Tips and Tricks: GPC/SEC The Column www.thecolumn.eu.com April 2009 Chris Stein/Getty Images Tips & Tricks: GPC/SEC How to Choose a Static Light-scattering Technique for Molar Mass Determination Daniela Held and Peter Kilz, PSS Polymer Standards Service GmbH, Mainz, Germany. Light scattering is one of the few absolute methods for the characterization of macromolecules and biopolymers. On-line light-scattering detectors are used in GPC/SEC systems to measure molar masses, the radius of gyration and to identify high molar mass content at low concentrations. There are several detectors available that differ in the provided light-scattering technique. It is necessary to select the correct technology for a specific application and to do this users need to know the basic principles. What is the Principle of Static Light Scattering from Polymer Solutions? In light scattering from polymer solutions, detectors measure the intensity of light scattered from the dilute sample solution. The light source is a laser; the scattered light is measured at one or more fixed detector angles (compare Figure 1). These angles can be, but must not be the true scattering angles. This depends on the cell design and geometry: • If the intensity is measured directly in a cell with cylindrical geometry the scattering angle and the detector angle are always the same. • If glass cells are used the interface solution/glass is responsible for the fact that all detector angles besides the Contact author: E-mail: Daniela Held Dheld@polymer.de 90° angle need a correction to obtain the true scattering angle (compare Figure 2). The term “static” does not refer to performing an on-line (GPC/SEC) or batch (stand-alone) light-scattering experiment, but to the fact that the time averaged scattering intensity is measured. “Dynamic” light scattering measures the light intensity fluctuations and can also be done in an on-line and batch experiment. Why is One Angle (e.g., at 90°) for Light Scattering Not Enough? If the dimensions of the particle are small compared to the wavelength of the incident beam this particle can be 28
http://www.thecolumn.eu.com
Table of Contents for the Digital Edition of The Column - April 2009