foreline pressure

Discussions about GC and other "gas phase" separation techniques.

9 posts Page 1 of 1
Hi There
I shut down the GC-MS over the break and turned it back on today.
The foreline pressure drops very slowly (from 136 mTorr to 83mTorr in 6h) is it normal because of the moisture trapped in the MS?
Please note the oven and inlet temps are sitting at 50C (to avoid column ox, when flow was off) does the low temp inlet can causes this?
Thanks in advance for your response.
Is this by chance an Agilent 597X?
Is the door closed and sealed? Have you leak checked it?
Have you performed an Air/Water Check?

If you cooled the MS inlet down to 50c it is possible there is a leak at the nut as the vespel/graphite ferrule will shrink when heated and when the nut cools it will then for a leak. Start with tightening that nut 1/8 to 1/4 turn and see if that helps.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Have you performed an Air/Water Check?

If you cooled the MS inlet down to 50c it is possible there is a leak at the nut as the vespel/graphite ferrule will shrink when heated and when the nut cools it will then for a leak. Start with tightening that nut 1/8 to 1/4 turn and see if that helps.



Hi James, thanks for your reply. I tried it and I think it fixed it or maybe I just needed more time as the next day it was at m59 Torr.
Something else that I notices is the H2O/N2 ratio. in my system N2 usually is the larger peak than water but it does pass tuning and leak check. this time water peak was larger. the oven was cold and I was thinking maybe the column attached to MS leads to a larger H2O peak. now the ratios are as before.
to be exact N2 4.96 E7 and the ratios are N2:H2O:O2 100:25:8.
Is this high N2 an issue? I use ultra high purity He and the color indication of filter is ok.
Sorry it might be long but I appreciate if you have any idea what is going on.
SherryGC-MS wrote:
James_Ball wrote:
Have you performed an Air/Water Check?

If you cooled the MS inlet down to 50c it is possible there is a leak at the nut as the vespel/graphite ferrule will shrink when heated and when the nut cools it will then for a leak. Start with tightening that nut 1/8 to 1/4 turn and see if that helps.



Hi James, thanks for your reply. I tried it and I think it fixed it or maybe I just needed more time as the next day it was at m59 Torr.
Something else that I notices is the H2O/N2 ratio. in my system N2 usually is the larger peak than water but it does pass tuning and leak check. this time water peak was larger. the oven was cold and I was thinking maybe the column attached to MS leads to a larger H2O peak. now the ratios are as before.
to be exact N2 4.96 E7 and the ratios are N2:H2O:O2 100:25:8.
Is this high N2 an issue? I use ultra high purity He and the color indication of filter is ok.
Sorry it might be long but I appreciate if you have any idea what is going on.


How does N2 (mz28) compare to mz 69 when the pftba valve is open? It should be less than 10% and is much better if below 5% 28 vs 69, anything above 10% and you have a leak that needs to be fixed.

Water will some times be higher due to it adsorbing into the column when it remains cold for a long period of time, but if you run the column up to about 95% of the max temperature for a few hours then cool it back down the water should go away for the most part.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
SherryGC-MS wrote:
James_Ball wrote:
Have you performed an Air/Water Check?

If you cooled the MS inlet down to 50c it is possible there is a leak at the nut as the vespel/graphite ferrule will shrink when heated and when the nut cools it will then for a leak. Start with tightening that nut 1/8 to 1/4 turn and see if that helps.



Hi James, thanks for your reply. I tried it and I think it fixed it or maybe I just needed more time as the next day it was at m59 Torr.
Something else that I notices is the H2O/N2 ratio. in my system N2 usually is the larger peak than water but it does pass tuning and leak check. this time water peak was larger. the oven was cold and I was thinking maybe the column attached to MS leads to a larger H2O peak. now the ratios are as before.
to be exact N2 4.96 E7 and the ratios are N2:H2O:O2 100:25:8.
Is this high N2 an issue? I use ultra high purity He and the color indication of filter is ok.
Sorry it might be long but I appreciate if you have any idea what is going on.


How does N2 (mz28) compare to mz 69 when the pftba valve is open? It should be less than 10% and is much better if below 5% 28 vs 69, anything above 10% and you have a leak that needs to be fixed.

Water will some times be higher due to it adsorbing into the column when it remains cold for a long period of time, but if you run the column up to about 95% of the max temperature for a few hours then cool it back down the water should go away for the most part.



Hi James
I missed this reply! Thanks for your response.
[img][img][][/img][/img][/img]
SherryGC-MS wrote:
James_Ball wrote:
SherryGC-MS wrote:


Hi James, thanks for your reply. I tried it and I think it fixed it or maybe I just needed more time as the next day it was at m59 Torr.
Something else that I notices is the H2O/N2 ratio. in my system N2 usually is the larger peak than water but it does pass tuning and leak check. this time water peak was larger. the oven was cold and I was thinking maybe the column attached to MS leads to a larger H2O peak. now the ratios are as before.
to be exact N2 4.96 E7 and the ratios are N2:H2O:O2 100:25:8.
Is this high N2 an issue? I use ultra high purity He and the color indication of filter is ok.
Sorry it might be long but I appreciate if you have any idea what is going on.


How does N2 (mz28) compare to mz 69 when the pftba valve is open? It should be less than 10% and is much better if below 5% 28 vs 69, anything above 10% and you have a leak that needs to be fixed.

Water will some times be higher due to it adsorbing into the column when it remains cold for a long period of time, but if you run the column up to about 95% of the max temperature for a few hours then cool it back down the water should go away for the most part.



Hi James
I missed this reply! Thanks for your response.
[img][img][][/img][/img][/img]


Looks like I can't attach a copy of the report. as per tuning report i have a history of 28/69 at 62% (if that's what you're asking).
this ratio was 210% while 32/69 was 40% (the leak check was 3% on the report). did another tune (didn't change anything) and 28/60 was at 1143% while 32/69 was 345%! leak check show 0.41%!!!
why leak check is so low but O2 and N2 content are high?
the filter color indicator is fine, I retighten column to Ms. what are other causes of this high N2? And why the report passes the leak check? I am frustrated and don't know what is happening. Your help is really appreciated.
Many Thanks
Depends on the tune setting you are using when you look at the ratio versus the one the air and water check uses. I believe the air and water check uses the Atune instead of Standard tune or any other targeted tune you may use for analysis.

If you have a can of the computer duster compressed gas handy you can spray that around each fitting and watch for a high spike in the 50-59m/z range. It will draw in the gas anywhere there is a leak.

I have had just a piece of lint on the o-ring seal around the door cause a large leak like this before. Always wipe the o-ring with a Kemwipe or other lint free cloth before closing the door, and I also wipe the same surface on the door itself just in case. There is also a sealing washer between the heated interface and the analyzer that can develop a leak, and I have had the interface itself have a crack in it causing a leak. The easy places to check are the column nuts but sometimes those are sealed and it is just hard to find where it is leaking.

If the air and water check shows low amounts of air and water, then maybe double check your tune file that you are using and make sure it is not optimized for the masses near 28m/z and depressed for higher masses as that would give the false impression that you have a massive leak. It is not common, but I have seen it happen before.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
Depends on the tune setting you are using when you look at the ratio versus the one the air and water check uses. I believe the air and water check uses the Atune instead of Standard tune or any other targeted tune you may use for analysis.

If you have a can of the computer duster compressed gas handy you can spray that around each fitting and watch for a high spike in the 50-59m/z range. It will draw in the gas anywhere there is a leak.

I have had just a piece of lint on the o-ring seal around the door cause a large leak like this before. Always wipe the o-ring with a Kemwipe or other lint free cloth before closing the door, and I also wipe the same surface on the door itself just in case. There is also a sealing washer between the heated interface and the analyzer that can develop a leak, and I have had the interface itself have a crack in it causing a leak. The easy places to check are the column nuts but sometimes those are sealed and it is just hard to find where it is leaking.

If the air and water check shows low amounts of air and water, then maybe double check your tune file that you are using and make sure it is not optimized for the masses near 28m/z and depressed for higher masses as that would give the false impression that you have a massive leak. It is not common, but I have seen it happen before.


I am so grateful of your responses James! You exactly know the issue! I'm going to evacuate the MS tomorrow and will check all the areas. Thanks
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