The second step involved centrifuging and filtering the contents of
the vials. The samples were centrifuged for ten minutes at RCF of
860 g. Fragments of the stones were also decanted into the sample
vials since the stones are already porous, and upon vortexing, particu-
lates were easily broken off. The samples were then filtered by transfer-
ring 4.0 mL from the centrifuged vial into a 5.0 mL syringe and filtering
(0.45 μm pore size) into a third 15.0 mL acid washed vial. This was to en-
sure that no fragments of the stones remained, since even micrometer-
sized particulate could clog the ICP–MS nebulizer.