Gas Chromatography vs HPLC

Gas chromatography (GC) and high-performance liquid chromatography (HPLC) are the two workhorses of modern separation science. Between them, they measure pesticides in food, drugs in blood, impurities in medicines, flavours in coffee and pollutants in water. Both push a sample through a column and detect components as they emerge, but they differ in one fundamental way: GC uses a gas as the mobile phase, HPLC uses a liquid. That single difference decides which samples each can handle.

Quick comparison

Feature	Gas chromatography (GC)	HPLC
Mobile phase	inert gas (helium, hydrogen or nitrogen)	liquid solvent mixture (e.g. water with methanol or acetonitrile)
Stationary phase	thin liquid or polymer film coating the inside of a long, narrow capillary column	small particles (often silica with bonded groups such as C18) packed in a short column
Sample requirement	must be volatile and thermally stable	must dissolve in the mobile phase; needn’t be volatile
Temperature	column in an oven, often up to 300–350 °C	usually near room temperature to about 60 °C
Column length	typically 15–60 m (coiled)	typically 5–25 cm
Main separation factor	boiling point and interaction with the stationary phase	polarity and interaction with the stationary phase
Common detectors	flame ionisation (FID), mass spectrometer, electron capture	UV-visible, diode array, fluorescence, mass spectrometer, refractive index
Typical molecules	small, volatile organics: solvents, fuels, flavours, fatty acid esters, many drugs and pesticides	larger, polar, non-volatile or heat-sensitive molecules: pharmaceuticals, sugars, proteins, vitamins, dyes
Running cost	lower (carrier gas is cheap)	higher (high-purity solvents, disposal)

How each works

Gas chromatography

1. A tiny volume (around 1 μL) of sample is injected into a heated port, where it vaporises.
2. The carrier gas sweeps the vapour into the column.
3. Components interact with the stationary film to different extents. Those that are more volatile and less attracted to the film emerge first.
4. The oven temperature is often programmed to rise during the run, so that high-boiling compounds eventually elute.
5. A detector records each component as a peak; the time taken is its retention time.

See gas chromatography.

HPLC

1. A pump forces the liquid mobile phase through the packed column at high pressure (often hundreds of bar), because the tiny particles resist flow.
2. The sample is injected into the flowing liquid.
3. In the most common mode, reversed-phase HPLC, the stationary phase is nonpolar (C18 chains bonded to silica) and the mobile phase is polar (water with an organic solvent). Polar compounds elute first; nonpolar ones are retained longer.
4. The solvent composition can be changed during the run (gradient elution) to elute strongly retained compounds.
5. A detector, most often UV-visible absorbance, records the peaks.

See HPLC.

The key question: can it be vaporised?

This is the fastest way to choose.

• Yes, easily and without decomposing → GC is usually simpler, cheaper and gives very sharp peaks.
• No (too large, too polar, ionic or heat-sensitive) → HPLC.
• Only with difficulty → GC may still work after derivatisation, a chemical reaction that makes the molecule more volatile (for example, converting –OH or –COOH groups into less polar derivatives). This adds a sample-preparation step, so HPLC is often preferred.

Resolution and speed

• GC capillary columns are very long and narrow, which gives extremely high resolution: hundreds of components in a fuel or essential oil can be separated in one run.
• HPLC columns are shorter; resolution is excellent but typically not as high as capillary GC for small volatile molecules. Modern ultra-high-performance systems (UHPLC), with even smaller particles and higher pressures, have narrowed the gap and cut run times.

Detectors

GC:

• Flame ionisation detector (FID): burns the organic compounds in a hydrogen flame and measures the ions formed. It responds to almost all organic compounds, roughly in proportion to their carbon content, so it’s excellent for quantification.
• Electron capture detector (ECD): very sensitive to halogenated compounds such as many pesticides.
• Mass spectrometer (GC-MS): identifies each peak from its fragmentation pattern. See mass spectrometry.

HPLC:

• UV-visible and diode array detectors: measure absorbance, using the Beer–Lambert law. A diode array records a full spectrum for each peak, which helps identification.
• Fluorescence detector: very sensitive and selective for fluorescent compounds.
• Refractive index detector: for compounds without UV absorption, such as sugars.
• Mass spectrometer (LC-MS, LC-MS/MS): now the gold standard for trace analysis in complex samples.

Real examples: which would you use?

Analysis	Technique	Why
Ethanol in blood	GC (headspace)	ethanol is small and volatile
Caffeine in energy drinks	HPLC	dissolves readily; UV-absorbing; simple aqueous samples
Composition of petrol	GC	complex mixture of volatile hydrocarbons
Active ingredient in a tablet	HPLC	many drugs are polar and thermally sensitive; standard in pharmaceutical QC
Fatty acids in cooking oil	GC, after conversion to methyl esters	derivatisation makes them volatile
Sugars in fruit juice	HPLC (refractive index detector)	sugars decompose on heating and don’t absorb UV strongly
Pesticide residues	both (GC-MS and LC-MS/MS)	different pesticides suit different methods
Proteins and peptides	HPLC	large, non-volatile molecules
Aroma compounds in coffee	GC-MS	volatile compounds, often at trace levels

See also how food is analysed and how drug tests work.

Sample preparation differences

• GC: samples are usually dissolved in a volatile organic solvent. Water is often avoided because it can damage some columns and detectors. For volatile compounds in solid or liquid samples, headspace sampling injects only the vapour above the sample, leaving the non-volatile matrix behind.
• HPLC: samples are dissolved in a solvent similar to the mobile phase and filtered, because particles would block the tightly packed column. Aqueous samples such as drinks can often be injected almost directly after filtering.

In both methods, cleaner samples mean longer column life and more reliable results.

Strengths and limitations summary

GC strengths: very high resolution; sensitive universal detector (FID); cheap mobile phase; ideal for volatile compounds; well-established libraries for GC-MS identification.

GC limitations: sample must be volatile and thermally stable; non-volatile residues can contaminate the column.

HPLC strengths: handles most compounds that dissolve, including large, polar and heat-sensitive ones; gentle conditions; can collect separated fractions (preparative HPLC).

HPLC limitations: higher solvent costs and waste; generally lower resolution than capillary GC for small molecules; some compounds lack a convenient detector response.

Key takeaways

• GC uses a gas mobile phase and suits volatile, thermally stable compounds; HPLC uses a liquid mobile phase and suits almost anything that dissolves.
• GC separates largely by volatility and gives exceptionally high resolution in long capillary columns.
• Reversed-phase HPLC separates by polarity using a nonpolar stationary phase and polar solvents.
• Both couple powerfully to mass spectrometry (GC-MS and LC-MS) for identification.
• Choose by asking whether the analyte can be vaporised without decomposing. For simpler planar methods, see TLC vs paper chromatography.