Gas chromatography

Basic principle of gas chromatography

1. Carrier gas
   Gas chromatography uses an inert carrier gas (usually helium, sometimes nitrogen or hydrogen) to transport the sample components through the system.

2. Injection of samples
   The sample mixture (often in liquid or gaseous form) is injected into the instrument. In most GC systems, injection is through a heated injection port. There, samples in liquid form are converted to gas phase (if they are not already gaseous) before they follow the carrier gas into the column.

3. Column
   The main component of a gas chromatograph is the column. It is usually a long and thin tube (capillary column) coated with a stationary phase on the inside:

   • Stationary phase: A thin layer of a liquid (or sometimes a polymer) attached to the inner wall of the column.

   • Moving phase: The carrier gas that continuously flows through the column.

4. Separation
   As the volatile components of the sample travel through the column, they interact differently with the stationary phase. Those substances with a higher affinity for the stationary phase are 'slowed down' and come out later, while those with a lower affinity move faster and elute earlier. In this way, the substances are separated in time.

5. Detection
   At the outlet of the column is a detector. Some common detector types are:

   • FID (Flame Ionisation Detector), which measures ionic current when organic compounds are burnt in a small flame.

   • TCD (Thermal Conductivity Detector), which measures changes in thermal conductivity as different substances pass through.

   • ECD (Electron Capture Detector), sensitive to halogenated and other electronegative compounds.

   • MS (Mass Spectrometer), provides more detailed identification of the compounds by mass spectra.

   The signal from the detector is recorded in a chromatogram where each peak corresponds to a substance, and the height or area of the peak is a measure of how much of the substance is present in the sample.

6. Evaluation
   From the chromatogram one can:

   • Identifying substances by the retention time (the time the substance takes to pass through the column) and, if coupled to a mass spectrometer, its mass spectrum.

   • Quantify levels by comparing peak areas or peak heights against a known standard.

Summary of work steps

1. The sample is injected and gasified.

2. The carrier gas carries the sample through a column where separation occurs via interaction between the substances and the stationary phase.

3. The components elute in turn and are detected.

4. A chromatogram is recorded and evaluated for qualitative and/or quantitative analysis.