Noise in 5971 blank run

[jkloepping]

I just replaced the electron multiplier on my 5971, and I wonder if someone with more experience can look at the attached images and tell me if the air and water check and the blank run look normal.

I'm pretty confident that the air and water are good, as I can hardly get less than zero! However, I can't say if the tune parameters are normal or not. They are certainly different from other tunes I have seen in the HP docs.

The other one I’m not so sure about. The system has been running for four days after changing the Electron Multiplier. The EM voltage dropped from near 3000 volts with the old EM, so the change has done some good, but I’m curious about the shape of the data that results when doing a blank run. It makes sense that the noise would rise as oven temperature rises, but I don’t understand why it is so high when I start the run? I’ve turned off the solvent delay in this case, as I didn’t actually inject anything into the system. The injector is at 220C, the transfer line is 240C, the oven starts at 60C and ends at 300C. I’m hooked to a hydrogen generator and the 5890 Series II is set for constant flow of .875ml, while the pressure is 0.2 psi. The indicated pressure on the ion gauge is 3.2x10^-5 and has been stable for days.

It seems to me that starting the run doesn’t change anything – the oven, injector and transfer line were being held at the start conditions for the previous 12 hours anyway, and so was the pressure.

The ion chromatogram is the average of the noise from 20:00 to 25:00. The peaks are roughly 15amu apart, so it appears to be hydrocarbons? Are these normal amounts for a good system, or is there contamination here? The total abundance of the peaks seems insignificant to me, but I’m new to this.

[James_Ball]

I see background like that when I run hydrogen as carrier, especially if it is on a new column or instrument that has run helium carrier for a long time. Crud builds up in the flow path and hydrogen will clean it out very effectively over time.

What is the EM volts on the new multiplier? It should be down near 1000-1200v if the system is good and clean with a new multiplier.

Also the ion gauge tube will read much higher when running hydrogen instead of helium as each gas has a different response to the filament, and if you look on the controller you will see it marked as being calibrated to pressure of nitrogen. You can actually run near the flow you had with helium and even though the gauge shows a much higher pressure you really are not as high as it appears.

Set it up to run blank injections all night and see if the background comes down, it may take several nights to get a good baseline.