How much mRNA can Truseq purification beads bind?

[woshiliangliang]

Recently I've got to know that 1ul of AmpureXP will bind over 3ug DNA (http://core-genomics.blogspot.com/20...eads-work.html),
I'm a little surperised because that I usually use more than 100ul AMPure beads to recover only ~100ng DNA.
And I think it's interesting to find out how much mRNA can 1ul Truseq purification beads bind.
Has it discussed before?

[kerplunk412]

mRNA purification beads are coated with oligo dT, which allows them to bind polyadenylated RNAs. Thus the capacity of the beads is determined by the amount of oligo dT linked to the bead. With AMPure beads the entire bead surface binds to DNA (at least that is my understanding), so the binding capacity is determined by the surface area of the bead.

Thus, I believe the binding capacity of these beads is what Illumina says it is in the manual. Also, since the beads are included with the kit, there is no reason why they would provide more beads than are necessary as that is extra cost to them.

Often the reason for using so much AMPure is because of the starting volume of the reaction and how AMPure size selection works on the basis of ratios. So if you want to perform a 1x AMPure cleanup on a 100 ul sample, you have to use 100 ul of beads even if there are only a few hundred nanograms of DNA present.