A method for introducing standard gas into a sample vessel is generally disclosed comprising providing a vessel containing a sample gas and a receptacle with a vessel port, such as sampling needle, pressurizing the vessel with a carrier gas, and introducing a volume of standard gas into the flow path of the carrier gas being used to pressurize the vessel when the vessel port of the receptacle is located within the vessel. In some embodiments, a rotary valve is loaded with the standard gas, and the valve is brought into fluid communication with the flow path of the carrier gas when the vessel port is within the vessel.