Laser-Enhanced Ionization (LEI) signals have been detected for Na, Li, and Ba analytes in a microwave-induced plasma (MIP). A 300-mW continuous-wave (cw) dye laser pumped by a 5-W argon-ion laser was used to promote measurably increased ionization rates for these elements. A low-power, high-efficiency microwave plasma at 1 atmosphere with nitrogen and nitrogen-containing support gas was employed as the atom reservoir. The effects of varying applied microwave power, support gas composition, electrode voltage, and geometry were studied and results are given. The experimental variables that most significantly affect LEI signal intensity are: (1) electrode geometry, spacing, voltage, and distance above the cavity; (2) applied microwave power; (3) gas composition in an argon and nitrogen mixture; and (4) laser intensity. Experimental results are presented from the studies of Na LEI signals as a function of each one of these variables. Preliminary analytical studies yield Na detection limits in the low ng/mL range, showing this method to be competitive with other laser-based ionization methods.
Richard S. Lysakowski, Raymond E. Dessy, and Gary L. Long, "Laser-Enhanced Ionization in Microwave-Induced Plasmas," Appl. Spectrosc. 43, 1139-1145 (1989)