Actually, it is ChIPmentation protocol of "ChIPmentation CeMM v1.14 (September 2016)" but ChIP-seq library protocol of PerkinElmer, Nextflex have same step in SPRI size selection.
What I cannot understand is, after right-side size selection and transfer sup to a new tube(0.65x beads), they add 12.5μl of magnetic beads and remove sup.
I understand SPRI as "low ratio of magnetic bead only bind with large DNA, a.k.a right-side size selection". But adding 12.5μl magnetic, almost 0.15x, and removing sup step after 0.65x right-side size selection confuse me. For me, 12.5μl do not have any DNA fragment.
Is there anyone who understand this step correctly? The protocol link is ChIPmentation_CeMM, page 5, size-selection step 4. PerkinElmer, Nextflex ChIP-seq library kit has same step in size selection protocol