Universal Calibration, size exclusion

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. 

Nave, R., Weber, K., and Potschka, M., Universal calibration of size-exclusion chromatography for proteins in guanidinium hydrochloride including the high-molecular-mass proteins titin and nebulin,. Chromatogr. A, 654, 229, 1993. 

Dubin, P. L. and Principi, J. M., Failure of universal calibration for size exclusion chromatography of rodlike macromolecules vs. random coils and globular proteins, Macromolecules, 22, 1891, 1989. 

Standard Test Method for Molecular Weight Averages and Molecular Weight Distribution of Certain Polymers by Liquid Size-Exclusion Chromatography (Gel Permeation Chromatograph -GPC) Using Universal Calibration" ASTM D 3593 80, Annual Book of ASTM Standards, ASTM, Philadelphia, PA, revised annually. 

A viscosity online detector in a size exclusion chromatography (SEC) instrument allows for a universal calibration for polymers with known K- and a-values. For polymers that are only soluble at high temperature, e.g., polyolefines, high-temperature detectors are available, which can be operated up to 200°C. In addition to molar mass measurements, viscosity detectors have also been employed successfully to obtain structural information of branched polymers [28]. 

Novolac molecular weights were measured in THF at 35°C by high pressure size exclusion chromatography using a Waters Model 510 pump (flow rate=1.0 ml/min), 401 differential viscometer detector and a set of Dupont PSM 60 silanized columns. A universal calibration curve was obtained with a kit of 10 narrow molecular weight distribution, linear polystyrene standards from Toya Soda Company. Data acquisition and analysis were performed on an AT T 6312 computer using ASYST Unical 3.02 software supplied with the Viscotek instrument. 

Size exclusion chromatography (SEC) polymer elution profiles yield information regarding the molecular size distributions of polydisperse macromolecules. Polymer molecular weight distribution (MWD) represents an intrinsic property which provides direct correlation with many end-use physical properties and a universal criterion for polymer characterization (1). In order to convert elution profiles or chromatograms into MWD information proper calibration methods are required. SEC molecular weight calibration techniques represent experimental approaches for transformation of polymer elution profiles into MWD information and are dependent upon instrumentation, columns, and the polymer/solvent system under study. 

Gruendling T, Junkers T, Gullhaus M, Bamer-Kowolllk C (2010) Mark-Houwink parameters for the universal calibration of acrylate, methacrylate and vinyl-acetate polymers determined by online size-exclusion chromatography-mass-spectroscopy. Macromol Chem Phys 211 520-528... 

The size exclusion chromatography for this study was done in the routine manner execept for the inclusion of an online viscosity detector called a Differential Viscometer <3> (Viscotek Corp., Porter, Texas, USAl. This instrument together with an RI concentration detector permits the calculation of intrinsic viscosities across the chromatogram. An IBM PC data system with software is also provided (5). The software acquires data from both detectors, and performs calculations of intrinsic viscosity and molecular weight distributions using the Universal Calibration Method. 

Sources of error in this approach arise from both experimental and theoretical grounds. Modern theories of SEC retention mechanism are based on the assumption that the size exclusion process uniquely determines the elution volume, and yet the possibility of reversible adsorption is difficult to dismiss and, where it occurs, errors in the interpretation may easily result. As a warning for the application of universal calibration methodology, Cas-sassa (29) indicates in a later paper that the quantity rj M is not a truly... 

Since size exclusion chromatography separates polymer molecules by their size (especially hydrodynamic size), plotting the molecular size vs. the retention volume should be universal, regardless of the polymer molecular weight. The universal calibration curve is given as ... 

In order to do this, experimental determinations of the intrinsic viscosities of both the standards and the fractions from the unknown polymer are required. It is possible to obtain commercial gel permeation chromatographs that will do this routinely, and hence to exploit the concept of universal calibration. Care must be taken, though, to ensure that the separation of the polymer molecules occurs purely as a result of size exclusion. If there are any other specific interactions, e.g. hydrogen bonding, between the polymer and the column packing, such as may occur with water-soluble polymers, Benoit s approach does not work and the universal calibration plot is not valid. 

Briefly describe how SEC (size exclusion chromatography) separates polymers according to size. Explain how the universal calibration curve can be used to determine molecular weight. 

Cotton fibers are single cells composed primarily ( 96%) of the polymer cellulose. In our laboratory (5), cotton fibers were dissolved directly in the solvent DMAC-LiCl. This procedure solubilizes fiber cell wall components directly without prior extraction or derivatization, processes that could lead to degradation of high MW components. MW determinations have been carried out by a size-exclusion chromatography (SEC) system using commercial columns and instrumentation with DMAC-LiCl as the mobile phase. Incorporation of viscometry and refractive index (RI) detectors (6) allowed application of the universal calibration concept (7) to obtain MW distributions (MWDs) based on well-characterized narrow-distribution polystyrene standards (5). The universal calibration concept used by incorporation of dual detectors bypasses the need for cellulose standards. There are no cellulose standards available. Polystyrene standards for a wide range of MWs dissolved readily in DMAC-0.5% LiCl with no activation necessary. 

LLTHOUGH SIZE-EXCLUSION CHROMATOGRAPHY (SEC) with the conventional or universal calibration technique in combination with viscosity detection is well established in nonaqueous media (i, 2), its application in aqueous media is more complicated because of unusual difficulties... 

By plotting the logarithm of solute size versus retention volume, the points of all different polymers will be represented by a unique curve — a universal calibration curve. Thus, by application of the universal calibration, average molecular weights (MWs) and MWDs of any type of polymer may be evaluated from the size-exclusion chromatograms, provided that the relationship between molecular size and polymer molecular weight is known. 

In order to calculate the molecular weight M) or molecular-weight distribution (MWD) of the polymer, the dependence of the Soret coefficient on M must be known. Because is virtually independent of M, at least for random coil polymers, the dependence of retention on M reduces to the dependence of D on M. The separation of molecular-weight components by D (or hydrodynamic volume, which scales directly with D) is a feature that thermal FFF shares with size-exclusion chromatography (SEC). In the latter technique, the dependence of retention on D forms the basis for universal calibration, as D scales directly with the product [rjjM, where [17] is the intrinsic viscosity. Thus, a single calibration plot prepared in terms of log([i7]M) versus retention volume (F,) can be used to measure M for different polymer compositions, provided an independent measure of [17] is available. In thermal FFF, a single calibration plot can only be used for multiple polymers when the values of for each polymer-solvent system of interest are known. However, a single calibration plot can be used with multiple channels. In... 

New electronics and improved computer data acquisition capabilities have permitted the development of SEC with on-line triple detection using LS, VISC, and re-fractometry. On-line triple detection is known as size-exclusion chromatography cubed (SEC ) with the three dimensions being defined by the three detectors [3]. The use of SEC eliminates the requirement for column calibration, unlike conventional and universal calibration, where a premium is put on control of variables such as flow rate, temperature, and column resolution. SEC can offer advantages in polymer production quality control as well as in research and development of new polymers. 

Universal calibration curve for size exclusion chromatography. Data points are polybutadienes in tetrahydrofuran at 25 °C in crosslinked polystyrene columns. There is a reasonably linear region of the calibration curve spanning the data (solid line) but the linear region has its limits (dotted curves). 

Stogiou, M. Kapetanaki, C. latrou, H. Examination of the universality of the calibration curve of size exclusion chromatography by using polymers having complex macromolecular architectures. Int. J. Polym. Anal. Charact. 2002, 7, 273-283. 

ASTM D-3593 Test Method for Molecular Weight Averages/Distribu-tion of Certain Polymers by Liquid Size Exclusion (Gel Permeation) Chromatography—GPC Using Universal Calibration (has also been adapted to the investigation of molecular weight distribution in petroleum fractions). 

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