Magnetite (Fe3O4) is coated onto this core particle and encapsulated with proprietary polymers. Streptavidin Magnetic Beads are nominally 1 μM microparticles (diameter) with highly active streptavidin bound to the surface. The beads are supplied at approximately 1% solids (10 mg/ml) in a storage buffer of 50 mM Tris, pH 8, 150 mM sodium chloride, 0.05% sodium azide and a stabilizing detergent at 0.1% concentration. The activity of bound streptavidin is measured by binding of biotinylated fluorescein (BF). Quantitative amounts of BF in the supernatant are measured on a fluorometer after incubating with and without streptavidin magnetic beads present. The biotin binding capacity of our Streptavidin Beads is approximately 2,500 to 3,500 pmol/mg of biotin. The binding capacity of our Streptavidin Beads has also been determined for biotinylated oligonucleotides. Quantitative amounts of biotinylated oligonucleotide in the supernatant are measured on a spectrofluorometer after incubating with and without Streptavidin Beads present. The A260 readings are used to calculate the biotin binding capacity of the beads. Biotin binding capacity is reported in picomoles of biotinylated oligonucleotide per microliter (μl) of streptavidin magnetic beads (pmol/μl). The binding capacity of the Streptavidin Beads has also been determined using the Poly T PNA Probe included in 's mTRAP mRNA Isolation Kits and with oligo dT. The binding capacity of the Streptavidin Beads is approximately 1 nmol of Poly T PNA Probe (or DNA oligonucleotide) per mg of beads, i.e. approximately 10 pmol of probe/μl of beads.