The easiest, but often the most difficult, way to collect a sample is by hand, placing a container directly in the gases. This method was perfected by the late Werner Giggenbach. Difficulty arises because of the high temperatures, dangers associated with being close to vents, and the possibility of contamination of the sample by the atmosphere. Left diagram: Evacuated-bottle sampling scheme. Direct samples are most commonly collected in solution-filled bottles (4N NaOH in a titanium or silica tube) and then returned to the lab for analyses. In the drawing on the left, the enlargement shows an evacuated glass bottle. The solution contains NaOH and collected H20), H2S, SO2, HCl, HF, and CO2. The headspace contains collected H20, H2, CH4, O2, CO, N2, Ar, Ne. Right diagram: Flow-through sampling scheme. Figure from Sutton and others, 1992).
Another way to measure gases released by a volcano is to collect fresh ash samples (before it rains) and pour distilled water through the ash. Then after the liquid passes through the ash it is collected. This is the leachate. The leachate is analyzed for Cl (chloride), F, SO4 and pH. The ratio of Cl to S increases prior to eruptions.
Leachates were measured prior to the 1980 eruption at Mount St. Helens (Nehring and Johnston, 1981). Cl and SO4 were measured in the field and the S/Cl ratio was observed to increase gradually from March 28 to May 18.
The S/Cl ratio increased 30 times over its initial value prior to an eruption of Asama volcano in Japan. At Fuego, in Guatemala, the S/Cl ratio increased 5 times over its initial value and the size of the change was proportional to eruption size.
Continuous direct sampling is a relatively new method to monitor gases. Results of measurements are telemetered to safe locations off the volcano. At Mount St. Helens, upward-moving fresh magma was detected 12 to 60 hours before it was extruded into the dome (McKee and Sutton, 1994).