Abstract
An inductively coupled oxygen plasma (O2ICP) using a 40.68MHz generator with a maximum radio-frequency (RF) power of 4kW was evaluated as an ion source for mass spectrometry (MS). Although the intermediate and central Ar flows are completely replaced with O2, a certain amount of Ar should be added to the outer gas for a stable plasma discharge. The proposed O2 ICP discharge is more stable than N2 ICP, because of the lower dissociation energy of O2. The sampling depth, RF power and carrier flow rate of O2 for maximum analyte signals were found to be 7mm above the work coil, 2.4kW and 1.41min-1, respectively. Under the optimized conditions, O+ is strongly observed in the mass spectra. The sensitivity of the proposed O2 ICP is inferior to those of Ar and N2 ICPs, and decreases with increasing the mass number of the analyte. Although the average kinetic energy of the analyte ions in the O2 ICP is almost equivalent to that of the Ar ICP, its distribution is wider. A wide energy distribution seems to lead to a higher ratio of doubly charged ions to singly charged ions. The ratio of monoxide ions to singly charged ions is much higher than those in Ar and N2 ICPs.
