Tetrafluoromethane (CF4, 1.01% by weight) was added to the argon support gas of a hollow cathode glow discharge to investigate the analytical utility of etch atomization. When a conducting copper cathode was analyzed, the sputtering rate (as measured by weight loss) was reduced by a factor of five compared to operation with pure argon. Copper atomic absorbance and copper atomic emission intensity were also reduced by factors of seven and two, respectively. When a nonconducting sample was analyzed, the stainless steel ring that held the sample acted as an auxiliary cathode, supporting the discharge processes. Radical fluoride species formed in this discharge reacted with the nonconducting substrate (silica) to produce volatile SiF4 that spontaneously evolved into the gas phase, carrying with it copper and uranium. This approach is analogous to plasma etching, a well-established technique for semiconductor processing. Atomic emission data were obtained with a pure argon discharge and an argon/CF4 discharge. With the addition of CF4, a 30% enhancement was observed for uranium in glass and a 50% enhancement for copper in glass. Scanning electron microscopy (SEM) was used to support the supposition that etching of the silica matrix on the inner surface of the hollow cathode contributed to this enhancement.
C. M. Barshick, K. R. Hess, A. L. Zook, R. E. Steiner, and F. L. King, "Investigation into the Analytical Utility of Plasma Etching in Reactive Glow Discharge Plasmas," Appl. Spectrosc. 53, 65-73 (1999)