Formation and identification of novel derivatives of primary amine and zwitterionic drugs

Abstract

Gas chromatography-mass spectrometry (GC–MS) is a “workhorse” in the analysis of controlled substances in forensic laboratories. However, many drugs are not amenable to GC–MS due to thermal instability, non-ideal interactions in the column, or both. To improve the suitability of a molecule for analysis by GC–MS, derivatization can be employed. Derivatization replaces a labile hydrogen in the analyte molecule with a more stable functional group. In this paper, three different derivatization agents were tested for effectiveness with two classes of drugs: primary amines (i.e., amphetamine and 2C-I) and zwitterions (i.e., gabapentin, lorazepam, vigabatrin, pregabalin, and clorazepate). Trifluoroacetic anhydride (TFAA) was used as an acylating agent and N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) was used as a silylating agent. Dimethylformamide-dimethyl acetal (DMF-DMA), which has not been previously used for derivatization of drugs, was used as an alkylating agent. DMF-DMA was found to form dimethylaminomethylene derivatives with several primary amines and zwitterions. Amphetamine, 2C-I, gabapentin, and lorazepam were all detected in their underivatized form but generally suffered from peak asymmetry and band broadening. Derivatization resulted in drastic improvements in their chromatographic behavior. Vigabatrin, clorazepate and pregabalin were not detectable in their underivatized form. However, the trimethylsilyl (TMS) derivative of clorazepate was readily detected by GC–MS, as were the TMS and trifluoroacetyl (TFA) derivatives of vigabatrin. Derivatization of pregabalin was not successful, resulting in multiple chromatographic peaks with each derivatization agent. The mass spectra of several derivatives were not found in commercially available mass spectral databases. Hence, those spectra are reported here with interpretation of their fragmentation.