Study On The Key Technology Of EDXFR Method For The Detection Of Trace Heavy Metal Elements In Rice

Rice is the major food crop in our country.More than 65% of population made rice as the staple food.In recent years,the events which heavy metal elements exceeded in the food occur frequently.It causes a high degree of public concern.Thus government departments and enterprises have strengthened the supervision inthe rice production circulation.Being on-site,fast,accurateto get the inspection result is the key to effective supervision.EDXRF(energy dispersive X-ray fluorescence)method has the advantages of simple sample processing,non-destructive detecting,fast and efficient,light instrument and low cost.It is suitable for spot rapid screening of heavy metals in food.The application key of detecting heavy metal elements in rice by EDXRF method is to reduce the detection limit of the detection system and to simplify the equipment and technology.Around this goal,in this paper,the theoretical research,MC simulation and physical experiments for the main key technologies of the geometrical structure of the excitation device,the optimization of the shape of the sample and the measurement methods were carried out.The optimal analysis mode of Cd,As,Se,Pb in rice was set up which effectively reduces the detection limit and improves the measurement accuracy,that has high academic significance and application value.The main achievements and innovations of the paper are as follows:(1)The mathematical models of X-ray fluorescence analysis of wedge,spherical and cylindrical shaped samples wereestablished,which broadened the theoretical basis of EDXRF method.The best physical parameters of the primary filter,the secondary filter and the secondary target were calculated.(2)The three geometries of "source-probe vertical","source-probe edge" and "source-probe contour" were simulated by MC software.The results show that the P/B(peak-to-back ratio)of "source-probe vertical" is biggest.The best wedge angle of As and Se is 85o,Pb is 55o and Cd is 70o.The cylindrical and spherical samples of "source-probe vertical" structure were also simulated,but the P/B is worse than the wedge-shaped sample.(3)Through MC simulation,the optimal tube voltages of As and Se are 38 kV,Pb is 40 kV.In order to detectconveniently,the tube voltage of As,Se,Pb is set to 38 kV.The P/B of Cd increases with the increase of tube voltage,and the curve tends to be gentle after 60 kV.Therefore,the tube voltage of Cd is set to 60kV(the maximum voltage of X-ray tube for experiment is 65kV).In order to ensure the reliability of the simulation results,MC simulation of the number of sampling times was also been made.The result shows that the relationship of number of sampling times and the relative standard error isthe power function.The curve tends to be gentle after the number of sampling times more than 8 × 108.(4)Al,Fe and Cu were selected to be primary filters in MC simulation.The results show that the P/B of 1200?m thickness Al sheetfor As and Se is the biggest,and the P/B of 48?m thickness Cu sheet for Pb is the biggest.For the convenience of detection,1200?m thickness Alsheetis selected as the primary filter for As,Se and Pb.For the Cd element,although the 22?m thickness Cu sheet has a biggest P/B,the Cu is easy to oxidize,so 32?m thickness Fesheet(or 304 stainless steel sheet)is selected as the primary filter.(5)MC simulated the filtering effect of balanced filter on the characteristic fluorescence of As,Se and Cd.The results show that although the balance filter greatly improves the P/B,the counting difference of the characteristic peak becomes little and the statistical fluctuation error is very large,so that the analysis accuracy is poor,so corresponding physical experimentverification is not carried out.(6)The Cd analysis was simulated by using two kinds of excitation modes of reflection target and transmission target.The results show that the P/B of Ba and La targets with 6mm height and 4mm radiusis the biggest when the reflection target is used.When the target is excited by the transmission target,the P/B of 220?m thicknessBa target is the biggest which is greater than the reflection target.(7)Under the best mode conditions,the detection limits of Cd,As and Pb are 0.167,0.183 and 0.204 respectively.The averagerelativeerrorsarewithin±10%.That meets the need of rice heavy metal testing work.In this paper,a set of optimal detection scheme for detecting heavy metal elements in rice by EDXRF method was obtained from theoretical analysis and MC simulation,and verified by physical experiments to meet the expected requirements.However,due to the time limit,some problems in the study,such as the actual effect of balance filter,the selectionsof conversion target for As,Se,Pb have not yet been in-depth studied,whichexpect to be completed in the future work.