| Sumario: | Filter packs have been used to collect sulphur and oxygen from air for isotope analysis from many areas of the world. Here we discuss their use to determine the sulphur isotope systematics of SO₂ and the sulphur and oxygen systematics of sulphate particles from volcanic vents. Our field data suggest that fractionation of sulphur isotopes occurs during the collection of SO₂ from concentrated volcanic gas plumes on alkali-impregnated filter papers. The collection of volcanogenic HCI on the same filters may play a role in this fractionation. Comparison with previous studies, which sampled areas of cleaner atmosphere, suggests that isotopic fractionation only becomes significant where elevated filter loadings occur (e.g., when sampling gas streams with SO₂ concentrations greater than ~ 1μmol m⁻³, although this requires further investigation. Previous studies also suggest that for volcanic sulphate particle samples collected at the same time, oxygen isotopes are fractionated while sulphur isotopes are not. Minimising the filter loading reduces these effects, but there is a trade-off with the amount of SO₂ or SO₄⁻² required for isotopic measurements. Despite the convenience of the filter pack technique these fractionation effects limit the use of filter packs for determining δ³⁴S for SO₂ in volvanic gases and δ¹⁸O and δ¹⁷O for the sulphate in volcanic particles, and they will certainly not be useful for determining small variations in the source isotopic signature of the sulphur in the SO₂ or the oxygen in the SO₄⁻² being measured. This highlights the importance of collecting SO₂ quantitatively for isotopic analysis. These effects may also have implications if using similar methods to collect SO₂ in other environments, especially where concentrations are high.
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