Research On Analytical & Test Methods And Software Of Atomic Fluorescence Spectrometry Based On Digital Micromirror Device

Atomic fluorescence spectrometer(AFS)has become a common analytical instrument in China's laboratories because of its low detection limit and low cost.Hydride generation and atomic fluorescence spectrometry(HG-AFS)as a representative instrument have been widely applied in the fields of heavy metals analysis for food,environment and life sciences products in China.However,its nondispersive detection method can not effectively analyze and eliminate the spectral interference caused by impure light source or water vapor scattering,so AFS is largely limited by related international industrial standards in the European Union and the United States.The dispersive detection method can solve the above interference problems,but the traditional ones such as filter,rotary grating and CCD spectrometer are unable to meet the requirements of AFS for high sensitivity and fast detection.The pixels of the ultraviolet digital micromirror device(UV-DMD)can be subtly rotated to achieve a rapid selection of specific wavelengths,and the solar-blind photomultiplier tube(PMT)has high response intensity in the range of atomic fluorescence(180 nm ~ 320 nm).Based on the above characteristics,the UV-DMD spectrometer with concave grating as light splitter,UV-DMD as spatial light modulator and PMT as detector was used as a dispersion detection system to replace the detector in non-dispersive HG-AFS.The UV-DMD-AFS was developed,which can detect the atomic fluorescence spectrum and solve the problem of spectral interference.Compared with non-dispersive HG-AFS,UV-DMD-AFS with a dispersion detection system leads to the attenuation of the signal reaching the detector due to the increase of optical path,which affects the identification of weak signals including spectral interference and the analytical performances of the instrument.For UV-DMDAFS,wavelength accuracy is the basis of quantitative analysis,due to the wavelength selection function of UV-DMD,determining the relationship between pixel and wavelength is an effective means to ensure wavelength accuracy;Good spectral resolution is the guarantee of spectral interference identification.The spectral line bending problem introduced by the spectroscopic component will produce spectral line aliasing when UV-DMD uses the linear scanning mode,which will affect the spectral resolution and signal intensity.Scattering interference is an important kind of spectral interference,and it is also a key problem to be solved using UV-DMD-AFS;As the first kind of human carcinogen,it is meaningful to ensure the measurement accuracy of arsenic.The suppression method of scattering interference can be carried out from the study of arsenic after ensuring signal intensity and accuracy.In addition,UV-DMDAFS with new functions such as spectral detection and interference suppression has no specialized analysis and test software to realize the automation of the sample detection process.With the support of the National Key Research and Development Program of China,the main contents are as follows:(1)Atomic Fluorescence Signal Enhancement Technology based on DMDCompared with the directly received by PMT in non-dispersive AFS,the detected fluorescent signal of the UV-DMD-AFS is weakened due to the addition of the UVDMD spectrometer,resulting in a worse limit of detection(LOD)by nearly one order of magnitude and limiting the detection ability and analysis performance.In this paper,the signal detection intensity in the range of 180 nm ~ 320 nm was increased to more than 2.5 times and the spectral resolution was improved to 1.85 nm by expanding the photosensitive area(MEA)in PMT,installing the cylindrical mirror in front of PMT,and adding light traps.Due to the improvement of the dispersive detection system,the best parameters of the instrument may be changed.Optimizing the following parameters: the current and the coaxiality of the hollow cathode lamp(HCl)(affecting the emission intensity);the flow of carrier gas and shielding gas,the acid concentration in the sample solution,and the KBH4 concentration in the reducing agent(affecting the argon hydrogen flame state);and the atomizer height(affecting the position of argon hydrogen flame).The fixed-location rotation mode of DMD was developed,and the analysis methods included DMD detection mode(scanning/fixed-location),spectral peak calculation mode(peak width/full width at half maximum)and spectral peak selection mode(maximum/all)were optimized.The light source emission and excitation fluorescence spectra of cadmium(Cd),tellurium(Te),tin(Sn)and zinc(Zn)were detected and analyzed.The LODs of 10 elements commonly measured by UVDMD-AFS were improved by 2.03 ~ 8.94 times,and the detection ability has met the requirements of toxicological index limits in the Standards for Drinking Water(GB 5749-2006).The spiked recoveries of real food samples(pork,fish and orange juice)and life drinking water(tap water and purified water)were 95% ~ 106% and 93% ~ 101% respectively,and the relative standard deviations(RSDs)were 0.9% ~ 3.4% and 0.2% ~ 4.1% respectively,and the LODs were 0.058 ng/m L and 0.055 ng/m L respectively.(2)Wavelength Calibration and Spectral Bending Correction for UV-DMD SpectrometerThe rotation of the specified pixels of DMD determines the spectral signal entering the detector,and the relationship between pixel and wavelength is the key to accurate detection.Based on the optical model and the parameters of concave grating and DMD,the theoretical relationship between the central row pixel of DMD and wavelength was studied.253.65 nm of Mercury(Hg)was used as the initial adjustment benchmark,and Bismuth(Bi)and Selenium(Se)were the error correction sources,which realized the wavelength error of less than 0.5 nm.The DMD block rotation mode was developed by extending y-direction parameters to analyze the detailed spectral distribution on the DMD surface,and the obvious spectral bendings of Bi,Lead(Pb),Antimony(Sb)and other elements caused by optical aberration were measured.The theoretical imaging state of incident light was studied by ray-tracing method,and the existence of spectral bending was determined.The bending span corresponding to 206.16 nm of Bi was theoretically calculated as 2.5 pixels and 306.77 nm as 4 pixels,that is to say,the curvature increases with the increase of wavelength,so the variable curvature correction scheme was determined.Taking Bi with uniform spectral distribution and strong intensity as the correction source,the variable curvature bending template was established.The DMD was controlled to flip in the bending mode,to correct the spectral bending,improve the spectral resolution,and increase the signal detection intensity by 36%.(3)Scattering interference analysis and suppression of Arsenic using UV-DMDAFSThe scattering interference caused by water vapor or aerosol will be directly superimposed on the resonance fluorescence line and lead to a fake measurement result,which can not be identified and deducted by non-dispersive AFS,and the fast wavelength selection ability of UV-DMD-AFS is helpful to realize the real-time suppression of scattering interference.The influence of blank solution drift on the measured concentration was studied,which showed the necessity of real-time elimination of interference.the scattering interference spectrum of As was drawn,and the 228.80 nm was determined as the non-fluorescent line.The ratios of 193.76 nm,234.98 nm and 245.65 nm to the non-fluorescent line were 1.08,1.44 and 0.53 respectively,and the stability of each ratio within 200 minutes were 0.79%,0.57% and 1.42% respectively,indicating that the ratio of fluorescent line to non-fluorescent line was relatively stable.The external factors such as instrument parameters,chemical generation parameters and interference ions in sample have an obvious influence on the intensity of scattering interference but have little effect on the ratios.Combined with the wavelength selection and fast flip(> 10000 frames/s)characteristics of DMD to propose a method named non-fluorescent line ratio method.The fluorescent lines and non-fluorescent lines were measured in real time,and the scattering interference was calculated according to the ratios,to complete the real-time suppression and avoid the concentration deviation caused by drift.The suppression proportion for scattering interference was 98.50% ~ 102.43%,showing a good suppression effect.Compared with the mixed solution addition method and desiccant addition method,the nonfluorescent line ratio method was superior in the quality of the standard curve(r = 0.9996)and measurement stability(Sblank=1.01),and the analytical performance was good(LOD = 0.025 ng/m L).By comparison with the traditional method of deducting constant intensity of blank solution by non-dispersive AFS,the accuracy of the nonfluorescent line ratio method was the same but more stable in the short term,and the long-term accuracy is higher.(4)Development of UV-DMD-AFS analysis and test softwareThe analysis and test software is the technical guarantee to realize the orderly work of components of UV-DMD-AFS and obtain accurate quantitative results.This paper used C# programming language to complete code development,MVVM architecture to optimize program structure,Ribbon menu control and WPF application control to realize a visual style similar to Microsoft Office.The communication protocols related to component control and data acquisition were formulated.Based on the functions of data acquisition,data processing,spectrum drawing and data storage,a multi-threading scheme was designed to ensure the accuracy of the workflow and the smooth execution of the interactive interface.As for the requirements of the sample measurement method and result classification storage,the SQLite database structure framework was built.In addition to the basic functions of atomic fluorescence such as parameter setting,sample measurement and performance analysis,special functions such as DMD control,atomic fluorescence spectrum detection,wavelength calibration and optimization,real-time interference suppression,etc.were also developed.The automation of key components orderly working,spectral data collection and spectrum real-time drawing,and results analysis and preservation were realized.The methods and results can be viewed,exported and reused as files.The concentration deviation of As measured by the software was only 0.02 ng/m L,and the LOD was 0.0342 ng/m L,indicating that the software has complete functionality and good analytical performance.