3.5. Chromatographic conditions
Preliminary esperiments carried out using HCOOH as organic modifier added to ACN/water mixture, showed a too short retention time of MA-DAN and, therefore, HFBA was then used. In fact, due to the hydrophobic feature of the ion-pair that heptafluorobutyrate anion formed with the naphtodiazepinium ion, the retention time of MDA-DAN on a reversed phase column was satisfactorily increased. Additionally, the use of DAN deserves a note of caution, since available commercial samples contain many impurities that could not be completely eliminated, even after repeated crystallizations. Nevertheless, a satisfactory separation of MA-DAN peak from interferences was possible by properly adjusting the eluent composition. In these conditions, MA-DAN was clearly detected between two impurities of DAN side products of the reagent, while IS was completely separated at the end of the chromatographic run. In Fig. 4 typical HPLC profiles of a standard solution (A) and of a mackerel sample (B) are shown.