What Is the Right Way to Make Gas Connections?
Three gas connection types are encountered in a GC system. Occasionally chromatographers encounter other fitting types, but
these three are the most common. First is the high-pressure tank fitting, identified by a letter and number designation such
as CGA-580, DIN 477-6, or BS-341-3 for inert gases. With few exceptions no additional sealing is required — just assemble
the fitting to the cylinder and tighten to seal. Most regulator fittings used in GC have a torque specification of 40–60 ft-lb
(54–81 Nm). Some types of high pressure fittings, notably for liquid CO2 in the United States, require a plastic washer that is usually suppled with each tank. If a washer is present then assume
it must be used, and the tightening torque must be reduced by half. If not in place, the absence of a washer will immediately
be evident upon opening the cylinder valve! Never use polyfluorocarbon tape or liquid sealant anywhere on a high-pressure
tank fitting; this will just make it leak. If either side of the fitting is scratched or deformed then replace the fitting.
The second type of fitting found in GC systems is the pipe-thread type. This fitting involves matching internally and externally
threaded sides with no washers or ferrules. After making sure the threads are clear of old sealing tape, wrap two layers —
no more — of polyfluorocarbon pipe sealing tape (available from instrument suppliers) onto the externally threaded side of
the fitting. Holding the exit end of a right-hand threaded fitting toward you, smoothly wrap the tape in a clockwise direction
while stretching it slightly. Then thread the taped piece into the internally threaded side and tighten.
The third type of GC fitting is the swaged tube fitting. This fitting consists of a threaded receiving piece with an internally
beveled surface, a matching hexagonal internally threaded nut, and a one- or two-piece washer set. The swaged fitting directly
connects the end of a tube to one side, and then includes one or two additional connections that may be pipe-thread, another
swaged fitting, or a tube. These fittings can be of different sizes so that a swaged fitting can be used with different diameter
tubes. Swaged fittings are available from several companies as well as in brass and stainless steel. Always use matched parts
of the same metal from the same company.
Connecting a new metal swaged fitting is slightly more complex than the other two types. First, the tubing end must be cut
squarely and free of burrs, scratches, or cutting debris. Check that the hexagonal nut threads onto the fitting smoothly,
especially if reusing the nut or fitting. Using the tube as a guide, first slide the nut onto the tubing with the threads
facing outward, then the circular washer (if needed) with the narrow side outward, and finally the conical washer with the
narrow end facing outward. Fit the parts together and hand-tighten.
Now, comes the dexterous part: while holding the tubing all the way into the fitting with one hand, take the right-sized wrench
in the other hand — such as a 7/16-in. wrench for a ⅛-inch swaged fitting. Hold the hexagonal part of the fitting with the
wrench. Now, take another wrench in the other hand and . . . wait hold on, that's three hands! In the absence of a vise, chromatographers
soon learn how to hold a wrench, the fitting, and the tubing in one hand while tightening the assembly with another wrench
in the other hand. They didn't teach that in instrument class.
A metal swaged fitting must be tightened a certain number of turns, and not to a particular torque. For a common type of this
fitting, the ⅛-inch size is tightened ¾ turn when new while the ¼ size is tightened 1¼ turns. The fitting manufacturer may
make available a maximum-clearance tool that helps gauge when the fitting is sufficiently tightened. In any case, instructions
are available from the manufacturers and should be followed closely. Overtightening these fittings will reduce the number
of times they can be reconnected or even cause them to fail to seal altogether.
Common capillary column connections are also of the swaged type, but instead of hard steel or brass washers a soft metal or
polymer ferrule is used. Sometimes the same polymer ferrule type is used to connect small diameter tubes inside of an instrument
Regulator and pipe-thread fittings may be reused unless damaged. Swaged fittings, if treated correctly, also can be reused
but must be carefully examined beforehand. Do not reuse if the nut does not thread smoothly onto the fitting or the existing
tube end is distorted or bulging out of the ferrule. Instead, substitute all new parts and recut the tubing to start over.
Always leak-check fittings after making the connection. In the case of the regulator or pipe-thread fitting types, additional
tightening up to the maximum specified level may help secure the seal, but never exceed that amount of torque; check the regulator
on another cylinder and replace as needed.
Many More Questions
I've gotten only halfway down my list of questions that are or should be asked about gases for GC. This discussion will be
continued in an upcoming installment next year. In the meantime, readers are encouraged to submit their questions about gases
or anything else GC-related to email@example.com
John V. Hinshaw
"GC Connections" editor John V. Hinshaw is a Senior Scientist at BPL Global, Ltd., in Hillsboro, Oregon, and a member of LCGC's
editorial advisory board. Direct correspondence about this column to the author via e-mail:
John V. Hinshaw