Abstract: An analytical instrument and associated method for ascertaining gas transmission rates of a target-analyte through a capped mouth of a bottle. The instrument employs a unique fixture that includes (1) a selectively openable and closeable enclosure defining a chamber, and (2) a mounting post extending into the chamber. The post is configured and arranged to sealingly engage an inner surface of a capped bottle neck, wherein mounting of a capped bottle neck onto the post sealingly separates the chamber into a first compartment inside the mounted capped bottle neck and a second compartment outside the mounted capped bottle neck. The fixture is configured and arranged with passageways for introducing a pressurized target-analyte-containing fluid into the first compartment of the chamber and flushing the second compartment of the chamber with a target-analyte-free fluid.