Estimation of Hydroxyl Free Radicals Produced by Atmospheric Air Cold Plasma with Salicylic Acid Trapping

Salicylic acid is used as a scavenger to quantify free hydroxyl radicals (•OH) produced from a cold plasma process. High-performance liquid chromatography revealed that salicylic acid was decomposed to 2,3-dihydroxybenzoic acid (2,3-DHB) and 2,5-dihydroxybenzoic acid (2,5-DHB). The concentrations of 2,3-DHB and 2,5-DHB were the highest when the concentration of salicylic acid was > 8,000 mg/L. Quantifying 2,3-DHB and 2,5-DHB concentrations revealed the association between the •OH concentration and the reaction time by using the following first-order equation: [•OH] = 0.01713t + 0.0311. Additionally, the rate of •OH formation in relation to different cold plasma conditions was also determined.