Simultaneous Determination Of Total Nitrogen And Total Phosphorus In Water By Capillary Electrophoresis Combined With Capacitive Coupling Contactless Conductivity Detection Online Digestion

Capillary electrophoresis(CE)combined with capacitive coupling contactless conductivity detection(C~4D)was a new method for field detection,which could detect multiple important parameters in a single device at the same time.Its instrument had the advantages of small volume,high sensitivity and easy-to-use.However,application of CE-C~4D method was limited to the detection of ionic compounds.To the best of our knowledge,there was no reports on the determination of total nitrogen and total phosphorus.Total nitrogen and total phosphorus were significant monitoring indicators reflecting water environment quality,which need to be detected online simultaneously without any reported.In order to expand the application scope of CE-C~4D method in water quality detection,it was momentous to develop a new method for simultaneous detection of total nitrogen and total phosphorus.(1)Potassium persulfate method was the most commonly used digestion method for detecting total nitrogen and total phosphorus.The water sample digested by potassium sulfate contains high concentration of sulfate,which seriously affected the electrophoretic separation and the detection of nitrate and phosphate produced by digestion.In order to solve this problem,after optimizing the conditions,a high concentration mixture of 500mmol/L 2,2-Bis(hydroxymethyl)-2,2’,2’’-nitrilotriethano(Bis Tris)and 500 mmol/L3-morpholinopropanesulfonic acid(MOPS)was used as the CE background electrolyte,which improved the stacking of trace ions in the salty sample;an inner wall modified capillary of 10μm inner diameter was adopted as the CE separation channel to reduce the influence of excessive Joule heat caused by the high concentration background electrolyte.Based on it,a new CE-C~4D method for the determination of trace ionic compounds in high concentration sulfate water samples were established.The maximum sulfate content that could be tolerated by the method was 18.5 mmol/L.At present,nitrate and phosphate was baseline separated,and the signal peak shape was not affected as well.Within 15 days,the relative standard deviation(RSD)of nitrate and phosphate standard solutions,both of the concentration as 500μmol/L,were 5.1%and 4.6%(n=30),respectively.The detection limits for nitrate and phosphate were 2.3μmol/L and 2.8μmol/L,respectively.(2)Based on the ce-c4d method established in the previous chapter,a new method was developed and combined with the off-line digestion method of potassium persulfate,for the simultaneous determination of total nitrogen and total phosphorus.After optimization,the diluted samples(dilution ratio of 25%-50%)was mixed with a high-purity alkaline potassium persulfate(concentration of 18.5 mmol/L sodium hydroxide)were digested in a high-pressure vessel at 120℃.The digestion solution was detected by the CE-C~4D method and standard method respectively.The results obtained by the two methods were well matched.In recovery testing,1 mmol/L 2-aminoethylphosphoric acid standard solution was added to wastewater samples,and recoveries of total phosphorus were 90%-110%,the recoveries of total nitrogen were 62%-83%.(3)A set of UV catalytic thermal digestion flow-through digestion system was built and connected with the capillary electrophoresis instrument to build an on-line digestion capillary electrophoresis instrument.In the flow-through digestion system,the sample was mixed with oxidant(5 g/L potassium persulfate solution and 1.5 g/L sodium hydroxide solution)in a 1:1 ratio online,digested under 18 W ultraviolet light for 15 minutes,and at90℃for another 15 minutes.The digested solutions would be mixed with internal standard,followed by bubble removing and cooling down in a water bath.Finally,objectives would be detected by CE-C~4D method.After the optimization of the flow-through digestion system,the digestion efficiency of most nitrogen and phosphorus compounds were over80%.Compared with the standard photometric method,the results of the two methods were agreed.However,the recoveries of some organic compounds was not satisfactory.Further improvement was necessary.