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Hydroxyl radical (•OH) and singlet oxygen (1O2) were detected by electron spin resonance (ESR) spectroscopy in a direct current He/O2 (2%) non‐thermal plasma microjet‐water system. ${}^{ \bullet }{\rm O}_{{\rm 2}}^{- } $ is shown to be the precursor of •OH. The concentrations of 1O2 and •OH are evaluated to be around 6 × 10−4 and 1.2 × 10−5 M, respectively. The survival rates of S. aureus exposed to plasma for 20 s in 1 ml H2O, SOD (100 U, for scavenging ${}^{ \bullet }{\rm O}_{{\rm 2}}^{- } $), D‐Man (0.15 M, for scavenging •OH), and L‐His (0.15 M, for scavenging •OH and 1O2) solutions were 0.7, 1.6, 13.4, and 40.9%, respectively, indicating that 1O2 contributes the most to the inactivation.

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contributed equally to this work.Bacillus subtilis spores suspended in distilled water was effectively inactivated in 6 min by a direct-current atmospheric pressure non-thermal air plasma microjet. Scanning electron microscopic images show clear distortion and debris of spores after plasma treatment. Direct contribution by temperature and pH change of water as well as relatively long lived species in plasma activated water (PAW) is excluded. Short lived species (such as OH, O À 2 and O 2 ( 1 D g )) are detected in the plasma-water system by electron spin resonance spectroscopy, and are considered to be the most important agents in the inactivation process.