It has often been noted that longer the oligonucleotide, the more difficult it is to isolate a pure, biologically active product. No chemical reaction can be accomplished in 100% efficiency. In an oligonucleotide synthesis, there are usually four chemical reactions in play - coupling, capping, oxidation and deblocking. A flaw in any of these reactions, will lead to the generation of failure sequences. As failure sequences accumulate, it becomes virtually impossible to obtain pure oligos. This results in a mixture of n-1 deletion mutations, which is potentially disasterous in biological applications. it must be stressed that the reagents used for long oligo syntheses should all be fresh and of the highest purity. Diluent and activator especially should be dry and fresh. Inferior reagents will lead to deletion mutations, as described above, but also potentially insertion mutations and chain branching. if the long oligo synthesis has been carried out with the best efficiency possible, purification can be simply accomplished by DMT-ON purification.