Asymmetric charge transfer with hydrogen ions — An important factor in the “hydrogen effect” in glow discharge optical emission spectroscopy

It is now well known that traces of hydrogen or nitrogen in the argon plasma gas in glow discharge optical emission spectrometry (GD-OES) may affect the sputtering rate. More seriously, such traces can also selectively affect the absolute and relative intensities of individual lines, and thereby have a major effect on the accuracy of analytical results. This problem is becoming more severe as the potential of GD-OES as an analytical tool is steadily increasing, and the technique is now used for the analysis of more complex samples.The results presented form part of an extensive study of the effects of hydrogen and nitrogen on the spectra of a number of elements — Fe, Ti, V, Ni, Zn etc. Two systems have been used to record the spectra for the present results — high resolution Fourier transform spectroscopy at Imperial College, London, and the LECO GDS500A, with CCD spectrometer at LECO Instrumente, Plzeň. The two approaches have yielded consistent results.During the analysis of the data for Fe II and Ti II spectra, it has become clear that asymmetric charge transfer involving hydrogen ions (H-ACT) is a very important selective excitation mechanism for spectral lines with a total excitation energy close to 13.6 eV, the ionisation potential of hydrogen. Detailed evidence for this mechanism is presented. The magnitude of the effect varies for different elements and spectral lines but great care must be taken before choosing ionic lines with a total excitation energy of between 12.5–14 eV for analytical use.