Research On Key Techniques Of Measurement Stability Of Mini Quadrupole Mass Spectrometry And Application Test Of VOCs In Water

Mass spectrometry is widely used as a highly sensitive and specific detection technology.However,the research on mass spectrometry technology in China is relatively backward,and there are still many scientific and technical problems to be solved,especially the key technology of measurement stability needs to be systematically studied.Volatile organic pollutants(VOCs)are an important type of pollution indicator in water and an important part of water quality safety testing.Mass spectrometry is a widely used detection method.However,the effectiveness of the detection and monitoring results of VOCs and other mass spectrometers in water,especially the large errors caused by the unstable measurement of the instrument,is one of the key problems of domestic instruments.This thesis deeply analyzes the scientific and technological problems caused by the space charge effect in the ionization chamber behind the system stability,and studies the effect and influence of the ion source electron quantity control on the system stability.The main research content of this article:(1)Taking volatile organic compounds in water as the detection object,build a quadrupole mass spectrometry system based on electron bombardment ionization source,and use it with a gas chromatograph.The mass resolution of the test system is better than the unit mass resolution.The minimum detection limit of Octafluoronaphthalene reaches the ppm level.In the long-term stability test of the instrument(48-hour continuous mass spectrometry scan on a vacuum background),it was found that the relative standard deviation of the total ion current of the mass spectrometer is 4.77%,which is higher than the usual 3%specified by commercial instruments,and the system stability needs to be improved.According to the"Tabletop Gas Chromatography-Mass Spectrometer Calibration Specification(JJF1164-2006)",the Octafluoronaphthalene-isooctane solution was tested,and the intra-day repeatability relative standard deviation RSD range was 4.46%～12.55%.The relative standard deviation RSD range is 10.42%～14.16%,and the measurement repeatability is higher than the RSD required by the calibration specification is not higher than 10%.(2)In order to further analyze the reasons for the relatively poor system stability,this paper experimentally explores the ionization process of the sample in the ion source,including the use of ion optics simulation software SIMION8.0 to simulate the electron trajectory inside the ion source,and the electron collector in series Resistance experiments and so on.The research results show that the space charge effect caused by the suspended configuration of the electron receiving electrode makes the electrons rebound,and there is a contradiction between the stable emission control circuit of the ion source filament,which makes the number of electrons in the ionization chamber of the ion source unstable,resulting in system stability.Sex is poor.In response to the above findings,in order to improve the stability of the system,this paper further connects the electron receiving electrode to a positive voltage to allow the electrons to flow smoothly.The 48h vacuum background mass spectrometry scan test showed that the relative standard deviation of the total ion current was reduced from 4.77%to1.47%.On this basis,according to the calibration specifications of the mass spectrometer,the intra-day and inter-day repeatability comparison experiments were performed on the Octafluoronaphthalene-isooctane solution.The experimental results showed that the RSD range of the system’s intra-day repeatability relative standard deviation was from 4.46%～12.55%was reduced to 1.02%～2.86%,and the RSD range of the day-to-day repeatability was reduced from 10.42%～14.16%to 7.76%～9.66%,all in line with the"Tabletop Gas Chromatography-Mass Spectrometer Calibration Specification(JJF 1164-2006)stipulates that the measurement repeatability RSD should not be greater than 10%within a day,indicating that the instrument accuracy and sample repeatability of the optimized mass spectrometry system and GC-MS system have been significantly improved compared with the previous ones.(3)Connect the optimized system to the purge and trap instrument to test the standard samples of VOCs in water.Through orthogonal experiments to optimize the chromatographic method,as well as the optimization of the relevant parameters of mass spectrometry,the detection of 9 components of TVOC in water was completed.The quantitative test repeatability is better than 20%,and the correlation coefficient(r~2)in the(5-150)μg/L concentration range is 0.9674～0.9999.