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Types of Liquid Stationary Phases for Gas Chromatography
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 liquid 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
100% Dimethylpolysiloxane
InertCap 1, InertCap 1MS Non-polar
5% Diphenyl (equiv.) -Dimethylpolysilphenylene siloxane
InertCap 5MS／Sil, InertCap Pesticides Low polarity
5% Diphenyl 95% Dimethylpolysiloxane
InertCap 5MS/NP, InertCap 5 Low-polarity
6% Cyanopropylphenyl 94% Dimethylpolysiloxane
InertCap 624, InertCap 1301 Medium-polarity
25% Diphenyl 75% Dimethylpolysiloxane
InertCap 25 Medium-polarity
35% Diphenyl 65% Dimethylpolysiloxane
InertCap 35 Medium-polarity
14% Cyanopropylphenyl 86% Dimethylpolysiloxane
InertCap 1701 Medium-polarity
50% Diphenyl 50% Dimethylpolysiloxane
InertCap 17MS, InertCap 17 Medium-polarity
50% Trifluoropropyl 50% Methylpolysiloxane
InertCap 210 Medium-polarity
50% Cyanopropylmethyl 50% Phenylmethylpolysiloxane
InertCap 225 Medium-High polarity
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
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.
Polyethylene Glycol
InertCap Pure-WAX, InertCap WAX, InertCap WAX-HT
Nitroterephthalic acid modified Polyethylene Glycol
InertCap FFAP High-polarity
Image 1. Relative Polarities of GL Sciences GC Stationary Phases
Special liquid phases
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.
| Column | Phase | Polarity |
Application |
| InertCap for Amines | Special liquid phase | － | Amine Compound, Alcohol |
| InertCap CHIRAMIX |
Special liquid phase | － | Optical Isomer |
| InertCap AQUATIC |
25% Diphenyl | Middle | VOC, Organic Solvent |
| InertCap AQUATIC-2 |
25% Diphenyl 75%Methylpolysiloxanee | Middle | VOC, Organic Solvent |
Challenges of liquid stationary phases
Silica- and WAX-based phases have some limitations. Residual metals, halides, and silanols from incomplete polymerization can cause unwanted interactions with components, leading to peak degradation. These residues can also cause phase degradation when exposed to moisture or oxygen.
Comparison of High-Adsorptive Samples
Image 2. Residues leading to peak degradation in WAX-based columns. GL Sciences' proprietary processing technology eliminates residues, providing highly inert columns with excellent peak symmetry for the analysis of polar, basic, and acidic compounds, as well as metal ligands.
Column bleed is another concern. Insufficient bonding, cross-linking, and residues can cause stationary phases to decompose and be eluted, especially at high temperatures. This leads to significant increases in the baseline until the column loses all retentive capability.
Image 3. Column bleed causing phase degradation. GL Sciences InertCap GC columns are highly cross-linked and surface-deactivated to ensure ultra-low bleed performance.
Summary
There are different types of liquid phases for capillary GC columns. Compatibility with components is primarily determined by polarity. Commercially available stationary phases face production challenges, making it important to choose the right column for your process.
We hope you've gained a deeper understanding of liquid stationary phases for capillary GC columns. Check back for the next article in our series on capillary columns for gas chromatography, where we will discuss column specifications.
For more information, take a look at GL Sciences' selection of capillary GC columns.
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