Atmospheric pressure capacitively coupled plasma source for the direct analysis of non-conductive solid samples

Abstract

An atmospheric pressure capacitively coupled device for the rf sputtering and direct analysis by AES of non-conductive solid samples was developed. It operates at 13.56 MHz, with Ar flow rates lower than 1 l min ^–1 and rf input powers between 20 and 50 W. With the aim of studying the expulsion mechanism and determining the analytical performance, the following materials were used: ZnO with known content of trace elements (Si, Pb, Cd and Na); four andesite standards with certified contents of Pb, Cu and Cr; and a synthetic (laboratory made) andesite standard. The expulsion mechanism depends on the rf power. Below 38 W, the sputtering rate is nearly constant and the atomisation is produced only by sputtering. At powers higher than 38 W, thermal evaporation will be present in addition to rf sputtering. All measurements for evaluating the analytical performance were made at optimum working parameters (rf power=36 W, Ar flow rate=0.5 l min ^–1 ). The detection limits are 0.3, 0.7, 1.0 and 0.9 µg g ^–1 for Na, Pb, Si and Cd, respectively, in the ZnO matrix and 0.8, 1.0 and 0.5 µg g ^–1 for Pb, Cr and Cu, respectively, in the andesite matrix. The dynamic range for Pb is about three orders of magnitude. The relative standard deviations for Pb in the certified standards are between 1.7 and 12.7% and the recoveries compared with the certified values are between 95 and 104% for the concentration range 5.8-35.1 µg g ^–1 .