-MEKC method is developed for the simultaneous determination of the antihistaminic drugs loratadine and desloratadine (the major metabolite and an impurity of loratadine). The tendency of these drugs, as hydrophobic basic analytes, to be adsorbed onto the inner capillary wall and the difficulty to separate them due to the extremely high retention factors make the present study challenging. The use of a low pH sample solution overcome problems associated with the low solubility of the studied analytes in aqueous solution while having advantages with regard to online focusing. In addition, the use of a basic BGE and the presence of cyclodextrin reduce the adsorption of these analytes in the separation compartment. The separation is achieved in less than 7 min using a BGE consisting of 10 mmol L-1 sodium borate buffer, pH 9.30 containing 40 mmol L-1 SDS and 20 mmol L-1 hydroxypropyl-β-CD while the sample solution is composed of 10 mmol L-1 phosphoric acid, pH 2.15. All validation parameters are thoroughly investigated. The developed method is successfully applied to the analysis of the studied drugs in tablets and in spiked human urine. Moreover, desloratadine is detected at the stated pharmacopeial limit (0.1%) as an impurity in loratadine bulk powder. The obtained results are compared with those of the official liquid chromatographic method and are found in a good agreement.|