Detection Of Typical Heavy Metal Elements In Rice By Coaxial Double Pulse LIBS

Laser induced breakdown spectroscopy(LIBS),which worked as a new kind of material composition analysis method,has been applied in many fields and with great development potential.But its development was restricted by the low detection sensitivity,poor stability,worse detection limit and so on.This study was committed to reducing the detection limit of LIBS,and the typical heavy metals elements in polluted rice were acted as the research target,single pulse laser induced breakdown spectroscopy,coaxial double pulse laser induced breakdown spectroscopy and microwave assisted coaxial pulse laser induced breakdown spectroscopy was carried out respectively and compared.Firstly,LIBS experiment was been done on the laboratory polluted rice samples by cadmium,and the characteristic lines of cadmium element can be determinated clearly at the wavelength of 226.502 nm and 228.802 nm,but the intensity of characteristic spectrum of Cd I 228.802 nm was stronger,therefore,which was chosen as analysis spectrum.Then,experimental condition parameters such as delay time,laser energy,interval delay time between two lasers,pressure,were optimized.And the result showed that the optimal delay time is 1580 ns,laser total energy is 350 mJ which the double laser beams were also 70% of single pulse total energy,interval delay time between two laser was 60 ns,and the best physical pressure is 30 MPa.Secondly,single pulse laser induced breakdown spectroscopy(SPLIBS),coaxial double pulse laser induced breakdown spectroscopy(Co-DPLIBS)and microwave assisted coaxial double pulse laser induced breakdown spectroscopy(MA-Co-DPLIBS)tests were carried out on the 135 laboratory contaminated rice samples respectively at the optimal experimental condition parameters.The consequence displayed that the characteristic spectrum of Cd I 228.802 nm in rice can be detected by SPLIBS which the lowest content is 13.69?g/g,while 2.32?g/g can be checked by Co-DPLIBS,and the detection limit reduced 56.85 times.However,due to the probe mainly material were iron and copper,abundant characteristic lines of iron and copper were contained,and serious interference was engendered to the characteristic spectra of Cd II 226.502 nm and Cd I 228.802 nm,MA-Co-DPLIBS cannot be taken effect during the determination of Cd characteristic lines in rice samples,but with the microwave assisted,the characteristic spectra relative intensity of nutrient elements magnesium and calcium in rice samples were enhanced effectively compared with MA-Co-DPLIBS.Therefore,signal can be heightened by microwave,which just not suitable for the test of Cd element in this research,and the sensitively and detection limit of Co-DPLIBS were superior to SPLIBS for the detection of Cd in rice samples.And then,traditional Schiebe G-Lomakin uni-variate linear regression calibration curve and partial least square calibration curve between Co-DPLIBS intensity and the real concentration determinated by anodize stripping voltmeter were constructed respectively.The study results proved that uni-variant linear regression can not meet the requirement of quantitative analysis,while with 15 points smoothing and centralization to the spectra data,PLS model showed that the relative correlation coefficient,root mean square error of cross validation of calibration model were 0.9984?7.63?g/g respectively,and the relative correlation coefficient and root mean square error of prediction of validation model were0.9979?8.77?g/g respectively,and the average relative error was 9.32%.The results showed that the model constructed by PLS with higher accuracy and more reliable.Last but not least,the practicability of Co-DPLIBS which was utilized to detect the typical heavy metals in rice and rice contamination situation around Po-yang lake in Jiangxi Province were discussed.Co-DPLIBS was employed to test the heavy metal of cadmium,chromium,lead and copper in coarse rice and its corresponding rice husk and polished rice of 41 natural rice samples,which No.1 to No.36 were the rice in the husk samples from paddy field and No.37 to No.41 were polished rice samples by rice mill from different farmers' home that last season harvest.And wet digestion combined with flame atomic absorption spectroscopy and anodize stripping voltammetry were used to determinate the true concentration.The consequence displayed that although Co-DPLIBS detection limit of Cd was 2.32?g/g in this study,the highest content was 1.94?g/g,in account of that,Cd element characteristic lines can not be detected in anyone sample by Co-DPLIBS,and it is also to copper and lead element,but chromium element characteristic spectra in coarse rice samples can be determinated occasionally,and chromium characteristic lines can be detected clearly in all of the rice husk samples.However,constrained to the test conditions,valence state of chromium in 41 natural rice samples can not be decided,its toxic or not still need to combined with other disciplines detection method to confirmed,and look forward to the team to reduce the detection limit to fulfill the requirements of practical application in the future work.