1. Principle of Gas Chromatography

Gas Chromatography is based on the principle of partitioning of components between a mobile phase (inert carrier gas) and a stationary phase (solid or liquid supported on a solid).

• Mobile phase: An inert gas (e.g., helium, nitrogen, hydrogen) that carries the sample through the column.

• Stationary phase: Coated inside the column (capillary or packed) — either a solid adsorbent or a liquid film on an inert support.

Separation Principle:
Each compound in the injected sample interacts differently with the stationary phase and travels at different speeds through the column.

• Components with less interaction with the stationary phase move faster and elute earlier.

• Components with more interaction move slower and elute later.

The time taken for a component to travel through the column and reach the detector is called the retention time (tR) — characteristic for each compound under fixed conditions.

2. Working of Gas Chromatography

The GC process occurs in six main steps:

1. Sample Injection

   • The sample (liquid or gaseous) is injected into the injector port using a microsyringe.

   • The injector is kept at a high temperature to vaporize the sample instantly.

2. Vaporization & Mixing with Carrier Gas

   • The vaporized sample mixes with the carrier gas (mobile phase) and enters the column.

3. Separation in the Column

   • The column (in an oven) contains the stationary phase.

   • Oven temperature is controlled — either isothermal or temperature programmed — to optimize separation.

   • Components partition between the mobile gas phase and stationary phase, separating based on volatility and polarity.

4. Detection

   • As separated components exit the column, they pass through a detector (e.g., Flame Ionization Detector [FID], Thermal Conductivity Detector [TCD], Electron Capture Detector [ECD], etc.).

   • The detector generates a signal proportional to the concentration of each component.

5. Signal Processing

   • The signal is sent to a computer/data system, which records it as peaks on a chromatogram.

   • The position (retention time) identifies the compound, and peak area/height quantifies it.

6. Data Interpretation

   • The chromatogram is analyzed for qualitative (identification) and quantitative (amount) purposes.

Schematic Flow of GC:

Sample Injection → Vaporization → Carrier Gas Transport → Column Separation → Detection → Data Output

3. Applications

• Pharmaceuticals: Residual solvent analysis, purity testing

• Environmental: Air pollutants, pesticide residues

• Food: Flavor and fragrance profiling

• Forensic: Alcohol and drug analysis in biological samples

• Petrochemical: Hydrocarbon analysis

✅ Key Takeaway:
Gas Chromatography separates compounds based on differences in volatility and interactions with the stationary phase, using an inert gas to transport the sample through a temperature-controlled column, with detection at the end.