Various magnetic particles (magnetic carrier) coated with silica are often used as silica coated beads 
Maghemite particle (γ-FeO) and magnetite particle (FeO),
as well as an intermediate iron oxide particle thereof, are most suitable as magnetic carriers.
Generally, the quality of the magnetic beads is characterized by
following parameters:
* saturation magnetization (～10-80 A m2/kg (emu/g):Superparamagnetic),
* coercive force (～ 0.80-15.92 kA/m),
* size diameter (～ 0.1-0.5 μm),
* mass of each particle (～ 2.7 ng),
* ease of collection (to be mentioned later),
* capture ability (to be mentioned later),
* Sedimentation rate (～4% in 30 min),
* Area ratio (> 100 m/g),
* Effective density (～ 2.5 g/cm), and
* Particle counts (～ 1 x 10 particles/mg).
Here, "ease of collection" is defined and compared by while capture ability are defined and compared by