Since you work for the Feds you'll fall under the regulations set by SAMHSA. Those regulations, for the most part, mirror those of DOT. As an example:

- The Federal workplace drug test still does an initial IA screen on the sample, Cutoff limit set at 50ng. The GC/MS is used only to confirm any positives on the IA screen.

-They cannot perform gender tests, pregnancy tests, or the like. They can only test the creatinine levels, the pH, the specific gravity, certain oxidants, surfacants, and excessive nitrites.

And theres more.

In fact here are the regulations, found in this link: Drug Testing


Another thing, the GC/MS is used to test for drugs and 2 types of adulterants (pyridine and Glutaraldehyde). It is not used to determine dilution or substitution.

Take some time and play with that link. Lots of important info for you.