Ferrules13-14_1-22 - page 4

Gas chromatography
relies on tubing, plumbing for gas flows and ferrules for sealing connecting fitting
and would not be a practical routine technique without ferrules
these essential consumables that provide a pressure seal between the atmosphere and the gas flow path.
Ferrules are used in junctions that can be disconnected and remade. Proper selection and use of ferrules
ensures a reliable installation. Incorrect selection and/or implementation can cause problems and can affect
the quality of analytical results.
Chromatogrphy Plumbing
The need for a leak-free and reliable flow path is essential in gas chromatography.
In particular the requirements of GC fittings are quite demanding compare to normal plumbing
They must be able to withstand high temperatures for long periods of time and wide variations as
experienced in GC ovens for example. Connections outside the oven and ar near amient conditions
are far less demanding or where constant temperatures are use such a valve oven, inlet or detector.
Ferrules are used to provide a seal between a tube and a fitting; union, a valve, a transfer line, or device
requiring attachmentt of a tube or chromatography column. For permamnent connection, then the tubes can be
soldering, braising, cementing, etc. can be used.
A ferrule is used to make a resusable connection The simplest ferrule seal uses some easily deformable material
such as a rubber gasket or O-ring commom in many industrial and domestic situations.
Unfortunately, such a connection cannot satisfy the special requGC and liquid chromatography is more
demanding requiring sealing material with unique characteristics.
Use of gases with very high diffusivities these gases (He and H2) can be prone to leakage
Air can also back diffuse through leaks, even against a pressure gradient
- Many of the detectors used in GC can detect air leading to raised detector baseline and increases drift
- Many of the stationary phases used in GC are sensitive to air through oxidation which will lower column
and increase column activity to polar compounds, as well as increasing detector noise adur to bleed of liquid
phase decomposition products
Larger leaks of course will waste gas, which can be quite expensive (e.g., when using High Purity grades
required for some sensitive detecors; GC-MS, Helium Ionisation and even TCD
Leaks in the sample flow path will vent sample (inlet side) or effluent
(detector side) causing errors in the a result in loss of sample and cause errors and inconsistenies in results
Where mass flow control is required , leaks can cause serious errors in retention time (packed column inlets
but more so with low flow rate capillary columns), with variations in split ratios.
Exacerbated byvariations in temperature, vibration, and time. Some of these effects can be offset by using
chemical standards where such leaks leaks are constant,
Metal Ferrules
come in two basic designs: one- and two-piece.
Swagelok
is the premier manufacturer of two-piece ferrule designs and the defacto standard over many years.
The two-ferrule design has some advantages (at a price premium) over single ferrule designs.
Swagelok fittings are ubiquitous in the GC laboratory.
Single-ferrule formats serve the same purpose as two-ferrule designs. Parker and VICI-Valco are examples
It is MOST important to match both the ferrule and fitting with those from the same manufacturer
and NOT to mix them..
Hard metal ferrules
(#316SS) are only used with similar hard tubing material.
The seal is actually made over a relatively narrow region near the tip of the ferrule.
The cone design of the ferrule directs the tightening force from the nut to the back of the ferrule to its cone tip which
in turn compresses onto the underlying tube.
If the tube is a glass or fused-silica column, it would break.
Soft metal (#304SS) or polymeric tubes, in contrast, will deform, allowing the ferrule to bite into them permanently.
Metal ferrules can be used at high temperatures and can withstand wide temperature cycling, and are ideal
for fixed installation in heated zones such as valve ovens and heated inlets and detectors.
However, when used for extended periods at high temperatures, the ferrule can sieze into the fitting.
If overtightend they can also distort both the ferrule and the fitting so that on reuse orientaion may be different
requiring even more excessive force to re-swage them into a good seal
- In some situations they just won't reseeal and in fact may need to be cutoff the tubing and replaced
Hard metal ferrules usually come in either brass or stainless steel.
Stainless steel is more inert than brass and is required for normal sample paths.
Outside the sample path, use the same ferrule material as the fitting or fixture.
One should never use a stainless ferrule with a brass fitting or fixture becuse steel is harder and can permanently
damage the fitting. A brass ferrule can be used with a stainless fitting if you are prepared to cut off the ferrule
if it becomes scored.
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