Methods for molecular weight determination

This expression is consistent with the analysis of each of the lines in Table 5.1 as presented above and provides a general answer to one of the questions posed there. It is often a relatively easy matter to monitor the concentration of functional groups in a reaction mixture as we saw in discussing end group analysis as a method for molecular weight determination in Sec. 1.7. Equation (5.4) is... 

Such data can provide a calibration curve and allow the constants (E) and F ) in equation (20) to be determined. The value of the molecular weight of an unknown solute can then be obtained from its (H) value by reading the value directly from the curve or by calculation using the predetermined constants (E) and (F ) in equation (20). It should be pointed out that an error of up to 30% may not appear to be very useful but, in fact, such precision can be extremely valuable in the preliminary examination of many biochemical substances where only very small quantities of material are available. It is also an ideal method for molecular weight determination before more accurate, labor-intensive and time-consuming methods are considered. 

This was exactly the measurement you made in Experiment 6—it is called the vapor-density method for molecular weight determination. To... 

Exactly the same type of behavior is found for the freezing point of a solution except that the freezing point is lowei than that of the pure solvent. Thus we have two methods for molecular weight determination which are applicable to compounds with such low vapor pressure or which decompose so readily that the vapor density method cannot be used. 

It provides a very reliable method for molecular weight determination. 

Molecular Weight Dependence and a Method for Molecular Weight Determination. It is anticipated, even by qualitative considerations, that the longer molecules will be increasingly readily extendable, and more quantitatively, that (5)... 

TABLE 3.13 Empirical methods for molecular weight determination of biomacromolecules... 

An absolute method for molecular weight determination is matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF) (Kona et al., 2005 Creel, 1993 Nielsen, 1999 Cho et al., 2001). The sample is dispersed in a UV-absorbing matrix (e.g., trans -cinnamic acid or 2,5-dihydroxybennzoic acid). Irradiation with a UV laser induces evaporation of ionized polymer chains, which are then detected using TOF. The technique requires relatively narrow MWD samples. Alternative ionization methods have been employed, such as electrospray ionization mass spectrometry (ESI-MS), which may have advantages for certain polymer end groups (Vana et al., 2002). IFFF and MALDI-TOF can be coupled to analyze polydisperse samples and polymer mixtures (Kassalainen and Williams, 2003). 

A possibility of using the viscosimetiy and sedimentation and diffusion methods in studying the process of enzyme chitosan hydrolysis is discussed in the chapter. It is shown that a change in the intrinsic viscosity of chitosan may be determined by both the hydrolysis process in the glycoside bonds and transformation of the supramolecule stmcture of the polymer. Thus, for setting the hydrolysis process it is necessary to use an absolute method for molecular weight determination. 

Although flow birefringence has been used many times for determination of molecular weight of macromolecules, the method depends upon assumptions regarding the molecular geometry which Involves an appreciable degree of uncertainty. Moreover it requires evaluation of empirical constants. These serious drawbacks have limited the application of this method for molecular weight determination. 

J. M. Machado, F. R. Denton III, J. B. Schlenoff, F. E. Karasz, and P. M. Lahti, Analytical methods for molecular weight determination of poly(p-xylyl-idene dialkyl sulfonium halide) degree of polymerisation of poly(p-phenylene vinylene) precursors, J. Polym. Sci. Part B Polym. Phys. 27 199 (1989). 

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