A few seasons into its run, the show hired Dr. Donna Nelson, a professor of organic chemistry at the University of Oklahoma, as an advisor after she approached the creators and offered her help. According to Gilligan, Nelson "vets our scripts to make sure our chemistry dialogue is accurate and up to date. We also have a chemist with the Drug Enforcement Administration based out of Dallas who has just been hugely helpful to us.” In early episodes, a once common clandestine route, the Nagai red phosphorus/iodine method, is depicted, which uses pseudoephedrine as a precursor to d-(+)-methamphetamine. Later in the first season, Walt changes methods and is shown using a reductive amination reaction, relying on phenyl-2-propanone and methylamine. On the show the phenyl-2-propane, otherwise known as phenylacetone or P2P is produced from phenylacetic acid and acetic acid using a tube furnace and thorium dioxide or ThO2 as a catalyst. P2P and methylamine form an imine intermediate, and Walt can be seen creating a reduction of this P2P-methylamine imine intermediate using mercury aluminum amalgam.