Volume, elution, molecular weight

Apart from one compound (II), the lignin model compounds that had free phenolic groups eluted at close to the retention times predicted by the calibration curve from the polymer standards and not from the derive tized model compounds. This could simply be a result of the underivatized models having a similar variation in hydrodynamic volume with molecular weight as the polymer standards. However, it is to be expected that solvation of the underivatized model compounds should occur with THF as solvent (10), with hydrogen bonding of one THF molecule to each under-... 

All polycondensation reactions were followed by FT-IR-spectroscopy. Polyamide formation could be demonstrated by the disappearance of the ester C = 0 stretching mode and simultaneous formation of amide C = 0 stretching modes32>. Additional proof for polymer formation is given by gel permeation chromatography. From the elution volumes a molecular weight between 2,000 and 10,000 can be estimated. 

The calculation library includes programs to process all data encountered in normal GPC operation for standards and unknowns. It includes routines to decode data, correct for baseline drift, interpolate for flow variation, calculate mean elution volume, calculate molecular weight averages, and calculate corrected molecular weight averages and distribu-... 

By appropriate calibration, the chromatogram is converted to a tme (fingerprint) molecular weight distribution curve, from which all pertinent averages are easily calculated. The most reliable calibration is based on the relationship between the product [ft]M (intrinsic viscosity multiplied by molecular weight) and elution volume (or time), following Flor/s equation (3-7) which relates intrinsic viscosity to hydrodynamic volume and molecular weight. 

For many commercial polymers the columns cannot be calibrated because well-characterized standards are unavailable. The situation is further complicated for branched polymers or copolymers, for which there is no single calibration curve relating elution volume to molecular weight (1). 

The calibration curve can be divided into three sections. In the first, there is no separation of the small molecules as they are all totally occluded within the pores of the gel and there is no separation. The second has an approximately linear variation of the elution volume with molecular weight over a wide range of elution volume. The columns separate the polymer molecule over this particular range of molecular weights very well. In the final section of the... 

The maximum concentrations which can be injected without peak broadening and change in elution volume are listed for various injection volumes and molecular weights in Table 4.1(d). These values vary with column characteristics and are not transferable, but they illustrate how the column loading must be substantially reduced as the molecular mass of the sample increases. It arises from the reduction in pores within the gel which separate higher-molecular-mass species, and also because of localized viscosity effects. 

A major part of the development of SEC in the study of the molecular mass distribution of polyethylene has been concerned with developing effective calibration procedures. This requires correlation of elution volume and molecular weight. There are, however, no well-characterized narrow-distribution linear polyethylene standards for determining the elution... 

The complexes were further purified by size-exclusion HPLC. In this way colorless material was removed, and from the elution volume a molecular weight of approximately 335 kDa could be estimated for both complexes. The height of the protein absorbance in the UV as compared to that of other pigment-protein complexes suggested a BChl g content of 10 - 15 % by weight, corresponding to about 40 BChls per complex. This would indicate that the complexes contain the antenna complement... 

The proteins are the p subuiut (O), the 35-kDa protein (A), the 9-kDa protein ( ), and the 6-kDa protein ( ).(V) is a plot of apparent molecular mass vs elution volume for molecular weight standards. 

Calibration of a packed column also presents difficulties when using SEC to characterize EOR polymers. No large molecular weight, monodispersed water-soluble polymer standards are available to relate elution volume to molecular weight. Consequently, the usual SEC calibration methods are not applicable and special calibration techniques must be applied. 

Typically, the elution of small molecules has been described as a linear function of log M and the elution volume, Vg. Molecular weight calibration curves obtained with members of a specific homologous series often give very good linearity with remaining members of the series, within the limit of the included volume of the column system used. An example of this application with oligosaccharides and several monosaccharides is shown in Figure 1. The predictive ability of such log M curves decreases as the solute studied... 

Today, data acquisition and processing are usually computer controlled. There are four transformations required of the raw chromatographic data to provide results as usually reported see Figure 3.20 (79) (a) conversion of elution time to elution volume, (b) conversion of elution volume to molecular weight, (c) conversion of detector response to polymer concentration, and (d) conversion of polymer concentration to weight fraction. Quantification of plate count and resolution, as well as calibration are discussed further in ASTM D5296-97 (89). 

In SEC, mass is not measured so much as the hydrodynamic volume of the polymer molecules, that is, how much space a particular polymer molecule takes up when it is in solution. However, the approximate molecular weight can be calculated from SEC data because the exact relationship between molecular weight and hydrodynamic volume for polystyrene can be found. For this, polystyrene is used as a standard. But the relationship between hydrodynamic volume and molecular weight is not the same for all polymers, so only an approximate measurement can be arrived at. Another drawback is the possibility of interaction between the stationary phase and the analyte. Any interaction leads to a later elution time and thus mimics a smaller analyte size. 

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