Excessive pressure error

[doctorante]

Hi folks,

I have inherited an 5971 MSD fitted to an HP 5890 GC. I am using the auxiliary line of the GC, therefore i am not injecting anything. The gas flow will have to take the sample from a trap on line, but at the moment is not in that configuration yet. I have tried to check the system joining the gas outlet from the EPC to a capillary column (30 m, 0.25 mm id). I try to work at 0.75 ml/min, but all the time i am obtaining this excessive pressure error. The highest pressure i can set in the EPC is 3 psi, which is clearly insufficient to obtain that flow rate. I have check the system looking for leaks (regulator, gas lines, fittings to the column...) but i have found nothing, and i have replaced several times columns, nuts, ferrules. I am obtaining at the moment over a 25% water and 300% Nitrogen and 150% oxygen (not in an atmospheric ratio...which should indicate no leaks): where are they coming from then?. I have checked the seals of the vacuum chamber, and filled the diffusion pump with new oil, and checked the rough pump as well... nothing. The joke is that when I cap the end of the transfer line with a blank ferrule, I have no problem of water or nitrogen, but as soon i let the gas flow through the system all appears again (the pressure error and the high content of water , N2 and O2). I bypassed the EPC and used a external pressure regulator fitted directly to the column resulting the same.
At this moment i haven't got a clue what's going on with that machine and i am going crazy... Do someone have a clue about all these problems? (avoiding me to join a mental institution soon).

(needless to say that i have tried all the possible causes referred in the manuals and in the website of agilent)

Thank you very much indeed.

Jose

[Consumer Products Guy]

I have one setup same as yours, but I'm a little confused. If your column head pressure is really only 3 psi you souldn't be getting enough column flow to the 5971 to result in an excessive pressure error from just the carrier gas. Personally, I'd isolate to see if the issue lies with the GC or with the MSD. From HP Flow Calculator I see 7 psi head pressure required to produce a column flow of 0.75 ml/min. with an MSD (vacuum). I'd vent the MSD, remove the column outlet from the MSD, and see if I could get a head pressure of 7 psi, then measure the column flow which should be 0.4 ml/min then becuase the column is not under vacuum any more, just to atmosphere. How to measure the flow rare through the column without specialized equipment? I'd push column end through a cylindrical septum, then shove that into bubble meter tubing or flow meter, and see if 10.0 minutes of flow gave me 4.0 ml of carrier gas. If that checks doesn't check out OK, you have a leak or a flow path probem in the GC. If that's OK, I'd plug the MSD inlet and pump down, see if that is OK, maybe you have an MSD leak. Only then would I install the column into the MSD with the 7 psi head pressure.

[doctorante]

Thank you for your quick answer. I tried to measure the flow through the column 3 days ago, using swagelok connection to a flowmeter but the response i obtained was completely absurd. Agilent service phoned me this morning, and they told me something that can be useful for people with my same problem. In order to erradicate the error you can use the CI mode (there is a switch just on the MS top plate). According to them will override the security system that avoid the system to work under high pressure. I tried it, and it works. Now I can set 7 psi without any error, and magic or not, the N2 and O2 have dropped under 5%. I still have some water, but is going slowly and i am doing cycles to 200C (the top limit of my column) to eliminate it. I dont know what was the problem, but as far as I can work and te system gives me the sensitivity I want (ppb) i will be happy. That's another story, because if i have an excess of gas going into the vacuum chamber i know i will have a loss of sensitivity, but perhaps that excess of gas is not real and the electronic is playing games.
Anyway, i wanted to share this with you and thank you your help in such a bad moment.

Jose

[Ron]

If you have an overpressure error and when you try to measure the column flow you get an unreasonable value it sounds like you may have a problem with your flow controller. Setting the toggle switch to CI mode does not solve the problem if you truly do have too high a flow rate, it just allows you to potentially damage your intrument by operating under improper conditions.

[doctorante]

Hi Ron!

Sorry for the delay in my reply but i was away. I thought that i could have a problem in the EPC, and i bypassed the EPC using a proper regulator. The pressure that i obtained in the head of the column was the same that using the EPC. I know that the problem could be still there and the only i have done is more or less avoid it because the system is not giving me an error.
My problem now is that i have a very high signal of water, N2 and O2 and the sensitivity is not so good. Sometimes the ratio N2/O2 is in atmosferic ratios and sometimes not. It is making me crazy. That's why i dont know if i could have a leak or not. I have checked all the connections and seem to be OK.
I have a doubt about the electron multiplier but i am not sure. I obtain a really high signal of N2/O2 (4000/2000) and almost no water when i cap the transfer line and no gas is flowing. is that normal?

thank you for your time.

Jose