The discharge of sulfur into the atmosphere during volcanic eruptions results in the formation of sulfuric acid droplets. The amount of sulfur given off, or degassed, during an eruption can be determined from the difference between the amount of sulfur in the magma before the eruption and the amount remaining in the erupted rocks or volcanic ash once the eruption has occurred. It is also necessary to know the total volume of magma erupted in order to make the calculation.

The amount of sulfur in a magma prior to eruption can be determined by analyzing magma inclusions that are trapped in crystals that form in the magma deep within the Earth. Magma inclusions do not lose their sulfur during an eruption because it is locked in by the host crystal. The magma inclusion in the photo above is only about 0.08 mm in diameter! It is now glass.
  An electron microprobe can be used to analyze volcanic rock samples for sulfur and many other elements. The instrument using a beam of electrons to analyze a spot on a sample less than 0.01 mm diameter. Learn more.  
  Conduct Microprobe Analyses of Laki Fissure Eruption Samples:    
1. Click on each of the two sample localities shown on the map to the right (red dots).
  2. Carry out the electron microprobe analysis of magma inclusions in the pop-up window for each sample.
3. Record the results in your fieldnotebook.