A multisyringe flow injection method for the automated determination of sulfide in waters using a miniaturised optical fiber spectrophotometer.

In this paper, a fully software-controlled multisyringe flow injection (MSFIA) spectrophotometric system is proposed for the determination of sulfide in environmental and waste waters. The implementation of ancillary solenoid valves into the flow network allows a multitude of injection modalities to be explored, the selected modality being directly dependent on the aim of the assays. The multicommuted sandwich-type approach is introduced in this work as an efficient means to warrant high sensitivity for the particular assay with excellent repeatabilities and a considerable reagent saving. Moreover, a high injection frequency may be easily attained by carrying out a multiple injection modality during a single forward displacement of the piston driver bar. The interfacing of the robust and versatile multisyringe piston pump with an optical fiber plug-in spectrophotometer furnished with a light emitting diode enables the miniaturization of the flow analyzer, which is thus readily adaptable to in-situ and real-time monitoring schemes. The flow method is based on the coupling Fischer's reaction of sulfide with N,N-dimethyl-p-phenylenediamine in the presence of Fe(III) as oxidizing reagent in a 0.7M HCl medium. Careful selection of the physical and chemical variables enabled coefficients of variations better than 1.5% (n = 10) at the 1mgl(-1) level for both injection modalities. Dynamic working ranges of 0.2-2.0 and 0.5-5mgl(-1) sulfide for sandwich and multiple injection techniques, and detection limits of 0.09 and 0.15mgl(-1), respectively, were obtained. Furthermore, the sandwich modality featured an average slope of 0.43 +/- 0.02lmg(-1) calculated from 10 day-to-day calibration plots. This result reveals better sensitivity than other flowing stream methods described in the literature. The multiple injection technique allowed an improvement of the injection throughput up to 80h(-1), although a decrease of sensitivity was concomitantly observed (average slope of 0.17 +/- 0.01lmg(-1)). The multisyringe flow method was successfully applied to the determination of sulfide in different spiked water matrices (namely, mineral, tap, freshwater, seawater and wastewater) with recoveries ranging from 96 to 104%. Good agreement was also found in water samples between the MSFIA results and those of the batch APHA-standard method.