Calibration secondary molecular weight

Using the intrinsic viscosity data and the universal calibration curve( ) a secondary molecular weight calibration curve can be constructed for the polymer of interest as shown by the following equation ... 

This "secondary" molecular weight calibration curve was fit to a polynomial over the retention volume range of the sample. Then the molecular weight distribution statistics are calculated from this "secondary" calibration curve and the DRI trace of the sample under... 

Figure 10. "Secondary" Molecular Weight Calibration Curve for a PMMA Sample (Eastman 6041).

Figure 1. Illustrative method for constructing a secondary molecular weight calibration curve from an HDV calibration curve and intrinsic viscosity values of GPC polymer fractions...

Using the PMMA direct molecular weight calibration curve shown in Figure 4 and the Mark-Houwink parameters for PMMA in TFE shown in Equations 26 and 27, an HDV calibration curve can be constructed as described in the theory section. Then secondary molecular weight curves can be constructed for other polymers of interest by the methods discussed in the theory section as was done in Ref. 1, using the indirect PMMA molecular weight calibration curve. 

Electrostatic and adsorption effects conspire to make aqueous GPC more likely to be nonideal than organic solvent GPC. Thus, universal calibration is often not obeyed in aqueous systems. Elence, it is much more critical that the standard chosen for calibration share with the polymer being analyzed chemical characteristics that affect these interactions. Because standards that meet this criterion are often not available, it is prudent to include in each analysis set a sample of a secondary standard of the same composition and molecular weight as the sample. Thus, changes in the chromatography of the analyte relative to the standards will be detected. 

In SEC, universal calibration is often utilized to characterize a molecular weight distribution. For a universal calibration curve, one must determine the product of log(intrinsic viscosity molecular weight), or log([7j] M). The universal calibration method originally described by Benoit et al. (9) employs the hydro-dynamic radius or volume, the product of [tj] M as the separation parameter. The calibration curves for a variety of polymers will converge toward a single curve when plotted as log([7j] M) versus elution volume (VJ, rather than plotted the conventional way as log(M) versus V, (5). Universal calibration behavior is highly dependent on the absence of any secondary separation effects. Most failures of universal calibration are normally due to the absence of a pure size exclusion mechanism. 

The technique described here is finding increasingly widespread use for the resolution and molecular weight estimation of naturally occurring polypeptides. Its intended function is to supplement the classical exact techniques of molecular weight analysis and is not intended as an alternative procedure. Although we are aware of no instance in which invalid data have been obtained, the method can only be considered an approximate one since measurement involves calibration with secondary standards, and at present it is not suitable for precise theoretical analysis. 

Though the absolute methods for the determination of molecular weights are well established, both theoretically and experimentally, the absolute measurements are difficult to carry out, are time-consuming, and often require expensive apparatus. For these reasons, for routine determinations of moleculcu- weight, the much faster secondary methods, such as solution viscosity and gel permeation chromatography, are commonly used. These methods require prior establishment of empirical relationships that relate the molecular weight to the viscosity of the polymer solution or to the retention times in a gel-permeation column. Once such calibration has been done, the secondary methods provide a fast, simple, and accurate... 

High-molecular-weight aliphatic amines are used extensively in many industries. The total primary- and secondary-amine content of aliphatic amines can be determined easily and rapidly by functional-group analysis in the near infrared [9], using chloroform solvent and 5-cm fused-silica cells. Primary amines have characteristic absorption maxima at 2.02 /um and 1.55 m, whereas secondary amines absorb only at 1.55 fim. Quantitation is achieved by the calibration-curve method using a series of standard solutions of primary and of secondary amines. Most other methods for the determination of total primary, secondary, or tertiary amine in a mixture are lengthy or inaccurate, or are unsuitable for small samples. 

This chapter first discusses the characteristics of silica as it pertains to size exclusion chromatography. Next, several methods for molecular weight calibration in SEC are examined and the effects of secondary retention discussed. The chapter concludes with an overview of practical aspects associated... 

When the secondary effects are eliminated, a SEC system may be calibrated using standards of known molecular weight. After cahbration the system may be used to very aeeurately determine the moleeular weight and moleeular weight distribution of a polymer sample. Calibration is always required unless a molecular weight sensitive detector is used to convert measured elution volumes to a molecular weight or mass. 

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