Development and application of a shipboard method for spectrophotometric determination of nanomolar dissolved sulfide in estuarine surface waters using reverse flow injection analysis coupled with a long path length liquid waveguide capillary cell

Portable instrument based on color sensor chip for on-site rapid detection of dissolved sulfides in environmental water samples

To ensure real-time validity of the detection of unstable toxic environmental pollutants, such as dissolved sulfides, we developed a portable on-site rapid analysis instrument. Through novel design of the color sensor chip-based core sensing components and the conversion between color signal and absorbance by Lambert's law, the instrument showed great performance for rapid (within 3 min) and sensitive on-site detection of sulfides in the environment. It is easy to achieve user-friendly, sample in-answer out, one-stop operation due to the touch-screen-integrated user interface of the instrument's data terminal. The detection limit of this method is 2.24 μg/L, the linear operation range is 0-1000 μg/L, and the coefficient of determination is 0.999. This instrument has been successfully applied to the on-site determination of sulfides in the Yellow River Delta and the Yantai Guangdang River in China. The portable instrument showed excellent anti-interference, good stability, and simple operation, which showed great prospects for the on-site rapid analysis of unstable targets in the environment.

On-site and high-resolution spectrophotometric measurement of total dissolved sulfide in natural waters

Reliable high-resolution data is essential for understanding the aquatic sulfur biogeochemical processes. However, the accurate quantification of total dissolved sulfide (TDS) remains challenging due to its low concentration and vulnerability to oxidation. Furthermore, the frequency and the spatial coverage of TDS measurements are constrained by the cost of the laboratory analysis. In this study, an automated portable system was developed for on-site real-time measurement of trace TDS in natural waters. This system was based on the classic methylene blue (MB) spectrophotometric assay combined with on-line solid phase extraction (SPE) and flow injection analysis (FIA). A commercially available weak-cation-exchange cartridge was used as the SPE sorbent. Experimental parameters affecting the performance of the proposed system were optimized. Under the optimized conditions, linear calibration range of 0.02-2.50 μmol L^-1 was obtained with a sample loading volume of 5.0 mL and a sample throughput of 12 h^-1. The limit of detection could be lowered to 0.003 μmol L^-1 by pre-concentrating 10.0 mL sample. The precision, determined as the relative standard deviation (RSD), was <2.75 % (n = 11) and the recoveries from spiked samples ranged from 54.4 % to 97.5 % with RSDs of 1.1-2.3 % (n = 3). Furthermore, the FIA-SPE-MB system was successfully deployed in the Taihu Lake for continuous 48 h monitoring of variations in TDS, demonstrating the applicability of this system for on-site TDS measurement in natural waters.

Improving the measurement of total dissolved sulfide in natural waters: A new on-site flow injection analysis method

Total dissolved sulfide (TDS) plays multiple important roles in the aquatic environments. However, the determination of trace levels of TDS in natural waters is challenging because TDS is vulnerable to oxidation and volatilization. In this study, a fully automated flow injection analysis spectrophotometric system, incorporating a hollow fiber membrane contactor (HFMC) and a long path length liquid waveguide capillary cell, was fabricated to facilitate the on-site measurement of trace TDS in natural waters. The HFMC was used for matrix separation and analyte preconcentration. The measurement was based on the reaction of sulfide and N,N-dimethyl-p-phenylenediamine in the presence of FeCl₃ under acidic conditions to yield methylene blue (MB). The proposed method was highly sensitive, with detection and quantification limits of 0.57 and 1.90 nmol L^-1, respectively. The linear working range was from 1.90 to 150 nmol L^-1, with a correlation coefficient of 0.9995. The repeatability, expressed as the relative standard deviation, was less than 0.86% (n = 15) and the recoveries varied from 76.2 ± 0.1% to 103.9 ± 0.6% (n = 3) for spiked samples. This method was applied to conduct a field analysis of TDS in a reservoir, giving results aligned with those obtained using a standard MB method. This work demonstrates that the new method for determining TDS was effective for both laboratory analysis and on-site measurement.

Flow-Injection Methods in Water Analysis-Recent Developments

Widespread demand for the analysis and control of water quality and supply for human activity and ecosystem sustainability has necessitated the continuous improvement of water analysis methods in terms of their reliability, efficiency, and costs. To satisfy these requirements, flow-injection analysis using different detection methods has successfully been developed in recent decades. This review, based on about 100 original research papers, presents the achievements in this field over the past ten years. Various methodologies for establishing flow-injection measurements are reviewed, together with microfluidics and portable systems. The developed applications mostly concern not only the determination of inorganic analytes but also the speciation analysis of different elements, and the determination of several total indices of water quality. Examples of the determination of organic residues (e.g., pesticides, phenolic compounds, and surfactants) in natural surface waters, seawater, groundwater, and drinking water have also been identified. Usually, changes in the format of manual procedures for flow-injection determination results in the improvement of various operational parameters, such as the limits of detection, the sampling rate, or selectivity in different matrices.