Validation of an enzyme-linked immunosorbent assay (ELISA) for the determination of 4-nonylphenol and octylphenol in surface water samples by LC-ESI-MS

4-Nonylphenol (NP) and octylphenol (OP) were measured by direct ELISA in both laboratory-fortified and surface water samples collected monthly from 10 rivers. In this procedure, samples were concentrated by solid phase extraction (SPE) using Lichrolut RP-18 sorbent with good recoveries obtained for both LC–MS and ELISA, giving a low level of detection (LOD) at the range of low μg L−1 and good reproducibility. Analysis of 40 surface water samples demonstrated that the ELISA may be a useful screening tool for the determination of the alkylphenols in surface water matrices. The concentration of NP and OP in surface waters ranged from 0.11 to 6.58 μg L−1. A good correlation of results obtained by ELISA and LC–MS within the concentration range of 0.08–6.86 μg L−1 was found in the river samples [R2 = 0.924, n = 39]. The influence of various factors on assay determination was also discussed.