| Natural medicine refers to pharmacologically active natural products extracted from animals,plants,and minerals.Chinese natural medicine is one of the developed herbal systems of the world,and its research level and quality control heavily dependent on the separation and purification of the effective ingredients.In order to reduce the loss of active ingredients to improve the yield,the monitoring on concentration of the targeting substance is an important part during the whole chromatography process.In this paper,several common ingredients such as flavonoid,alkaloid,and terpenoid are investigated with electrochemical approach.By studying the response on different working electrodes in the situation that supporting electrolyte exists,ingredients of high electrochemical activity are screened out.Afterwards,Nafion membrane is assembled into two electrochemical flow cells.By take advantage of the solid acid feature of Nafion,the active ingredients in electrolyte-free organic solution are detected directly,and keep the bulk solution not affected.Thus,non-destructive testing purpose is achieved,and the specific results are showed as follows:(1)Flavonoid has a good response on the gold electrode and has the strongest response on the graphite-liquid paraffin electrode with volume ratio 1 g/1 mL,and the detection limit reaches as low as 1.0×10-8 mol·L-1.The effect of accumulation time and potential on peak current is slight.As paraffin can form compact hydrophobic films on graphite powder,it inhibits the oxidation of myricetin by selectively blocking the second electron transfer,and yields the semiquinone radical as main product,which adsorbs strongly on the electrode surface so the myricetin to semiquinone radical reaction nearly achieves reversibility.The 3-hydroxy group on the C ring is then oxidized due to semiquinone disproportionate directly to a quinoid(quinone methide)product.However,the electrochemistry responses of matrine,ursolic acid and other substances are so weak on carbon paste electrode that carbon paste electrode is not fit for detecting these ingredients.(2)Two micro-gap flow cells with Nafion N-324 membrane as solid electrolyte are designed and employed successfully for the determination of myricetin(80%ethanol)and AOB(antioxidant of bamboo leaves,90%ethanol)in the absence of electrolyte.Compared to Nafion N-117,Nafion N-324 with PTFE enhanced structure is more stable in the organic solvent and is more suitable to be used as a unit installed in the flow cell when the supporting electrolyte is absent.Peristaltic pump is utilized to stimulate the chromatographic process.Setting the sampling potential to 0.53 V,amperometric i-t curve of eletroactive substances in the eluent are obtained by using 80%(v/v)ethanol-water as mobile phases.And then by setting the sampling potential to 0.68 V and sampling interval to 0.5 s,this assembly is utilized to monitor the concentration of AOB in the purification process with macro porous resin.The dependence of the amperometric response on flow rate and sample concentration has been characterized.Experimental results show the paraffin-graphite-Nafion assembly is superior for being used in electrolyte-free solution. |
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