Methods for molecular weight

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. 

ASTM D 5296 - 92 (1994). Standard Test Method for Molecular Weight Averages and Moleeular Weight Distribution of Polystyrene by High Performance Size-Exclusion Chromatography. Annual Book of ASTM Standards, Vol.08.03, pp. 419-431. 

ASTM D-3536-91, Standard Test Method for Molecular Weight Averages and Molecular Weight Distribution by Liquid Exclusion Chromatography (Gel Permeation Chromatography-GPC), ASTM Annual Books of ASTM Standards, Vol. 08.02, P. 349—359. This method was deleted in the 1997 edition of the ASTM book. 

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. 

Previous studies (5-7) have clearly demonstrated that gel permeation chromatography of reduced linear randomly coiled polypeptide chains in GuHCl provides an accurate, dependable method for molecular weight estimation. 

An alternative method for molecular weight measurement is the Archibald or approach to equilibrium method [22]. In this, rather than establishing an equilibrium... 

Moore, J.C. Gel permeation chromatography. I. A new method for molecular weight distribution of high polymers,/. Polymer. Sci., Part A, 1964, 2, 835. 

In the test method for molecular weight (ASTM D-2503) a small sample of wax is dissolved in a suitable solvent, and a droplet of the wax solution is placed on a thermistor in a closed chamber in close proximity to a suspended drop of the pure solvent on a second thermistor. The difference in vapor pressure between the two positions results in solvent transport and condensation onto the wax solution, with a resultant change in tempera-... 

ASTM D-3593 Test Method for Molecular Weight Averages/Distribu-tion of Certain Polymers by Liquid Size Exclusion (Gel Permeation) Chromatography—GPC Using Universal Calibration (has also been adapted to the investigation of molecular weight distribution in petroleum fractions). 

Methods for molecular weight measurement are also included in other more comprehensive standards (ASTM D-128, ASTM D-3712), and several indirect methods have been proposed for the estimation of molecular weight by correlation with other, more readily measured physical properties (Speight, 2000,2001). They are satisfactory when dealing with the conventional type of crude oils or their fractions and products and when approximate values are desired. 

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