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

摘要

A direct aqueous supercritical fluid extraction (SFE) system was developed which can be directly interfaced to an infrared spectrometer for the determiantion of oil in water. The technique is designed to provide an environmentally clen, autommated alternative to establised 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 establshed IR method and the SFE-FTIR method indicate that oil levels calcuated using an accepted equation which includes coefficients derived from reference hyderocarbon standards may results 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-lin silica gel clean-up prodcedure incorporated within the SFE-FTIR system enables the same water sample to be analysed for total oil content including vegetable oils and selectivley 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.