Dual‐Solvent SPE‐UPLC–MS/MS Method for Quantifying 17 Organophosphate Flame Retardants in Environmental Waters

Rationale Organophosphate esters (OPEs) have emerged as dominant flame retardants following the phase‐out of brominated alternatives, highlighting the need for accurate quantification due to their associated toxicity. Comprehensive analysis of OPEs, however, is challenging due to the wide polarity range of congeners and issues with co‐elution. To address these challenges, we developed an ultra‐performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) method for the simultaneous quantification of 17 OPEs. Methods Using an ACQUITY UPLC BEH C 18 column with sub‐2‐μm particles, we achieved baseline separation of critical isomer pairs, including TPrP/TiPP and TBP/TiBP. Furthermore, a novel dual‐solvent sequential elution strategy, employing methanol and ethyl acetate for solid‐phase extraction (SPE), significantly improved recovery rates. Results Recovery of the highly polar TMP increased from 14.0% to 33.2%, while that of the hydrophobic TEHP improved from 41.0% to 103.2%. This dual‐solvent approach enabled quantitative recovery (33.2%–153%) for all 17 OPEs, compared to successful recovery of only six compounds using traditional single‐solvent protocols. The method demonstrated detection limits ranging from 0.01 to 2.61 ng/L, with 82% of compounds achieving limits below 0.2 ng/L. Validation across three concentration levels (4–40 ng/L) yielded mean recoveries of 73.5%–109%, with relative standard deviations (RSDs) ranging from 1.0% to 13.0%. Application of this method to surface water and wastewater treatment plant (WWTP) samples detected 11 OPEs at concentrations ranging from 0.27 to 11.41 ng/L, with TBEP exhibiting a 100% detection frequency. WWTP removal efficiencies demonstrated property‐dependent trends: hydrophobic compounds (log K ow > 3.5) were removed by > 99%, whereas hydrophilic OPEs, such as TMP and TCEP, showed limited removal, with 88%–100% persistence. Conclusion This newly developed method provides a robust analytical foundation for environmental monitoring and risk assessment of OPEs.