Continuous sample recirculation in an opened-loop multicommutated ¯ow system Rui A.S. Lapa a,* , Jose  L.F.C. Lima a , Boaventura F. Reis b , Joa Äo L.M. Santos a a CEQUP/Departamento de Quõ Âmica-Fisica, Faculdade de Farma Âcia, Universidade do Porto, Rua Anõ Âbal Cunha, 164, 4050 Porto, Portugal b Centro de Energia Nuclear na Agricultura, Universidade de Sa Äo Paulo, Piracicaba SP, Brazil Received 27 April 1998; received in revised form 14 July 1998; accepted 9 August 1998 Abstract A continuous circulating multicommutated ¯ow system based on an opened-loop con®guration is proposed. The sample is inserted into the ¯ow system and continuously recirculated until a suitable dilution level is attained, which is permanently surveyed by including the detector in the circular reactor. A continuous removal of the highly dispersed front and trailing zones of the sample plug improves dilution ef®ciency. The chromogenic reagent used as carrier solution is permanently renewed, which contributes to a constant renewal of the reaction zone increasing the reaction rate by reducing its dependence on the dispersion at the sample/reagent interface. The developed methodology was tested in the determination of chloride in parenteral solutions by the formation of the iron (III) thiocyanate complex with expansion of the linear range of determination up to 10 000 mg l 1 . Results were reproducible (RSD <1.8%) and in agreement with those obtained by the conventional procedure. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Opened loop; Multicommutated ¯ow system; Sample recirculation; Dilution; Chloride determination 1. Introduction Continuous ¯ow techniques are based on the inser- tion of a sample plug into a continuous carrier stream of an appropriate composition, which is then trans- ported toward a suitable detector [1] and from there to waste. Typically, a ®xed-length reaction coil is used and a single analytical signal is attained with height or area related to the analyte concentration. These essen- tial and valuable aspects of the ¯ow methodologies have also some major limitations: an ineffective con- trol of dispersion and the placement of the detector at a de®ned site where it behaves as a static component. Consequently, the dilution level attained is ®xed and a single data point per determination is obtained. Several solutions were proposed to circumvent these problems, like the utilisation of closed loop con®gurations, multidetectors placed in serial or par- allel, relocation of the detector in the ¯ow manifold or exploitation of reversal ¯ow. Recirculation of the sample plug was accomplished by using ¯ow mani- folds with a closed loop con®guration, mainly for continuous solvent extraction or kinetic measure- ments. Ruzicka et al. [2] proposed an in-line concen- tration system by continuous extraction into an organic solvent in a closed loop whereas Iob and Mottola [3] and Roehring et al. [4] developed closed Analytica Chimica Acta 377 (1998) 103±110 *Corresponding author. Fax: +351-2-2004427; e-mail: laparuas@ff.up.pt 0003-2670/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0003-2670(98)00561-3