The Column - June 2008 - (Page 22)

Clarke The Column www.thecolumn.eu.com June 2008 ZenShui/Michele Constantini/Getty Images Analysis of Macromolecular Branching by Triple Detection GPC/SEC Paul Clarke, Viscotek European Operations, Crowthorne, Berkshire, UK. The measurement of the molecular weight distribution of macromolecules by gel permeation chromatography (GPC) or size exclusion chromatography (SEC) is a well established technique with many areas of application. These include natural polymers such as polysaccharides and proteins, which find applications in foodstuffs, medicines and drug delivery systems, as well as the huge array of synthetic polymers, co-polymers and polymer blends that are so important in many products and processes. The quality of the products will depend highly on having both the correct value of the molecular weight and in measuring its consistency during material or product manufacturing. Of equal importance is determining if the molecules are structurally consistent as this may have an even greater effect on the end properties. This article will focus on how advanced GPC can be used to measure branching, one of the most common structural changes in macromolecules. Triple Detection GPC In its basic form the technique of GPC, which is based on a size separation, delivers only relative molecular weight data and furthermore is unable to differentiate between polymers of different structure. The drive for more accurate molecular weight information has led to the development of advanced GPC systems containing three detectors which correctly resolve the effects of both molecular weight and structural differences. In these triple detection systems the three detectors measure different, but complementary parameters to give a complete view of the macromolecules as they elute from the GPC/SEC column. The concentration is most commonly Author: Paul Clarke Contact: paul.clarke@viscotek.com measured by either a differential refractive index (RI) or a UV detector; the molecular weight by a light-scattering detector (LALS); and the viscosity by a differential viscometer detector. Software can then turn this array of data — concentration, molecular weight and viscosity with respect to GPC elution volume — into valuable information about the sample, including very accurate “absolute” molecular weights, irrespective of the structure of the molecules, thereby overcoming the limitation of the basic GPC technique. This includes very accurate, directly measured “absolute” molecular weights from the LALS detector. Figure 1 shows a typical output from an advanced triple detection GPC system. 22 http://www.thecolumn.eu.com

Table of Contents for the Digital Edition of The Column - June 2008

The Column - June 2008
Contents
Sensory Quality Control of Food and Packaging Products
News
Incognito Conferences and Exhibitions – Worthwhile or Not?
Market Trends and Analysis
Tips and Tricks: GPC/SEC How Do I Calibrate a GPC/SEC System?
Analysis of Macromolecular Branching by Triple Detection GPC/SEC

The Column - June 2008