Derivitization agents are added in excess to overcome unknown quantities of other things that might react. Thus an uncontroled level of variability right up front. Next, an internal standard should be added at the beginning of the sample processing, allowing for losses and concentration effects to be found in both analytes and the internal standard. Derivitization agents are not added this early in the process - typically after the last transfer of sample into the GC vial. Third: the large excess of derivatizing agent can create overloaded peaks - these quantify badly. And Fourth, an internal standard should be chemically similar to the analytes and in a similar concentration range. The similarity between derivatizing agent and derivitized analyte may be a bit limited, particulalry in reactivity toward active sites and such.
Probably not a good idea.
