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

The analytical potential of radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES), keeping constant the delivered power and the dc-b ias 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 c ommon in-depth profiling mode of operation (constant current intensity, mai ntaining fixed the voltage at the expense of modifying the pressure). Under the selected operation conditions (40 W of delivered power and -400 V of d c-bias for rf-GD; -690 V and 11 mA for dc-GD), sputtering rates for differe nt 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 be tween +/-2.3 and +/- 10.8% for rf-GD and +/-1.9 and +/-7.2% for dc-GD. Reli able values for the emission yields of each analyte emission line under stu dy (Zn, Fe, Ni, Si, Cu, Al and Pb) in the different matrix reference sample s selected were achieved. Better correlation coefficients of the plots of e mission intensity versus the product of sputtering rate times and analyte c oncentration in the case of dc-GD-OES were observed. The conversion from qu alitative profiles (emission intensities versus time of acquisition) into q uantitative profiles (concentration of the elements versus sputtered depth) was attempted by rf-GD-OES as well. Good results were obtained using the s elected operation conditions for materials such as electroplated ZnNi, galv anneal 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.