Structural elucidation of dicarboxylated metabolites of nonylphenol polyethoxylate surfactants in surface waters by gas chromatography/tandem mass spectrometry

Persistent dicarboxylated metabolites of nonylphenol polyethoxylates (NPEOs), carboxyalkylphenoxy ethoxy carboxylates (CAPECs), have periodically been detected during gas chromatography/mass spectrometry (GC/MS) characterization of NPEO metabolites in samples of surface water and sewage effluent. An electron impact ionization (EI) MS study was conducted on two esterified CAPEC metabolites, CA₄P1EC isomers (where A₄ denotes a 4-carbonalkyl group plus one carboxylic group (C)). The most abundant characteristic ions were produced by benzylic cleavages to yield [M-87]⁺ and [M-101]⁺ ions, corresponding to ions of m/z 249 and 235, each of which is formed from one of two isomeric dipropyl esters CA₄P1EC. These two base-peak ions were selected as precursor ions to help elucidate the fragmentation mechanisms and support the proposed structures via tandem mass spectrometric (MS/MS) experiments. The two isomers have been tentatively identified as 4-(2-ethyl-2-methylacetic acid)phenoxyacetic acid and 4-(3,3-dimethylpropionic acid)phenoxy acetic acid, which give rise to the fragments at m/z 249 and 235, respectively.