Capping B (acetonitrile26-lutidineacetic anhydride 503020 VVV) is commonly used as a capping reagent in solid-phase oligonucleotide synthesis particularly during automated synthesis cycles.

Its purpose is to acetylate and deactivate unreacted reactive groups on the growing oligonucleotide chain so that they cannot participate in subsequent coupling reactions thereby reducing the formation of truncated sequences.

Capping B is a solution composed of 50% acetonitrile 30% 26-lutidine and 20% acetic anhydride by volume. Acetic anhydride acts as the acetylating agent 26-lutidine functions as a basic component that facilitates the capping reaction and acetonitrile serves as the organic solvent.