Solvent-free oil-in-water analysis

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Non-ionic surfactants can homogenize oily, produced water samples, creating optically clear microemulsions that are ideal for fluorescence measurements [1]. The use of surfactants to reduce fouling and improve the performance of on-line, fluorescence-based, oil-in-water monitors has been reported previously [2,3]. This paper describes the use of surfactants to prepare discrete water samples (“grab-samples”) for analysis, eliminating the need for hazardous, organic extraction solvents. Two methods are described. The first is designed to report the dispersed oil content of a water sample. The second is designed to report dispersed oil concentration, and to allow the user to trend the concentration of water-soluble organics (WSO). Both procedures are optimized for the TD-500D Oil-in-Water Analyzer (Turner Designs Hydrocarbon Instruments, Inc.)

Introduction

Aromatic fractions of dispersed oil and water-soluble organics (WSO) found in produced water can be stimulated to emit fluorescent light. The process is illustrated in Figure 1. Excitation light is directed to a sample at a wavelength, λEX. The aromatic molecules in the sample absorb the excitation light and jump from their normal energy level (E0, ground state) to an excited energy state, E2. The excited molecules then lose some of their absorbed energy by a variety of mechanisms (relaxation) and go to a lower energy state, E1. The molecules then drop back down to E0 by emitting a photon of fluorescent light at a wavelength λEM. The energy emitted by fluorescence (E1 – E0) is lower than the energy gained by absorption (E2 – E0). Since light energy is inversely proportional to wavelength, the wavelength of the fluorescent light, λEM, is always longer than the wavelength of the excitation light, λEX. The intensity of the fluorescence emission, IF, is proportional to the concentration of the fluorescent molecules in the sample.

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