Finding the Best Carrier Gas Average Linear Velocity
Determining the best average linear velocity is fairy easy and only involves a small amount of trial and error. Hydrogen provides the best resolution in the shortest amount of time. Helium provides similar resolution, but at a longer analysis time. Nitrogen is not recommended for use with capillary columns due to the extremely long analysis times.
When using helium as the carrier gas, try an initial average linear velocity of 30 cm/sec. If better resolution is desired, reduce the velocity to no less than 25 cm/sec; however, the analysis time will be increased. If a shorter analysis time is desired, increase the velocity to 35 cm/sec up to 40 cm/sec. Beware of potential resolution losses at these higher linear velocities. Minor adjustment to the oven temperature may also be needed. Average linear velocities of 30-35 cm/sec are used for many analyses when using helium as a carrier gas.
When using hydrogen as the carrier gas, try an initial average linear velocity of 60 cm/sec. If better resolution is desired, reduce the velocity to no less than 50 cm/sec; however, the analysis time will be increased. If a shorter analysis time is desired, increase the velocity to 70 cm/sec up to 80 cm/sec. Be aware of potential resolution losses at these higher velocities. Minor adjustment to the oven temperature may also be needed. Average linear velocities of 60-70 cm/sec are used for many analyses when using hydrogen as a carrier gas.
Upon comparing the chromatograms at the various average linear velocities, retention and resolution differences will be noticeable. Sometimes different average linear velocities are best for different peaks within the same chromatogram. In these cases, a compromise velocity is usually selected. Except with nitrogen, small changes in the average linear velocity (<2 cm/sec) rarely result in significant changes in resolution. When experimenting with average linear velocities, try values that are different by at least 3-4 cm/sec.
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