Research Of Detection Technology For Inductively Coupled Plasma Optical Emission Spectrometer Based On DMD

Inductively coupled plasma optical emission spectrometer(ICP-OES)has become one of the necessary analytical instruments in laboratory because of the characteristics including wide linear range,simultaneous determination of multiple elements,high sensitivity and high accuracy.It is widely used in environment field,food safety,metallurgical industry,chemical industry,geological survey and other fields.Currently,there are three different kinds of ICP-OES: multi-channel direct-reading ICP-OES uses concave grating as the dispersive device,multiple photomultiplier tubes(PMT)or linear array charge-coupled devices(CCD)as the detector.This type of instrument is limited in the analytical number of elements and spectral lines due to the space constraint.Besides,the instrument cannot be used for qualitatively analysis;singlechannel scanning ICP-OES uses a plane grating as the dispersive device and a photomultiplier tube as the detector.The mechanical part for wavelength scanning of the instrument requires high mechanical precision.The complicated operation results in a relatively long time for analysis;full-spectrum direct-reading ICP-OES uses an echelle grating and a prism as dispersive elements to form a two-dimensional spectrum.The area-array detector such as CCD or charge injection device(CID)is used as the detector.However,the quantum efficiency of front-illuminated area-array detector is relatively lower due to the materials.The expensive back-illuminated area-array detector results in the high cost of the instrument.In recent years,digital micromirror device(DMD)has been successfully used as spatial light modulator in various kinds of spectrometers.The paper of DMD using in ICP-OES has not been reported yet.In this paper,the detection technology for inductively coupled plasma optical emission spectroscopy based on DMD was proposed.After the cross-dispersion of the echelle grating and the prism,the twodimensional spectrum is imaged on the DMD.The state of each micromirror can be changed according to the digital signal and the selection of the spectral signal can be realized.Finally,the spectral signal is detected by PMT.Based on the obtained wavelength and signal intensity,ICP-OES can realize the qualitative and quantitative analysis of elements.Research contents include the following parts:(1)Development of the measurement and control system of the ICP-OES based on DMDAs one of the important parts of the ICP-OES based on DMD,the measurement and control system is mainly consisted of the DMD controller and signal acquisition system.In this paper,according to the requirements of the experimental system,the controller of 0.95 1080 p DMD was designed and the control method of DMD used for the qualitative and quantitative analysis was developed.By setting control parameters such as the beginning rotating row and column,the simultaneous rotating row and column,the interval rotating row and column and the DMD period,global rotating mode or block rotating mode was realized.The refresh rate of DMD can reach more than 17,000 frames per second.The signal acquisition system based on the AX845 core board is developed.Therefore,the spectral signal selected by DMD can be acquired rapidly.The acquisition rate can reach more than 2Msps.Both the DMD controller and signal acquisition board realize the data communication with the personal computer based on the network chip CH395 Q.The experimental system adopts SPI communication interface.The maximum speed can reach 30 Mbps,which meets the high-speed transmission requirements of control commands and collected data.(2)Development of the echelle grating spectrometer based on DMDAfter the selection and acquisition of spectral signal can be achieved by the measurement and control system,the design of the echelle spectrometer is carried out.The echelle grating is used as the main dispersive element while the prism is used as the cross-dispersive element.A group of optical lenses including a cylindrical lens and a spherical mirror are placed between the entrance slit and collimating mirror,which are used to compensate astigmatism and spherical aberration.The backend optical lenses correct aberration and converges the two-dimensional spectrum and make sure that the two-dimensional spectrogram is not imaged out of the range of DMD.The mechanical structure of each optical element is designed according to the adjustable requirements of pitch,deflection and roll.According to the structure of the echelle grating spectrometer,the laser and mercury lamp are employed as light sources,the optical component is adjusted one by one to ensure that the performance of the spectrometer meets the design value.Finally,the experimental results of the onedimensional spectrum of the mercury lamp and two-dimensional spectrum show that the error of wavelength extraction meets the requirements of the spectrometer.The spectral resolution is 0.01nm@253.7nm.(3)Research on the spectral reduction model of the echelle grating spectrometer based on DMDThe geometric model between the imaging mirror,the backend optical lenses and the DMD is established.Therefore,the theoretical spectral reduction model of the echelle grating spectrometer is obtained.Next,the refractive index of the prism is refitted based on the wavelength and actual coordinates of the electrodeless discharge lamp.Then,the coordinate equations are corrected to obtain a corrected spectral reduction model.The corrected coordinates and the actual coordinates of the eight characteristic wavelengths of the mercury lamp are compared.It is found that the correction of initial model can effectively reduce the deviation as a whole.The dispersion direction of the prism is the x-axis,while the dispersion direction of the echelle grating is the y-axis.The deviation of x coordinates is less than half of the corresponding interval pixels between adjacent orders,which means the wrong extraction of order won’t appear.The deviation of y coordinates multiplied by corresponding pixel resolution is less than spectral resolution.It is determined that the corrected spectral reduction model satisfy the requirement of the order judgment and wavelength extraction.(4)Research of the detection technology and the development of ICP-OES based on DMDThe experimental system of inductively coupled plasma optical emission spectrometer was built based on digital micromirror device,which was consisted of light source system,sample introduction system,echelle grating spectrometer and control system.The working parameters of the experimental system including the control parameters of DMD,power of high frequency generator and flow rate of carrier gas were optimized.According to the signal to noise ratio and the experimental requirements,the optimized control parameters of DMD were obtained.The simultaneous rotating column is 6 and the simultaneous rotating row is 14.The interval rotating column is 1 and the interval rotating row is 7.According to the signal to background ratio,the optimized power of high frequency generator is 1.2k W.The optimized flow rate of carrier gas is 0.8L/min.The optimized negative high voltage is450 V.In the condition of optimized working parameters,the emission lines of Mg and Ca are detected and analyzed.The corresponding micromirrors of DMD of different wavelength are selected based on the corrected spectral reduction model.Each spectrum only needs 0.615 s.Mg at 279.55nm: the relative coefficient r is 0.996;the limit of detection(LOD)is 0.05μg/ml;the relative standard deviation(RSD)is0.9%～1.8%;the spiked recoveries is 98%～103%.Ca at 396.85nm: the relative coefficient r is 0.997;the limit of detection(LOD)is 0.01μg/ml;the relative standard deviation(RSD)is 0.8%～0.9%;the spiked recoveries is 89%～93%.The experimental results showed that the ICP-OES based on DMD with the characteristics of flexible measurement and fast detection had preliminarily realized the function of detection and analysis.