Are you generating Gel Permeation Chromatography/Size Exclusion Chromatography (GPC/SEC) data for comparison to historical results?  Do your novel samples complicate multi-detector analyses?  Or are you simply feeling nostalgic for calibration curves?  Whatever your reason, the OMNISEC software now offers a variety of analysis options for your samples!

Introducing version 11.20 of the OMNISEC software, which now contains the following analysis method options:

  • Conventional calibration
  • Universal calibration
  • Mark-Houwink calibration

These analysis methods join the multi-detection option based on light scattering data already available with OMNISEC.  Below, I’ll provide a brief summary, discuss the detectors involved, and describe the data available for each method. 

Conventional calibration

The simplest analysis method, conventional calibration is how many users are introduced to GPC/SEC.  First, the analyst creates a conventional calibration curve by running standards of known molecular weight.  This calibration curve assigns a molecular weight to each retention volume.  Then, when an unknown sample is analyzed, its molecular weight is calculated based on its retention volume and the corresponding molecular weight at that point on the calibration curve. 

Conventional calibration requires the least amount of equipment and knowledge of sample and standard molecular characteristics.  As a result, it is the most accessible GPC/SEC configuration. A setup requires the presence of a single concentration detector only; often a refractive index (RI) or ultraviolet (UV) detector.  Since molecular weight results are based on retention volume, the resulting data are described as relative molecular weight values.  The single-detector arrangement limits the calculated data to molecular weight moments; no structure-based information is available (e.g. intrinsic viscosity).  The accuracy of the calculated results depends on the similarity of the sample’s structure to that of the standards.

Universal calibration

The addition of a viscometer detector to a concentration detector makes universal calibration possible.  Universal calibration capitalizes on the relationship between hydrodynamic volume (Vh), molecular weight (MW), and intrinsic viscosity (IV), where Vh ∝ MW•IV. Recognizing that GPC/SEC separates samples by size, or Vh, Benoit et al. showed that plotting MW•IV against retention volume (essentially Vh) generated a single, universal calibration curve for polymers of different types and structures.

In addition to providing intrinsic viscosity and related size information, a universal calibration can offer structural insight through Mark-Houwink parameters and branching information.  Another advantage is that a universal calibration eliminates the need to have standards that are structurally identical to your sample in order to calculate accurate molecular weight data.  As a result, these molecular weights are sometimes referred to as true or accurate molecular weights, as the term absolute is generally reserved for describing molecular weights determined by light scattering methods

Mark-Houwink calibration

A Mark-Houwink calibration is similar to a conventional calibration in that it only requires a single concentration detector.  But, if you know the Mark-Houwink parameters for your standards and samples you can build a MW•IV vs. retention volume calibration curve like the one used for universal calibration.  Consequently, this will allow you to calculate IV, size, and accurate molecular weight values for your sample without having a viscometer.  

The Mark-Houwink calibration is the least common of these three methods, mainly because many users don’t have Mark-Houwink parameters available for their samples.  But when that information is accessible, this method allows you to calculate more data with less equipment.

Final thoughts

In conclusion, three analysis methods utilizing calibration curves are now available in the OMNISEC software!  To find out more on how these methods can work for you, please contact us or reach out to me individually at kyle.williams@malvernpanalytical.com.  Furthermore, stay tuned to Materials Talks for additional information on the topic!

Further reading: