Mass spectrometric detection of short-living radicals produced by a plasma needle

Abstract

A plasma needle is a radio-frequency (rf) micro-discharge operated in a mixture of helium and air at atmospheric pressure. This source is designed for medical treatment of living tissues. Therapeutic effects of plasma treatment depend on generation of short-living active radicals: reactive oxygen and nitrogen species (ROS/RNS). In this work we determine the concentration of several ROS and RNS (atomic oxygen, nitrogen and hydroxyl radical) by means of threshold ionization mass spectrometry. It is shown that molecular oxygen and nitrogen are substantially dissociated in the plasma. Atomic nitrogen and oxygen are the most abundant radicals: the densities are on average few times 10−4. Hydroxyl radicals are less abundant (10−5 fraction of the total gas density). As expected, the densities of active species increase with increasing plasma power. Spatial (axial) distributions have been determined; the radical density reaches a maximum at 2.5 mm away from the rf powered electrode, and it decreases at distances larger than 3.5 mm. The amount of active radical species is reasonably high, which explains the effectiveness of plasma in bacterial inactivation and tissue treatment.