Investigating ANFO Lattice Vibrations After Detonation with Raman and XRD

A recent study published in the Journal of Raman Spectroscopy analyzed ammonium nitrate (AN) lattice vibrations in ammonium nitrate fuel oil (ANFO) residues from explosive events using confocal Raman microscopy and single-crystal X-ray diffraction (XRD) (1). In this study, two improvised explosive devices (IEDs) were detonated, and crystalline material was found on post-blast substrates. The Raman analysis revealed lattice vibrations differing from intact ammonium nitrate, including spectral shifts and new spectral bands (1).

Geraldine Monjardez, the lead author of this study, is an Assistant Professor in the Department of Forensic Science at Sam Houston State University (2). Along with her coauthors, including Christopher Zall, an Associate Professor in the Department of Chemistry at Sam Houston State University, and Jared Estevanes, a Forensic Scientist at Microtrace LLC, they determined that it is possible to detect thermal changes in AN through microscopy and low-frequency Raman spectroscopy, highlighting its importance in identifying crystalline salts in explosive residues (1).

Geraldine Monjardez (left) is an Assistant Professor at Sam Houston State University. Christopher Zall (center) is an Associate Professor at Sam Houston State University. Jared Estevanes is a Forensic Scientist at Microtrace LLC. Photo Credits: Geraldine Monjardez, Christopher Zall, and Jared Estevanes.

Spectroscopy recently sat down with Monjardez, Zall, and Estevanes to discuss their findings and their implications for explosive analysis.

Can you explain the significance of characterizing ammonium nitrate lattice vibrations in ANFO post-detonation, and how this contributes to forensic investigations?

Geraldine Monjardez: This study does not approach the topic from a forensic standpoint but rather focuses on a more fundamental investigation of the crystal structure of AN following exposure to an explosive event. Given the widespread use of AN in the chemical industry, our research adds to the existing body of literature on its phase transitions. Although the 400–1700 cm⁻¹ region of the Raman spectrum is commonly utilized for the analysis of inorganic oxidizers like ammonium nitrate, this study highlights the importance of low-energy phonons in determining the material’s crystal morphology.

How did the combination of confocal Raman microscopy and single-crystal X-ray diffraction enhance the identification and characterization of ammonium nitrate in the post-blast residue?

Monjardez: The integration of the optical properties obtained through microscopy with the spectral data provided by Raman spectroscopy and XRD was essential in determining the structural state of the AN crystals. The crystalline AN material obtained post-blast exhibited isotropic optical properties, while still retaining Raman scattering in the lattice vibration region of the spectrum. This indicated that the crystalline residues exhibit a slightly altered crystal structure compared to pure Phase IV ammonium nitrate. Single-crystal X-ray diffraction facilitated the determination of the unit cells and molecular structures of the crystals formed following the explosion and clarified our findings regarding the detailed atomic arrangement within a single crystal.

Christopher Zall: These XRD results strengthened our overall Raman and PLM data by confirming the crystallinity and overall Phase IV structure of the ammonium nitrate crystals. The crystals were (surprisingly, for post-blast residues) of X-ray quality (that is, single-crystalline). The unit cell metrics are generally consistent with a Phase IV structure, although there are statistical differences with previously measured structures. There is no clear connection between these unit-cell differences and any potential stresses or alterations to the Phase IV structure. It is more likely that they are because of experimental differences such as the ambient temperature used in data collection. However, these unit cell metrics do clearly rule out other phases of AN, such as Phase I. Similarly, the bond distances and bond angles in the molecular structure are in line with those of the known Phase IV structure, although there are again some statistical differences. Altogether, the X-ray data support the conclusions from the Raman and PLM data regarding the crystallinity and phase behavior, but the inability of XRD to identify any clear structural changes or stresses within the material highlight the utility of low-frequency Raman spectroscopy in analyzing these post-blast ammonium nitrate molecular and crystal lattice changes.

What were the key differences in the lattice vibrations of the isotropic crystalline material compared to the intact ANFO reference, and what do these changes indicate?

Monjardez: In two of the isotropic crystalline material fragments, a shift of several bands to higher wavenumbers was observed, along with the disappearance of the B_3g coupled translational and librational mode (139.62 cm⁻¹) in one of the crystals. Furthermore, the Raman spectrum of three crystals displayed a blue shift in several bands, including the B_1g libration, the N-O in-plane deformation, as well as the emergence of peaks at 109.5 cm⁻¹, 107.08 cm⁻¹, and 221.21 cm⁻¹, respectively. A general trend of peak shifting to higher wavenumbers was observed for the crystalline material in this study, which indicates a history of stress, either from heat or pressure, both of which are involved in an explosion. The small structural changes that give rise to these shifts may be the underlying reason for the observed abnormal optical properties of the post-blast residues.

Your study observed minor yet statistically significant variations in the unit cell dimensions and O–N–O molecular angles of Phase IV ammonium nitrate. What factors do you believe contributed to these structural changes?

Zall: It is unclear from the X-ray data whether these statistical differences correlate in any meaningful way to structural changes in the material. The statistical differences are merely based on the estimated errors in the bond distances and angles. Compared to recent, high-quality structures, we observe some differences that are outside the customary limit of three estimated standard deviations (ESDs), whereas other metrics are within the limits of “statistically equivalent” bond distances and angles. We certainly do not observe any qualitative changes in the XRD data that could be correlated with post-blast stresses to the crystal structure or phase. It seems likely that these statistical differences indicate limits to the use of arbitrary tests of “statistical equivalence” in structural analysis. More broadly, the limited utility of X-ray diffraction in assessing these post-blast structural changes highlights the importance of the low-frequency Raman spectroscopy in identifying changes within the crystal lattice. More work will have to be conducted to better understand the reasons behind the observed information.

How does the presence of stressed state Phase IV ammonium nitrate in the post blast residue impact our understanding of ANFO detonation chemistry and its forensic implications?

Jared Estevanes: The information obtained in this study suggests that the detonation of ANFO as an explosive charge can potentially affect the crystalline structure of any post-blast residue remaining after detonation. Future studies should evaluate if there is a correlation between the reaction conditions of the detonation and any structural stresses observed within post-blast residues, taking careful consideration of the optical properties of any crystalline material remaining.

Could you discuss the role of low-frequency Raman spectroscopy (10–250 cm⁻¹) in detecting phase changes in crystalline salts, and why it was particularly important in this study?

Monjardez: Previous literature has demonstrated that internal crystal strain and atom replacement within the crystal structure induce peak shifts in the Raman spectrum of any crystal analyte analyzed.

Estevanes: These structural changes were only suggested by a few data points from the XRD information that was outside of the statistical differences for the XRD data, but by no means was it confirmed. The low-frequency Raman data was particularly important in giving additional evidence to support the assertion of slight structural changes observed, as well as correlate the optical observations of the crystalline materials.

What challenges did you encounter in analyzing the post blast residue, and how did you ensure the reliability of the data obtained from the Raman and XRD analyses?

Monjardez: One of the challenges encountered in this study was ensuring that the crystals analyzed were representative of the entire sample and determining whether the crystalline phases were consistent throughout the entire residue. Although less than 2 mg of crystalline residue was analyzed, it was sourced from multiple fragments of the toolbox, rather than from a single location. Additionally, the XRD data was obtained from six distinct crystals, each sourced from different areas of crystalline residue observed on the toolbox. The XRD analysis revealed that the phase identified was consistent across all six crystals.

Estevanes: Another significant challenge was verifying that the crystalline material was optically isotropic, while simultaneously ensuring its suitability for single-crystal XRD analysis. This process involved extensively selecting many optically isotropic crystals and meticulously assessing their suitability. Although optically isotropic crystals gave clean diffraction patterns, without signs of twinning or polycrystallinity, only the largest crystals gave sufficiently strong data for analysis.

How do your findings contribute to the broader field of explosives analysis, and what future research directions would you suggest based on your results?

Monjardez: Previous literature on the phase transitions of ammonium nitrate has primarily focused on analytical methods conducted under controlled in-situ conditions. We are not aware of any published studies replicating the conditions described in our study, specifically those occurring after detonation.

Furthermore, no studies to our knowledge have described the detailed opticalchanges that may occur during heating or increased pressure, prior to the completion of the full phase transition. A potential direction for future research with forensic relevance could involve correlating optical and chemical properties of ammonium nitrate post-blast crystals with varying detonation conditions and fuel/oxidizer ratios, providing forensic examiners with valuable insights into the bomb's construction.

References

1. Estevanes, J.; Jernigan, N.; Zall, C.; Monjardez, G. The Characterization of the Lattice Vibrations of Ammonium Nitrate in ANFO Mixture After Authentic Detonations Using Confocal Raman Microscopy and Single Crystal X-Ray Diffraction. J. Raman Spectrosc. 2024, 56 (2), 146–154. DOI: 10.1002/jrs.6752
2. Sam Houston State University, Geraldine Monjardez. SHSU.edu. Available at: https://www.shsu.edu/academics/criminal-justice/about/directory.html?mode=view&item=343 (accessed 2025-02-19).