Step 5: Sequencing by mass spectrometry. Identification of sequences, including location and type of modification, was achieved by reverse-phase liquid chromatography coupled with secondary ion mass spectrometry (MS/MS, Fig. ). Fragments are produced by breaks in the oligonucleotide chain caused by the collision-induced dissociation. In the 5' direction the c-notation is the nucleotide molecular weight and the -ab(n) notation is c(n-1) + (B(n) -Base MW). In the 3' direction y notation is the nucleotide molecular weight and w(n) = y(n) + PO 3 H molecular weight (80 Da). The most abundant ions observed were c-and y-and w-fragments, with ab-also observed. Fifteen sorted MinE07Library aptamers were chosen at random from the 170 sorted aptamers, of which three gave good MS/MS data and were named MinE07UA, MinE07UB and MinE07UC. Given that there are just 10 17 strands per bead on average, we are operating near the limit of detection of the instrument.