Talanta 68 (2005) 245–252 Autoadaptative sequential injection system for nitrite determination in wastewaters M. Baeza a,b,∗ , J. Bartrol´ ı a , J. Alonso a a Sensors and Biosensors Group, Department of Chemistry, Facultat de Ci` encies, Edifici Cn, Universitat Aut` onoma de Barcelona, 08193 Bellaterra, Spain b Escola Universit` aria Polit` ecnica del Medi Ambient, Rbla. Pompeu Fabra 1, 08100 Mollet del Vall` es, Spain Available online 15 September 2005 Abstract A novel autoadaptative sequential injection system for the analysis of nitrite is described. The automatic determination uses a direct spectrophotometric method, based on the Griess–Ilosvay reaction. In this method the absorbance of the purple azo dye formed is measured at 555nm. In the sequential injection operation, the sample and the reagent are aspirated and mixed by reverse flow. The sequencing and overlapping of stacked (reagent) zones as well as selection of volumes have been studied in detail. The proposed analytical system is intelligent, simple and robust, allowing for nitrite determination in a double concentration range, by a simple and automatic programmable operation change. These two ranges are 0.0–3.0 and 0.0–20.0 ppm with detection limits being 0.048 and 0.4 ppm, respectively. Next surroundings have been developed allowing autocalibration and independent monitoring of nitrite concentration. The experimental set-up has been evaluated applying it to real samples analysis of very diverse concentration samples coming from a WWTP. The throughput of the method was 12 samples per hour. © 2005 Elsevier B.V. All rights reserved. Keywords: Autoadaptative analytical system; SIA; Intelligent system; Ion nitrite; Spectrophotometry; Schlieren effect 1. Introduction The increasing demand on process monitoring has led to new analytical systems, which can work during long periods of time with little maintenance and low reagent consumption. Flow injection analysis (FIA) fulfils those requirements and its simplicity and robustness helped in its introduction into the design of monitoring systems [1]. Additional improvements can be obtained exploiting the autoadaptative potentialities of the continuous flow techniques that can automatically modify its operational parameters according with the sam- ple requirements. However, conventional FIA analysers have been designed as closed and dedicated systems useful to work with very well defined sample compositions. When modifi- cations in the system features are needed, they can only be introduced after considering physical changes of the system parameters. This limitation prevents its operation in unat- ∗ Corresponding author. Tel.: +34 93 5796791; fax: +34 93 5796785. E-mail address: mbaeza@eupma.uab.es (M. Baeza). tended automatic conditions when high variability in analyte concentrations is expected. This disadvantage can be circum- vented easily by the use of an alternative flow technique called sequential injection analysis (SIA), which confers versatil- ity, robustness and high degree of autoadaptative abilities [2]. SIA techniques are based on sequential aspiration of small volumes of sample and reagents into a single channel fol- lowed by mixing during their transport to the detector. A simple manifold including a selection valve, a pump and a suitable detector can be adapted for many different chemi- cal determinations. All the operations related to the liquid handling can be automated using an automatic burette as liquid driver and a selection valve. Forward and backward movement of the syringe piston produces the aspiration and impulsion processes while the length of the piston stroke regulates the volumes of the liquids. The sequence of sam- ple/reagent zones used as well as additional unitary opera- tions (e.g., injection, washing) was controlled with the aid of the selection valve [3]. 0039-9140/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2005.07.038