Determination of weakly acidic endocrine-disrupting compounds by liquid chromatography–mass spectrometry with post-column base addition

A sensitive analytical method based on liquid chromatography–electrospray ionisation mass spectrometry (LC–ESI-MS) has been developed for the determination of seven endocrine-disrupting compounds: 4- n-nonylphenol (NP), 4- tert-butylphenol ( t-BP), bisphenol A (BPA), 2,4-dichlorophenol (DCP), 2,4,5-trichlorophenol (TCP), pentachlorophenol (PCP) and 4- tert-butylbenzoic acid (BBA) in water samples. To achieve a good LC separation, acidification of the LC mobile phase was necessary, but this led to MS signal suppression for the less acidic compounds. In order to enhance the sensitivity for these analytes, post-column addition of different bases such as ammonia, trimethylamine, and 1,8-diazabicyclo-(5,4,0)undec-7-en (DBU) was evaluated. The post-column addition of base is proposed here to raise effluent pH, helping in the ionisation process of the compounds with higher p K a values ( t-BP, BPA, DCP and NP). The use of DBU, diluted in MeOH, proved to be the most efficient post-column reagent for enhancing the MS signal. The signal-to-noise ratios for t-BP and NP increased by more than 200-fold and 35-fold, respectively, whereas for DCP and BPA an increase of about 10-fold was achieved. This strategy permitted direct determination of the seven compounds at low ppb levels. For application to real water samples, an extraction and preconcentration step using the solid-phase extraction (SPE) technique was carried out. The applicability of three solid-phase materials—Bond Elut C 18, and two polymeric sorbents: LiChrolut EN and Oasis HLB—and the optimization of other SPE parameters such as the elution solvent and sample volume used, were studied in order to maximize extraction efficiency. Oasis HLB provided the best results, obtaining—with the proposed SPE procedure—satisfactory percentage recoveries for all compounds (70–110%) with the exception of NP, for which a recovery of 54% was achieved. Application of the whole method, SPE–LC–(ESI)-MS, to natural waters permitted low nanogram-per-liter determination of all seven compounds.