Flash Column Chromatography Silica Gel: Particle Size and Flow Rate

Flash column chromatography is ordinary column chromatography with a little pressure applied so the mobile phase moves faster. The one choice that governs how fast it runs is the silica gel particle size: finer particles pack tighter and give sharper bands, but raise back pressure; coarser particles flow freely but resolve less. In our range the XCL grades are specified to your mesh on agreement, and macroporous silica is offered from 2–250 µm. Choose the particle size that the flow rate and resolution of your separation can live with, and state it on the order.

Why Particle Size Is the Flow-Rate Control

When you push the mobile phase through the bed, the resistance it meets comes from the gaps between the particles. Smaller particles leave smaller gaps, so the pressure needed to drive a given flow rises — which is exactly the back pressure a flash column has to be built to take. The reward is a tighter, more evenly packed bed and less band spreading, so closely eluting compounds separate more cleanly. Larger particles open the gaps, cut the back pressure and let you run faster, at the cost of broader bands.

Choosing the Grade

Particle size and pore structure are separate decisions. First pick the pore grade for the molecule you are separating, then pick the particle size for the flow you want:

• XCL-A — narrow pore (20–40 Å), highest surface area (≥ 600 m²/g). For small molecules.
• XCL-B — medium pore (50–80 Å), surface area 400–600 m²/g. The general round.
• XCL-C — coarse pore (80–100 Å), largest pore volume. For larger molecules.

These are the grades of our column chromatography silica gel; particle size (mesh) is set on agreement with your requirement. Where the molecules are larger still, macroporous chromatography silica gel gives an average pore diameter of 10–18 nm with a particle size from 2–250 µm, customisable.

Packing the Bed

A flash column is only as good as its packing. Load the silica evenly so there are no channels for the mobile phase to short-circuit through, and keep the bed level as you add the sample. Any crack or uneven zone lets solvent rush past the rest of the bed, which spoils the resolution you paid for with a fine particle size. Our guide to choosing a silica gel grade for column chromatography covers the selection side of this in more detail.

What to Specify

• Grade — XCL-A, XCL-B or XCL-C, or a macroporous grade.
• Particle size — the mesh range your column and pump can handle.
• Grade class — industrial or reagent, by the purity your work needs.
• Quantity and packing — the XCL grades ship in a 20 kg composite bag, other packaging on request.

Tell us the column diameter, the flow rate you want and the compounds you are separating, and we will match the grade and particle size. Prices are quoted on request.

For small-scale analytical work instead of a running column, see silica gel TLC plates and H plates and the TLC silica gel range.