Molecular size fractionation

In homopolymer analysis this meant a closer study of the accuracy and reproducibility of data from GPC to see how resolution correction techniques could be either circumvented or practically applied. In copolymer analysis the limitation of conventional molecular size fractionation emerged as the fundamental difficulty. An orthogonal coupling of GPCs operated so as to achieve the desired cross fractionation before detection is presented as a novel approach with considerable potential. 

Bunzl K, Kracke W, Agapkina GI, et al. 1998. Association of Chernobyl-derived 239+240Pu,241 Am, 90Sr and 137Cs with different molecular size fractions of organic matter in the soil solution of two grassland soils. Radiat Environ Biophys 37 195-200. 

Because NMR spectroscopy is a nuclei-specific technique and has the ability to distinguish between similar compounds, it is an excellent method for identifying similar species in complex matrices. Thus, 31P FT-NMR spectroscopy is ideal for the identification and characterization of the hydrosphere DOP. Even so, NMR spectroscopy is fairly insensitive and requires high sample concentrations. Low DOP concentrations are increased to 31P FT-NMR detection limits by using ultrafiltration and reverse osmosis membranes. Not only is the DOP concentrated, but it is fractionated according to its molecular size. Compared to other concentration and molecular size fractionation techniques for DOP, ultrafiltration and reverse osmosis are relatively rapid and easy. 

Fig. 2 Two methods of sample preparation selectivity by polarity or by molecular size. Fractionation by polarity, e.g., using Florisil (vertical fraction), selects a limited range of the pesticides but does not remove high-molecular-weight materials of similar polarity. GPC (horizontal fraction) removes primarily material of high molecular weight, leaving all the pesticides (and other compounds of similar molecular weight) in the fraction. (From Ref. 19.)...

Dick, D. P, and Burba, P. (1999). Extraction kinetics and molecular size fractionation of humic substances from two Brazilian soils. J. Braz. Chem. Soc. 10(2), 146-152. 

Richard, C., Trubetskaya, 0.,Trubetskoj, O., Reznikova, O., Afanaseva, G. V., Aguer, J. P, and Guyot, G. (2004). Key role of the low molecular size fraction of soil humic acids for fluorescence and photoinductive activity. Environ. Sci. Technol. 38, 2052-2057. 

Sargentini, E., Jr., Rocha, J. C., Rosa, A. H., Zara, L. F., and dos Santos, A. (2001). Aquatic humic substances Molecular size fractionation and characterization of inner rearrangements after metal ions complexation. Quimica Nova 24(3), 339-344. 

Shin, H.-S., Monsallier, J. M., and Choppin, G. R. (1999). Spectroscopic and chemical characterizations of molecular size fractionated humic acid. Talanta 50, 641-647. 

Distribution of Molecular-Size Fractions (Dalton) of DOM Derived from Various Organic Wastes (% of Total DOM)... 

Figure 2 shows the size distributions for a typical GPC prep separation. A gradual decrease in mean sizes for Cuts 1-5 is observed, where Cut 1 is eluted first from the column and contains the largest molecules. As the figure shows, the preparative method gives a good separation of the sample into different molecular size fractions. 

Reuter J. H. and Perdue E. M. (1981) Calculation of molecular weights of humic substances from colligative data application to aquatic humus and its molecular size fractions. Geochim. Cosmochim. Acta 45, 2017-2022. 

Figure 4. The behavior of molecular size fractions vs. time. The integrals were determined by cutting and weighing the GPC traces of Figure i, (left column) in the six fractions shown and setting the sum of each 150-19,000...

Electrophoretic separations of solutes are determined primarily by the mass-to-charge ratio of the solute. However, certain electrophoretic separations use gel or paper supports for the electrolyte, and adsorptive and molecular-size fractionations may be as significant as electrophoretic fractionation in these systems. Hall (1970) found that polyacrylamide gel electrophoresis of aquatic humic substances gave size fractionations corresponding to Sepha-dex gel fractionation however, individual fractions were poorly resolved in polyacrylamide gel electrophoresis. Clapp (1957) and Mortensen and Schwendinger (1963) used electrophoresis with electrolyte supporting curtains to separate colorless polysaccharides from humic substances in soil-water extracts. 

Sephadex is a cross-linked dextran polysaccharide, which is prepared by reacting dextran with epichlorohydrin. It is available in several different grades depending on the degree of cross-linking. These different grades of Sephadex allow one to carry out molecular size fractionation experiments over a wide range of molecular sizes. 

Hong, J. K. and Yamane, I. (1981). Distribution of inositol phosphate in the molecular size fractions of humic and fulvic acid fractions. Soil Sci. Plant Nutr. 27, 295-303. 

Range corresponds to molecular size fractions of organic ligands obtained by ultra-filtration. 

In sequential ultrafiltration successive volume reductions occur (Figure 6). Since volume is reduced by 50% at each stage, both the filtrates and retentates contain components of other molecular size fractions consequently, the components contributing to each measured fraction must be taken into account in a mass balance. 

Shand, C.A., Smith, S., Edwards, A.C. and Fraser, A.R. (2000) Distribution of phosphorus in particulate, colloidal and molecular-sized fractions of soil solution. Water Research 34, 1278-1284. 

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