Determination of polyethoxylated non-ionic surfactants using potentiometric flow injection systems. Improvement of the detection limits employing an on-line pre-concentration stage

In this paper, the universal determination of non-ionic surfactants (NIS) of the polyethoxylate type is attempted using flow-injection potentiometry. Two systems are proposed which use specifically developed tubular flow-through ion selective electrodes (ISEs). These are sensitive to NIS with an hydrophilic chain between 6 and 18 ethoxylate units, which are predominant species in the environment. An on-line pre-concentration system is designed with the aim of a possible application for the unattended monitoring of NIS in surface waters. This on-line pre-concentration is achieved by employing a column packed with a commercial solid phase extraction (SPE) sorbent for the enrichment and purification of the target analytes. The procedure outlined improves the detection limit of a direct system, decreasing it from 1 × 10-4 to 3 × 10-6 M by use of a pre-concentration volume of 40.0 ml and 200 μl of 75% acetonitrile in water as the eluent. Precision was estimated as 4% relative standard deviation (R.S.D.) (n = 25) for a 1 × 10-6 M (0.7 ppm) nonylphenol polyethoxylate with 12 ethoxylate units when 10.0 ml of sample are pre-concentrated. Finally, the on-line pre-concentration system is applied for the total NIS determination in environmental samples from Llobregat river basin of Barcelona area (NE Spain). © 2002 Elsevier Science B.V. All rights reserved.