Oligonucleotide synthesis is a complex process that requires more than one hundred sequential
chemical reactions to make a single 25‐base sequence. Contemporary phosphoramidite
synthesis chemistry is robust and modern solid‐phase synthesis platforms are reliable and
highly automated. However, each synthesized oligonucleotide must be evaluated for quality
prior to use in order to ensure that the correct sequence was made and that purity meets or
exceeds IDT standards. The best method available to assess oligonucleotide purity is analytical
capillary electrophoresis (CE). The best method available to assess compound identity in a high
throughput environment is mass spectrometry (MS).

If the substitution was there in the primer dC which is 289Da and G which is 329 Da,

the expected and observed MW will differ by 31 DA,

which is a huge difference that cannot escape detection in ESI mass spectrometry.