The effect of hydrogen and nitrogen on emission spectra of iron and titanium atomic lines in analytical glow discharges

It is now well known that traces of molecular gases such as hydrogen or nitrogen can affect significantly the electrical characteristics, sputtering rates and relative intensities of emission lines in glow discharge optical emission spectrometry (GD-OES). These changes, caused by the molecular gases which are very often present in the discharge for various reasons, have a serious impact on the accuracy of analytical results. Therefore, it is important to describe these effects in detail and to try to understand the processes involved. The results presented in this paper focus on the effects of hydrogen and nitrogen on intensities of atomic emission lines of iron and titanium. In the case of hydrogen, when intensity ratios (intensities measured in argon-hydrogen relative to those measured in pure argon) are plotted against the excitation energies of the lines, these intensity ratios increase with the excitation energy between approx. 3 and 5 eV for both elements studied. Furthermore, in the case of iron, several emission lines with the excitation energy between 5.3 and 5.6 eV are strongly enhanced in the presence of hydrogen. It has been also observed that this effect is more pronounced at lower currents. On the other hand, it has been found that nitrogen does not have any similar effect: the gradient of the intensity ratios of both elements is negligible or even negative and no emission lines are enhanced in the presence of nitrogen. A comparison was also made between direct current and radiofrequency powered glow discharges and very similar trends have been obtained.