Recovery and detection of urea nitrate in traces

Urea nitrate (uronium nitrate, UN) is a powerful improvised explosive, frequently used by terrorists in the Israeli arena. It was also used in the first World Trade Center bombing in New York in February 1993. It is difficult to identify UN in postexplosion debris, because in the presence of water it readily decomposes to its original components, urea and nitric acid. A method for the recovery and detection of urea nitrate in traces is described. The residues are extracted with hot acetone, and the extract chromatographed on chromosorb G-HP. The eluent is analyzed by liquid chromatography mass spectrometry using atmospheric pressure chemical ionization (APCI). By applying this technique, we were able to identify urea nitrate in actual exhibits. It was found that UN can be also formed during the analytical procedure, by certain combinations of urea, nitrate ions, and a source of protons and, hence, the presence of the characteristic adduct ion does not necessarily indicate an "authentic" urea nitrate. Several solutions are suggested to overcome this ambiguity.