A Design Of Portable Testing System For Inferring Of The Time Of Death By Detecting The Concentration Of Potassium And Sodium Ions In Vitreous Humor

The chemical changes of many substances in vitreous humor are relatively stable and more slowly than that in the blood and cerebrospinal fluid after death. Therefore, analysis of compositions in vitreous humor has been considered to be one of the important methods in the study of forensic pathology.Development status of portable ion concetration meter domestic and broad was analysed in this paper. A type of portable measuring equipment was designed using ion selective electrode(ISE) to measure the ionic concentration of chemical composition in human vitreous humor, which can cosequently be used to infer the time of death of the corpse at the crime scene.The major work of this paper are as follows:1 As for the high impedance of ion selective electrode, a differential amplifier was employed as the signal preamplifier with a high impedance.2 In order to reduce the influence of temperature on the output of the electrode, a temperature compensating circuit was designed. Temperature signals was sent from the sensor of digital thermometer DS18B20 to MCU so as to achieve the effect of temperature compensation3 The hardware circuit of ion concentration meter was designed and put into practice based on the combination of MCU and CPLD.4 The logical design of CPLD was achieved; the validity of CPLD was verified through a software simulation; the software of each module of the detection system was also programmed.Systemic analysis was made on ion selective electrode signal, noise and temperature compensation, and then a design proposal was put forward to decrease ion selective electrode signal drift and increase the accuracy of signals and stability of the ion concentration meter.Through preliminary tests, the instrument was found to be able to immediately measure the concentration of potassium and sodium ions in vitreous humor at the scene. The concentrations of other different ions can be obtained only by changing the ion selective electrode and op amp's feedback resistors, and then the instrument can quickly measure the ions concentrations of the chemical components in human vitreous humor after death.