Development Of Low Cost Portable Capillary Electrophoresis Instruments For The Determination Of Ions In Environmental Waters

The study of new methods and technologies related to water quality detection is one of the hotspots of environmental science research,which is also closely related to human life.Traditional water quality detection has shortcomings such as cumbersome process,lack of on-site“multi-purpose"instruments and others.Capillary electrophoresis technology can simultaneously achieve detection of multiple key parameters of water quality on site,because of its unique advantages.In this paper,with capillary electrophoresis technology as the core,a portable low cost capillary electrophoresis instrument for on-site ions detection in water environment is developed,which is combined flow injection analysis with contactless conductivity detection technology.The instrument includes four parts:a capillary injection module,a DC high-voltage power supply module,a contactless conductivity detection module and an electronic control system.The capillary injection module is connected by a micro peristaltic pump,a micro solenoid valve,a unilateral valve,a three-port connector and some other components.This system can adopt a pressure injection method to realize reproducible and controllable injection by precisely controlling the start and stop of the pump and valve.Electrodynamic injection can be completed by applying DC high voltage as well in the system.The DC high-voltage power supply module is mainly composed of two miniature high-voltage power supply modules with opposite polarities and two high-voltage relays.The voltage output of the high-voltage power module from-10 k V to+10 k V is controlled by the corresponding control circuit,which realize the purpose of detecting the anions and cations in the water sample respectively.The contactless conductivity detection module uses a commercial conductivity detector,which has the merits of a small noise signal,sensitivity and an accurate detection results.The electronic control system is the control hub of the whole instrument operation.The electronic control system is mainly including the hardware circuit design,the software design of the low computer and the host computer.The hardware circuit is with STM32F103RCT6 as the core and design the corresponding drive circuit and conversion circuit to achieve capillary sample injection,high voltage controlling,A/D acquisition,D/A conversion and serial communication and other functions.The low computer software realizes capillary sample injection controlling by the hardware circuit,data acquisition and communication with the host computer.While the host computer software realizes the serial data transmission and reception,voltage data display and storage and other functions.The hardware circuit and the host and low computer software design coordinate to complete the overall function of the system.Finally,the instrument framework was made by 3D printing technology,and the capillary electrophoresis system was integrated.The circuit part of the instrument was placed in a 30 cm×25 cm×10 cm box,and the flow path and electrophoresis capillary were placed on the box.The system only use the commercial device in the contactless conductivity detection part,and the rest parts are made of low cost components.Therefore,the development cost of the instrument is greatly reduced,and the cost of the whole instrument is controlled within 30,000 yuan,which is far lower than the commercial capillary electrophoresis instrument.Capillary electrophoresis system for continuous injection test,and the relative standard deviations（n=11）of the ion peak time and peak height were 1.53%and 1.16%,respectively,which proved that the instrument had good stability.The standard solutions of anions and cations(K⁺、Mg²⁺、Ca²⁺、Cl^-、NO₃^-and SO₄²)with the concentration of 0～500μmol/L were detected respectively.The peak height of electrophoresis signal was proportional to the concentration.R² values of the linear fitting equation of each ions was between 0.972 and0.991,the detection limits of each inorganic ions was between 2.96μmol/L（K⁺）and 5.36μmol/L（Na⁺）.Finally,by switching the polarity of the high voltage power supply,the anions and cations in the laboratory tap water were detected in two consecutive electrophoresis experiments,and the results were consistent with the trend of the laboratory ions chromatography analysis results.It is proved that the instrument can be used for the detection of ions in actual water samples.