Trace analysis of urea nitrate in post-blast debris by GC/MS

Urea nitrate (uronium nitrate, UN) is an improvised explosive that looks very much like sugar and is easily made from accessible starting materials, urea and nitric acid. During the last decade it has been frequently used by terrorists in the Israeli arena and in other countries as well. It is difficult to identify urea nitrate in post-explosion debris, since only a very small fraction survives the blast. Also, in the presence of water, it readily decomposes to its original components, urea and nitric acid, two ubiquitous substances with relatively low evidential value. By further modifying McCord's recent version of Clark's method for the detection of minute amounts of urea, we were able to identify with high degree of certainty traces of uronium ion, the main characteristic factor of urea nitrate, in post-blast residues by GC/MS. The analytical process is based on the initial formation of xanthenyl urea by the reaction of uronium cation with xanthydrol, followed by reaction with alcohol to form xanthylurethane, which is readily identified by GC/MS. The reaction mechanism was corroborated by the use of labeled ¹⁵N-urea. By applying the technique to residues collected from scenes of controlled firing experiments, 4 out of 16 samples showed the presence of uronium cation as indicated by the formation of the corresponding xanthylurethane. Potential interferences such as urea and ammonium nitrate did not respond under standard conditions. However, under strongly acidic conditions (pH < 2), urea is converted into uronium ion, which is a nuisance, since it behaves as an authentic uronium cation. Such conditions, however, do not prevail at common crime scenes.