Headspace-GC low response

[adsfo]

I've been running alcohols on Headspace GC and since recently my peak areas started to go down across the board but especially the methanol peak which is alost non-existent now. No changes to the method had been done. Could it be the column? (although the peak shapes are fine).

[alemaggot]

If all of your peaks has suffer area decrease it's possibile that you have some leak into GC system. Have you perform leak test?

[chromatographer1]

The most likely problem is in the sealing of your HS vials. If the vials are not sealed well, leaks and loss of volatiles will occur.

Injection losses from a leaking septum or valve seal are also possible sources/causes of your problem.

best wishes,

Rod

[aldehyde]

The other possibility is the vent or vial pressure solenoid is broken. This is usually the case when your peaks are suddenly very small, only a few pA tall on FID.

[adsfo]

The other possibility is the vent or vial pressure solenoid is broken. This is usually the case when your peaks are suddenly very small, only a few pA tall on FID.

Is there any way to check if it's broken? HP 7694
The pressure reading does respond when turning vial pressure knob...

[aldehyde]

Cap an empty vial, open the oven, and stick it in the position under the probe (3 oclock). Look on the back of the HS for the vent (a metal tube sticking out next to where your gases are plumbed), you need to connect a piece of tubing to that because to test the valves you stick the vent into some water and we're going to check for bubbles. The headspace comes with a piece of tubing but you may have lost it, if you have an LC the waste tubing is a similar size.

Once you have some tubing press the advanced function button, then 5, then enter. You'll use the arrow keys to go back and forth, and the 1 and 0 buttons to change settings (1 up, 0 down for the lifters and 1 on, 0 off for valves).

First press the arrow until you see the sample lifter, press 1 in order to push the empty sealed vial up onto the probe. Next press the arrow until you see the vent valve, the vial pressure valve, and the sample valve.

The first thing you do is pressurize the vial (vent closed, vial pressure open, sample closed) and make sure you don't see bubbles. If you do your vent valve is stuck open.

Once the vial has pressurized for 15 seconds or so, turn vial pressure off and turn vent on. You should see a nice stream of bubbles which comes to a stop in 5-10 seconds. If it continually bubbles your vial pressure is stuck open, if you get no bubbles your vial pressure is probably stuck closed and you're only getting tiny peaks because when the vial heats in the oven a bit of pressure builds and a small amount of sample makes it into the sample loop. I suppose its also possible that no bubbles could indicate a vent valve that is stuck closed--if you have vent and vial P on at the same time you should see a constant stream of bubbles which would prove you're getting vial pressure. I usually just replace both valves if the test fails.

If everything looks good go ahead and repeat the process with the sample valve open.

Are you using an EPC or manual pressure control?

One time I saw a headspace where the vial pressure had recently been replaced but they were still seeing very small peaks, and it passed this test. I took the probe out and stuck it through a sealed vial half way filled with alcohol and inverted, solvent dripped through the needle just fine--proving that the probe wasn't plugged. Next I examined the zero dead volume union that is on top of the probe, and it was fine. Then I took an autosampler needle and unscrewed the sample loop, I injected 5 uL of alcohol from the needle directly into the loop and then screwed it back on. When I turned the pressure back on I got a nice gigantic peak after about 5-10 seconds, indicating that there were no plugs from the loop all the way through to the detector (and it was a huge peak, and system already passed leak test so no leaks!)

This indicated to me that the fault was the piece of tubing going from the probe to the 6 port, I replaced it and the problem was gone. My guess is that when the vial pressure failed they did a bunch of troubleshooting, trying to inject samples. Each sample would heat up, and 'burp' a small amount of sample when the loop was opened in order to sample. Some of the sample somehow solidified in the tubing, despite it being close to the various heated zones.

edit: and don't forget to take the empty vial out of the oven! Otherwise once you get things fixed you'll go to run some samples and the first 3 or 4 vials will go in, then it will try to put one on top of the empty vial and screw up your sequence .

[adsfo]

My current vial pressure is set to 22 PSI, decreasing it to 16PSI lowers the peak areas even more. Wouldn't that also mean that the presurizer does work?
The areas get progressively lower as the sequence runs. I installed a new column, doesn't seem to change anything. Increasing headspace injection time didn't do antyhing either...
I started to have this problem after Nitrogen tank was switched but maybe just a coincidence...

[Johnny Rod]

I would check your flows all over, manually.

[adsfo]

I would check your flows all over, manually.

Can't find any leaks. Wouldn't there be retention time shifts if leaks are present?

[aldehyde]

If your vent valve is stuck open you'll be losing tons of sample during the injection but it would still be proportional to vial pressure.

Its possible you have a tank of bad gas, do any other instruments share the gas?

[adsfo]

I connected a piece of tubing to the vent port and stuck it to a vial with water. I didn't observe any bubbles/flow during either vial pressurization or venting...

If your vent valve is stuck open you'll be losing tons of sample during the injection but it would still be proportional to vial pressure.

Its possible you have a tank of bad gas, do any other instruments share the gas?

[aldehyde]

Did you put a capped vial in the oven and raise it onto the probe? If not you'll see no bubbles b/c the test relies on generating a bit of pressure in the vial. When you first have vial pressure on and vent closed you are generating pressure (and if you see bubbles that would be bad), then when you close vial P and open vent you want to see the generated pressure bubble out, and then stop (which is why a constant unending stream of bubbles is bad.)

If you had a vial on the probe and got no bubbles either your gas supply has a problem or the solenoid is bad. Even with a big leak you should see a few bubbles.

[adsfo]

Did you put a capped vial in the oven and raise it onto the probe? If not you'll see no bubbles b/c the test relies on generating a bit of pressure in the vial. When you first have vial pressure on and vent closed you are generating pressure (and if you see bubbles that would be bad), then when you close vial P and open vent you want to see the generated pressure bubble out, and then stop (which is why a constant unending stream of bubbles is bad.)

If you had a vial on the probe and got no bubbles either your gas supply has a problem or the solenoid is bad. Even with a big leak you should see a few bubbles.

I was running my regular sequence. In the method, vial pressurization time is 0.2 min following by venting. No bubbles during either cycle. I should probably order a solenoid vent valve

[aldehyde]

To be safe order both of them if you can.