Detector response size-exclusion

The 1000 A column did not show any resolution between 312 nm and 57 nm particle sizes. Shown in Fig.2 are the calibration curves for the 2000 A and 3000 A columns and for their combination. The 57 nm particle standard appears to have been erroneously characterized by the supplier. This was subsequently confirmed by electron microscopy. The 2000 X column exhibited a sharp upturn in its calibration curve close to the exclusion limit. It is to be noted that while data points corresponding to 312 and 275 nm diameter particles appear on individual column calibration curves, they are not indicated for the calibration curve of the combination. This is because these larger diameter particles were completely retained in the packed colimms, generating no detector response. The percentage recovery for these particles from individual columns was considerably less than 100 resulting in their complete retention when the columns were combined in series. 

Four primary problems must be considered in performing GPC studies of asphaltenes and maltenes. These are poor weight recoveries from GPC columns through sample adsorption, nonuniform detector responses for different sample sizes and/or types, uncertainty in selection of appropriate column calibrations, and non-size-exclusion behavior because of abnormal aromatics retention on Styragel. All these problems will be addressed in the following sections. 

Local Properties. Local properties are values such as detector response, molecular weight, or polymer size at a particular retention volume of a size-exclusion chromatogram. They are denoted by the subscript i. The excess Rayleigh scattering, Rei, at each retention volume Vi of an SEC is related to the concentration at each retention volume, Ci, and angle, 6, by... 

Determination of the average chemical composition and polymer composition by size-exclusion chromatography (SEC) has been reported in the literature. Two different types of concentration detector or two different absorption wavelengths of an ultraviolet or an infrared detectors are employed the composition at each retention volume is calculated by measuring peak responses at the identical retention points of the two chromatograms. 

An analytical TREF system was first described by Wild and Ryle [8], Their system (Fig. 8) used components taken from a Waters model 200 size exclusion chromatograph. The solvent reservoir, degasser and pump from the Waters unit was used as was the refractive index detector system. In place of the SEC oven a temperature programmed oil bath provided the temperature gradients. A 0.2 g polymer sample was loaded in a hot trichlorobenzene solution into a small column packed with 40-60 mesh Chromosorb P. Crystallization was achieved by slow-cooling the polymer solution (0.2 g in 5 ml) in the packed column at a rate of 1.5 K/hour down to room temperature. The temperature rising elution was carried out at a flow rate of 6 ml/min and a rate of temperature rise of 8 K/hour. The refractive index response and the separation temperature were recorded continuously on a two-pen recorder. A calibration curve of methyl content vs. elution... 

The primary information obtained from SEC is not the molar mass, but the apparent concentration at an elution volume. Only with the matching SEC calibration and the concentration profile from the concentration detector can the molar mass average and the MMD be obtained. Size exclusion chromatography is therefore a relative method if no absolute detection is employed [1]. The SEC calibration is based on assigning a molar mass to an elution volume (calibration of x-axis). This is in contrast to HPLC, where the detector response (signal intensity and peak area) is calibrated and assigned to a concentration (calibration of y-axis). 

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