Chromatography Forum

I am analyzing sediment and tissue samples by GC/MS using a Gerstel PTV large volume injector. I am only using the PTV for the temperature programming feature, and only inject 1 uL (extract is in hexane). I sonicate the PTV head in hexane after every analytical sequence, as part of the instrument maintenance (it helps restore the sensitivity of the instrument. The other day, I reassembled the head, and when I removed a syringe needle from the PTV it would not hold pressure. When I disassembled the head, there was a white powdery substance on the Kalrez o-ring -- appeared to be some kind of salt. I cleaned this residue off and the PTV again worked fine. One of my coworkers said he has seen salt residues in the PTV. Is there a better solvent for cleaning the PTV heads for removing salts (distilled water -- is that OK)? Further sample cleanup is not an option.

Thanks

There are not many (any ?) salts that will dissolve in hexane. It is a worry that the head needs cleaning; if components of the sample are getting up to the injection head gas flows must be going in directions that they should not. You might be getting flash back - what kind of liner are you using ?, are you working splitless ?, what is the column flow and head pressure ?

Peter

First, I want to apologize for not responding to the reply to my original post. After some "soul searching", I determined that the foreign substance in the PTV was not a salt. It was a flake of "white out" from the "white out" used to position the column nut. The "white out" flakes all over the place when it dries out.

As an addition to this post, I want to say that as I have gained more experience with the PTV, I have found it to be very reliable and have had minimal problems with leaks. I am running some really nasty samples (such as fish parts -- ie livers, brains, ova, gonads, etc.) and the extracts sometimes still contain a significant amount of lipids. It appears most of the lipids and other high temperature contaminants end up in the top of the injection port liner, and not on the head of the column. I haven't had to trim the head of the column in quite a while, and am still getting good results.

littlepeaks,

I have a leaking problem with PTV: the head doesn't leak when syringe is inserted (Teflon ferrule is good), but the head leaks without a syringe inserted. The leak is small enough to not affect the flow or pressure, but you can see it clearly with a helium leak detector. I cleaned the head and replaced the kalrez seal, but that didn't fix the problem. Any ideas?

Assuming that you have a Gerstel PTV with a septumless head that seals with an angled metal plunger, you probably have some dirt between the plunger and its seat. Try ultrasonication and flushing with a series of solvents. If that fails, and your nerves are good, you can dismantle it, but I have to admit that I've never tried.

Having said that, leak seekers are so sensitive that they detect leaks which are of no practical consequence as long as they are not in the flow path to the column.

Peter

An alternative method for column installation is to quarter an old septa and push the column through it then add the nut and septa. Trim the column end and set the insertion depth. The septa will hold the distance so you do not need white out.
It took my fumble fingers some time to get used to this method but it works.