Abby identifies the nerve agent as 3-quinuclidinyl benzilate (BZ), a genuine organic compound. Later Hogan incorrectly calls it an enzyme. Enzymes are (mostly) proteins, not simple organic compounds. He said that he denatured it so that it could only work by contact, not as a gas. Even if the agent had been an enzyme, proteins are not volatile and lose their activity completely if they are denatured.
When NCIS agents rush to the "Alexandria, VA. Amtrak station," palm trees can be seen outdoors in the background. There are no palm trees outdoors in Alexandria, VA.