Calcium oxalate monohydrate
(COM, CaC2O4.H2O, 99%,
Alfa Aesar),
sodium oxalate (Na2C2O4, 99%, Sigma-Aldrich),
calcium chloride dihydrate (CaCl2.2H2O, 99%, Sigma-Aldrich), H217O (∼90% 17O enrichment, CortecNet), H218O (99.3% 18O enrichment, CortecNet), D2O (≥99% 2H enrichment, Sigma-Aldrich), and HPLC-grade water (Acros
Organics) were used as received. COD was prepared as previously described
in the literature37 (link),41 and stored under vacuum before
any further use.