Gas chromatography (GC) — Which of the following materials is NOT used as the stationary phase coating inside a GC analytical column?

Introduction / Context:
In gas chromatography (GC), the stationary phase is a thin film coated onto the inner wall of capillary columns or onto solid supports in packed columns. Knowing what truly constitutes the stationary phase versus what is simply the column substrate is essential for correct method selection and troubleshooting.

Given Data / Assumptions:

• Capillary GC columns are typically made of fused silica tubing.
• The stationary phase is a chemically distinct, immobilized liquid or ionic liquid polymer coating.
• Chiral separations may employ derivatized cyclodextrin stationary phases.

Concept / Approach:
The stationary phase in GC is a film of a liquid-like polymer (e.g., polysiloxanes, polyethylene glycol) or specialized phases (e.g., ionic liquids, derivatized cyclodextrins). The tubing itself (fused silica) is not the stationary phase; it is the inert support that carries the coating. Thus, bare silica is not used as the stationary phase coating in standard GC columns.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer datasheets describe columns by their stationary phase chemistry (e.g., 5% phenyl–95% dimethylpolysiloxane), while the tubing is specified separately as fused silica with polyimide coating for strength.

Why Other Options Are Wrong:

• Cyclodextrins: widely used as chiral GC stationary phases.
• Polysiloxanes and PEG-type phases: the most common GC coatings.
• Ionic liquids: modern stationary phases offering unique selectivity and thermal stability.

Common Pitfalls:
Confusing the support (fused silica) with the stationary phase coating. The support is inert; the stationary phase is the film interacting with analytes.

Final Answer:
Silica (bare, uncoated silica used as the phase itself).