Sources of error in the determination of non-ionic surfactants in environmental samples

Abstract

The analytical procedures for the determination of non-ionic surfactants (NS) consist of sampling, multistage separation and final determination. Each of these stages is a potential source of error. 33% of an initial amount of 100 μg NS may be lost due to adsorption on the walls of glass vessels during storage or 55% in the case of polyethylene vessels. Ineffective preservation of samples leads to very quick biodegradation of NS and incorrect results. Chloroform, frequently used for this purpose, is ineffective, whereas formaldehyde gives satisfactory results. Also during the separation procedure substantial fractions of NS are lost. The currently used methods enable the determination of NS having 5–30 oxyethylene subunits only. In the extraction with ethyl acetate NS having long oxyethylene chain remain in the water phase or are caught at the interface. If looked at carefully these analytical procedures show serious drawbacks. The washing of the precipitate in the BiAS procedure causes a dramatic loss of precipitate. The recommended use of G4 glass filters also leads to loss of precipitate. Instead, the use of G5 gives satisfactory results. The choice of one surfactant as the standard, necessary for the determination of the total concentration of a mixture of unknown composition, possibly consisting of several hundred of individual compounds, can be also a serious source of error. The higher is the difference between the slopes of the calibration curves of the mixture components, the higher is the error. Two newly developed techniques, namely the indirect tensammetric method (ITM) and the BiAS procedures combined with the ITM give better results than the procedures currently recommended.