As far as biotin we actually use Biotin TEG, a 15 atom mixed polarity spacer arm based on a triethylene glycol. The spacer acts to minimize steric hindrance between the biotin moiety and the oligo, thereby providing streptavidin easy access to the biotin for capture and immobilization of the oligo. We highly recommend using this longer spacer instead of C6 which require NHS labeling, a post synthetic conjugation.