Development Of A Non-target Screening Method For VOCs Based On GC-QTOF

Volatile organic compounds(VOCs)are an important class of pollutants in the ambient air,which can result in a series of environmental effects,including reacting with nitrogen oxides to produce ozone,triggering the formation of the photochemical smog,endangering human health,etc.The monitoring and analysis of VOCs in the environment is a prerequisite and basis for their effective control.To date,gas chromatography-mass spectrometry(GC-MS)is widely used in the analysis of VOCs as a mainstream instrument.In recent years,due to the impartant impact of VOCs on the quality of ambient air,the monitoring requirements for VOCs have changed from monitoring only a few representative pollutants in the past to monitoring hundreds of substances in key control areas now.Due to the different sources and wide variety of VOCs in the environment,some VOCs might be left out in the analysis of complex samples only with a defined VOCs target list,which can not reflect the true pollution situation.Based on the improving of analysis requirements of VOCs and the limitations of the current target analysis,the purpose of this study is to develop a non-target screening method for VOCs in air,and to conduct a preliminary analysis of the VOCs in the ambient air.In this study,the adsorption tubes were applied in sampling,and a combination of thermal desorption and GC-MS was used to develop an analytical method for VOCs in ambient air.Environmental samples from an industrial park and nearby residential areas were sampled and analyzed using the established analytical method.The results showed that 12 of 36 suspected compounds were confirmed by standard samples,and the benzene compounds accounted for the largest proportion of the VOCs in the area,followed by phenols and esters.The environmental sample analysis demonstrates that,although GC-MS is equipped with a matching database,the qualitative analysis of VOCs for unknown environmental samples is still very challenging.For example,due to the small molecules of VOCs,different VOCs sometimes contain the same fragment ions in mass spectrometry analysis,and qualitative errors can occur if the chromatographic retention times are also similar.GC-MS possesses relatively low mass resolution,which make it difficult to overcome the problem of qualitative errors.Gas chromatography-quadrupole time-of-flight mass spectrometry(GC-QTOF)not only retains the separation function of gas chromatography,but also has the relatively higher mass resolution capability of TOF mass spectrometry than GC-MS,which can effectively reduce the probability of qualitative errors of VOCs.In this study,a non-target screening methods for VOCs was established using GC-QTOF.In order to optimize the mass spectrometry signals,the effects of ion source temperature and electron ionization energy on molecular ion peaks were investigated.The results showed that the ion source temperature at 200?,230?,270? had no significant effect on the mass spectrometric signal,but the electron ionization energy demonstrated a significant effect on the molecular ion peak at 15 eV,30eV,70 eV.Ionization at 70 eV facilitates the matching with the mass spectrometric library,and ionization at 15 eV has better performance in the acquisition of molecular ion peak.Based on the boiling point of VOCs,a retention time prediction model was established,which can be applied in the analytical characterization of substances with the retention time.The accomplished non-target screening workflow includes procedures as following:(1)Firstly,VOCs are analyzed in the sample at 70 eV,and the obtained mass spectrums are matched with the spectral library of the instrument to screen possible candidates.(2)The analytical ion peaks are obtained at 15 eV and the compounds are identified based on the accuracy error of the candidate molecular ion < 5 ppm as threshold.(3)Retention time prediction model is applied to further identify compounds.(4)Finally,the identified compounds can be confirmed using standard samples.Using 108 standard substances as unknown samples for non-target screening method testing,it was found that 94 substances can be correctly identified,with an accuracy rate of 87%.VOCs in the real environment were also analyzed using the constructed non-targeted screening method,59 suspected compounds were found by spectral library matching,and 30 VOC species were identified by molecular ion peak screening.Finally,23 VOCs were certificated by standard samples and the remaining 7 compounds were finally identified as suspected compounds due to the lack of standard samples.Environmental sample analysis indicates that the non-target screening method based on GC-QTOF can be applied for screening analysis of VOCs in unknown ambient air.