The present invention provides a viral inactivation and sampling apparatus for use in a process of purifying a liquid drug substance. A closed circulation line is formed by an outlet port 11 of a bag 10 containing a liquid drug substance, a first line 41, sampling means 20, a second line 42 (pH meter 30) and an inlet port 12 of the bag 10, and an acidic aqueous solution is fed through an acidic aqueous solution line 45 and circulated to inactivate viruses in the liquid drug substance.
