| Capillary electrophoresis(CE) and liquid-phase microextraction(LPME) are important separation and sample preparation techniques. They have some common characteristics such as low solvent-consumption and friendly to environment, thus their combination has broad application prospect in many fields including bioanalysis, food and environment analysis. However, water-immiscible LPME extraction solvents are always incompatible with aqueous CE buffer. The main mediate routines to couple LPME with CE are evaporation of immiscible extractants and backextraction of analytes. But the former is not applicable to extractants with high boiling points, the latter being effective only for acidic or basic analytes, both of which limit the further application of LPME-CE. Based on these above, direct introduction of extractants is regarded as an ideal strategy. With good tolerance to high levels of organic solvent and salt, in this study, acetonitrile(ACN) stacking was chosen to overcome the imcompatiblity of extractants and CE buffer and achieve the direct combination of LPME and CE. The research results were summarized as follows.(1) In this study, a novel method based on acetonitrile stacking was proposed to accomplish large-volume injection of extractant diluted with a solvent-saline mixture before micellar electrokinetic chromatography. With 1-octanol as a model LPME extractant and six phenols as model analytes, Brij-35 and β-cyclodextrin were employed as pseudostationary phases for for improving the compatibility of sample zone and aqueous running buffer. A short solvent-saline plug was used to offset the adverse effect of the water immiscible extractant on focusing efficiency. The sweeping effect of Brij-35 and β-cyclodextrin after ACN stacking was free from the introduction of octanol. Under the optimized conditions, with ~30-fold concentration enrichment by DLLME, the diluted extractant(8 ×) was then injected into the capillary with a length of 21 cm(42% of the total length), which yielded the overall improvements in sensitivity of 170-460. Limits of detection and qualification ranged from 0.2 to 1.0 ng/mL and 1.0 to 3.4 g/mL, respectively. The average recovery rates were in the range of 89%~113%. Acceptable repeatability lower than 3.0 % for migration time and 9.0 % for peak areas were obtained. The developed method was successfully applied for analysis of the phenol pollutants in real water samples.(2) With seven highly-substituted chlorophenols as hydrophobic model analytes, dual stacking mechanism of ACN stacking was firstly revealed — pseudo-isotachophoresis and induced pH-junction. Based on this, we developed two direct combination modes of LPME and micellar electrokinetic chromatography by utilizing ACN stacking and succeeded in introducing two water-immiscible extractants, diethyl carbonate(DEC) and the mixture of DEC and 1-hexyl-3-methylimidazolium hexafluorophosphate(2:1, v/v), into CE for analysis of the chlorophenols. Different cosolvents played a key role in dissolving extractants. The buffer plugs deprived of pseudo-stationary phases imporved the separation of analytes and sweeping effect. DEC-based LPME combined with CE exhibited better analytical performance: with ~40-fold concentration by DLLME, the diluted DEC(8×) was then injected into the capillary with a length of 20 cm(33% of the total length) to perform stacking, and 260–791 folds enrichment was achieved finally. Limits of qualification ranged from 5.5 to 16.0 ng/mL. The average recovery rates were in the range of 88%~106%. Acceptable repeatability lower than 1.8% for migration time and 8.6% for peak areas were obtained. The proposed method was applied for analysis of the chlorophenol pollutants in 15 red wine samples.(3) 12 anionic compounds with the pKa values ranging from 2.97 to 10.09 or logP values ranging from 0.80 to 5.12, respectively, were selected to probe the ACN stacking rules of anionic analytes. The effect of composion and pH of buffer and sample on stacking were investigated. Results indicated that the stacking of different analytes was closely related to buffer pH. The analytes with higher pKa than buffer pH showed single focused peak as soon as ionization, however, the lower pKa analytes showed the splitting peaks in the lower p H buffer relative to pKa, while in higher pH buffer to pKa the splitting peaks merged. Sample pH had an effect on the ionization of analytes with higher pKa and suppressed anodic stacking in the sample zone whether the pH was higher or lower than analytes pKa. |
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