gc-headspace problem

[che313]

I,m working with gc- headspace from perkin-elmer
the gc is clarus 500 and the sampler is turbomatrix 40
and the detector is fid

this insturment is utilized in quantifing the amount of ethanl in blood and urine.

in this instrument I'm facing the following problem
when the vail is heated and it is the time for sampling, the carrier gas presuure - He - in gc dropedd from its setting value and gc becomes not ready!!

Any body can help me??

[shaun78]

I've observed this problem with mine and spent a very long time trying to figure it out while I waited for the instrument to be serviced.

When PE came out, the identified the problem to be a break in the transfer line from the HS to the GC injection port. Naturally, this puzzeled me because the "old" transfer line was only in use for less than six months and we never once removed the transfer line in that duration.

Anyway, it might be the same for you.

[che313]

as I expect

is there any way to check while PE come?

[chromatographer1]

Questions from someone who does not own a PE:

Is the transfer line something that only PE can service?

Can't the user replace the transfer line?

If it is a fused silica piece of tubing you might consider replacing it with a fused silica coated SS piece of tubing.

thanks for any replies.

best wishes,

Rod

[che313]

Questions from someone who does not own a PE:

Is the transfer line something that only PE can service?

Can't the user replace the transfer line?

If it is a fused silica piece of tubing you might consider replacing it with a fused silica coated SS piece of tubing.

thanks for any replies.

best wishes,

Rod

We have to ask them becuase of the Lab policy.
yse, it is fused silica.

[chromatographer1]

I never broke a SS fused silica lined transfer line. Not once in several years. And I only had to replace it once due to contamination.

best wishes,

Rod

[GaryR]

We have two PE headspace samplers, an HS40XL, and the newer Turbomatrix.

Changing the transfer line is described in the user manual, and is not particulary difficult to do.

You can perform a leak test to see whether or not the transfer line is broken, also described in the manual.

It would be unusual, but not impossible, to break the transfer line inside the insulated tube.
We usually see the transfer line becoming unusably short at the GC inlet end from continually trimming the end after piercing the septum.

[Peter Apps]

Hi Gary

You can save trimming the end of the silica by using a hypodermic needle as a guide through the septum.

First find a needle that the end of the transfer live will fit into, and smooth off the cutting edges so that it does not gouge great lumps out of the septum. Rinse it out with solvents.

Remove the septum nut and septum, and stick the needle through the septum from the underside. Thread the transfer line through the septum nut, then through the needle. Pull the needle back out of the septum and the transfer line will be through the septum, but with no bits of rubber jammed up the end.

Peter

[shaun78]

For what it is worth, our transfer lines seem to break in our PE system (HS40 and Clarus 500) about every six months. I've had ours replaced three times in the past year and a half. I would just do it myself as I have done in the past, but the company I now work at does not take kindly to me doing so.

Like I said, we never disconnect them. Heck, we never even touch them. The silica tube simply breaks down somehow and disintegrates.

I've run headspace applications on other systems for years and have never once seen this problem that I am having with the PE's.

I don't mean this post to start a war against PE systems, but I am getting teh indication from the use of our system that something else is going on that had to be causing this breakdown. Replacing the transfer line every six months simply is not right.

[GaryR]

Peter

Great idea! thanks, we will do that.

Shaun

I agree, there must be something else going on there, it is not right for the transfer line to just break like that. We have never had such problems.
How far inside the tube is it breaking? Is it the same place each time? If your temperature cycle is extreme, the polyimide coating will deteriorate and become brittle over time. Our transfer line temperature rarely goes over 150C, usually around 110 - 120C.

Gary

[chromatographer1]

The transfer line being discussed is often a piece of fused silica tubing with a polyimide coating. Any place within the heated sheath where the FSOT tubing touches the sheath will wear away the coating, exposing the fused silica and a break will occur. This is a common occurance and there is nothing exceptional about it.

If of course, your piece of tubing has an extra thick coating this will extend the lifetime. FSOT Tubing from Quadrex often has a very thick coating in my experience.

If however, you are interested in rugged service with a VERY long service life then buy a piece of fused silica coated stainless steel tubing and replace the FSOT polyimide tubing. This tubing is available from Restek.

As I have previously stated, this was an excellent choice for a transfer line and only required replacement once in more than 5 years of service.

best wishes,

Rod

[Ron]

I was in a lab when a new PE instrument was being installed, and the first thing the operator did after the service engineer left was to replace the fused silica transfer line with SilcoSteel. As far as I know there haven't been any transfer line issues in the last 3+ years.

[shaun78]

GaryR

We generally keep the headspace operating at 110-120C. If we feel carry over is present, we may bake the line at 150C for 30 min to an hour; however, this is very rarely done. The line generally breaks around (and use around loosely) the area where it enters the HS. However, one time the entire line just fell apart (though I suspect that this just did not magically happen).

Ron and Chromatographer1

Thanks for the tip on the ss coated transfer lines. I have a feeling that the next time the line breaks it will be replaced with one of those!

[Peter Apps]

Headspace analysis of aqueous samples is pretty rough on fused silica transfer lines (in the days before silcosteel I used to thread fused silica through the old-fashioned nickel line of a Varian Genesys) because the silica is exposed on the inside to hot saturated water vapour, and it does not even have a layer of stationary phase to protect it a bit. Silcosteel is mechanically more robust - but I would be surprised not to see a deterioration in surface inertness.

Peter

[chromatographer1]

Peter

Your concerns are not unfounded, but I think you might be surprised at how well the fused silica coating will hold up over time. Granted I only used routine solvents in most of my analyses but as long as one did not try to elute strong bases through the line the tubing held up quite well. I had DMAc vapors on most samples due to spiking of samples and acids did not bother the coating. Only when ammonia was used did the degradation of the surface take place and activity of the surface increased. I was not required to use excessive temperatures and maintained a 110°C on the transfer line. When the activity increased a quick replacement was warranted.

It was a pain to feed the tubing through the shealth heating but once done it was quite reliable for years of service at a time.

I never exposed the outside of the polyimide tubing to water vapor. I used connections that kept all sample vapors inside the tubing. The FSOT was only exposed to atmosphere like any column inside a GC oven.

Highly reactive analytes of course are a different matter but then the issue of metal needles and sealing O rings also enter into the concerns.

best wishes,

Rod