Pressure not increasing when increasing T at constant Flow

[saioa]

Hello all,
I have an unusual problem with the GC-MS. Let´s see if someone has experienced the same.
I have a GC program starting at 60 degree. When I start the temperature program it often (only recently) stops after some minutes because the inlet is not reaching the setpoint pressure (and therefore flow decreases a little of what it should be). The machine suts-down as a consequence. The temperature when it reaches this sut-down point varies within runs.
Before I start the temperature program I have checked for leaks and everything seems fine. However when the temperature increases I can see that the nitrogen content increases (even the leak test gave less than 5% of nitrogen what is the recommended).
I have read the troubleshooting and the most probable things from my understanding is to have:
- A large leak is present somewhere in the system
- Since I am using split/splitless inlet that the inlet´s proportional control valve is stuck open or closed because of contamination or another fault.

Can anyone help me? How can I know if it is the vale? Or if it a leak, where could it be to be depended on temperature?

Thanks,
Saioa

[danicrd]

Hi
more info please! What kind of GC you have?
What is your type of injection system? Direct, p&t, Headspace... What are your parameter? Total flow, mode of injection, etc
I think you have Agilent GC...and the error is "pressure shutdown".
Most probably cause is a leak...have you changed your septa? Or problem with total flow: when the injector requires high pressure (ie oven is rising your temp) but total flow is very low the pressure can't rise...so the error appear.
Hope this help.

[saioa]

Hello, Thanks for your answer.
Yes, I have an Agilent GC-MS. Injection system: split/splitless. Flow: 1.2 mL/min He.
Yes, I changed the septa a week ago.
How can I know that the total flow is the right one? or high enough?

[GasMan]

As mentioned, one of the causes could be a leak. Another cause could be that the inlet pressure of the helium to your GC is not high enough. It should be about 20 psi higher than the pressure needed for the flow at your highest temperature.

Gasman

[danicrd]

Hi
agilent s/sl need at least 10-15 ml of total flow to work right, but this value depend also from your column. Small diameter or long colum require higher pressure...
In split mode at lower split ratio this value sometimes is not achieved. Try higher split ratio.
If you work in splitless mode look at "gas saver": if at...5 min(for example) the value is very small can be a problem. Gas saver is the total flow
Bye

[saioa]

Thanks for the answers.
Some more details about the system. I work with splitless for 2 min.
Purge flow to split vent is 50 mL/min in 2 min
Total flow is set to 54 mL/min to give a constant column flow of 1.2 mL/min
Gas saver 20 mL/min in 5 min.
This morning I have been doing some tests with the gas saver. When I have it on I get the problems with not reaching the right pressure. If I do not click gas saver on after 5 min I do not have the problems.
The method has worked before and I had the gas saver on before. With gas saver off I do not have high amounts of nitrogen in the system. Is this normal?

[danicrd]

oK. The total flow seems enough to S/SL needs. So I think you would investigate for leak.
Try injector leak test without column installed.
bye

[Peter Apps]

Actually this problem is not unusual. Check back through the forum, or search the archives and you will find several other very simialr cases - and thie cures.

Peter

[saioa]

Thanks to all.
Finally after checking everything once more I found a leak in the injector. As I read in another post I solved the situation by replacing all the component and tighten them again. Now there is no leak based on an electric leak detector. Before I just performed the air/water test from the machine giving me no leaks.