Abstract: A vented cap and capillary tube assembly is disclosed together with a method of use of such assembly. The assembly includes a capillary tube having a bore extending therethrough and a cap slidably mounted to one end of the tube. The cap includes one or more vent grooves therein which allow air to escape therethrough when the cap is in a first slidable position. The walls of the capillary tube prevent air from escaping through the vent when the cap is more fully inserted within the tube. The method provided herein includes the steps of providing such a pre-assembled capillary tube and vented cap assembly, maintaining the cap in the first position while the opposite end of the capillary tube is inserted within a liquid sample, allowing the liquid to enter the tube through capillary action, thereby displacing air within the tube through the vent, and sliding the cap to the fully inserted position, thereby sealing the vent.