Rapid and specific detection of urea nitrate and ammonium nitrate by electrospray ionization time-of-flight mass spectrometry using infusion with crown ethers

Urea nitrate (UN) and ammonium nitrate (AN) are fertilizer-based explosives that are commonly used in improvised munitions and can have highly destructive effects. Because they are in the form of salts, their relatively low volatility makes them difficult to detect at trace levels. In addition, these salts readily undergo metathetic reactions in water to form urea, ammonium and nitrate, which are ubiquitous in the environment. Thus, selective methods are needed for their detection. In this study, a procedure was developed to detect UN and AN in non-aqueous environments by positive ion electrospray ionization mass spectrometry through the addition of 18-crown-6. The method is sensitive, with detection limits under 2 μM, and selective. The procedure is capable of differentiating urea from uronium ions (protonated urea) and a mixture of urea and AN did not interfere with the UN signal. The procedure is quite versatile and the addition of crown ethers to the sample matrix does not interfere with the detection of high explosives in the negative ionization mode. Experimental results are presented on the utilization of the method in the detection of UN and AN on various surfaces. Semi-quantitative studies showed that AN and UN can be detected at trace levels following finger transfer and a series of studies were performed to demonstrate the effect of various interferences. The results show the method to be a quick and robust procedure for trace detection.

Colorimetric detection of urea nitrate: the missing link

Traces of the improvised explosive urea nitrate can be characterized by a sensitive colorimetric reaction with p-dimethylaminocinnamalaldehyde (p-DMAC, UN-1 reagent). As recently shown, the dark red product has a structure of a protonated Schiff base. The unprotonated free base, previously postulated in the literature to be the colored product, was now prepared and fully characterized. It shows totally different spectroscopic properties from the dark red compound. Similarly, the analogous free base of the reaction between urea nitrate and the one-vinyl shorter reagent, p-dimethylaminobenzaldehyde, was synthesized and characterized. Similar differences between the free and protonated forms were observed. The protonated Schiff base spontaneously decomposes to the free base over time. This study provides the missing link in the elucidation of the colorimetric reaction between urea nitrate and UN-1. Both colored products show characteristics of typical acid-base indicators.