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.

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 as well.

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 lcgcedit@lcgcmag.com
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John V. Hinshaw