Separation

Frequently asked questions

Should I set centrifuge speed by RPM or RCF?

Set it by RCF (relative centrifugal force, expressed as x g) whenever you can, because that is the actual force the sample feels and it makes runs reproducible across machines. RPM alone is ambiguous: two rotors of different radius spinning at the same RPM apply different g-force. If a protocol gives RPM for a specific rotor, convert to RCF for your own rotor's radius.

What is the difference between a fixed-angle and a swing-bucket rotor?

A fixed-angle rotor holds tubes at a set angle with a shorter sedimentation path, so runs are quicker and it pellets efficiently. A swing-bucket rotor lets tubes swing to horizontal during the spin, giving a longer path that suits density-gradient separations and produces a pellet at the tube bottom. Choose based on whether you are pelleting solids or separating layers by density.

When should I use vacuum filtration instead of centrifugation?

Use vacuum filtration when you want to physically remove particulates or recover a solid precipitate by pulling liquid through a membrane or filter paper, as with a Buchner funnel. Use centrifugation when you need to separate by density, split phases, or pellet very fine solids that would clog or pass a filter. The two are often complementary steps in a purification sequence.

How do I size separation equipment?

Start from your batch volume and throughput. For centrifuges, confirm the rotor and tube or bottle capacity, the maximum g-force, and whether you need refrigeration for heat-sensitive samples. For filtration, match funnel and flask size to your batch and pick a filter pore size or membrane suited to the particle size you are removing. Make sure the vacuum source can sustain the flow you need.