After his Los Alamos stint, Weissman went to the Washington University in St. Louis in 1946 becoming a full professor in 1955.

[2] At Washington University, Weissman worked with others developing the use of electron spin resonance.

[1] He was one of the first, probably in parallel with Clyde Hutchison, to measure the hyperfine splitting of the ESR line caused by the interaction with nuclear spins.

Much of his work concerned the way this interaction term behaves as molecules tumble around in solution or undergo chemical reactions.

He also investigated the special non-equilibrium ESR effects which are found as reactions take place.