ASTRA has native support for viscometry data collection and analysis from the ViscoStar®. The viscometry module may also be utilized for high temperature GPC applications. Several different types of viscometry analysis are performed. Coupled with Wyatt Technology's experience in measuring molar mass using light scattering, macromolecular characterization via viscometry has reached a new level with ASTRA.
Instrinsic viscosity calculations
Using the ViscoStar and a concentration detector in series, it is possible to calculate the intrinsic viscosity of a sample as a function of elution volume. Analysis is a snap. Set a peak region and enter a dn/dc or UV extinction value for the sample if using an RI or UV concentration detector respectively. Most important of all, ASTRA's powerful band broadening correction makes accurate analysis of viscometry data possible for the first time. Consider the intrinsic viscosity for a BSA oligomer sequence with and without the band broadening correction. The constant, and accurate, value for the intrinsic viscosity of each oligomer can be determined and reported using ASTRA.
Using a combination of the ViscoStar, DAWN or miniDAWN, and a concentration detector, ASTRA can determine the Mark-Houwink-Sakurada constants K and a. The level of accuracy in determining these parameters is unparalled. Coupled with the usual measurement of intrinsic viscosity, the molar masses are measured directly using light scattering. Consider the Mark-Houwink-Sakurada plot for the 706 polystyrene standard. ASTRA provides a new benchmark for determining the Mark-Houwink parameters, and can accurately reveal instances where deviations occur from the classical Mark-Houwink-Sakurada equation behavior.
Molar Mass: Mark-Houwink-Sakurada Procedure
ASTRA can take data from the ViscoStar and a concentration detector to calculate the molar mass based on the Mark-Houwink-Sakurada (MHS) equation. To use this procedure, it is only necessary to know the Mark-Houwink K and a coefficients. These can be taken from the literature, or determined with unparalleled accuracy using ASTRA in combination with a light scattering detector, ViscoStar, and concentration detector. Once the molar mass has been calculated, ASTRA's powerful distribution and moment procedures are used to calculate the molar mass moments (Mn, Mw, and Mz) and molar mass distributions for your sample. Most importantly, the MHS procedure benefits from ASTRA's band broadening corrections, making it possible to determine the molar mass accurately for each eluting volume.
Molar Mass: Universal Calibration
Unversal Calibration creates a single calibration curve for a given column using just the ViscoStar and a concentration detector (Optilab or UV). The elution time is directly related to hydrodynamic volume which, in turn, is related to the molar mass and intrinsic viscosity. Assuming the sample is a random coil polymer, intrinsic viscosity and elution time are combined to determine molar mass. Universal Calibration results can be turned into distributions and moments, just like MHS procedure results.
Coupled with the DAWN or miniDAWN and a concentration detector, the ViscoStar can determine an effective size of a macromolecule from its excluded volume. When compared with the rms radius from light scattering measurements, or the QELS hydrodynamic radius, the viscometry size measurements shed new light on macromolecular conformation and properties. Consider the effective size, or hydrodynamic radius, of random coil polystyrene standards in THF. The Rh vs. volume plot shows how well viscometry can determine sizes, and accurately reveal the chromatography of the system. These types of size measurements with viscometry have never been seen before; the band broadening that occurs with a viscometer and two additional detectors is so extreme that only ASTRA's band broadening correction can reveal the true nature of the system.
Distributions and moments
An additional advantage of coupling a viscometer with a light scattering and concentration detector comes from the ability to calculate distributions and moments of the intrinsic viscosity. For example, the traditional number, weight, and z-averaged intrinsic viscosity moments can be calculated for each peak. Also, ASTRA's powerful distribution features make it possible to view accurate plots of cumulative and differential intrinsic viscosity, or the corresponding effective sizes.
Why the band broadening correction is essential for viscometry
Viscometers have an additional source of broadening beyond mixing (see band broadening correction for details). When a sample enters the second stage of the capillary bridge, it creates a differential pressure that shows up as the viscometer signal. The volume of one arm of the bridge is about 30 µL. Because the sample flow is split in the bridge, the detected elution volume is thus approximately 60 µL for a viscometer. Contrast this with the < 1 µL detection volume of a light scattering or concentration detector.
An initially sharp peak will therefore be broadened by approximately 60 µL for the viscometer alone, without even taking into account the broadening due to mixing. This makes the band broadening correction essential for determining intrinsic viscosity accurately, since intrinsic viscosity is calculated by taking the ratio of the viscometer and concentration peaks.
ASTRA's powerful band broadening correction accounts for the type of broadening present in viscometers, as well as the broadening due to mixing. The results are amazing. For the first time, accurate intrinsic viscosities can be measured for narrow standards.