Development And Applications Of Portable TOFMS Equipped With A Cryo-Trap For The Rapid Enrichment Of Volatile Organic Compounds

Volatile organic compounds?VOCs?are a class of organic chemicals having a high vapor pressure at room temperature,and they are the main precursors of secondary organic aerosols.It has a great harm to human health and ecological environment,so it is crucial to develop rapid and efficient detection methods of VOCs.Time-of-flight mass spectrometer?TOFMS?takes the advantages of simple structure,fast analysis speed,wide range of mass to charge ratio and so on.TOFMS can depict the contour of VOCs in the atmosphere with variation in the time and space distribution,and provide an effective analytical tool to measure organic pollutants,which is potential for their treatment from the emission sources based on the obtained data.However,the concentrations of VOCs in the atmosphere are pretty low?at the level of pptv to ppbv level?,and it is a great challenge to directly on-line detect their levels with a portable TOFMS.In this study,a cryo-trap coupled to a portable TOFMS has been designed for on-line monitoring trace VOCs in the atmosphere.In this study,the geometry and working principle of portable TOFMS are introduced,and the key factors determining the performance of TOFMS are analyzed.Based on the above,a portable TOFMS equipped with a vacuum ultraviolet?VUV?photoionization has been developed,and its size is 37 cm×33 cm×20 cm?length×width×height?.The total weight is17 kg,and the mass resolution is better than 1100 at m/z 92,and the detection limit of benzene is as low as several ppbv.The portable TOFMS has two ionization modes:single photon ionization mode?SPI?and photoelectron ionization?PEI?.In the SPI mode,the sample molecules absorb the photon?10.6 eV?emitted by VUV lamp and was softly ionized,the molecular ion is the characteristic peak in the spectrum,and is beneficial for the analysis of complex samples.In the PEI mode,photons radiation to the surface of metal electrode and photoelectron emits from the surface with the photoelectric effect,then photoelectron was accelerated by the electrostatic field and acted as electron ionization source.This instrument can quickly switch between SPI and PEI to realize the ionization of different substances.The sensitivity of portable TOFMS is not high enough to directly detect-trace VOCs in the atmosphere,so the VOCs need to be enriched first.In this study,based on the principle of low temperature enrichment and high temperature thermal desorption,a cryo-trap was designed to on-line enrich the VOCs.The cryo-trap is realized by electric refrigerator,which takes the advantages of a minimum temperature up to-196 ^oC,high cooling efficiency,no refrigerant consumption,small size,working continuously for a long time and so on.The cryo-trap mainly includes sampling system,enrichment system,refrigeration system,thermal-desorption system and transmission system.The working process of it mainly includes:washing,sampling,sample injection and aging.Cryo-trap combined with online mass spectrometer was used to analyze benzene,toluene and xylene?BTX?mixtures,the results showed that the signal strength of benzene,toluene and xylene,with the cryo-trap enrichment,has improved 212-fold,254-fold and 242-fold,the limit of detection is 7 pptv,8pptv and 5 pptv respectively,and the recovery rate is 98%,87%and 87%,respectively.The analytical time for single sample is only 14 min.The electric driven cryo-trap combined with on-line mass spectrometer is used to detect 39 VOCs listed in TO-14 method of EPA both for standard gas sample and real air sample,the sensitivity of the newly designed instrument is better than the threshold of national regulation for indoor VOCs.The electric cryo-trap coupled with online mass spectrometry can rapid detection of trace VOCs in atmosphere and indoor air,and it has important potential applications for in-site and real-time monitoring of volatile organic pollutants.In order to predict the failure of substation,the cryo-trap portable TOFMS is developed to analyse fault decomposition products in high voltage gas-insulated switched?GIS?with SF₆.Using cryo-trap and portable TOFMS successfully analyzed the decomposition compounds of SO₂,CO₂,SO₂F₂ and CS₂ inside the GIS.The lowest limit of detection is much lower than the threshold of SF₆ decomposition products inside of breakdown GIS with the cryo-trap.The combination of cryo-trap and TOFMS can meet the requirements of in-site detection in terms of portability and sensitivity,and it has a wide application prospect in the online fault detection of substation.