Quantitative Characterization of Homemade Ammonium Nitrate Explosives Based on Powder X-ray Diffraction (PXRD) Patterns with Rietveld Refinement

Industrial explosives are usually strictly controlled, with many sales links and high prices, which bring great profits to the homemade explosives. In recent years, cases of illegal manufacturing, trading, transportation and storage of ammonium nitrate explosives have frequently occurred. The raw materials for homemade ammonium nitrate explosives are widely available and easy to obtain. These result in the complex compositions and formulations. Accurate qualitative and quantitative characterization of its composition is of great significance for comparison, tracing and case investigation. Powder X-ray diffraction (PXRD) has become an efficient method to identify the component. However, it is still in the qualitative stage, i.e. matching of the phase and lack of quantitative method to illustrate the content of each substance in mixtures. In this paper, a quantitative method based on PXRD is described, including pre-processing method, sample preparation, instrument parameter settings and full spectrum fitting method. Firstly, the crystalline phase and amorphous phase components contained in the sample are accurately judged. Subsequently, Rietveld refinement method is used in which multi-dimensional factors such as structural parameters, peak parameters, instrument parameters are all included. The differences between the measured pattern and the calculated pattern is the target function and the optimization algorithm is used to obtain model parameter. After refinement fitting, the best structural parameters and other parameters including content of each component are all identified. The method can realize the quantitative characterization of powder, granular, or block solids. It can provide an important basis for the comparison and tracing of such evidences, so as to further correlate the cases.