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 ¼-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!)