Purification of ammonium nitrate via recrystallization for isotopic profiling using isotope ratio mass spectrometry

Abstract

Stable-isotope analysis of ammonium nitrate (AN) plays an important role for comparing and tracing the sources of AN samples. Numerous studies have reported the application of isotope ratio mass spectrometry (IRMS) for the stable-isotope analysis of AN. However, in real cases, AN is often mixed with liquid hydrocarbons, organic explosives, wood flour, and sodium chloride. Therefore, the purification of AN samples prior to IRMS analysis is essential. Based on the different solubilities of AN at different temperatures, AN samples were purified by recrystallization. The results show that recrystallization can effectively purify AN, and the isotope fractionation effect caused by recrystallization increases the stable-isotope ratio (δ15N increased by 0.07–0.14‰, δ18O increased by 0.15–0.33‰, and δ2H increased by 4.61–18.16‰). The change of δ15N and δ18O were close to the standard deviation values, and did not affect the differentiation of AN. δ2H was deemed unsuitable for AN differentiation, owing to isotopic exchange during recrystallization. Eight AN samples from different cities and four AN samples produced in different years and batches from the same manufacturer in China were analyzed after recrystallization. Both groups of AN samples could be distinguished by combining the nitrogen and oxygen isotope ratios. Therefore, recrystallization can be used as a means of AN purification for the accurate determination of AN in ammonium nitrate fuel oil and explosive residues.