Development And Application Of An Electromagnetic Induction Heating On-line Sample Pretreatment System

Sample preparation,which is often considered as the bottle-neck of an analytical procedure,is developing to be more rapidly,exactly and automatically.In this thesis, different electromagnetic induction heating(EMIH) systems based on EMIH technique were developed and applied to on-line sample digestion as well as on-line sample preconcentration.In chapter 1,the theory and development of flow injection analysis(FIA) technique as well as the detection methods which were often coupled to FIA were firstly introduced.Then,different sample digestion techniques and sample preconcentration methods were presented in detail.At last,the theory of EMIH was introduced and the relevant knowledge about the magnet design and circuit design was also illustrated.In the following chapters,different EMIH systems were established and the parameters which influence the performance of the proposed system were optimized.In combination with different detection methods,on line sample preparation and detection could be accomplished.The method was safe,high efficient,environmental-friendly,low cost and easy to operate.Additionally,it was in accordance with the requirement of green analytical chemistry.These exploratory studies provided reference information for further studies.In more specific terms,the main work in this thesis was listed as follows:In chapter 2,a simple,rapid method was developed for on-line determination of arsenic in tea.The method employed an electromagnetic induction oven as heating source,followed by pre-reduction of As(â…¤) to As(â…¢) by means of 25 g L^-1 KI and 25 g L^-1 ascorbic acid and then determined by hydride generation-atomic fluorescence spectrometry(HG-AFS).The parameters such as the temperature and acidity of on-line sample digestion,the length of the digestion coil,sample flow rate,the concentration of the reductant,the kind and concentration of carrier,etc.were optimized.The interference of coexisting ions such as Se⁴⁺,Co²⁺,Cd²⁺,Hg²⁺,Al³⁺, Fe³⁺,Pb²⁺,Cr³⁺ was studied.Under optimal conditions,the detection limit(3σ) was evaluated to be 0.063μg L^-1,and the relative standard deviation(RSD) for 11 replicate determinations of 5μg L^-1 As was evaluated to be 2.5%.The proposed method was proved to be accurate and reliable.In chapter 3,an electromagnetic induction heating column(EMHC) was designed and another EMIH system was developed to perform on-line sample digestion.In this system,the heat generated inside the sample solution in EMHC and concentrated just in a thin-layer region beneath the heating material,faster heating energy transfer and higher heating efficiency were obtained and the sample solution was quickly digested at a relative lower heating power.The size of EMHC as well as the on-line digestion parameters was investigated systematically.The proposed method was compared with microwave digestion,and the results proved its accuracy and reliability.The developed method was successfully applied to the determination of zinc and manganese in tea leaf by flame atomic absorption spectrometry(FAAS).In chapter 4,a new low power EMIH device was designed to save heating power and was applied to on-line oxidation of organomercury in seafood.The heating temperature could be easily controlled by adjusting the output of heating power.For total mercury determination,the organomercury in sample solution was quickly oxidized by potassium persulfate in EMHC at a relatively lower heating power(15 W).Inorganic mercury determination was performed by keeping EMHC at room temperature.In combination with atomic fluorescence spectrometry(AFS),inorganic mercury and organomercury in seafood could be determined quickly and accurately. In the proposed method,it was easy to control the heating temperature and to accomplish homogeneous heating.Additionally,the heating efficiency was improved and energy consumption was reduced.In chapter 5,the EMIH system was further improved and was employed to perform solid sample decomposition in acid medium.In combination with FIA,an on-line pressurized EMIH digestion system was developed.In the experiment,the parameters which influenced the performance of the proposed system were optimized and the system's performance was evaluated for on-line digestion of edible seaweeds. The efficiency of acid digestion was improved by a pressure of 0.36 MPa and 135℃reaction temperature.Therefore,the proposed system could be applied for solid sample digestion and the result was satisfactory.By on-line coupling the digestion system to cold vapor generation atomic fluorescence spectrometry(CVAFS),the developed method could be satisfactorily applied to the determination of mercury in edible seaweeds.The proposed method reduced energy consumption,analysis time and Hg emission.It was a safe,convenient,low-cost method for on-line solid sample digestion.In chapter 6,the EMIH technique was introduced to chemical oxygen demand (COD) determination and opened a new frontier in EMIH application.EMIH was employed instead of conventional reflux heating,and an on-line pressurized oxidation flow injection system for COD evaluation was developed.When samples flowed through the EMHC,they were on-line oxidized by 0.004 mol L^-1 K₂Cr₂O₇+5.4 mol L^-1 H₂SO₄ within 10 min under approximately 145℃reaction temperature and 0.44 MPa manifold pressure,and the corresponding heating power was just 40 W.The reduction in dichromate absorbance due to oxidation of the organic compounds was measured at 445 nm by spectroscopy.The proposed method improved the heating efficiency,reduced energy and reagent consumption,analysis time,hazardous emission,external contamination and tedious operation.EMIH technique was used for on-line sample digestion in the above-mentioned work.In chapter 7,we applied it to cloud point extraction(CPE) for on-line sample preconcentration.A new on-line EMIH-CPE system was designed to preconcentrate Cr(â…¥),Cu and Zn in water samples followed by FAAS detection.In the experiment, the parameters which influenced the performance of the proposed system were optimized.And the method was validated in terms of enrichment factor,limit of detection,recovery and RSD.The results proved the sensitivity and reliability of the proposed method.