An isocratic HPLC method for the determination of phenol and nitrophenols (4-nitrophenol, 2-nitrophenol, 4,6-dinitro-o-cresol and 2,4-dinitrophenol) has been developed and validated using 2-chlorophenol as internal standard (IS) and a monolithic column in tap water samples. Prior to HPLC, the method requires solid-phase extraction (SPE) using polymeric Lichrolut EN cartridges. The method development involved the study of methanol and acetonitrile as organic modifiers, pH and flow-rate using a Chromolith RP-18e (150 mm × 4.6 mm I.D.) column. After comparing the performance of the separations obtained with both organic modifiers, the optimum separation of these compounds was achieved using 50 mM acetate buffer (pH 5.0)-acetonitrile (80:20, v/v) as mobile phase, 3 mL min^-1 flow-rate and UV detection at maximum absorbance wavelength. Under these conditions, all analytes were separated (Rs > 2.0) in an analysis time of less than 3.5 min and the most important validation parameters were evaluated. The recoveries obtained in the accuracy test for all phenols studied were in the 90–112% range using a preconcentration factor of 40, and the intraday and interday precisions [expressed as coefficient of variation (CV)] were smaller than 15%. Finally, the proposed method was applied to wastewater samples from several industries.

Phenol and nitrophenols are important pollutants in water because of their wide use in many industrial processes as pesticides, insecticides, herbicides and synthetic intermediates.¹ Owing to their toxicity both the United States Environmental Protection Agency (EPA) and the European Union (EU) have included some phenols in their lists of priority pollutants. In addition, the 80/778/EC directive states a maximum concentration of 0.5 μg l^-1 for total phenols and individual levels under 0.10 mg l^-1 for drinking water.

Analytical techniques for phenol determination are mainly high performance liquid chromatography (HPLC) and capillary electrophoresis (CE) using ultraviolet (UV), electrochemical, fluorescence or mass spectrometry (MS) detection.^2–3 Gas chromatography (GC), usually after phenols derivatization, has also been used.⁴ However, it is very difficult, in general, to reach the quantification limits using the above combinations required for the direct determination of phenols in drinking water, and a preconcentration step in the analytical procedure is generally required.

For the SPE of phenols silica-based sorbents, such as C18, C8 and cyclohexil were used. Phenyl and cyano are other silica-based sorbents that have also been used for SPE of phenols in water samples, but none of them give breakthrough volume for phenol higher than 100 mL. Polymeric sorbents based on polystyrene-divinilbenzene (PS-DVB) have higher capacity for polar analytes because of the higher surface area exhibited by polymers and many of the commercially available have areas of 1000 m² g^-1 . In particular, the LiChrolut EN material allows at least 1 L of water with quantitative recoveries for phenol to be concentrated.⁵

The need for fast, high-resolution separations is sometimes required because of the increase in the number of samples analysed in routine analysis. It has made the columns evolve from a bed packed with porous particles to a straight rod of highly porous silica with a bimodal pore structure (monolithic columns). These columns possess a unique combination of very large internal surface area because of mesopores (13 nm) together with significantly higher total porosity (2 μm macropores) to transport mobile phase and analytes, reducing the diffusion path and providing high permeability (and thus low pressure). This behaviour allows the use of monolithic columns at flow-rates close to 9 mL min^-1 without problems and enables faster separations than with a standard column.^6–7 In addition, efficiency for monolithic columns does not decrease significantly when the flow-rate is increased because of their flow-through pores, thus diffusion path is reduced, resulting in a reduction in mass transfer effects. However, for traditional particulate columns, using high flow-rates, the efficiency decreases.⁸

In a previous article, a comparison of the performance of conventional microparticulates and monolithic reversed-phase columns for the HPLC separation of 11 pollutant phenols was reported.⁹ Taking this study into consideration, an HPLC method for the determination of phenol (P) and some nitro-phenols (NPs) using 2-chlorophenol (2CP) as internal standard (IS) and a monolithic column (Chromolith RP-18e) has been developed. In the method development, the effect of acetonitrile and methanol as organic modifiers, pH and flow-rate have been studied. From the comparison of the performance of the optimal separations obtained using both organic modifiers, acetonitrile was finally selected. This method was validated in tap water samples using SPE LiChrolut EN cartridges. Finally, the proposed method was applied to wastewater samples from automotive, paper mill and petrochemical industries.