How to make a good fused silica seal using PressTight type connections

Column coupling remains a very interesting topic. Whether its for connecting a guard column, a retention gap, column extension, transfer line s to one or multiple detection systems, or just repairing a broken column.  For fused silica coupling the PressTight has found wide application.  For ease-of-use the universal PressTight was developed.

Fig 1 PressFits must be correctly deactivated. Activity is always seen first on polar compounds

To enhance response and peak symmetry these PressFits are also deactivated. The difference is best seen when looking at the peak shape of a polar component, like hexane diol see fig.1.  Besides the standard siloxane deactivations, Restek also offers a Siltek deactivation which shows remarkable inertness for sulfur and organo-metal type compounds.

Making the connection si not always easy. We get complete opposite reactions varying from customers that cannot make a leak tight seal at all to people that make hundreds of seals using mass spectrometry, and never have a leak.

When making a connection with fused silica its important to cut a 90 degree cut. This can be done with wafer, a diamond needle pen or even a “shortix” cutter.  Pushing that end in to the Presstight with a little force will give you already a seal. You can see the seal by the formation of a brown-ring all around the top of the column.  Better is to wipe the outside polyimide after cutting with some solvent (methanol, acetone) and then make the connection. Fig 2 shows the difference. Using a solvent wipe, we remove finger fats and it will make the contact surface a little larger which will result a better and more durable seal.

This sealing area is important as we always damage the poly-imide a little when we use the cutting tools, see fig 3.  The more sealing-surface we have in thw connector, the more chance we have to maintain a good seal.

Fig. 3 cutting with wafer always damages the polyimide a little.

Fig 2 Wiping the fused silica end will remove fats and will make a larger contact surface resulting in a better seal

To reduce the stress on the press tights there are also PressTights with an angle, or one can choose for the VU2 union where the ends are secured by an extra connection.

Alternate one can use some polyimide to secure that the ends of the column do not slip out easily. It more messy, than a VU2 union, but it also works.

The maximum temperature for PressTight type connections is about 330C. Above that we see often appearance of leaks due to hardening of the polyimide outside coating.