Using OMNISEC to Understand the Effect of Molecular Weight and Structure on the Performance Characteristics of Cellulose Derivatives

Introduction

A key family of molecules of interest to the pharmaceutical industry is those derived from cellulose such as hydroxyethyl cellulose and others. These macromolecules are typically used as viscosity modifiers for various products such as eye drops and moisturizing creams, and as excipients in many other applications. Typical analysis of cellulose derivatives has focused on determining the molecular weight distribution and intrinsic viscosity of the sample. The main goal of this is to understand how the addition of the cellulose derivative to an existing process or solution will affect the viscosity. Miscalculating the amount of cellulose needed to modify the viscosity will either not achieve the desired increase (on the low end) or drastically increase the pumping and other processing costs (on the high end). By analyzing the cellulose before mixing into the existing solution, the user can know how much of the cellulose to add, how to blend or grade the sample, or whether to seek out another source of cellulose if the analyzed batch does not meet the user’s specifications.

Gel permeation chromatography (GPC) or, equivalently, size exclusion chromatography (SEC) is a widely used technique to characterize a wide variety of macromolecules, from bulk manufactured polymers to proteins. This technique can be used to measure the molecular weight moments, molecular weight distribution, the intrinsic viscosity, and/or hydrodynamic size of these macromolecules. Figure 1 shows a complete setup for a GPC system.