The Determination of Molecular Weight

The range of the molecular weights of the polysaccharides is very great and almost all the conventional methods of determination have been applied. The more usual of these methods and their limits of application are summarized in Table I. As reviews of the methods1 and the techniques1 involved are available it will not be necessary to describe these in detail, but a short outline will be given below. 

Method Molecular weight range most suited for method Molecular weight average 

Chemical methods Molecules containing more than one end-group per one hundred units. Number-average 

Cryoscopic and ebullioscopic methods Molec. wt. less than 20,000 N umber-average 

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Beckmann thermometer A very sensitive mercury thermometer with a small temperature range which can be changed by transferring mercury between the capillary and a bulb reservoir. Used for accurate temperature measurements in the determination of molecular weights by freezing point depression or boiling point elevation. 

Molecular Weight Determinatioos. Details of the determination of molecular weights on a semi-micro scale by the Freezing-point method are given on p. 436, and by the Boiling-point method on p. 440. 

Before doing this, however, it is informative to compare the sensitivity of the four colligative properties in the determination of molecular weight. In the following example this is done by making the appropriate numerical calculations. 

These results show more clearly than Fq. (8.126)-of which they are special cases-the effect of charge and indifferent electrolyte concentration on the osmotic pressure of the solution. In terms of the determination of molecular weight of a polyelectrolyte by osmometry. ... 

This returns us to the problem of the determination of molecular weight. Avogadro s Hypothesis provides one of the convenient ways of measuring molecular weight if the substance can be vaporized. 

Note that for the determination of molecular weight, the charge-state distribution is not of great importance as it does not affect the m/z value of the ion involved and thus the calculated molecular weight. If the conformational state of the biopolymer is of interest, however, the distribution of charged states is a fundamental consideration and any parameter likely to change this distribution must be carefully controlled. 

Absolute calibration is of critical importance for the determination of molecular weights by the light-scattering method. The scattered intensity is so low compared with the incident beam at a distance of a few cm. being of the order of 10 lo) that a reliable direct comparison of the two beams using the same photocell is impractical. Several methods have been used, with results which are not always comparable, however. 

Experimental variables such as temperature, flow rate, sample concentration and mobile phase composition can cause changes in the elution volume of a polymer [439,457,460-464]. Chromatographic measurements made with modem equipment are limited more by the errors in the absolute methods used to characterize the molecular weight of the calibration standards than any errors Inherent in the measurements themselves, since the determination of molecular weights by SEC is not an absolute method and is dependent on calibration [462]. The Influence of temperature on retention in SEC is not very great, since no strong sorptive interactions are involved in the retention mechanism. Temperature differences between the column and solvent delivery... 

Podzimek, S., The use of GPC coupled with a multiangle laser light-scattering photometer for the characterization of polymers. On the determination of molecular weight, size, and branching, /. Appl. Polym. Sci., 54, 91, 1994. 

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],... 

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. 

NMR allows the determination of molecular weight by near end [97] or end groups [98]. The method is limited to rather low molecular weights below 3 x 104 g / mol, because for higher molecular weight the number of end groups eventually... 

V. Properties of the Starch Components in Solution 1. The Determination of Molecular Weight... 

However, before proceeding to the determination of Molecular weights of polymers we will take up an elementary discussion of the use of x-ray diffraction, spectroscopic techniques and electromicroscopic techniques, etc. in determining the structure of polymers. 

It is the simplest method for the determination of Molecular weights of the polymers. Determinations by this methods require that the polymer contain a known number of determinable groups per molecule. 

Methods for the determination of Molecular weight based on colligative property are vapour-pressure lowering, boiling point elevation (ebulliometry), freezing-point depression (cryoscopy), and the Osmotic pressure (osmometry). 

Such an osmometer was first designed by Herzog and was further modified by Fuoss and Mead. This type of osmometers is widely used for the determination of Molecular weight. 

For the determination of Molecular weight of macromolecular compounds, the viscosity method was introduced by H. Staudinger... 

Characterisation of A/B/C terpolymers is still in its infancy, but there are already clear indications that, in a compositionally heterogeneous terpolymer, the determination of molecular weights MA, MB, Mc and M may prove an intractable problem in the general case. Apart from some semi-quantitative studies, most effort in this direction has been made by Kambe, Kambe and collaborators162,163, who extended the original approach of Bushuk and Benoit164. ... 

ESI has become the most commonly used interface for LC/MS. It was recognized by John Fenn and co-workers as an important interface for LC/MS immediately after they developed it as an ionization technique for MS. ESI transforms ions in solution to ions in the gas phase and may be used to analyze any polar molecule that makes a preformed ion in solution. The technique has facilitated the ionization of heat-labile compounds and high-molecular-weight molecules such as proteins and peptides. ESI is a continuous ionization method that is particularly suitable for use as an interface with FiPLC. It is the most widely accepted soft-ionization technique for the determination of molecular weights of a wide variety of analytes and, has made a significant impact on drug discovery and development since the late 1980s. 

Cabannes factor analy chem An equational factor to correct for the depolarization effect of the horizontal components of scattered light during the determination of molecular weight by optical methods. ko banz. fak-tor cacodyl orgchem (CH3)2As A radical found in, for example, cacodylic acid, (CH3)2A-sOOH. kak-3 dil ... 

Mass Spectrometry (MS) Approaches Used in the Determination of Molecular Weights of Oligomeric and Polymeric Materials... 

What principle is used in the determination of molecular weight by vapor pressure osmometry ... 

Knowledge of the molecular weight and of the molecular-weight distribution of a polymeric material is indispensable for scientific studies as well as for many technical applications of polymers. They effect the solution and melt viscosity, the processability, and the resulting mechanical properties tremendously. Therefore, we will give a short introduction into methods that allow us to determine the required information. Roughly, the methods developed for the determination of molecular weights are subdivided into absolute and relative methods ... 

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. 

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