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The easiest, but perhaps the most dangerous way to collect a gas sample is by hand, placing a container directly in the gases. This technique is used to produce a detailed chemical analysis of a specific fumarole or vent, where a scientist can actually insert a tube into a hot opening. This method is ideal for long term study of volcanoes rather than for monitoring rapidly changing conditions. The technique requires days to weeks of laboratory analysis following sampling in order to get data.
Gas sampling from Baker, 1981
Direct sampling requires a scientist to insert a chemically inert and heat resistant tube into a hot opening like a fumarole or vent. It takes about 5 minutes for the tube to heat up to a point where any condensation within the tube has reached equilibrium with the escaping gases. Then, either by attaching an evacuated-sample bottle or a flow-through sample bottle to the collection tube, the gases will be gathered for analysis.
USGS geologists collect gas samples around the dome of Mount St. Helens
The evacuated-bottle method is shown in the image to the right. The device includes a glass bottle with a sample port and a high vacuum-stopcock. Before arriving at the collection site, the bottle must be partially filled with concentrated aqueous sodium hydroxide (NaOH) that has been carefully weighted and evacuated with a vacuum pump. Once the tube is inserted into the fumarole or vent, the gases will bubble through the solution and gases like CO2, H2S, SO2, HCL and HF will dissolve into the liquid. Those gases that remain like N2, O2, H2, CO and He will rise further and collect in the headspace of the bottle.
This technique involves collecting the gases at the site where the gases are being emitted and then returning to the laboratory for analysis. This method is used because of its good analytical precision that stems from its ability to concentrate the gases in the solution and the headspace. Those gases that rise into the headspace are analyzed by gas chromatography. Those that dissolve into the liquid are analyzed by ion chromatography or traditional wet-chemical techniques.
Flow-through bottle method
The flow-through bottle method is shown in this image to the left. The device includes a glass bottle but with a stopcock at each end and a hand-operated pump attached to the sampling tube. The purpose of the hand pump is to flush out the air while entraining the gases into the bottle. This method is not as precise as the evacuated-bottle method, but is utilized in situations where sampling must be done rapidly due to hazardous environments and conditions.
A detailed analysis has that advantage that it can provide the information necessary to reconstruct the conditions of the magma at depth, which is the source region for the emitted gases.
USGS website: Direct gas sampling and laboratory analysis
Sutton, A.J., McGee, K.A., Casadevall, T.J., and Stokes, B.J., 1992, Fundamental volcanic-gas-study techniques: an integrated approach to monitoring: in Ewert, J.W., and Swanson, D.A. (eds.), 1992, Monitoring volcanoes: techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90: U.S. Geological Survey Bulletin 1966, p. 181-188.