no inlet pressure after installing headspace transfer line

[MestizoJoe]

Hi guys,

The part that was broken on the transferline was the fused silica deactivated column. I replaced it with agilent's prosteel transferline which was about $100. This is an Agilent 6890N.

I'm still new at troubleshooting GC problems, but I'm fairly sure that the flow was blocked somewhere in the transfer line. I think something may have clogged it when I reassembled the inlet after replacing the gold seal, liner, septum, etc. I still don't know if it was a leak however.

Today I'm running system suit with ETO standards. So far, everything is working fine. I think we will be purchasing a leak seeker as well.

[Peter Apps]

Now I am properly confused; was it the transfer line or the chromatographic column that was broken ? NB that a piece of dectivated fused silica tube that is transferring the sample from the headspacer to the GC is not a column. And a 6890 is a GC, not a headspacer - was it a part of the GC that was broken ?, something in the inlet perhaps.

If you are connecting the transfer line to the inlet by pushing a needle on the end of it through the ordinary injection septum then there is a chance of blocking the needle with a little plug of septum. This is especially a problem if you are trying to put a silica transfer line into the inlet - it is nearly impossible to prevent the sharp edges of the silica cutting a core out of the septum and getting plugged. That is why I asked how you are connecting the transfer line - and you have not answered yet. There is a way of getting silica through septa without coring, but if you are using another connection method it is not going to help.

Peter

[MestizoJoe]

It was the transfer line. Sorry about that forgot to mention the 7697A.

The transfer line does go through the septum and take a core of the septum. I pulled that little plug out each time though. How do you avoid coring?

[Peter Apps]

If you are frequently connecting and disconnecting then you might need one of these:

APPS, P. and MMUALEFE, L. 2011. An injection cannula that enhances the versatility of split–splitless inlets. Chromatographia 73: 1229–1231.

But if you leave the headspacer connected for several days at a time then do as follows;

Find a hollow needle that is just wide enough to allow the fused silica line to thread into it.

Remove the inlet nut and septum from the inlet and stick the needle through the septum from below and up through the nut. This is much easier if you have already made a hole in the septum with the nut screwed onto the inlet.

Thread the end of the silica into the needle far enough to get through the septum.

Pull the needle back through the septum and off the silica.

Voila ! silica threaded through a septum without blocking itself.

Screw the nut onto the inlet.

Check for leaks.

Peter

[Bigbear]

I still say it's best to cut the line supplying the injector and put the headspace in series. When you need to un install the headspace all you need do is re connect the cut pieces with a zero dead volume union.
The #1 benefit is that you get the sample to the GC at 50-70 cc/min ( dependant on the split ratio).

[Peter Apps]

I still say it's best to cut the line supplying the injector and put the headspace in series. When you need to un install the headspace all you need do is re connect the cut pieces with a zero dead volume union.
The #1 benefit is that you get the sample to the GC at 50-70 cc/min ( dependant on the split ratio).

The problem with this is that it puts a piece of cool, undeactivated stainless steel in the sample path. You can out the same flow through the transfer line if it goes in through the septum, with no cold spots.

Peter

[Bigbear]

The "cold spot" issue is a non issue for my P&T volatiles. It may have been a problem when we desorbed at column flow. With about 5cm exposed to 20C room air I have no noticeable band broadening. I use a 150:1 split so my desorb flow is about 100 cc/min through a 1/16" tube. My gases have good peak shape as well as do my polar compounds.

[Peter Apps]

The "cold spot" issue is a non issue for my P&T volatiles. It may have been a problem when we desorbed at column flow. With about 5cm exposed to 20C room air I have no noticeable band broadening. I use a 150:1 split so my desorb flow is about 100 cc/min through a 1/16" tube. My gases have good peak shape as well as do my polar compounds.

All good if it works for you, but the OP is one of those who leave us guessing for significant details like what is he trying to analyse. In general I would avoid cold spots and bare metal if I could, and putting the transfer line through the septum is an easy way of doing that as long as the gas flows can be directed in the right directions and set to sensible values that give high flows through the line.

Peter

[larkl]

Excuse me for bumping an old topic, but I feel I must add the following. I find it best to cut the inlet pressure line and tie the headspace sampler into this line. This works fine, but you need to be sure that you cut the inlet pressure and not the spetum purge line. On Agilent GCs, the inlet pressure line is 1/16", while the septum purge line is a metric line. Don't listen to anyone that tells you "the septum purge is the one in the front or rear". This did NOT work for me and I was down for a day waiting for the metric union to be FedEx'ed so I could repair the septum purge line.

Get a piece of 1/16" line and either use a micrometer or your well calibrated fingers to decide which line going to the inlet is the 1/16". This is the inlet pressure line.

[merilon]

Hello,

Let me refresh this topic and ask you for help/advice. I'm working with Agilent 7697A headspace sampler. I have a problem with GC inlet pressure after headspace sampler is on. At the beginning I've noticed only the lack of pressure on front inlet, but after I've noticed that pressure is going down immediatelly after headspace sampler is "on". I've change liner, o-ring and septa and replugged transfer line again, but it didn't resolve the problem. I've tried to perform leak test, but the equipment can't make it --> I've got these messages on the screen:
"Leak test failed, Unable to set pressure zero" and
"Restriction test failed"
I have also:
"GC Carrier control --> Pressure: 0.360; Flow: -27.3"

The bottle with helium is full.
A few days ago we had problems with the leaks of helium and we've replaced the new valve at the supply line of helium to headspace.
Someone has an idea what might be causing these problems ?

[Bigbear]

If you are inserting your transfer line through the injection port septa, column flow will be controlled by the head space unit. It's not surprising that your GC inlet shows no pressure.

[ingcargar]

Look at vent valve or sample needle:"Restriction test failed"