best pressfit or meltfit connectors ?

[rherman]

Looking for recommendations on pressfit or meltfit connectors for joining 0.1 mm ID capillary tubing. Have tried pressfits from Supelco and Agilent, with mixed success -- IF a leak free seal is achieved after heating in the GC oven to anneal the surfaces, it is not robust following a few weeks of oven cycles during sample analysis.

I heard the newer "meltfit" fittings from Chromedia are good -- deactivated with negligible loss of resolution or sensitivity. Has anyone had good/bad experience with these ? What vendor distributes them in the US ? Any other suggestions ?

Thanks,
Rob

[ChromatographyDude]

I like the Varian universal presstight connectors, they hold up really well.

[Peter Apps]

I use Restek presstights, with Siltek deactivation.

Peter

[chromatographer1]

When I connected tubing/columns for Supercritical Fluid Chromatography I used a polyimide glue to permanently seal the connections using presstight connections. It is similar to the coating on the outside of the tubing/column. This glue is available from many vendors, Supelco is one of them I believe.

It takes a little bit of practice to perfect the technique but it can be learned, and isn't that hard. I learned it when I took a class on SFC in Salt Lake City some years ago. It isn't that hard and is required for anyone using SFC.

This might not be the best solution for you but I thought I would mention it as it might be cheaper, and to give you another option to consider.

best wishes,

Rodney George

[chhubert]

I guess Meltfit technique may be a good option and provide better sealing than Pressfit one, especially the connection is near high vacuum. I have bad experience trying to merge two cap columns near the MSD (for some experiments) by using a Pressfit Y- connector and polyimide glue but air/water could still be detected by MSD. Eventually I found a two-hole ferrule and two cap columns could pass through the transferline in parallel. However, it limited the use of the columns to <0.25mm i.d.

Hubert

[AICMM]

rherman,

I looked at the Meltfit and think it is a great way to connect. Unfortunately, the initial capital cost is a bit steep and I don't do enough of that to make it worthwhile. Therefore, I have stayed with the press fit or the larger Y's with backing ferrules (such as offered by Restek.) If you use the press fit, I would certainly agree with Rodney that glue is a necessity and it should be done on the system with no head pressure to ensure a really good seal.

Best regards.

[Schmitty]

It takes a little bit of practice to perfect the technique but it can be learned, and isn't that hard. I learned it when I took a class on SFC in Salt Lake City some years ago. It isn't that hard and is required for anyone using SFC.

Rodney,

Can you summarize this technique? I have an on-column system that is limited to 0.53mm at the inlet, but a 0.25mm column is recommended for separation. I want to join the two as reliably as possible. I did buy the Supelco polyimide resin last week, but it hasn't arrived yet.

Any help would be appreciated.

Thanks,
Schmitty

[chromatographer1]

Schmitty,

The goal is to get the column butted up to the narrow neck of the connector while filling the space away from the neck completely full of resin.

The first connection is not too difficult. Cut a good straight end on the column and use a light vacuum to draw up the resin toward the neck just before pressing the column into the narrows. Let it cure completely before connecting the second side.

The hard part is the second connection. Wetting the outside of the second column with resin and getting the resin to wet the inside of the union without leaving any voids inside is difficult. The syrupy resin doesn't flow easily. Gravity and patience is required.

How do you get to Carnegie Hall ? ? ? Practice Practice Practice.

I have considered using a gentle use of vacuum to draw the resin into the other side of neck might be possible and press fit the column just before the resin passes by the column end into the narrows. Of course, I mean the vacuum is drawn through the column and the other end of the other column is closed off.

Does any of this make sense?

It is more difficult to describe than to do. I hope I have been helpful.

Perhaps there is an on-line help web page for making SFC column connections?

It took a couple attempts before I got a good seal for SFC. Keeping the pressure on the columns into the union while the resin cures is required.

GOOD LUCK.

Using metal capillaries and a metal union might be easier if the end cuts are made precisely. Will you have enough deactivation? Try it and see. Restek might be able to assist you.

best wishes,

Rodney George
consultant

[Schmitty]

Thanks Rodney. That is a bit more involved than I had imagined. I guess we'll find out how well it works when the resin arrives.

[Peter Apps]

Remember that the quality of the cut end that seals to the tapered surfaces is absolutely critical.

Peter

[chromatographer1]

Good call, Peter.

Peter is quite right. The seal of the column end using the polyimide coating of the column is critical and performs the 'work' of sealing primarily.

Diligence is required and care. The resin seal inside on either side of the union essentially holds the columns in place and gives a rigidity to the whole construct.

You can imagine the pressures used in SFC in comparison to GC. Yet, these materials are capable of keeping a good seal. It just takes a little practice. Did you learn to drive a car in a single day? Or learn to swim?

best wishes,

Rodney George
consultant

[Peter Apps]

When Egmont Rohwer first developed press fits we made them by hand from glass capillary. To test their maximum burst strength we made up a joint and then hung weights on the end until it pulled apart. I don't recall the exact figures, but they corresponded to pressures several orders of magnitude above anything in GC. The current precision-made commercial versions are probably even better, and they can survive SFC pressures, so when they come apart it is probably not due to system pressure, but more likely to flexing or vibration. Using polyimide glue protects the seal from this kind of mechanical stress.

Does anyone have any thoughts on whether oxygen could diffuse axially along the seal, letting oxygen into the system even though the seal is leak-tight ?

Peter

[philippemottay]

Hi Rob,

Very good results have been obtained with NLISIS Meltfit one. It is distributed in the US by Brechbuehler, Inc. (www.brechbuehler.com)
Send me an e-mail at philippemottay@brechbuehler.com and i will send you some documentation on it

Philippe

[chromatographer1]

Peter,

Your question is a good one. My hunch is that unless metal components are used (as I mentioned as a possibility previously above) there will always be SOME leakage or diffusion of air if these fittings are in a sub-ambient pressure MS environment. A purged container holding the unions might be necessary to minimize air entering the MS from the connections.

Not using the connections until the glue has fully cured is highly recommended, but a little leakage is still a possibility.

best wishes,

Rodney George
consultant

[Peter Apps]

Hi Rodney

So, no matter what kind of connection is used, it still seals to the polyimide coating of the column, and the only way to decrease the diffusion is to make the seal longer. In a metal compression fitting the seal between ferrule and column is 2-3 mm long compared to say 0.5 mm in a presstight. That corresponds to more than an order of magnitude less diffusion. With a glued presstight the seal can be even longer.

Peter