Development of a predictive model for calculation of molecular weight of humic substances

Size exclusion chromatography (SEC) is a powerful tool for the determination of molecular size of compounds. However, the conducted SEC analysis of 35 humic substances (HS) samples demonstrated, that it has some severe pitfalls fur the molecular weight characterization of HS resulting from a lack of appropriate standards. To overcome this problem. a new approach to calculation of molecular weight from the SEC data was developed. It is based on the correction of the experimentally determined distribution coefficient (K-d) values according to the predictive equations derived tither from the data on charge density and hydrophobicity of the analyte or from the values of its characteristic structural parameters. Verification of both predictive equations using model compounds has displayed much better prognostic potential of the polynomial equation developed with a use of integral structural parameters of the analyte. Application of this predictive equation to synthetic polyelectrolytes gave satisfactory estimates for all the polymers with K-d>0.15. The model calculation for an averaged sample of HS allowed to estimate a difference between the ideal and experimental values of K-d(Delta K-d) by a value of 0.02. The correspondent correction of the SEC-data of 35 HS samples showed overestimation of the values of molecular weight of HS determined by the standard data handling technique. The obtained model estimations are to be verified along with the further development of the model based on the adjustment of the model assumptions to specific polyelectrolytic properties of HS. (C) 1998 Elsevier Science Ltd. All rights reserved.