The needle assembly is the core component of the closed liquid sampler. It consists of a housing and two needles. The sample medium enters the sampler through one needle (the process connection), while the displaced gas or vapor is expelled through the other needle (the vent connection).

The vent connection serves to prevent pressure buildup inside the sample container. The vent line must always remain unobstructed; therefore, no valves or other devices that could cause blockage should be installed at the vent outlet. To prevent backflow from the waste system into the sampler when the unit is not in operation, a check valve may be installed in the vent line.