Multi-Syringe Flow Injection System with In-Line Pre-Concentration for the Determination of Total Phenolic Compounds

Abstract.

A multi-syringe flow injection system for the determination of total phenolics based on the 4-aminoantipyrine reaction is proposed. In-line preconcentration using solid-phase extraction was implemented, offering an environmentally friendly alternative to organic solvent extraction performed in the batch procedure and in previously described flow methodologies. Therefore, phenolic compounds were retained in Amberlite XAD4 resin using acidic conditions. The elution was carried out using 0.10 mol L⁻¹ NaOH solution, promoting the ionisation of the retained phenolic compounds, and thus their desorption.

Since different elution profiles were obtained for several phenol derivatives, the calibration curves were established using the peak area as analytical signal. Moreover, a dynamic calibration range was applied by using a variable sample volume (0.500–6.00 mL) and calibration curves based on the mass of phenol retained in the pre-concentration column. A methodology based on a fixed sample volume (24.0 mL) was also proposed for determination at concentration levels below 50 µg L⁻¹. A limit of detection of 52 ng of phenol (9 or 2 µg L⁻¹) was achieved with a sampling frequency of 10–16 h⁻¹ for the variable volume method and 4 h⁻¹ for the fixed volume method. The application of the proposed methodology to reference material provided results that were within the certified acceptance limits.