|
|
|
|
|
|
Tetra Detector Array
A revolutionary, integrated multiple-detector device designed for the characterization of natural and synthetic polymers and copolymers, proteins, protein conjugates, excipients and other macromolecules. Comprised of modular Refractive Index, Ultra-Violet, Low Angle Light Scattering and Viscometer detectors.
|
HTGPC
The Viscotek High Temperature GPC (HT-GPC) System is a revolutionary advanced detector system specifically designed for the characterization of polyolefins, natural and synthetic polymers, nanoparticles and other large molecules.
|
"I have purchased 7 GPC systems from Viscotek in Europe, US and Canada. I am very satisfied. The systems are robust and affordable. OmniSEC software is very user friendly and highly versatile."
- R.N., Academic Institution
|
News USA:
Check out the latest USA News involving Viscotek. |
|
|
|
|
|
|
|
|
GPC Theory: Conventional Calibration |
In Gel Permeation Chromatography - as we have seen in Separation
-
molecules are separated according to their hydrodynamic volume and then characterized by a detector or series of detectors as they elute from the GPC
column. Conventional Calibration
is a widely used GPC technique that employs a concentration detector, typically
a Refractive Index (RI) Detector
and/or UV/VIS, and a calibrated column to obtain
relative molecular weight, molecular size and polydispersity.
In Conventional Calibration, the
molecular weights (MW) and molecular weight distribution are
determined from the measured retention volume (RV) by means of a
calibration curve (log MW vs. RV), which must be set up with the
aid of a number of
standards of known molecular weight.
The process of conventional calibration is as follows:
- Inject a series of Narrow Standards of known Molecular Weight.
- Measure the Retention Volume (RV) of the resulting peak apex.
- Construct a calibration curve of Log(MW) vs. Retention Volume.
Note:
Retention Volume is measured instead of Time to eliminate dependence on flow rate,
and you must select standards that cover the entire Molecular Weight range of the sample to be analyzed.
Having gathered this calibration data, we now create a calibration curve to describe
the relationship between Retention Volume and Molecular Weight; this is usually
done by fitting the data to a polynomial:
Now when the unknown sample is analyzed, it is a simple process to determine its
Molecular Weight. Typically, polymer scientists are interested in at least
3 properties of the Molecular Weight distribution. These are called the Weight,
Number and Z-Average
and are defined as follows:
Where Mi is the Molecular Weight at the ith data point, and Ci is the concentration (or weight-fraction)
at the ith data point.
The drawback with Conventional Calibration is that you may not
have available standards of the same type as your sample. Remember,
GPC separation is based on Hydrodynamic Volume, not Molecular Weight;
therefore, in attempting to determine Molecular Weight by Conventional Calibration we are making the assumption that the sample is of the
same density as the standards. This is often not the case, and hence
we often refer to the results from Conventional Calibration as Relative Molecular Weight or
'Polystyrene Equivalent' Molecular Weight (Polystyrene being the most
common standard type).
To overcome this problem, we need to construct a calibration curve
where the y-axis is proportional to the Hydrodynamic Volume, instead
of Molecular Weight. Fortunately, the addition of a Viscometer Detector, and utilization of the technique of Universal Calibration
allows us to do precisely that. For more information on Universal
Calibration,
please access the navigation on the left.
|
|
|
|