Gases Q & A - - Chromatography Online
Gases Q & A

LCGC North America
Volume 30, Issue 11, pp. 978-981

How Often Should the Gas Delivery System Be Checked?

Given the slight chance of a contaminated cylinder, check indicating filters every time a tank is replaced or at least once every two weeks if using a gas generator. That's also a good time to check the gas lines for unusual bends or kinks. Check for leaks any time a fitting is disconnected and reconnected, and of course on all new connections.

It's also a good idea to check regulator function at least once a year. The easiest way to do this is to establish a flow of 500 mL/min or greater on each gas cylinder. Note the pressure output, then turn the tank valve off. Wait until the outlet pressure starts to drop off, and then turn the tank valve back on. The same pressure should be reestablished. Next, turn the regulator pressure knob down and back up while observing how smoothly the knob moves, how the output pressure gauge reacts, and that the knob is not screwed in all the way when the original pressure is restored. Don't forget to set the flow back to normal at the gas chromatograph.

Check that the correct cylinders are connected through to the correct fittings on the instrument. Some cylinders, such as helium, argon, and nitrogen, use the same gas fitting (at least in the United States); hydrogen and high-level combustible gas mixtures also may share a fitting type. Thus, it is possible to connect the wrong cylinder. Chromatographers should not rely on the uniqueness of cylinder fittings to ensure the correct gas type.

What Kinds of Regulators Should Be Used for GC?

Generally, dual-stage high-purity regulators with stainless steel diaphragms are the correct choice in all cases. For economy, less expensive dual-stage regulators may be used for detector air, hydrogen, and make-up gas if separate from the carrier supply. The added cost of a dual-stage compared to a single-stage regulator does not justify the potentially poor pressure regulation of a single-stage regulator as the cylinder pressure decays.

A regulator outlet valve is a handy accessory that I like to order on all regulators. However, in laboratories where the downstream connections remain in place permanently, the outlet valve can be omitted.

Always dedicate a regulator to its intended use. Never change the cylinder fitting on a regulator to switch it from inert-gas to detector-gas service or the other way around. Making the cylinder fitting connection correctly so that it will maintain high pressures is best left to the regulator manufacturer.

What Types of Fittings and Tubing Are Required?

The fittings and ferrules used in any GC installation should all be new, as should the tubing. The fittings should be of a suitable type for high-purity gas lines, such as those that are available from instrument company and supplier catalogs. I always like to order some spare fittings to have on hand for the usual problem connection that refuses to seal, as well as for later on when the gas setup needs some modifications.

Only metal tubing should be used for GC gases, with one exception. Copper tubing is the easiest to install, while stainless steel tubing is more robust and generally can be more organized visually. Either type of tubing must be precleaned before installation, and can be ordered that way as "GC" or "Chromatography" cleaned tubing. "Refrigerator" designated tubing is not suitable. For installations with tanks or gas generators dedicated to one or two instruments, ⅛-in. or 3-mm o.d. tubing is appropriate. If multiple instruments share a tank or generator then consider using -in. or 6-mm o.d. tubing up to the point where the flow path splits to the individual instruments.

Polymer tubing is appropriate only in one case. If the GC system uses air-actuated sampling valves then that air supply can be connected with polymer tubing. But if the air tank is shared with a flame ionization detection (FID) system — which is not such a good idea, but it is done — then metal tubing is required throughout. Polymer tubing may allow air and airborne contaminants as well as monomers from the plastic into the gas stream. This is unacceptable for carrier and detector gases, even for FID air.

A nice touch when installing tubing is to mark either end of each tube with differently colored electrical tape. This makes it clear which tube goes to which gas inlet and regulator and avoids some of the more hazardous mix-ups such as swapping FID hydrogen and air. (Yes, I have seen that situation and the result of igniting the flame was, well, exciting!)


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