Trace analysis of urea nitrate by liquid chromatography–UV/fluorescence

In this paper we have adapted a technique, previously used to determine the presence of urea in aqueous samples of wine and urine, to detect trace levels of urea nitrate explosives. The procedure involves the reaction of the uronium ion (protonated urea) with a fluorophore, xanthydrol. By modification of the procedure to utilize non-aqueous reagents, in neutral conditions, it can be made specific to the presence of the urea nitrate ion pair. The procedure includes selective detection of derivatization products by UV and fluorescence following separation by High-Performance Liquid Chromatography (HPLC). Analytical method development included optimization of HPLC conditions (solvent, gradient), UV and fluorescence wavelengths, and derivatization parameters (xanthydrol amount, reaction times, temperature). The extraction of urea nitrate from surfaces was also investigated and optimized. For best quantification, it was shown that an internal standard was required; this resulted in a quantification limit around 0.17 mM (21 mg/L). The entire procedure could be performed in less than 30 min per sample and potential interferences such as ammonium, nitrate, and urea did not produce a response under standard conditions.