Development of cost-effective and sensitive spectrophotometric detection devices in flow analysis systems for ammonium/ammonia and sulfide/hydrogen sulfide determinations

Abstract

Commercially available instruments with high analytical performance are often expensive and limited to use by high budget users. Thus, the development of a cost-effective, sensitive, and rapid analytical device is important and necessary, especially for developing countries such as Thailand where budget limitations may be an issue for instrument use. Nowadays, environmental pollution is a significant concern that affects not only humans, but also a wide range of other living things. Inorganic pollutants, such as ammonium/ammonia, sulfide/hydrogen sulfide, can be found in either water or air. These pollutants often result from human activities. It challenges analytical chemists and environmental scientists to monitor the levels of these pollutants. In this research, cost-effective, sensitive, and rapid analytical devices were developed for determination of these inorganic ions or gases in environmental and clinical applications. In the first part, an environmentally friendly modified flow analysis carrier with a high refractive index (RI) liquid was developed as a cost-effective long-path liquid core waveguide (LCW) for absorption spectrophotometry. This home-made LED-photodiode based long-path colorimeter allows for highly sensitive chemical analysis without requiring a preconcentration unit. A low toxicity carrier such as syrup or ethanol was chosen to improve the sensitivity for determination of NH4+ using a butterfly pea flower extract reagent in a gas-diffusion flow injection (GD-FI) system. The proposed GD-FI system based on the syrup-modified LCW has a linear calibration range for NH4+ ion determination of 10-500 μM, with a detection limit (LOD) of 9.8 μM, repeatability of less than 2% RSD, and a throughput rate of 12 injections h-1. The proposed method was applied for the determination of NH4+ in wastewater and surface water sample with the % recovery in range of 99–104% and has the potential to be used with other natural or synthetic chromogenic reagents for the green chemical analysis approach. In the second part, a cost-effective long-path absorption LCW device was fabricated using a Teflon tube and a high RI medium comprising high concentration acid. This device was integrated into the GD-FI system for the sensitive detection of sulfide using the methylene blue (MB) reaction. The gas diffusion unit was able to eliminate interferences from several coexisting analytes in the samples. The proposed device provides a linear range of 0.4–20 μM sulfide with the LOD of 0.13 μM without preconcentration. It was successfully used to determine sulfide levels in canal water samples with 89-103% recoveries. In the third part, the fluorometric determination of NH4+ in air and exhaled breath condensate (EBC) were developed based on the fluorescence emission from a windowless falling drop (WFD) cell at the end of the tube, via a sensitive and selective reaction of NH3-OPA-SO32- in a flow injection analysis (FIA) platform. A low-cost USB microscope was used as a simple and cost-effective fluorescence sensor for the WFD cell. The proposed device provides a linear range of 0.5-10 μM of NH4+, with the LOD of 0.5 μM NH4+, and repeatability of 5.5% RSD (n=11). The signal to noise ratio for the fluorescence (FL) signal obtained by the USB microscope was approximately the same as that from a photodiode detector coupled to a flow-through cross cell. The device was successfully applied for the determination of NH4+ in EBC and air samples extracted into water, with 98-110% recoveries. In the fourth part, a non-suppressed open tubular capillary ion chromatographic (NS-OTIC) system with online dialysis was developed for a robust, easy-to-use, and cost-effective method for simultaneous small ions determination in drinks. The AS18 Latex columns on 25-μm bore fused silica capillaries were applied for the separation of mixed anions (Cl-, Br-, NO2-, and NO3-). High concentrations of anions (up to 100 mM) could be directly introduced into the NS-OTIC system without typical off-line dilution while maintaining chromatographic resolution of 1.1 or better for adjacent peaks. The linear relationship of the peak area and concentration of anions can be appreciated for the linear coefficient of determination (r2) of 0.995, with peak area reproducibility of 6% RSD or better. The method was successfully applied for the determination of anions in juice and tea samples with 90–119% recoveries.