Determination of molecular weight distribution

NEW APPROACH TO INCREASING OF ROBUSTNESS FOR METHOD OF DETERMINATION OF MOLECULAR WEIGHT DISTRIBUTION IN DEXTRANS... 

The main problem of determination of molecular weight distribution (MWD) of dextrans (polysachaiides which ai e used as active substances for infusion medicines) is low robustness of the existing method. It means that obtained results are strongly dependent on controlled and uncontrolled pai ameters of chromatographic system standai d substances for calibration loading on columns etc. It has been shoved on practical examples. 

Occasionally there is the need for simultaneous determination of MW, MWD of polymers and identifica-tion/quantilication of additives [38]. This was the case for polymer and additive analysis of SBR/(softeners, flavour agents, stabilisers) (chewing gum) [41]. The many constituents of the SBR portion of the sample were not resolved, since adjacent components were similar in size. It should be stressed, however, that the need for simultaneous determination of the molecular weight of polymers and the identification/quantification of additives is exceptional rather than the rule. The determination of molecular weight distributions by SEC has indicated that oligomer fractions analysed by dissolution and (Soxhlet) extraction methods may differ essentially [42],... 

Polydispersity of molecular weights of heparin was recognized in early studies (reviewed in Refs. 9 and 183). However, the polyelectrolyte nature and heterogeneity of charge distribution have hampered accurate determinations of molecular-weight distribution. 

Hunt, S. M. Sheil, M. M. Belov, M. Derrick, P. J. Probing the effects of cone potential in the electrospray ion source Consequences for the determination of molecular weight distributions of synthetic polymers. Anal. Chem. 1998,70,1812-1822. 

Beckett, R., Jue, Z. and Giddings, J. C. (1987). Determination of molecular weight distributions of fulvic and humic acids using flow field-flow fractionation, Environ. Sci. Technol., 21, 289-295. 

Sieck, L.W. Determination of Molecular Weight Distribution of Aromatic Components in Petroleum Products by Cl-MS with Chlorobenzene As Reagent Gas. Anal. Chem. 1983,55,38-41. 

Figure 6 shows calibration curves for three other two column combinations, each representing 30,000 to 40,000 plates per set. The 10 A° plus 10 A° curve can be interpreted to show a deficiency in relative pore population in the range equivalent to about 50,000 to 600,000 molecular weight. The other two, properly calibrated, can conceivably be used for determination of molecular weight distributions. However, utility for resolution of specific polymodal mixtures is too difficult to assess from calibration curve alone. How much curvature of a calibration curve translates into utility or non-utility Calibration curves indicating pore size populations all have the same shape for given column combinations whether the plate count level is 5000 plates or 20,000 plates or 80,000 plates. 

The molecular weight distribution obtained from SEC analysis was also shown in Fig. 8. In order to check the effect of the estimated exponent a(-0.55) on molecular weight distribution for Ei branched PVAc, we used another a(-0.58) value to compute a new calibration curve as shown in Fig. 9. The two calibration curves almost overlapped with each other. The results are listed in Table 2. In both cases, we obtained the same weight-average molecular weight and the polydispersity index (M /M ). Thus, we could confirm that in using a two-point (Bq and %l) estimate for a, we have not introduced an appreciable error in the determination of molecular weight distribution of branched PVAc. 

Size-exclusion chromatography (SEC) has been also used as complementary technique for characterizing changes in the composition of synthetic resins due to aging. In particular, it enables the determination of molecular weight distributions and the glass transition temperatures of resins. 

Lorry, D. and Vedrines, M. (1985). Determination of molecular weight distribution of gelatins by H.P.S.E.C. In Photographic Gelatin, H. Ammann-Brass and J. Pouradier, eds. Proc. 4th IAG Conf., Fribourg, Germany, 1983, 35-54. 

A wider choice of solvents to be used as developer is possible, and in certain cases, the TLC method is much more sensitive to a small amount of satellite component (cf. Section V.l.). This section is devoted to a summary of some results on the determination of molecular-weight distributions by the phase-separation mechanism in TLC. 

Figure 1. Determination of molecular weight distribution for narrow polystyrene standard. Curve I, calibration of Rf vs. M curve II, densitogram of two dimensionally developed sample curve III, molecular weight distribution Mw/Mn = 1.02 (12).

DIRECT DETERMINATION OF MOLECULAR WEIGHT DISTRIBUTION WITHOUT CALIBRATION... 

Polymers with well-defined chemical structures are a prerequisite for direct determinations of molecular weight distributions (MWDs) [1-3]. Ute and coworkers [6] have used on-line SEC-NMR coupling to determine the molecular weight distribution in isotactic poly(methyl methacrylate) (PMMA) samples. 

S.W. Provencher, J. Hendrix, L. DeMayer and N. Paulussen, Direct determination of molecular-weight distributions of polystyrene in cyclohexane with photon correlation spectroscopy, J. Chem. Phys. 69 (1978) 4273-4276. 

Fodor JS, Hill DA (1994) Determination of molecular weight distribution of entangled cis-polystyrene melts by inversion of normal-mode dielectric loss spectra. J Phys Chem 98(31) 7674-7684... 

Berth, G., Dautzenberg, H., Lexow, D., and Rother, G. (1990). The determination of molecular weight distribution of pectins by calibrated GPC. Part I. Calibration by light scattering and membrane osmometry. Carbohydr. Polym. 12 39-59. 

Determination of Molecular Weight Distribution. A 1.6- X 95-cm column of Sepharose C1-4B in 50% tris(ethylenediamine)cadmium dihydroxide (1 1 with water) was prepared and equilibrated by elution with six column volumes of 50% tris(ethylenediamine)cadmium dihydroxide. Downward flow with a pressure head of 125 cm was used. 

Here, V° is the volume of fluid required to flush a sample that is not affected by the field (U = 0). In most FFF subtechniques, the parameters in Eq. (1) are, in fact, constant throughout the channel. In thermal FFF, however, these parameters vary across the channel because they depend on temperature, which varies between the hot and cold walls. As a result, Eq. (3) is only an approximation in thermal FFF. Fortunately, the approximations associated with Eq. (3) are inconsequential for the determination of molecular-weight distributions, so that the only concern is variations in T, as outlined earlier. For measuring transport coefficients, on the other hand, the approximations can lead to significant errors. 

Phenylurethane (carbamate) derivatives of insoluble polysaccharides, produced by reaction with PhNCO in neutral organic solvents, are used in determination of molecular weight distributions (see Section 4.5.1) carbamates are acid stable and base labile and have better alternatives as protecting groups in synthesis. 

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