Magnetic particles
Magnetic particles are a completely different type of affinity support from beaded agarose and other porous resins. They are much smaller (typically 1-4µm diameter) and solid (non-porous). Their small size provides the sufficient surface area-to-volume ratio needed for effective ligand immobilization and affinity purification. Magnetic beads are produced as superparamagnetic iron oxide particles that are covalently coated with silane derivatives. The coating makes the beads inert (i.e., to minimize nonspecific binding) and provides the particular chemical groups needed for attaching ligands of interest.
Affinity purification with magnetic particles is not performed in-column. Instead, a few microliters of beads is mixed with several hundred microliters of sample as a loose slurry. During mixing, the beads remain suspended in the sample solution, allowing affinity interactions to occur with the immobilized ligand. After sufficient time for binding has been given, the beads are collected and separated from the sample using a powerful magnet. Typically, simple bench-top procedures are done in microcentrifuge tubes, and pipetting or decanting is used to remove the sample (or wash solutions, etc.) while the magnetic beads are held in place at the bottom or side of the tube with a suitable magnet.