Design, characterization, and application of spectrometers for determination of low-level dissolved oxygen with small-volume sampling methodology Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5712m855c

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  • New analytical methods and instrumentation were developed for the determination of dissolved oxygen (DO) at low concentrations in water. The methods were based on monitoring the absorbance increase due to the oxidation of reduced indigo carmine (IC) by oxygen in the sample in different types of spectrometer cells. Spectrophotometer cells included a 1.2-cm vial cell and liquid core waveguide (LCW) cells with path lengths from 20 to 100 cm. The LCW based spectrophotometer consisted of a miniature peristaltic pump, a multi-port reactor, and a LCW cell (24-cm, Teflon-AF tubing) which were connected together in a flow loop and enclosed in a unique housing filled with a reductant solution. This configuration allows double containment of the components and purging of the solutions to reduce the oxygen contamination. Unique sampling methodology based on syringes or a unique micro-pump sampler and on double-septum caps enabled transferring of small-volume samples (1 mL or less) from septum-sealed bottles into the spectrometer cells containing IC with minimal atmospheric contamination. The new methodology was applied for the determination of low DO concentrations in prepared standards and anaerobic cultures under Fe(III)-reducing, sulfate-reducing and methanogenic conditions. A detection limit of 0.03 mg/L DO for a 0.5-mL sample was achieved with the 1.2-cm vial and N2 purging of the headspace in the cell. The novel use of hydrogen purging of the solution circulating through the LCW cell for continual reduction of IC with a Pt catalyst was demonstrated. This purging made it possible to negate the effects of oxygen diffusing into the reagent and oxidizing IC, to keep the absorbance on scale with the long pathlength, and to achieve a DL for DO of 0.03 mg/L with a 0.1-mL sample volume.
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