Application of approach to equilibrium analytical ultracentrifugation (Archibald method) to the determination of molecular weights of humic substances.

Huniic substances (fulvic and humic acids) are the decomposition products of plant material and constitute a major part of the earth's organic carbon [ 11. Their presence in aquatic ecosystems is important because of their binding capacity for protons. metal ions (geocycling) and organic micropollutants such as pesticides and herbicides [2]. The characterisation of humic substances in ternis of niolecular weight and molecular weight distribution is a difficult problem due both to the width of the molecular weight distributions in humic samples, (several hundreds up to millions) [3], and to the tendency of huniic substances to associate in aqueous media. In a recent study [4] the technique of analytical (UV scanning) ultracentrifugation was applied to the characterisation of a range of humic samples from peat and surface water. This technique usefully exploits the large extinction coefficients of hutnic substances in the UV spectral region allowing iiieasurenients to be made at very low concentrations (45 nig 1-l). comparable t o those found in natural waters. The method has limitations for samples of high molecular weight as not all the saniple is contained in the measurable concentration gradient (OD 0-1 ), and hence the information relates only to the lower end of the molecular weight distribution. In an attempt to overcome this problem we have investigated the application of the approach to equilibrium (Archibald) method to humic samples. This method depends on the fact that the conditions for sedimentation equilibrium are realized at all times at the ends of a fluid column. Hence if a measurement of the concentration (c) and concentration gradient (dcldr) can be made at those positions the weight average molecular weight (MB,) can be calculated after a short period of centrifugation [S]. Thus from measurements made at the meniscus (m) , M,. can be calculated from the equation :-