Column Switching in Gas Chromatography

Abstract

In spite of the wide variety of stationary phases that are used for gas chromatography, analysts frequently encounter situations where it is not possible or practical to analyze a given mixture on a single column. The situation may be as simple as in the analysis of fixed gases requiring the resolution of O2, N2, and CO2. Except for the use of porous polymers at subambient conditions, the molecular sieves (5A or 13X) are the packings of necessity for such samples. However, carbon dioxide and/or low molecular weight hydrocarbons are not quantitatively eluted from molecular sieves even at elevated temperatures. To perform this analysis on a single sample injection thus requires the use of two columns, typically a porous polymer and a molecular sieve column. The columns can be connected in parallel and, with proper sizing of the lengths of the columns, the desired resolution can be achieved. Gradual deterioration of the performance of the molecular sieve column will be experienced due to the buildup of noneluted material, particularly so with samples containing water. The column must then be periodically removed and reactivated by heating. Column performance can be maintained by preventing the noneluting components from ever reaching the molecular sieve column by connecting the columns by the use of a switching valve [20,97]. Even in this seemingly simple situation the analyst must use a multi-column approach to the solution of the problem. This is only one of the ways that column switching can be used in gas chromatography.