Infrared Reflectance Spectroscopic Evaluation of Inkjet Printed Standards of Cyclotrimethylenetrinitramine (RDX) on Aluminum Substrates

The Department of Defense (DOD) and first responder communities are evaluating and developing optical systems for the detection and identification of explosives and components used for assembling homemade explosives (HMEs). Emerging detection technologies must be evaluated with authentic hazard material concentrations to ensure their accurate and reliable use in the field. In this work, infrared (IR) reflectance spectra over the spectral rage of 1000–1700 cm^–1 were collected for different concentrations of inkjet-printed RDX (cyclotrimethylenetrinitramine) samples deposited onto aluminum substrates. A plot of the integrated area of both the symmetric and asymmetric NO₂ vibrational bands for RDX on aluminum exhibited good linearity over the concentration range 20–500 µg/cm². Detection limits for RDX on an aluminum surface were calculated to be 10.7 µg/cm² for the symmetric NO₂ vibrational band and 1.4 µg/cm² for the asymmetric NO₂ vibrational band. Evaluation of the NO₂ vibrational band areas at different locations of the RDX array demonstrated that the samples exhibited good homogeneity across the surface. The concentration of an unknown sample of RDX on aluminum was determined using the fitted equations; results showed good agreement between the calculated and actual RDX surface concentration. The lot-to-lot variation of RDX on the aluminum surface was compared using the long wavelength infrared (LWIR) spectral band areas for two different lots of standards printed at the same RDX surface concentration. Results showed excellent lot-to-lot agreement indicating good reproducibility of the standards for RDX.