17 Widespread use of organophosphorus flame retardants (OPFRs) and their ubiquity 18 in waters results in the need for a robust and reliable monitoring technique to better 19 understand their fate and environmental impact. In situ passive sampling using the 20 diffusive gradients in thin-films (DGT) technique provides time-integrated data and is 21 developed for measuring OPFRs here. Ultrasonic extraction of binding gels in methanol 22 provided reliable recoveries for all tested OPFRs.

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... coefficients of TCEP, TCPP, 23 TDCPP, TPrP, TBP, TBEP and TPhP in the agarose diffusive gel (25 o C) were obtained. 24 The capacity of an HLB binding gel for OPFRs was >130 μg per disc and the binding 25 performance did not deteriorate with time up to 131 d. DGT performance is independent 26 of typical environmental ranges of pH (3.12-9.71), ionic strength (0.1-500 mmol L -1 ) 27 and dissolved organic matter (0-20 mg L -1 ), and also of diffusive layer thickness (0.64-28 2.14 mm), and deployment time (3-168 h). Negligible competition effects between 29 OPFRs was found. DGT-measured concentrations of OPFRs in a wastewater treatment 30 plant (WWTP) effluent (12-16 d) were comparable to those obtained by grab sampling, 31 further verifying DGT's reliability for measuring OPFRs in waters. 32