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Solutions to the Problem of Substitution of ERL 4221 for Vinyl Cyclohexene Dioxide in Spurr Low Viscosity Embedding Formulations

Spurr low viscosity embedding medium was introduced in 1969 and 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). This low viscosity formulation has been used extensively for embedding plant materials, microrganisms and other difficult specimens. Over the years numerous concerns about the toxicity and carcinogenicity of VCD have been raised. VCD (Fig. 1) was finally removed from the market in 2005 and replaced with ERL 4221 (Fig. 2), a cyclic epoxide with a higher molecular weight and higher viscosity. Vendors instructed that ERL 4221 be substituted for VCD at the same rates as used in the original Spurr formulation. This direct substitution resulted in a more viscous resin and blocks were extremely hard and brittle and lacked the sectioning qualities of the original formulation. Vendors then suggested modifications to the dehydration schedule and reductions in the amount of DMAE to improve the quality of the blocks.