A robust method for the determination of mineral oil in water samples - Pat Sandra and Frank David  

Introduction

Mineral oil is a complex mixture of semi-volatile hydrocarbons, including alkanes, alkenes and aromatics. These compounds can be determined in water samples by gas chromatography after liquidliquid extraction. The hydrocarbons are not measured as individual solutes, but as a sum parameter, often called the hydrocarbon oil index (HOI). In the past, methods based on liquid-liquid extraction using a halogenated solvent followed by quantitative determination by Fourier Transform Infra-Red spectroscopy (FTIR) were used for the determination of mineral oil in water. These methods are currently being replaced by liquid-liquid extraction using a volatile hydrocarbon solvent, followed by clean-up on Florisil and capillary GC-FID analysis. All hydrocarbons are measured that are not retained on Florisil (non-polar fraction) and that elute on an apolar stationary phase between ndecane (C10H24, boiling point 174°C) and ntetracontane (C40H82, boiling point 525°C). This fraction is defined as mineral oil. Typically concentrations between 0.1 mg/l and 1 mg/l are determined. This method complies with ISO 9377-2 and ISO 9377-4 [ISO 9377-2, Water Quality, Determination of hydrocarbon oil index, Part 2: Method using solvent extraction and gas chromatography, reference number ISO 9377-2:2000, ISO, Geneva, Switzerland (www.iso.ch)]. The method can be applied to drinking water, surface water, waste water and effluents from waste water treatment plants. The method is not applicable to the analysis of volatile hydrocarbons (boiling point < n-decane).

General procedure and analysis flow scheme

The water sample is extracted with an non-polar hydrocarbon solvent. Polar compounds that are coextracted are removed by column chromatography over Florisil. The non-polar fraction, not retained on Florisil, is concentrated and analysed by capillary GCFID. The total peak area between n-decane and ntetracontane is measured and the hydrocarbon oil index is calculated using an external standard consisting of a mixture of two specific mineral oils.