Analysis And Traceability Of Typical Soil Evidence Based On Ion Chromatography

Soil is one of the most widely encountered substances and is a type of trace evidence often collected by criminal technicians during the investigation of a scene.However,soil evidence and its analysis techniques are rarely used in actual cases.The reasons for this are,on the one hand,the difficulty of tracing soil evidence and the few tracing techniques;on the other hand,the vast territory of China and the insufficient collection of soil samples by public security organs in various regions.Ion chromatography is a technique for the detection of anions,cations and small molecule polar organic compounds.Compared with traditional detection techniques,it has the characteristics of fast detection speed,high precision and simple operation,and has been widely used in the field of court science in recent years..Ion chromatography can detect inorganic ions in soil,so as to carry out soil distribution pattern research according to the difference of soil inorganic ion composition in different environments.The analysis of soil ion content in different regions and environments can provide traceability clues for case investigation and provide scientific basis for court proceedings.This paper focuses on the analysis of soil physical evidence and traceability methods by ion chromatography and carries out the following studies:(1)Established a soil sample library.In this paper,105 soil samples were collected from Beijing,Hangzhou,Nanjing,Dancheng County,Henan,Yucheng County,Dongping County,Shandong,Maizhi District,Tianshui City,Gansu,and Hualing Town,Jiangxi,covering East China,Central China,North China,Northwest China and other regions.(2)Established a soil physical evidence ion chromatography detection method.CS12 A was selected as the cation column and Ion Pac AS19 was selected as the anion column.Methane sulfonic acid was selected as the mobile phase for cation separation,with isocratic elution and retention time of 30 min,and KOH was selected as the mobile phase for anion separation,with gradient elution and retention time of 40 min.14 anions and cations were used as the ions to be measured.The pretreatment conditions of soil samples were selected as soil-water ratio of 1:10,ultrasonic shaking for 120 min and stirring for 30 min.The methodologies showed good correlation coefficients(R2>0.999)for the 14 anion and cation mixtures with relative standard deviations ranging from 0.17 to 0.98%,and the recoveries of the sample cations ranged from82.20 to 118.00%,and the recoveries of the anions ranged from 94.75 to 114.01%.The method is simple in operation,convenient in sample pretreatment,fast in detection and reliable in determination.(3)The 105 collected samples were tested to establish a database of inorganic ion content of soil samples and analyzed in both temporal and environmental dimensions.Temporally,the inorganic ion content of soil samples within 1 day and 3 months had different distribution characteristics.Environmentally,soil ions in four types of environments,namely,farmland type,woods type,riverside type,and roadside type,have different characteristics,and there is correlation between inorganic ion pairs in the four types of environments.Two chemometric analysis models,support vector machine and nearest neighbor,were used to classify and trace the analysis of these four types of environmental soils,and the prediction accuracies were95.4% and 97.7%,respectively.These two models have high classification accuracy and can be combined with soil inorganic ion data to evaluate the 4 types of environments and help public security authorities trace different environmental soil physical evidence.(4)Combined with X-ray fluorescence spectroscopy for elemental content analysis of soil samples,the detection results showed that the distribution of sodium,magnesium,calcium and chloride matched with the detection results of ion chromatography,which can assist ion chromatography to detect soil samples.Applying the method of this paper to cases where the crime area is farmland and woods,the results show that the soil ion content at the case site differs greatly from the surrounding environment,which can accurately determine the soil differences in the case and help the public security authorities to identify and analyze the soil physical evidence at the scene.