A Green Analytical Tool for In-Process Determination of RDX Content of Propellant Using the NIR System

The conventional approach (high performance liquid chromatography, HPLC) used for determination of the hexahydro-1,3,5-trinitro-1,3,S-triazine (RDX) content of a solid propellant intermediate consumes toxic reagents, produces hazardous wastes, and cannot afford in-process monitoring at military plants. This work investigated the feasibility of using a near-infrared (NIR) system as a green alternative in lab scale. All raw materials were obtained from a propellant plant, and a fiber optic probe interfaced with a Fourier transform NIR (FT-NIR) spectrometer was directly inserted into samples that were continuously stirred to record in situ spectra. A quantitative chemometrics model based on the characteristic spectral regions of RDX was established using a partial least squares (PLS) regression algorithm. The developed model was assessed by external validation with the coefficient of determination (R-2 = 0.948) and root-mean-square error of prediction (RMSEP = 0.236). The proposed method was confirmed with samples covering the full concentration of a calibration set over a period of several months, and it exhibited good performance on reproducibility and repeatability. In addition, HPLC analysis was carried out to compare with the developed NIR method, and no significant difference between the NIR and HPLC values was obtained according to the result of a paired t-test. Using our proposed NIR method, there was no chemical reagent consumed and no waste formed during the overall prediction procedure. The results show that the NIR system is inherently rapid, clean, capable of preventing hazardous residuals originating from traditional analytical procedure, and has the potential for monitoring RDX content during solid propellant processing.