  Hydroxyl (OH) radical is the most important reactive species produced by the
plasma-liquid interactions, and the OH in the liquid phase (dissolved OH
radical, OHdis) takes effect in many plasma-based applications due to its high
reactivity. Therefore, the quantification of the OHdis in the plasma-liquid
system is of great importance, and a molecular probe method usually used for
the OHdis detection might be applied. Herein we investigate the validity of
using the molecular probe method to estimate the [OHdis] in the plasma-liquid
system. Dimethyl sulfoxide is used as the molecular probe to estimate the
[OHdis] in an air plasma-liquid system, and the partial OHdis is related to the
formed formaldehyde (HCHO) which is the OHdis-induced derivative. The analysis
indicates that the true concentration of the OHdis should be estimated from the
sum of three terms: the formed HCHO, the existing OH scavengers, and the OHdis
generated H2O2. The results show that the measured [HCHO] needs to be corrected
since the HCHO destruction is not negligible in the plasma-liquid system. We
conclude from the results and the analysis that the molecular probe method
generally underestimates the [OHdis] in the plasma-liquid system. If one wants
to obtain the true concentration of the OHdis in the plasma-liquid system, one
needs to know the destruction behavior of the OHdis-induced derivatives, the
information of the OH scavengers (such as hydrated electron, atomic hydrogen
besides the molecular probe), and also the knowledge of the OHdis generated
H2O2.
