Gas Chromatography

Updated Apr 17, 2026

Gas chromatography (GC) separates volatile compounds in the gas phase. It is used for small, volatile organic molecules that can be vaporized without decomposition. GC is fast, sensitive, and highly automated - one injection can resolve dozens of peaks in minutes.

How GC Works

A small volume of sample (typically 1 μL in solvent) is injected into a heated port where it vaporizes.
A carrier gas (usually helium, sometimes nitrogen or hydrogen) sweeps the vapor through a long, coiled capillary column inside a temperature-controlled oven.
The column’s inner walls are coated with a liquid stationary phase.
Compounds with higher affinity for the stationary phase are retained longer; compounds with higher volatility pass through faster.
At the end of the column, a detector (FID, TCD, or mass spec) registers each compound as it emerges.
The output is a chromatogram: peaks vs. time. Each peak is a compound; its retention time and peak area are the key data.

What Gets Separated

Only volatile, thermally stable compounds can be analyzed by GC. Typical targets:

Solvents and alcohols.
Hydrocarbons, fatty acid methyl esters.
Small organic compounds (MW < 500 usually).
Volatile flavors and fragrances.

What does NOT work:

Large biomolecules (proteins, DNA) - too heavy, non-volatile.
Ionic salts - do not vaporize.
Thermally labile compounds - decompose at injection temperature.

For these, use HPLC or another technique.

Retention Time

Retention time (tR) is the time for a compound to pass from injection to detection. It depends on:

Boiling point: low bp = faster movement (less time in stationary phase’s liquid film).
Column interactions: polar column retains polar compounds more strongly.
Temperature: higher oven temperature = faster movement for everything.

Programmed temperature GC (temperature ramps up during the run) is standard for complex mixtures.

Common Detectors

Detector	Abbreviation	What it measures	Use
Flame ionization	FID	Carbon ions in a flame	Most organic compounds
Thermal conductivity	TCD	Heat transfer in carrier gas	Simple, universal
Electron capture	ECD	Electronegative atoms	Halogens, nitro groups
Mass spectrometry	MS	Mass-to-charge ratio	Full identification (GC-MS)

GC-MS combines separation (GC) with identification (MS). It is the gold standard for analyzing complex mixtures - the GC gives you retention time, the MS gives you the molecular structure of each peak.

Quantification

Peak area is proportional to the amount of compound (when using an appropriate detector like FID). Calibration with known standards allows quantitative analysis:

Drug testing (forensic analysis).
Food quality control.
Environmental analysis (pesticides, pollutants).

GC vs. HPLC

Feature	GC	HPLC
Mobile phase	Gas (usually He)	Liquid (usually water/MeCN mix)
Temperature	40-350°C	Room temp (or slightly heated)
Volatile compounds	Yes	Yes
Non-volatile compounds	No	Yes
Thermally labile compounds	No	Yes
Speed	Fast (minutes)	Slower (15-60 min typical)
Resolution	Very high	Very high

For MCAT: GC = volatile small molecules. HPLC = non-volatile, large, or thermally labile.

A forensic analyst wants to identify volatile compounds in a blood sample. She has two options: GC-MS or HPLC. Which technique should she choose and why?

Click to reveal answer

GC-MS. For volatile compounds in a biological sample (like alcohol, toxic solvents, or some drugs of abuse), GC-MS is ideal: GC separates the volatile components quickly, MS identifies each peak by its mass spectrum. HPLC could work but is slower and less sensitive for small volatile molecules. For non-volatile analytes like proteins or large drug molecules, HPLC (often HPLC-MS) would be preferred. The volatility of the target analyte decides the choice.