Molecular weight, determination calibration curve

To determine molecular weights, a calibration curve like that in Figure 9.21 is first prepared. This is a semilog plot like the general one shown earlier (Figure 9.19) and is linear from C9 to C57, including both hydrocarbons and triglycerides. 

Table 10.1 shows the KV) values obtained on the same gels for seven standards of known molecular weight. The calibration curve used to determine the molecular weight of hemagglutinin is then constructed, and the molecular weight of hemagglutinin is determined by interpolation of this curve, shown in Figure 10.2. 

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. 

Determining Calibration Curves frcm Polydisperse Samples. In conventional SEC interpretation, narrow molecular weight distribution standards are needed for calibration purposes. Nonlinear regression has enabled polydisperse scimples to be used. A variety of methods... 

The GPC analysis of block copolymers is handicapped by the difficulty in obtaining a calibration curve. A method has recently been suggested to circumvent this difficulty by using the calibration curves of homopolymers. This method has been extended so that the calibration curves of block copolymers of various compositions can be constructed from the calibration curve of one-component homopolymers and Mark-Houwink parameters. The intrinsic viscosity data on styrene-butadiene and styrene-methyl methacrylate block polymers were used for verification. The average molecular weight determined by this method is in excellent agreement with osmometry data while the molecular weight distribution is considerably narrower than what is implied by the polydispersity index calculated from the GPC curve in the customary manner. 

The combination of the differential refractive index (RI) detector and on-line viscometer allows the direct use of the universal calibration and thus true molecular weight determination. The RI detector is concentration-sensitive, and the viscometer records specific viscosity. The ratio of the specihc viscosity to the concentration is equivalent to intrinsic viscosity (as discussed in Section 6.1), and the continuous dependence of this ratio versus the retention volume could be related to the universal calibration curve, thus allowing the correlation of each point on the chromatogram with the true molecular weight. 

Molecular Weight Determination. As mentioned earlier, the GPC results for our samples are based on a polystyrene calibration curve. As such, all numbers must be regarded on a rel ati va basis. PS based weight average molecular weights, Mw, were about 10b for highly reacted samples. Actual Mw values may be lower even by a factor of 2, based on calculations for an stepwise polycondensation with 99% conversion (18). Mw versus DBTDL concentrations for a typical series are shown inTigure 13. As observed the effect of a hotter mold wall... 

Figure 10.2. Calibration curve for hemagglutinin molecular weight determination. [Reprinted, by permission, from D. J. Holme and H. Peck, Analytical Biochemistry , Longman, New York, 1998. Addison Wesley Longman Limited 1998.]...

A series of polystyrene standards were purchased from Waters Corporation and American Polymer Standards Corporation for creating a calibration curve and determination of molecular weight by elution time based on polystyrene standards number. The calibration curve was created using Chemstation GPC module software. The molecular weight determined by this method was used for comparison with literature data, since polystyrene standards are commonly used for reference. 

Relative methods measure properties that depend clearly on molecular weight, for example, the hydrodynamic volume of the polymer coils (GPC, viscosimetry) or their solubility as a function of chain length. However, these measurements can only be evaluated with respect to the molecular weight of the macromolecules if experimental calibration curves are available which were generated by comparison with an absolute method of molecular-weight determination. 

In order to achieve a calibration curve of optimum linearity and a maximum of precision In the molecular weight determination... 

These values were used to calibrate GPC colunms for the polymer. Number-average molecular weights determined with modified calibration curves agreed well with those determined by embulliometry. The value of 0.96 for a was consistent with the idea that poly(3-alkylthiophene)s formed relatively stiff polymer chains in solutions. 

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