The influence of added hydrogen to an argon direct current glow discharge for time of flight mass spectrometry detection

Abstract

The effects of controlled addition of hydrogen (from 0.5 to 10% v/v) to an argon direct current glow discharge (dc-GD)-time of flight mass spectrometer (TOFMS) have been investigated in terms of changes in the mass spectra features, the signal intensities of argon, hydrogen species, and sputtered analytes, the sputtering rates and the observed crater shapes. The dc-GD was operated at constant electrical conditions (15 mA and fixed voltages, in the interval 600–890 V) while the pressure was left as the free parameter. As a general trend it has been observed that the Ar²⁺ signals increased notoriously when 0.5 v/v hydrogen was added to pure Ar. However, such enhancements were not so important for the observed Ar₂⁺ signals, where even signal decreases were noticed in some cases due to hydrogen addition. The presence of hydrogen also gave rise to the appearance of a strong line at m/z = 3, corresponding to H₃⁺. Concerning signals from analytes, TOFMS results have shown that enhancements were obtained in most cases by the addition of hydrogen. On the other hand, a decrease of the sputtering rate was observed with increasing hydrogen additions to the argon discharge (although the electrical conditions were kept constant), this decrease being more steep in the interval from 0% to 1% v/v H₂. Besides, as expected from previous observations by optical emission measurements, the presence of hydrogen influences the shapes of the craters (as compared with pure argon). However, a proper optimisation of the operating conditions allowed us to tailor the crater shapes to obtain flat-bottom shapes. Advantages of both aspects, low sputtering rates and tailoring of crater bottoms by using such gas mixtures, are discussed.