In the second installment of our series about capillary columns for gas chromatography (GC), we focus on liquid stationary phases, the most popular type of phase for capillary GC columns. We examine the major types of liquid stationary phases, their features, and challenges.

Polarity is the principal factor determining the capability of a stationary phase to retain gas or volatile components in gas chromatography, as particles are drastically farther apart than in other phases, and interaction between particles is much more limited.

Liquid phases for general analysis are based on a strong skeleton, which is functionalized with different groups to modify their polarity. Liquid phases for general analysis are structurally divisible into silica-based phases and polyethylene glycol (PEG) phases, also referred to as WAX phases.

Silica-based phases are founded on a siloxane (Si-O-Si) skeleton. Group modification is done at the Si terminals. Silica-based liquid phases generally exhibit low to medium polarity. Common substituent groups include the following:

Table 1. Comparison of Capillary and Packed GC columns

Commercially available silica columns have produced standard naming conventions and ratios of functional groups for silica-based phases (–1, –5, –1701, –35, etc.).

Stationary phases used for capillary columns have two types: solid phases and liquid phases. Liquid stationary phases are much more common for general analysis. Capillary columns can be grouped into three types based on the method the stationary phase has immobilized on the inner walls: Wall-Coat Open Tubular (WCOT) column, Support-Coated Open Tubular (SCOT) column, Porous-Layer Open Tubular (PLOT) column. The different types are described below:

WAX-based liquid phases use a polyethylene glycol (-(CH2)2O-) polymer backbone. WAX-based phases are more polar than silica-based phases and less thermally stable, decomposing at around 280 °C. Free fatty acid phase (FFAP), which is a polymer of polyethylene glycol and nitro terephthalate, is very polar and is used in free fatty acid and fatty acid ester analyses.

Aside from these general-purpose phases, liquid phases for specialized applications are also employed. Examples include special siloxane-based phases for the measurement of volatile organic compounds and chiral stationary phases for separating enantiomers.