Monitoring:  | Gas | Ground Deformation | Hydrologic | Remote Sensing | Seismicity |

Direct gas sampling and laboratory analysis

Collecting gas samples from a fumarole on Mageik volcano, Alaska

Sampling volcanic gases is ideally suited for long-term study of volcanic systems because it produces a detailed chemical analysis of specific fumaroles and vents. This method is not well suited for monitoring rapidly changing conditions, however, because of the time necessary for completing a laboratory analysis--often days or weeks. Furthermore, this method is often an unsafe activity for scientists when a volcano becomes increasingly restless because of the necessity of sampling at or near the potential eruptive vent.

Sampling the gases

© Brad Lewis Evacuated-bottle sampling method

Volcanic gases are typically collected by inserting a chemically inert and durable tube (made of titanium, quartz, alumina, or mullite) into a hot fumarole. After allowing the tube to heat until condensation in the tube has reached equilibrium with the escaping gases, usually about 5 minutes, either a specially-designed evacuated-sample bottle or a flow-through sample bottle is attached to the collection tubing.

The evacuated-bottle method utilizes a borosilicate glass bottle with a high-vacuum stopcock and sample port. Before arriving at the sampling site, the bottle is partially filled with concentrated aqueous sodium hydroxide, carefully weighed, and evacuated with a vacuum pump. As gases from a fumarole  bubble through the alkaline solution, acid gases, such as CO2, H2S, SO2, HCl, and HF dissolve into the liquid. The remaining gases, such as N2, O2, H2, CO, and He, collect in the headspace of the bottle. Many liters of fumarolic gas can be collected in a single bottle, because volcanic gas is typically composed mostly of water and condensable acid gases. This method concentrates gases in the solution and headspace, and thereby promotes better analytical precision. The headspace gases are analyzed by gas chromatography while the gases in solution are analyzed by ion chromatography or traditional wet-chemical methods.

In the flow-through bottle method a glass bottle with a stopcock at each end and a hand-operated pump are attached to the sampling tube. The hand pump flushes out air and draws the fumarole gases into the bottle. Because this collection method is faster, it is used when a complete gas analysis is not necessary, or where field conditions are too hazardous to safely make an evacuated-bottle collection. Sulfur gases are stable for only a short time so this type of sample must be analyzed within a few hours of collection.

Analyzing the gases

Analyzing gas samples collected from fumaroles on Kilauea Volcano, Hawai`i

Both the Hawaiian Volcano Observatory and the Cascades Volcano Observatory are equipped to analyze gas samples for SO2 and CO2. If a complete and detailed analysis is required, samples are sent to an analytical laboratory at the USGS Western Region Center in Menlo Park, California. The advantage of a detailed analysis is that it provides the necessary information to perform thermodynamic calculations and construct models in order to provide insight into the condition of the magma at depth from which the gases originated.

References

Other methods of monitoring volcanic gases

• Measuring gas-emission rates in volcanic plumes
• Continuous on-site gas monitoring
• Soil-efflux measurements