In a recent paper on the revised formulation of Spurr low viscosity embedding medium with ERL 4221 the importance of maintaining an appropriate anhydride:epoxide (A:E) ratio was discussed. By understanding a few simple concepts about epoxy resin formulations and setting up a formulation table it is possible to create new resin mixtures with good sectioning properties and other desirable properties such as decreased viscosity and increased beam stability.

Before starting a formulation you need to know the molecular weight of the anhydride and the WPE (weight per epoxide equivalent) of the epoxy resin component. The molecular weights and WPEs are usually printed on the bottle or can be obtained from the vendor. An A:E ratio of 0.7:1.0 -1.0:1.0 is used for most biological specimens. Increasing the A:E ratio results in a harder block; decreasing the A:E ratio results in a softer block. Table 1 shows a basic formulation spreadsheet where the molecular weights of the anhydrides and the WPEs of the epoxy resin components can be entered. The A:E ratio is entered under the anhydride for the molar ratio and the molar ratios of the epoxy components are entered under the epoxy components. The calculations are done as shown in each column and row.