Customers frequently ask about the differences in ferrule materials: “Should I use 100% graphite, Vespel®/Graphite or 100% Vespel®? What’s the difference between 85/15 and 60/40 V/G? I have a non-GC application; what kind will work best?” While in most cases ferrules made from any of these materials can be used interchangeably, each has unique properties that can make it the preferred type for a particular application. Here we will list some of their properties, and explain how these can guide selection for use.
These are made from 100% Dupont polyimide, a hard polymeric plastic. They are excellent for creating seals on metal or glass and have long lifetimes. 100% Vespel is essentially non-porous to oxygen so it’s ideal for GC/MS interface use, where air permeation can raise the background signal and contribute to phase degradation. They’re rated for use up to 350°C.
Vespel ferrules tend to fuse permanently to the outer polyimide layer of fused silica GC capillary columns. After bonding they can’t be re-used, for example to reposition them after trimming the column. Vespel tends to shrink through repeated heating and cooling cycles so it works best in connections that are isothermal or insulated from oven temperature changes. It is the hardest of the polymeric ferrules so it needs firm torque to compress and seal.
These are made from DuPont polyimide/graphite blends, with either 15% or 40% graphite by weight, respectively. Graphite increases high-temperature tolerance, and reduces sticking and shrinking. They can be reused and repositioned along capillary columns if not over-compressed in the fitting. The 60/40 V/G blend will have the lowest chance of bonding. Because they are softer than 100% Vespel, only gentle force is needed to compress them and form a seal. They are both rated to 400°C.
Due to slight shrinking at high temperatures, they must be retightened after initial temperature cycles to avoid leaks. The 60/40 blend has enhanced wear resistance, lower friction, and improved dimensional and anti-oxidation stability compared with the 85/15 blend, which is harder. Both are good choices for routine GC/MS applications.
These are made from high-purity graphite. They seal with minimal torque and can be re-used many times if not over-compressed. They do not shrink in normal use so retightening is not needed. They are very soft and must be handled carefully. Graphite ferrules are commonly used in GC systems on the inlet and detector ends because they don’t stick to the column, and can be removed and reinserted easily. Graphite ferrules should not be used with GC/MS instruments because of permeability to air.
You should always trim a short piece of the column after replacing the ferrule to be sure that no graphite particles have plugged the column. Graphite ferrules are rated to 450°C.
They are made from 100% PTFE. They are soft, completely inert, and have very low friction. PTFE ferrules are hydrophobic and commonly used in liquid applications. PTFE Ferrules are rated up to 250°C.
It’s important to match the ferrule ID to the OD of the capillary or tube for the best sealing, although in a pinch a ferrule with a slightly larger OD than optimal can still be used. For example, a 0.5 mm ID ferrule can be used with a 0.25 mm ID capillary, which usually takes 0.4 mm ID ferrules. We’ve even sealed 0.25 mm ID columns using 0.8 mm ID ferrules just to see if this can be done, although it isn’t recommended. If the “nose” of the ferrule doesn’t close evenly, by the time it gets to the column it could exert a lateral force that snaps the column.
For non-GC uses, ferrule selection depends on the requirements of your application. Graphite ferrules can be used to very high temperatures, for example to seal around a thermocouple, while Vespel/Graphite ferrules are good for sealing around glass. 100% Vespel ferrules are hard enough to deform thin-walled steel tubing. Please note that Vespel, Vespel/Graphite and PTFE ferrules should not be used in high-pressure applications, especially if the ferrule is the only means of gripping and holding the tube or device in place.
If you still have questions on your specific application and your ferrule options, contact CRS for expert advice!