ANALYTICAL CHARACTERISTICS OF INDUCTIVELY-COUPLED OXYGEN-PLASMA MASS-SPECTROMETRY ASSISTED BY ADDING ARGON TO THE OUTER GAS

An inductively coupled oxygen plasma (O-2 ICP) using a 40.68 MHz gener ator with a maximum radio-frequency (RF) power of 4 kW was evaluated a s an ion source for mass spectrometry (MS). Although the intermediate and central Ar flows are completely replaced with O-2, a certain amoun t of Ar should be added to the outer gas for a stable plasma discharge . The proposed O-2 ICP discharge is more stable than N-2 ICP, because of the lower dissociation energy of O-2, The sampling depth, RF power and carrier flow rate of O-2 for maximum analyte signals were found to be 7 mm above the work coil, 2.4 kW and 1.41 min(-1), respectively. U nder the optimized conditions, O+ is strongly observed in the mass spe ctra. The sensitivity of the proposed O-2 ICP is inferior to those of Ar and N-2 ICPs, and decreases with increasing the mass number of the analyte. Although the average kinetic energy of the analyte ions in th e O-2 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 i ons to singly charged ions is much higher than those in Ar and N-2 ICP s.