Eurofins Genomics has many years of experience in oligonucleotide synthesis. Occasionally we can identify oligonucleotides that are difficult to produce, due to their sequences. For example, oligonucleotides with a high percentage of "G" residues, especially stretches of "Gs", can be difficult to synthesise.
Moreover, oligos with four or more "Gs" in a row tend to aggregate and form a "guanine tetraplex" (Poon and MacGregor (1998) Biopolymers 45:427-434). Oligos with self-complementary sequences tend to form aggregates, and this formation can complicate HPSF, HPLC and HYPUR purification.