Identification of Blue Discoloration in PBX 9404 using Ultrahigh Pressure Liquid Chromatography with Quadrupole Time‐of‐Flight Mass Spectrometry

Abstract

PBX 9404, composed of 94 wt% HMX (1,3,5,7‐Tetranitro‐1,3,5,7‐tetrazoctane), 3 wt% NC (nitrocellulose), 2.9 wt% CEF (tris(beta‐chloroethyl) phosphate) and 0.1 wt% DPA (diphenylamine), has often been observed with a slight to entire blue discoloration of the manufactured prills. This blue discoloration has been observed since the formulation development of PBX 9404 in the late 1950s. In order to determine the source of the blue discoloration in PBX 9404, ultraviolet/visible (UV/Vis) absorbance of PBX 9404 and two potential candidates for the discoloration, p‐nitrosodiphenylamine (p‐NDPA) and N‐nitrosodiphenylamine (N‐NDPA) were evaluated. An absorbance band at 590 nm in PBX 9404 sample was attributed to the blue discoloration while p‐NDPA and N‐NPDA had different absorbance bands. Using the knowledge gained from the UV/Vis analysis, a semi‐targeted approach using ultrahigh pressure liquid chromatography coupled to UV/Vis detector and quadrupole time of flight mass spectrometry (UHPLC‐UV/Vis‐QTOF) was implemented. Investigating the UHPLC‐UV/Vis‐QTOF chromatograms at a wavelength of 590 nm, a single compound was tentatively identified as being the source of the blue discoloration. Targeted tandem mass spectrometry (MS/MS) on the compound and subsequent analysis indicated that 4,4′‐{[4‐(Phenylimino)‐2,5‐cyclohexadien‐1‐ylidene]methylene}bis(N‐phenylaniline), also known as Spirit Blue Base, was the source of the blue discoloration, which was confirmed using a standard. A theory on the formation of Spirit Blue Base in PBX 9404 involving DPA, nitric acid (HNO3) / nitrogen oxide(s) NOx, and formaldehyde is proposed.