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Gas Chromatography
Gas chromatography (GC) is a key separation technique for volatile and semi-volatile compounds. Thanks to the combination of precise temperature control, optimised capillary columns and a defined carrier gas flow, even complex samples are reliably broken down into their individual components. The technology impresses with its high sensitivity, short analysis times and excellent reproducibility.
Technical Data
Basics
Stationary phase
The stationary phase is located as a thin film on the inner wall of capillary GC columns. It significantly determines the selectivity of the separation. Depending on the chemical polarity, film thickness and temperature stability, the analytes interact with the stationary phase to varying degrees. Non-polar phases mainly separate according to boiling point, while polar phases take into account specific interactions such as dipole-dipole or hydrogen bonds. The choice of the correct stationary phase is therefore crucial in order to optimally adapt resolution, peak shape and retention behaviour to the sample matrix.
PLOT columns (Porous Layer Open Tubular)
- The inner wall of the capillary is coated with a porous, solid layer
- Particularly suitable for permanent gases and very light, volatile components
- Provides high retention for small molecules through physical adsorption
- Robust against moisture, but more sensitive to overloading
WCOT columns (Wall-Coated Open Tubular)
- Thin liquid film of stationary phase applied directly to the inner wall of the column
- Widely used for general GC applications, including environmental, food and petrochemical analysis
- Offers high efficiency, good peak shapes and a wide range of polarities
- Standard type for many analytical methods due to reproducible performance
COT columns (Support-Coated Open Tubular)
- Column inner wall first coated with a solid support (e.g. silica gel), followed by the liquid stationary phase
- Higher sample capacity than WCOT, but lower efficiency
- Less commonly used today, but useful for samples with higher concentrations or demanding matrices
- Combination of high load capacity and moderate separation performance
Mobile phase
The mobile phase is an inert gas such as helium, hydrogen or nitrogen. It transports the sample components through the column and influences efficiency, analysis speed and detection sensitivity. The choice of carrier gas and the set flow or linear gas velocity determine the theoretical plate number, the Van Deemter minimum and thus the separation performance. Hydrogen enables fast analyses with high efficiency, helium offers good performance with safe handling, while nitrogen is designed for maximum efficiency but produces slower separations.
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