Development of an automated method for determining oil in water by direct aqueous supercritical fluid extraction coupled on-line with infrared spectroscopy

Abstract

A direct aqueous supercritical fluid extraction (SFE) system was developed which can be directly interfaced to an infrared spectrometer for the determination of oil in water. The technique is designed to provide an environmentally clean, automated alternative to established IR methods for oil in water analysis which require the use of restricted organic solvents. The SFE-FTIR method involves minimum sample handling stages, with on-line analysis of a 500 ml water sample being complete within 15 min. Method accuracy for determining water samples spiked with gasoline, white spirit, kerosene, diesel or engine oil was 81–100% with precision (RSD) ranging from 3 to 17%. An independent evaluation determined a 2 ppm limit of quantification for diesel in industrial effluents. The results of a comparative study involving an established IR method and the SFE-FTIR method indicate that oil levels calculated using an accepted equation which includes coefficients derived from reference hydrocarbon standards may result in significant errors. A new approach permitted the derivation of quantification coefficients for the SFE-FTIR analyses which provided improved results. In situations where the identity of the oil to be analysed is known, a rapid off-line SFE-FTIR system calibration procedure was developed and successfully applied to various oils. An optional in-line silica gel clean-up procedure incorporated within the SFE-FTIR system enables the same water sample to be analysed for total oil content including vegetable oils and selectively for petroleum oil content within a total of 20 min. At the end of an analysis the SFE system is cleaned using an in situ 3 min clean cycle.