John V. Hinshaw
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Chromatographers need to use appropriate fittings and ferrules to get the most out of their instruments; proper fittings help
ensure the best separation and detection performance while getting the most out of gas tanks and filters. Connections at the
tank regulator, the instrument bulkhead, the column, and finally the detector all have specialized requirements and assembly
procedures that should be followed. Not doing so risks wasting expensive gases and exhausting in-line filters prematurely,
as well as causing excessive peak band broadening and distortion, shortened column life, and poor detector performance. Problems
with gas fittings fall into two categories: leaks and band-broadening.
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Leaking connections cause a number of significant problems, including poor retention time and peak area reproducibility, loss
of accuracy, drifting detector baselines, high detector noise, shortened column life, and increased gas consumption. Hydrogen
leakage from flame-ionization detectors or carrier-gas supplies can be a potential explosion hazard.
Contrary to intuition, a leak is not just a wasteful one-way path from the high pressures inside the tubing to the atmosphere;
oxygen can diffuse from the atmosphere back into areas where its partial pressure is lower, which is always the case with
gas chromatography (GC) gases except for detector fuel air. High oxygen levels in the carrier gas can cause excessive stationary
phase bleed and greatly reduced column life, as well as increased detector or mass spectrometry (MS) noise and background
signal levels.
Column connections require specialized low-dead-volume fittings. Internal areas in a fitting that are exposed to carrier gas
but are not actively swept by carrier flow — so-called dead volumes — can cause peak broadening and tailing, especially at
the low flow rates of narrow-bore columns with inner diameters under 320 μm. GC instrument manufacturers and consumable suppliers
make available the specific types of fittings and ferrules for each inlet system and detector that will minimize peak broadening
and tailing. It might be tempting to substitute less expensive fittings and ferrules or to keep only a few types on hand,
but doing so almost compromises instrument performance.
Chromatographers always should choose nuts and ferrules that are in good condition, not worn or distorted. Proper assembly
and tightening are required, and the procedures are different for new parts than for reassembly of used parts. Fittings that
employ graphite or polymer composites need to be checked and maintained periodically because the soft ferrule material can
deform and loosen. And finally, leak-checking is always necessary when making up new or reused fittings. As I have repeated
more times than I can count, never use a soap solution to check for leaks. Liquid can flow back into a leak just as easily
as can oxygen; commercial electronic leak checkers are mandatory in my lab. In the few cases where an electronic device will
not respond, such as for detector air, use a pure liquid such as water or propanol.
A Word About Tubing
Three kinds of tubing material are found in gas chromatographs: copper, stainless steel, and plastic. Plastic tubing should
be used only in noncritical areas where small leaks and some contamination can be tolerated, such as for the air supply to
valve actuators or other pneumatically driven devices. Other than this, plastic tubing has no place in a gas chromatograph:
the main problem is that plastic tubing can contaminate a gas stream with plasticizers and residual volatile chemicals, as
well as with oxygen that diffuses in through the tubing walls.
All detector gases, including air, need to be provided from dedicated supplies. Otherwise, besides a contamination risk, the
large air pressure fluctuations that occur upon valve actuations will not affect the detectors' outputs. For the most part,
internal tubing in GC systems is made from stainless steel. Stainless steel tubing is best for the connections from gas tanks
as well, although copper is just as acceptable if the fittings are not to be opened and remade more than a few times.
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