Under the Sea: Detection of Explosives Underwater Using Raman Spectroscopy

Secondary explosives such as 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) and 2,4,6-trinitrotoluene (TNT) were commonly used in the production of many types of munitions during World War II and still continue to be used, for example, 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) and pentaerythritol tetranitrate (PETN). After the war, numerous remaining munitions were disposed of by dumping them into the Baltic Sea. The same was true for the Black Sea, but due to a lack of documentation about these dumpsites, the awareness of hazards and knowledge about the scale of the problem have significantly increased in the last 30 years. After decades spent in salty water, the shells of said projectiles corroded, and currently, they pose a risk of environmental contamination and explosion. Because of this, new underwater detection methods that are reliable, quick, remote and, at the same time, insensitive to the external interference of sea water are needed. The detection of explosives used in projectiles can be performed using field Raman spectrometers. Raman spectroscopy is an analytical method that is non-invasive: it allows one to perform analysis without taking samples out of the projectile. Field detectors are water-resistant due to the frequent need for decontamination. Therefore, in this study, a ThermoScientific Gemini analyzer (Waltham, MA, USA) was used. The purpose of the study was to determine how the Raman spectra of pressed explosives are influenced by two main factors: the distance between the Raman probe and the sample and the type of medium that stands between the Raman probe and the sample. For the purpose of this study, four explosives: TNT, RDX, PETN, and HMX were tested through various layers of distilled water, tap water, silica dispersion, Black Sea water and, finally, Baltic Sea water.