High resolution mass spectrometry（HRMS)was used to study and characterize the background ions under direct analysis in real time（DART) ionization conditions,and the possible sources of these signals were traced and assigned.The research showed that the background ions are mainly generated under the positive ion mode,which may derive mainly from plasticizers and polydimethylsiloxane in the lab environment.The polydimethylsiloxane may come from the pump oil vapor.Understanding and mastering of these background ions will be favorable in the research and testing by DART-MS technique.Firstly,these background signals are stable and do not produce significant interference in sample analysis of DART.Therefore,the stability and accuracy for HRMS spectrometer could be investigated with no need of any internal standard compounds added deliberately.Secondly,understanding of these background ions will also help to avoid erroneous judgment in DART mass spectrometry analysis on complex systems.