Corrected Formulation for Spurr Low Viscosity Embedding Medium Using The Replacement Epoxide ERL 4221

The Spurr low viscosity embedding medium [1] used vinyl cyclohexene dioxide (VCD or ERL 4206) as the low viscosity epoxy resin together with nonenyl succinic anhydride (NSA), the flexibilizer DER 736 and the accelerator dimethylaminoethanol (DMAE or S-1).VCD, a cycloaliphatic diepoxide with an average WPE (weight per epoxide equivalent) of 76 was removed from the market in 2005 due to its toxicity and replaced with ERL 4221, a cyclic epoxide with a WPE of 137.Microscopists have been instructed to use ERL 4221 as a direct replacement in the same amounts for VCD in the Spurr formulation.Using ERL 4221 at the same rate as VCD results in a harder, more brittle block which lacks the sectioning qualities of the original formulation.Modifications to the dehydration schedule and reductions in the amount of DMAE have been suggested to improve the quality of the blocks.However, the suggested modifications do not address the real problem with the replacement epoxide.The source of the problem is raising the anhydride:epoxide ratio from 0.88:1 in the original formulation to 1.6:1 in the formulation by directly substituting ERL 4221.The significance of maintaining an appropriate anhydride:epoxide ratio for a given embedding formulation [2] has not been appreciated in recent years; however, substituting ERL 4221 for VCD reiterates this importance.The corrected formulation presented here maintains the anhydride:epoxide ratio at 0.88:1 [1] and preserves the good sectioning qualities and beam stability of the original Spurr formulation.