Development of a Ship-Based Membrane Inlet Photoionization Mass Spectrometer (MI-PIMS) for Online Detection of Explosives in Seawater

Toxic nitro organic compounds, such as trinitrotoluene (TNT), released from dumped ammunition in coastal waters pose a threat to humans and the marine environment. Ship-based real-time screening of ammunition dumpsites requires online trace analysis of dissolved nitroaromatic explosives and their degradation products. We introduce a membrane inlet mass spectrometry approach (MIMS), using a laser based photofragmentation/(1 + 1)-Resonance-Enhanced Multi-Photon Ionization (REMPI) process, which forms nitrous oxide marker fragment ions (NO + ). The ionization process enables simultaneous detection of all dissolved nitro organic compounds via NO +; thus, organic nitro-groups not only from the bulk explosive parent molecule (e.g., TNT) themselves but also from propellants, primers, and TNT degradation products contribute to an integrated detection reading. Online sampling of dissolved organics is performed by a hollow polydimethylsiloxane (PDMS) fiber membrane inlet. Laser parameters for the photofragmentation/REMPI detection of explosives were tested by a tunable Nd/YAG OPO laser unit, and analytical figures of merit were determined. At a wavelength of 213 nm, detection limits in the suitable range of 300 ng/L–32 μg/L were achieved for nitro organics. The developed demonstrator unit was deployed for a field test in the framework of the AMMOTRACe EU project on board a German research vessel, targeting the “Kolberger Heide”, a known ammunition dumpsite in the Baltic Sea. A successful detection of explosives from the seabed was achieved, proving the feasibility of this detection technique. Furthermore, the results are paving the way toward the development of a submersible REMPI-MIMS system for in situ monitoring of underwater threads.