Graphite
has been a favorable ferrule material for capillary column use from the beginning of
gas chromatography. It is very forgiving because it is so soft and can deform and seal in almost any space.
Graphite is easy to identify because it can bedeformed, or scratched.
Incorrect size ferrules can sometimes be used easily either to expand the hole to accommodate
a larger column or to compress a larger ferrule just by putting it on the column and tightening
the fitting a little more than usual.
The graphite is easily reformed in both cases to create a near perfect seal.
However, this malleability can lead to one of the biggest problems with graphite ferrules as well.
This ability of graphite to reform can cause it to extrude through openings in fixtures into adjacent spaces.
Graphite pieces can end up contaminating areas like the bottom of the inlet or the detector jet.
These graphite pieces can interact with sample causing losses, tailing, and can become a constant source
of contamination as graophite is an absobent
A second major problem with 100% graphite ferrules is that they are very permeable to air due to
their semi-lamated structure.
So, when using air sensitive columns (e.g., carbowax) or detectors (
e.g., mass spectrometers or ECDs) Graphite/Polyimide are preferred - much more dense & impermeable.
Some inlet and detector designs use graphite ferrules wherein the graphite is contained within a
secondary metal tube.
This provides the benefits of graphite ferrule material, while greatly addressing the issues of deformation,
extrusion, and air diffusion.
Graphite Ferrules are resilent enough for glass and ceramic tube use.
Chromalytic have a unique market extening sizes to fit tube size from 1/16"x0.3
up to a Maximum od 2.0inches (50mm) ID
Polyimide
is a polymer with high temperature stability ( 310-35-degC max ) and low outgassing.
Because of its relatively high temperature stability, polyimide is the coating of choice for the outside
of fused silica columns. Polyimide ferrules are easy to identify because they are brown (see Figure 1).
Polyimide is very hard but is softened with admixtures of Graphite and under pressure in-situ can be
molded into shapes that match fitting profiles, and devices typically designed for metal ferrules.
Even though it has high temperature stability, polyimide softens when heated, problematic
in high-heat zones. It tends to bind onto the tube and can fix inside the fitting.
For this reason, 100% polyimide ferrules find most use outside the GC oven.
These are preferred over metal ferrules for room temperature or low temperature connections
since they are much more forgiving than metal ferrules when sealing against flawed tubing,
they can also be removed without having to be cut off the tubing.
They also have low air permeability.
Ferrules
can be made of graphite, Vespel® (polyimide), Vespel®-graphite mixtures,
and Teflon® (polytetrafluoroethlyene, PTFE) Even though graphite is not a polymer,
its characteristics are more similar to the synthetic polymers than metal ferrules.
Attributes of these ferrule materials are summarized in Table 1.
Graphite and polymeric ferrules have several advantages for use in gas chromatography
over metal (especially hard metal) ferrules:
They can seal against imperfect surfaces with little force
They can be used with virtually any type of tubing or column including glass
and fused silica
They can also be hand drilled with a pin vise to get the right size for any given
tube or column
However, because they are organic and porous, graphite and polymeric ferrules
do have some general weaknesses that constrain their uses as well:
Polymers have a limited temperature range (compared to metal ferrules)
They are more permeable to air infiltration (a function of polymer density)
They sometimes come out of their commercial packaging, or the lab
drawer, contaminated. They are then a source of ghost peaks and baseline disturbances.
Contamination comes from poor manufacturing processes as well as poor choices in packaging
(the contamination comes from the packaging).
A common practice is to leave ferrules in GC oven prior to use for conditioning
They can also interact with sample components or solvent causing tailing or losses,
especially at trace levels
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