Molecular weight measurement procedure

Conversely, if we know the molecular weight we can make inferences about the conformation of polysaccharides in solution using Eq. 13 and other power-law relations. We will consider this in more detail after we have considered further molecular weight measurement by absolute (i.e. without assumptions concerning conformation) procedures. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Another promising approach, in the sense that it avoids expensive fractionation procedures, is to promote the direct polymerization of dep-suberin mixtures under polycondensation or polytransesterification conditions, using dep-suberin mixtures issued from alkaline hydrolysis or methanolysis, respectively. Ongoing experiments prove that both types of systems are viable [85, 86], as shown by the FTIR spectmm of the product obtained from a polycondensation of a dep-suberin (Fig. 14.11), which clearly shows the feature of a polyester. This evidence was further corroborated by NMR spectroscopy and molecular weight measurements [85]. 

Measurement of molecular mass of biomolecules is now a suitable replacement for prior methods based on the use of gel electrophoresis. Molecular weight measurement for all classes of biomolecules is a relatively routine procedure, and the results obtained are typically of greater accuracy than those previously... 

Molecular weights of PVDC can be determined directly by dilute solution measurements in good solvents (62). Viscosity studies indicate that polymers having degrees of polymerization from 100 to more than 10,000 are easily obtained. Dimers and polymers having DP < 100 can be prepared by special procedures (40). Copolymers can be more easily studied because of thek solubiUty in common solvents. Gel-permeation chromatography studies indicate that molecular weight distributions are typical of vinyl copolymers. 

The composition of an oil and the progress of its hydrogenation is expressed in terms of its iodine value (IV). Edible oils are mixtures of unsaturated compounds with molecular weights in the vicinity of 300. The IV is a measure of this unsaturation. It is found by a standardized procedure. A solution of ICl in a mixture of acetic acid and carbon tetrachloride is mixed in with the oil and allowed to reac t to completion, usually for less than I h. Halogen addition takes place at the double bond, after which the amount of unreacted iodine is determined by analysis. The reaction is... 

The major problem in this procedure is not in obtaining an accurate value for the solute diffusivity from peak width measurements (which is relatively straightforward) but in identifying the best relationship between diffusivity and molecular weight to employ in the subsequent data processing. 

Thus, a practical procedure would be as follows. Initially the HETP of a series of peptides of known molecular weight must be measured at a high mobile phase velocity to ensure a strong dependence of peak dispersion on solute diffusivity. 

The procedure for determination of neutral oils in AOS measures petroleum spirit-extractable material from an aqueous alcoholic solution. Normally a solution of 1 1 ethanol/water is used as to dissolve the sample prior to extraction. For higher molecular weight materials 2 1 propan-2-ol/water is preferred. The petroleum ether is removed on a steambath and finally under vacuum. The amount of neutral oil is then determined gravimetrically. Due to the slight volatility of alkenes, alkanes, and alcohols, especially in the C,2 materials, the neutral oil tends to be underestimated by these procedures. 

Morton and Salatiello have deduced the ratio kpp/kp for radical polymerization of butadiene by applying the above described procedure, appropriately modified for the emulsion system they used. The primary molecular weight was controlled by a mercaptan acting as chain transfer agent, as in the experiments of Bardwell and Winkler cited above. Measurement of the mercaptan concentration over the course of the reaction provided the necessary information for calculating % at any stage of the process, and in particular at the critical conversion 6c for the initial appearance of gel. The velocity constant ratios which they obtained from their results through the use of Eq. 

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