Chromatography Forum

I have an application that requires high temperatures for headspace analysis. I am currently using a CTC CombiPAL attached to a GC. The syringe must be heated to 150C and I am seeing a constant bleed of high molecular weight materials from just the syringe by GC. I have tried heating the syringe for several hours, then cooling and washing with a polar and then non-polar solvent. The software will only allow a nitrogen purge of ten minutes between samples so I have set up a series of dummy runs so that I can purge in ten minute intervals for several hours. I then cool and wash and then heat and purge again. This conditioning cycle doesn't seem to eliminate the bleed. The syringe is a 1mL series 1001 Hamilton gas tight syringe that is sold with the CTC. Any suggestions would be appreciated.

Are you sure it's not the septa? How can you test the syringe without puncturing the septa?

Maybe it's some other component, like the inlet liner? I've seen dirty inlet liners that look pretty clean, when tested without an injection, ie- just push start and run. Problem is without an injection, there's no thermal load or volatilization in the inlet and then you get a clean run.

Maybe I'm all wet, but you might want to take a step back from the syringe theory.

All good questions. Prior to this experiment, I ran several liquid injections and observed no trace of the contaminates that I am currently seeing. I also ran blank headspace samples with and without septa to determine if that was an issue. I saw the same contamination in both types of samples. I changed the syringe to another newly purchased syringe that had never been heated. At that point the concentrations of the contaminates increased. Over time with conditioning the contaminates do decrease in concentration, but I can't seem to get rid of them altogether. I am wondering if the syringe is continuing to degrade even after conditioning, or if my cleaning process is flawed. Is there another syringe out there that can handle heat better (150C), or is there a better conditioning process that I need to go through?

Hi Nikko

All (?) gas tight syringes have a PTFE tipped plunger, and all GC syringes have their needles or needle hubs stuck to the glass barrel with glue. These things bleed, and since thete is a substantial chunk of PTFE on the plunger tip the bleed will go on for ever. 150C is really hot for a gas tight syringe - as well as the bleed you will get distortion of the plunger tip, which will then tend to jam or leak (or both ).

This application would really run better on a headspacer with a high temperature 6-port valve, but if you cannot go that route you need to look at any way available to reduce the syringe temperature.

Washing after purging might be counterproductive - the solvents (and their impurities) will soak into the PTFE and then bleed out again.

Peter

Hi

Yet it would seem strange that a company sells an instrument that is supposed to be operational with the standard set up up to a needle temperature of 150°C do give extra peaks as "standard", might be a good idea to contact them.
One would expect the shift to a new needle would solve the problem if only needle was the issue.
Or is this instrument set up/needle really not better than that?

At same time it is hard to see the whole picture as details are lacking.

Septa choice, yes can be an additional factor especially if headspace/incubator oven is 100°C or above. Mismatch with solvent is another.

As Peter indicated, this CTC has some limitations, needle temperature limit of 150°C in long run can lead to build up of residues from typical headspace solvents with higher boiling points especially if incubator/headspace oven is operated at 100°C or above. And as Peter also mentioned wear and tear on the plunger depending on solvents used.

As Krickos suggested, contact CTC or the distributor in your country.
Probably they will advise a syringe that has less bleeding at those temperatures.

I've heard that Gerstel MPS (a CTC-clone) had similar problems.

Are you sure the contamination is not coming from the headspace vials or septa? Most headspace applications aren't run at a temperature above 60 to 80C, and I have seen significant bleed from vial septa heated above 120C.