Volume, elution, molecular weight calibration

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... 

This behavior shows that the dimensions of these polymers are independent of pH, ionic strength (in the ranges studied) and presence or absence of Tergitol or polyethyleneoxide. This result is of considerable help in interpretation of GPC behavior since in the absence of polymer-glass substrate interactions, the molecular weight calibration curves (log MW vs. elution volume) should be independent of pH, ionic strength or the two nonionic surfactants investigated. 

Non-Aqueous SEC Evaluation. The SEC calibration report for a peak position method using a series of narrow MWD polystyrene standards is shown in Table I, As can be seen, a linear fit produces a high correlation to the data (r=0.9997). Figure 1 displays the molecular weight calibration plot of elution volume versus log molecular weight for the series of polystyrene standards. 

Optimization of Linear Calibration Methodology. The accuracy of linear calibration methods for utilization of polydisperse calibration standards depends upon (1) how well the column set approximates true linearity over the molecular weight calibration range and (2) the extent to which instrumental band broadening affects the elution volume profile of the polydisperse standard. 

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-... 

A universal calibration curve was established by plotting the product of the limiting viscosity numbers and molecular weight, Mw[iy], vs. the elution volume, EV, for a variety of characterized polymers. The major usefulness of the universal calibration curve was to validate individual molecular-weight values and to provide extended molecular-weight calibration at the ends of the calibration curve where fractions of narrow dispersion of the polymer being analyzed are not available. The calibration curve was monitored daily with polystyrene fractions certified by Pressure Chemicals. The relationship between the polyethylene fractions and polystyrene fractions was determined using the universal calibration curve. 

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... 

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... 

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. 

The void volume (Kg) pore volume (F,), elution volume (Fr), and slope of the molecular weight calibration curve (m) should remain constant in a column that does not develop mobile phase channeling or bonded phase degradation. To give results identical to those obtained with a previous column, a replacement column must have identical values of these characteristic constants. Because it is often impossible to obtain identical columns, especially over a long period of time, it is always best to determine these parameters for every new column and to make operational condition adjustments when necessary. 

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). 

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