Direct sampling

Continuous measurements of gases emitted from volcanic system would provide information of fundamental importance for surveillance purposes. However, the ‘survival’ of the analytical instruments is rather difficult due to the hostile environmental conditions for electronic devices related to the presence of corrosive gases. Attempts to direct field measurements of fumarolic discharges were successfully performed on specific gas species by using Dragger tubes (Tonani 1971), field gas-chromatographs (Le Guern et al. 1982, Zimmer and Erzinger 2003, Tedesco et al. 2005) and mass spectrometers (Naumann et al. 2001). However, the periodic ‘direct’ gas sampling still represent an inalienable approach for extensive geochemical surveys of active volcanoes, spanning from main, minor, trace and ultra-trace compounds to isotopic investigation (Cioni and Symond et al. 1994; Giggenbach 1992, 1996; Panichi and La Ruffa 2001; Jordan 2003; Schwandner et al. 2004). The presence of steam and acidic (highly reactive) gases, the latter being typically enriched in volcanic fumaroles, poses one of the most challenging problems of gas sampling and analysis (Giggenbach et al. 2001, Taran et al. 2006). A pre-concentration procedure directly in the field has to be adopted to avoid secondary chemical reactions among the different species present in the gas phase, as well as to enhance the “analytically available” concentrations of minor and trace compounds.

The basis for most “direct” sampling methods and analytical procedures currently on use was provided by Giggenbach (1975) and Giggenbach and Gougel (1989), which can be considered as an improvement of the old techniques developed with the use of KOH (De Fiore 1926) or NH3 (Sicardi 1955). Gas from the fumarolic vent are convoyed through “sampling lines” composed of non-reactive material into pre-evacuated and pre-weighed glass bottle (the Giggenbach’s bottle) equipped with Teflon stopcocks and filled with a certain amount of a highly alkaline medium (generally, 4 to 6M NaOH) solution, where acidic gases and steam dissolve. Those gases that do not react with the alkaline solution will be enriched in the headspace of the gas vial. Some modifications have then been proposed for different purposes, e.g. analysis of metals in trace amounts (Sortino et al. 2006) and speciation of S-bearing gases (Piccardi, 1982; Montegrossi et al. 2001).

 

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