Volcanic monitoring has as main goal the identification of typical phenomena of impending-eruption precursors. Therefore, recording a large amount of data of volcanic gases composition, during the quiescence period, is fundamental in order to identify baseline values to be distinguished from any anomalous geochemical signal. The gases emitted by most of the volcanoes are difficult to be sampled especially when the volcano is restless. Volcanic gases have traditionally been monitored by means of direct in situ sampling of fumaroles, followed by laboratory analysis [e.g., Symonds et al., 1994]. However, since direct sampling is often impractical and hazardous, particularly during eruptions, or of too low temporal resolution, efforts have been made (since the 1970s) to improve...