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Setting linear velocity

The linear velocity is an important parameter in chromatography. It has an influence on chromatography resolution and therefore setting the dead volume is basic part of method development.

Linear velocity measurement in GC

To set a dead time, inject 2 µl of a non-retained gaseous substance, which is compatible with the detector. Take a gas-tight syringe and draw the headspace over neat compound. Accurately mark the injection starting time and peak elution time.

Recommended compounds for dead volume determination
Detector Compound
FID methane, propane, butane
ECD methylen chloride, air (at lower oven temperatures)
TCD methane, butane, air (at lower oven temperatures)
NPD acetonitrile, air (at lower oven temperatures)
MS propane, butane, argon, air (at lower oven temperatures)
PID acetylene, ethylene

Note: Some compounds may be slightly retained on thick-film phase, however, they will be reproducible for similar column types.