Discrete opto-fluidic chemical spectrophotometry system (DOCSS) for online batch-sampling of heavy metals and fabrication of dithizone based evanescent wave optical fiber sensor

Justus Chukwunonso Ndukaife, Purdue University

Abstract

Dissolved heavy metals constitute a very dangerous class of water-borne chemical pollutants, consumption of which could threaten human lives and burden the health-care system. Therefore frequent monitoring of these pollutants is very vital to prevent consumption of contaminated water and ensure the sustainability of people. Current standard instruments such as inductively-coupled plasma mass spectrometer, atomic fluorescence spectrometer and high-performance liquid chromatographer are bulky, expensive, require trained personnel, and are thus not suitable for such frequent monitoring need. This thesis presents a low-cost discrete opto-fluidic chemical spectrophotometric system (DOCSS) for online monitoring of heavy metals in water. The DOCSS device is composed of three main components, fluidic, optical and automation. The fluidic system for reagent handling is based on gravity-driven flow, which was modeled using the Bernoulli's equation in fluid mechanics. Because under gravity-driven flow condition, the flow-rate decreases with decreasing liquid level, a novel time series formula was developed to ensure constant volume dispensing irrespective of fluid level. The dispensing time series can be used to ensure uniform volume of reagent was dispensed as sampling progresses, under gravity-driven flow. A special design was employed in the design of the flow-cell. Based on the design, it is possible to determine optically the volume of test sample introduced into the test cell. In essence the cell is a non-invasive volume sensor, and the volume sensing principle was based on light scattering by entrapped water droplets, and was modeled via the geometrics optics approach (GOA) of light scattering theory. Thus the need for high precision syringe pumps for sample introduction was eliminated, lowering the cost of the device. The DOCSS device was tested for detection of four selected contaminants namely: chromium (VI), Mercury (II), Vanadium (V) and nitrite using their respective sensing reagents. Further evaluation of the device performance using chromium (VI), revealed very good stability in the absorbance readings, and very good precision of 3.04% RSD at 50ppb Cr(VI). In summary, this technology is deemed to be a low-cost and reliable solution for in-situ sequential monitoring of the presence of multiple dissolved contaminants such as heavy metals in-situ in both static and dynamic water system. The thesis also reports the fabrication of a dithizone-based evanescent wave sensor for ammonia sensing in water and head-space. In the course of this work, dithizone- a widely used chelating reagent for heavy metals was observed to change color on protonating and deprotonating. This phenomenon was exploited in development of an evanescent wave sensor for ammonia sensing, and the sensor was found to be capable of discriminating its response to ammonia from its response to acidic gases based on a spectral red-shift in the optical intensity spectrum.

Degree

M.S.E.

Advisors

Nnanna, Purdue University.

Subject Area

Analytical chemistry|Electrical engineering|Optics

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