Purify the pyrophosphate by paper chromatography on Whatman No 3 MM paper in a system of isopropyl alcohol/isobutyl alcohol/ammonia/water (40:20:1:39), RF 0.77-0.82. Store it in the dark as the ammonium salt at 0o. The E-form crystallises in platelets from aqueous Me2CO, m ~120o. It dissolves in dry MeCN. Alternatively purify it through a column of Dowex AG 1x8(200-400mesh) equilibrated with 50mM NH4 formate, and elute with MeOH/H2O/NH4OH (95:5:05), then freeze-dry. [Dixit et al. J Org Chem 46 1967 1981, Beilstein 1 IV 3580.]
