In-depth profile analysis by radiofrequency glow discharge optical emission spectrometry using pressure as variable parameter

Abstract

The analytical potential of radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES), keeping constant the delivered power and the dc-bias while leaving the pressure as a free parameter, is investigated for in-depth profile analysis of conducting zinc-based coatings on steel. Results are compared with those achieved with a direct current (dc)-GD-OES in its common in-depth profiling mode of operation (constant current intensity, maintaining fixed the voltage at the expense of modifying the pressure). Under the selected operation conditions (40 W of delivered power and −400 V of dc-bias for rf-GD; −690 V and 11 mA for dc-GD), sputtering rates for different matrices (e.g., brass, stainless-steel, aluminium/silicon, nickel alloy) were of the same order when comparing both discharges. Precisions achieved in the measurement of sputtering rates for five replicates (burns) were between ±2.3 and ±10.8% for rf-GD and ±1.9 and ±7.2% for dc-GD. Reliable values for the emission yields of each analyte emission line under study (Zn, Fe, Ni, Si, Cu, Al and Pb) in the different matrix reference samples selected were achieved. Better correlation coefficients of the plots of emission intensity versus the product of sputtering rate times and analyte concentration in the case of dc-GD-OES were observed. The conversion from qualitative profiles (emission intensities versus time of acquisition) into quantitative profiles (concentration of the elements versus sputtered depth) was attempted by rf-GD-OES as well. Good results were obtained using the selected operation conditions for materials such as electroplated ZnNi, galvanneal and hot-dipped Zn. Such results were in agreement with those average values obtained in a dc-GD-OES laboratory intercomparison project for the same materials.