Evaluation of 5-(4-nitrophenyl)-2,4-pentadien-1-al (NPPD) as a tracer for shadowing pursuits

The chemical compound 5-(4-nitrophenyl)-2,4-pentadien-1-al (NPPD), called "spy dust," was used in the Soviet Union as a shadowing pursuit, the act of following someone secretly, for investigating the activities of diplomatic personnel. It is also useful for counter-terrorism, and some criminal cases in the forensic science field. In this paper, it was synthesized and evaluated as a tracer for shadowing pursuits. The method for utilizing this reagent was very simple: it was dissolved in methanol (1 mg/mL) and sprayed on the restricted area. If the suspect was to enter this area or touched the sprayed material, NPPD was attached to the suspect's shoe surface or hands. The color examination was a two-steps process: first was the addition of 1 mL of a 0.1% naphthoresorcinol methanol solution to the methanol extracts of a methanol-contained cotton swab used to smear some surfaces of the suspect, and second, the addition of 1 mL of concentrated hydrochloric acid, which turned the solution dark red. The gamma max of the colored solution was 510 nm, measured by ultraviolet-vis spectroscopy. Detection limits for three methods were determined: a visual method (detection limit 100 ng/3 mL), an ultraviolet-visible spectrometric method (detection limit 10 ng/3 mL), and a selected-ion-monitoring gas chromatographic/mass spectrometric method (detection limit 300 pg/injection). The forensic utility of NPPD was demonstrated for two simulated cases: a theft case and a case where NPPD was used as a tracer to prove that an automobile had entered a restricted area. These examinations prove NPPD is a useful shadowing pursuit (spy dust) for the forensic science field.