Study Of The Rapid Detection Of Lead In Tea With Bismuth-film SPCE Sensor Based On Differential Pulse Stripping Voltammetry

As one of the three most popular drinks in the world,tea is recognized as natural health drinks and loved by the world people.China is the second largest tea-producing country in the world,and also is one of the biggest export countries of tea.Tea is an important agricultural export,and its quality directly relates to our country's tea industry.Lead is a kind of heavy metal elements which has nerve toxicity and accumulative toxicity.Excessive absorption of lead will result in severe hazard of human health.There are strict restrictions of lead content in lots of food both at home and abroad.Tea also contains a certain amount of lead, which mainly derives from soil.The detection of lead content is one of the significant indicators of hygiene inspection in many types of food including tea.Therefore,it is very important to seek for a rapid detection method of lead in food. Electrochemical analysis,which is sensitive,accurate and fast,needs the instrumentation equipment with low cost and is easy to operate.The traditional heavy metal detection method has the lack of tedious procedure, expensive equipment and high cost of detection.Electrochemical analysis overcomes the defects above,and has already been a focus in the rapid heavy metals detection study.The voltammetry was the most common approach in electrochemical methods to detect heavy metals,and differential pulse stripping voltammetry was one of the highest sensitivity ways to detect heavy metals.The solid electrodes commonly used in electrochemical heavy metals detection have difficulties in mass production and widespread application, because of their complicacy of treatment and maintain.The study using a screen printed carbon electrode(SPCE) instead of common solid electrodes to overcome their lack,has successfully developed a new technique of rapid detection of trace lead in tea based on differential pulse stripping voltammetric(DPSV) methodology with screen printed carbon electrode (SPCE) situ-plated in mercury(â…¡).On this achievement,we adopt the non-toxic "green metal" bismuth to replace mercury in order to avoid the harm of conventional use of mercury electrodes,such as great toxicity, contamination to environment and hazard to health.The Bi-electrode which has high accuracy and reproducibility,was easy to operate and renew, therefore has great research value and practical application potential.Based on this,the study combines the differential pulse stripping voltammetry(DPSV) and SPCE modified by Bi-plating method,in order to construct a novel electrochemical rapid detection electrode of lead in tea.On the foundation of all these study obtain ideal result,we research and improve the pretreatment method of lead detection in food,trying to find a new sample pretreatment technique with the characteristic of fast,easy and widely applicable.This paper mainly contains the following aspects:The first part of the thesis summarizes the heavy metals detection methods in food,especially introduces the detail of electrochemical detection method.It also states the relative principle,material and data analysis.The second part presents a new rapid detecting technique of trace lead in tea based on differential pulse stripping voltammetry method with SPCE electrochemical sensor.Several factors such as conditions of Hg-film plating on SPCE,the concentration of buffer solution,and other parameters of electrochemistry were optimized.After the experiment,we choose the 3×10-4 mol/L HgCl2 solution as Hg-film plating reagent.A series of optimum parameter are as follows:Plating potential—-1.0V,E_step—0.05 V,Pulse amplitude—0.2 V,Pulse width—0.5s,Pulse interval—0.5 s,plating time—300s,balance time—30 s,base solution—25 mL 0.1 mol/L HCl. Under the optimized condition,the study result showed that the calibration curve was linear in the range of 10～300μg/L,the equation was Ip(μA)= 0.0205×X(μg/L)+0.8211 with regression coefficient of 0.9936,and the detection limits was 1.31μg/L(S/N=3)(equivalent to 0.065mg/kg lead content in tea).The interference of 150μg/L of lead was studied on the existence of a variety of other ions.The heavy metals of lead in tea samples were detected and the result was good,the standard deviation was less than 8%,and the recoveries were among 85.2%to 109.3%.There was no significant difference between the results of GB detection method with the paired t-test comparison. The method is sensitive,accurate,fast and simple,and worth to be further studied.In the third part,the method of rapid detection of lead in tea based on DPSV with SPCE situ-plated in bismuth(Bi) was built.Detailed description of the process optimization was reported.A series of best parameter are:Bi³⁺ concentration—1.0×10^-5mol/L,base solution—25 mL 0.1 mol/L acetic acid buffer solution,pH 4.0,Plating potential—-1.1 V,E_step—0.01 V,Pulse amplitude—0.1 V,Pulse width—0.1 s,Pulse interval—1.5 s,plating time—300 s,balance time—30 s.Under the optimized condition,the study result showed that the calibration curve was linear in the range of 50～300μg/L,and the detection limits was 2.35μg/L(S/N=3)(equivalent to 0.12mg/kg lead content in tea).The interference of 150μg/L of lead was studied on the existence of a variety of other ions.The heavy metals of lead in tea samples were detected under the optimized conditions,and the result was accord with GB detection result well.This part built a new rapid detecting technique of trace lead in tea with Bi-SPCE electrochemical sensor,which is accurate,fast,lower cost and suitable for disposable usage.In the fourth part,we chose tea as test sample to study on three kind of pre-treatment methods that are tea-infusion method,acid liquid immersion method and microwave digestion method.The result showed,tea-infusion method is simple,economical and safe,but the lead extraction effect is not good,so it is not suitable for the electrochemical sensor of rapid detecting lead in tea.Acid liquid immersion method is also simple and economical.The optimized conditions are as follows:reagent—20%nitric acid,water bath temperature—70℃,immersion time—30min.After centrifugation,we heat the supernatant solution and then dilute to constant volume.It can eliminate part of the influence induced by disadvantage factors,but shows low lead extraction rate.In the research of microwave digestion method,we improve the digestion process,and optimize the digestion reagent and equipment parameter.The working parameters of the microwave digestion procedure are: 0.4 MPa,4 min,0.8 MPa,1 min,1.2 MPa,1 min,1.6 MP,1 min,2.0 MPa,4 min.The detection result of sample treated under these conditions was precise and accurate,and has high conformity to GB detection result.This microwave digestion method has many advantages such as fast,convenient,low pollution, saving reagent and safe to operate,therefore it has great research value and practical application potential.In the fifth part there were summary of papers and prospects.A rapid and simple method for determination of heavy metal element lead in tea was developed.It was also a new detection means for detecting heavy metals individually and simultaneously in other kind of food.