The Study On Electrokinetic Flow Analysis System And Its Application On Separation And Determination Of Preservatives

The electrokinetic flow analysis(EFA)system based on the flow analysis techniques of sequencial injection analysis,multicommutation and electroosmotic pump and the chromatographic separation with silica monolithic column is developing to be a total portable analysis system.The EFA system is simple, convenient and easy to be controlled automatically.In this dissertation,the analytical methods of the separation and determination for preservatives in food and cosmetic products are proposed by combining the EFA system with capillary electrophoresis or reversed-phase electrochromatography.The preparation conditions of reversed-phase silica monolithic microcolumn filled with fine quartz sand are studied.And the prepared microcolumn has been employed in the separation of parabens by reversed-phase electrochromatography(EC)successfully.1.An analytical method of on-line pretreatment and determination of three parabens(i.e.methyl,ethyl and propyl p-hydroxybenzoate)in cosmetic products is proposed by using flow injection analysis(FIA),solid phase extraction(SPE)and micellar electrokinetic chromatography(MEKC).An improved homemade split-flow interface is used to couple solid phase extraction(SPE)of C₈ bonded silica with MEKC separation,which can avoid the separation buffer contamination and reduce the buffer consumption,especially,for the buffers containing expensive reagents.The samples are pretreated by the SPE and eluted with the eluent of 10 mmol/L sodium tetraborate buffer(pH 9.3)containing 60%(v/v)ethanol at 0.75 mL/min,and the parabens are separated on baseline with the running buffer of 20 mmol/L sodium tetraborate(pH 9.3)containing 100 mmol/L sodium dodecyl sulfate(SDS).Butyl p-hydroxybenzoate(BP)is used as an internal standard added into the standard and real sample solutions to improve accuracy of the method.The analytical characteristic of this FIA-SPE-MEKC method and the recoveries with and without the internal standard were compared.2.A combined analysis system of electrokinetic flow analysis,solid phase extraction and micellar electrokinetic capillary chromatography is proposed to determine six preservatives in food samples.The electrokinetic flow analysis system consists of one homemade electroosmotic pump,five solenoid valves and a solid-phase extraction(SPE)microcolumn.The electroosmotic pump and solenoid valves are controlled by a personal computer with a homemade interface card and a Visual C program written by our group.With a modified interface of EFA and CZE, the buffer consumption can be reduced to 130μL for each running.The running buffer of MEKC is 20 mmol/L disodium tetraborate(pH 9.3)containing 100 mmol/L SDS.p-Hydroxybenzoic acid(PBA)is adopted as an internal standard(IS).The proposed method can separate and determine the preservatives within 15 min,having the relative standard deviation(RSD)of peak area less than 3.4%(n=5),limits of detection(LODs)ranged from 0.04 to 0.1μg/mL(S/N=3,n=11)and recoveries in the range of 91.4%and 104%.3.A combined analysis system of electrokinetic flow analysis,ion-pair solid phase extraction and capillary zone electrophoresis is proposed to determine benzoic acid and sorbic acid in food samples.The electrokinetic flow analysis system consists of one electroosmotic pump,five solenoid valves and one on-line solid-phase extraction(SPE)unit.By using the SPE unit,the SPE operation and CZE separation can be performed simultaneously.Tetrabutylammonium bromide is adopted as an ion pair reagent to improve the retention of the preservatives on C₈-bonded silica sorbent. The preservatives are separated and determined under optimized conditions with p-hydroxybenzoic acid as an internal standard.The relative standard deviation(RSD). of peak area for each analyte is less than 3.1%(n=5)and the limits of detection (LODs)range from 10 to 20 ng mL^-1(S/N=3,n=11).4.Silica monolithic columns functionalized with octyl groups are synthesized by a two-step hydrolysis/condensation of tetrathoxyailane(TEOS)and n-octyltriethoxysiliane(C₈-TEOS)catalyzed with acid and base conditions.The influence factors in the sol-gel procedure,such as the reactant composition,hydrolysis acitity and time are investigated systematically.The optimal preparation conditions are as follows:0.5 mL TEOS,0.4 mL C₈-TEOS,0.6 mL ethanol,0.093 mL-0.33 mol/L HCl are mixed,hydrolyzed at 60℃for 4 h and cooled to room temperature.50 mg n-dodecylamine is added into 0.5 mL above solution,and then the mixed solution is introduced into a PTFE mold and polymerized for at 60℃48 h.The results of micromorphology and element analysis validate that the microcolumn can be employed in the chromatographic separation.Based on the preparation of the silica monolithic column mentioned above,the silica monolithic microcolumns are prepared in 2.0 mm i.d.fused silica tube filled with 100-120μm quartz sand.The microcolumn shrinkage in the fused silica tube can be avoided during the condensation process and the heating effect can also be eliminated by filling the fine quartz sand.5.A pressure-assisted separation system with the electrochromatographic monolithic microcolumn and on-line UV detection is designed and established to separate and determine parabens.The influence factors of the separation are explored, such as organic solvent and concentration,buffer pH and concentration,flow rate and electric field strength.Associated-pressure is adopted to drive the mobile phase in reversed-phase electrochromatography,which can improve the separation efficiency. On the other hand,the heating effect is limited in the separation column by using an appropriate separation voltage.The separation results were satisfying with the electrochromatographic microcolumns.The parabens are separated on baseline using the reversed-phase silica monolithic microcolumns with the mobile phase of 1.0 mmol/L Tris-HCl(pH 8.5)containing 20%(v/v)acetonitrile.