Sulphate extraction for isotopic analysis
Sulphate was extracted from bedrock samples following the method of Wynnet al. (2008). 300 mg aliquots of bedrock sample were dissolved
in 1.5 ml of 4M HCl to yield at least 350 μg of BaSO4for both δ34S-SO4 and
δ18O-SO4. The solution was filtered
using mixed cellulose 0.45 μm filters into 2 ml Eppendorf polypropylene
tubes.
Rainfall and dripwater samples were prepared for analyses following the
method in (Wynn et al. , 2015). Resin-filled syringes were
prepared by mixing a dry resin component with MilliQ water. This
uninitialized resin was poured into 5 ml polypropylene syringe tubes.
The resin was then initialised by dripping through 10 ml of 1M HCl and
then rinsed using 40 ml of MilliQ water. Aliquots of dripwater and
rainfall were taken with sufficient to yield 350 μg of
BaSO4 for both
δ34S-SO4 and
δ18O-SO4. Each aliquot was then passed
through a syringe tube filled with 50 W-X8 resin to remove the cations,
and through a syringe tube filled with AG2-X8 to load anions from the
solution onto the resin. The anion resin was then eluted with three
sequential 0.5 ml aliquots of 1M HCl, and the resulting solution
collected in 2 ml Eppendorf tubes.
For both bedrock and water samples, 0.2 ml BaCl2 was
added to each tube and thoroughly mixed. Solutions were refrigerated for
≥ 48 hours to allow the BaSO4 to precipitate. The
BaSO4 precipitate was separated from the eluant by
centrifuging for 20 minutes at 3500 rpm and rinsed three times with
MilliQ water. Once the solution pH was neutral (tested with pH paper),
the samples were dried at 40°C for ≥ 48 hours to evaporate the remaining
water, leaving behind the dried BaSO4 pellet.