Principles of Triple Detection GPC/SEC

Introduction

Triple Detection GPC/SEC is a well-established technique for the accurate and complete characterization of macromolecules. The technique combines both light scattering and viscometer detectors with a refractive index (RI) detector and is applicable to both natural molecules, such as proteins or polysaccharides, and synthetic polymers.

The resulting molecular weight and structure information provides an invaluable tool for macromolecular researchers. This document will help the practitioner of triple detection GPC/SEC to understand the theory behind the detector technology and have a greater appreciation of the strengths and limitations.

1. The RI detector 

1.1 Theory

The most common type of RI detector in use today is often called the deflection type.  As the name suggests, it is based on the deflection of a beam of light as it passes through a dual compartment flow cell, as shown in Figure 1(a). The deflection of the beam is measured by two photodiodes in a differential circuit. One side of the cell compartment contains the reference solvent of refractive index n₀, which is static during the measurement process. The other side contains the sample solution, i.e. the column eluent, having refractive index n. The beam is refracted at the liquid-glass interfaces that separate the two compartments and also at the liquid-glass interface on the exit wall, and again at the glass-air interface on the exit wall. A rigorous analysis must take each of the refractions into account. Fortunately, a simplified analysis that ignores the glass interfaces as shown in Figure 1(b) yields substantially the same result as the rigorous analysis, so that derivation will be reviewed instead.