Diethylene glycol (DEG) is extensively used on offshore gas platforms to prevent the hydrate formation during the gas-water separation process and to inhibit corrosion events. This chemical might enter in the marine environment via the produced formation water (PFW) discharge. In this study, a new approach was applied to the investigation of the DEG content in PFW discharges and seawater samples from four gas installation platforms in the Adriatic Sea (Italy). The method includes an off-line solid-phase extraction/pre-concentration technique, followed by a nanoscale flow injection/direct-electron ionization (EI) mass spectrometric analysis. Direct-EI is a novel and miniaturized interface for directly coupling a liquid chromatograph with an electron ionization mass spectrometer. The capability to acquire EI spectra, and to operate in selected ion monitoring mode during actual sample analyses, allowed a precise quantification of DEG with a method limit of detection of 31microg/l. In addition, a careful evaluation of the matrix effect showed that, as opposed to electrospray ionization, the response of the Direct-EI interface was not affected by sample interferences.