| Organophosphorus pesticides are generally difficult to degrade,It’s easy to remain on the surface of food and enter the human body.The most of organophosphorus pesticides have the P=O and P=S groups and show extremely high Raman activity.Therefore,Raman spectroscopy is a very efficient and non-destructive probe to detect organophosphorus pesticide residues.However,Surface-enhanced Raman scatting（SERS）,which is commonly used at present,is difficult to be applied to high-precision quantitative pesticide residue detection due to the lack of standardized substrate and pretreatment technology.The novel ultra-low frequency Raman spectroscopy can effectively detect the low-frequency Raman peak(0-500cm-1)that is more sensitive to the concentration of two commonly used organophosphorus pesticides,chlorpyrifos and trichlorfon.In this paper,we studied the Raman frequency and intensity changes of two representative organophosphorus pesticides chlorpyrifos and trichlorfon at different concentrations,and accurately identified each low-frequency vibration mode through Lorenz curve fitting.It was found that 12.98mg/kg chlorpyrifos sample has two low-frequency vibration modes,located at 125cm-1 and 145cm-1 respectively,the two modes are caused by respiratory vibration of P=O-C.There are four low-frequency vibration modes in the 1mg/kg trichlorfon sample,which are located at 81cm-1,184cm-1,254cm-1,431cm-1,and them are caused by the deformation of P=O-C and the tensile vibration of C-Cl.At the same time,we found that the Raman vibration modes of chlorpyrifos and trichlorfon at different concentrations were shifted.Among them,the P=O circular respiratory mode of chlorpyrifos appears covalent bond recombination or even break with the increase of concentration,resulting in blue shift;The respiratory model of O-C shows that covalent bonds between groups appear with the increase of concentration,resulting in red shift.There are long chain branches in chlorpyrifos,and the atoms are easily coupled locally.Only the 184cm-1 mode is blue shifted,and the frequencies of the other modes remain unchanged.In addition,we can accurately identify the concentration of organophosphorus pesticides by establishing the dependence of the concentration of two types of pesticides on the ultra-low frequency and intensity.The lowest detectable limit of ultra-low frequency Raman technology in the detection of chlorpyrifos is 0.012 mg/kg,and the lowest detectable limit in trichlorfon is 0.25 mg/kg.This analysis provides a novel and feasible experimental method for organophosphorus pesticide detection. |
