Trouble with methane flow rate in PCI

Has the methane supply pressure ever exceeded about 40 psi?  The CI Reagent Gas Flow controller will be damaged if the supply pressure is too high.

The symptoms you are seeing happen when the CI Gas Flow Controller has failed.  It is a 5977A, so quite old now, too!

Thanks for your response!

The regulator is set to 25 psi. We first started having this issue after switching from EI to CI. The methane tank was very low/empty and so we thought the problem was due to the empty methane tank. However the problem persisted after changing the tank. Could the CI gas glow controller have failed without exceeding the 40psi pressure? Is there a way to test if this is the problem other than calling out a service engineer? 

Many thanks!!

Is there a trap on the methane line? Has it been changed recently - within a year?  The tank may be 25psi, but if the trap is old/compromised then after the trap the pressure may be too low.  Try removing the trap, purge the line at 25 psi tank regulator at the back of the MS, and try it again. Getting rid of the air introduced during this may take a couple of purge cycles.  It also could be a methane tank that is not 100% methane. It would be worth trying a different tank, too.

Yes, parts fail sometimes.  The CI Reagent Gas Flow Controller is a delicate expensive part as it is a true forward flow controller calibrated to go to vacuum. The part is about $10,029 USD and needs two VCR gaskets replaced at the same time.   The flow controller fails more often than the solenoid selection and shutoff valves - it has a much harder job to do. 

You can monitor the methane a few ways. With the flow on and the setpoint stable, check the vacuum reading. Is it the same as it was before?  You could do this in manual tune like once an hour for a while to see.  Another way would be to either do a "Generate Report" -- it uses the currently loaded tune file and gets new tune ion peaks and a spectrum without changing anything. Or, make a CI run method that turns on the PFDTD every 30 minutes for a few minutes and look at the profile of the chromatogram and the spectra to see if it's turning on and off the same every time.

The trap we had on previously was quite old, but when the same problems persisted with the new methane tank we did put in a new trap to see if it helped, but with no luck. As this problem has carried over to a new methane tank, we feel it's unlikely that the methane tank is the issue, but we will keep this in mind.

The vacuum reading each time after source cleaning pumping down is initially very good, and as good as it was before. However, after the autotune (or after a methane flow set up) once the instrument starts to ramp up the methane flow above 20% to reach the desired ion ratio, the vacuum starts to get worse and is no longer as good as before.

We have tried baking the source further after a "bad" autotune, but this doesn't make any difference, the vacuum also remains the same in the proceeding tunes. We have also tried running a few autotunes one after another to see differences (with no changes inbetween), the methane flow may change 1 to 2% but that is it. If we do a full vent and source clean, after pumping down the methane flow rate has been somewhere between 28 and 40%, different each time, sometimes better, sometimes worse. 

I forgot to mention in my original post that the abundance of the lowest ion, m/z 41 is much higher than in the past, where as the abundance of the other two ions has slightly decreased.

We are currently pumping down again after another source clean. This time we have plugged the transfer line in the GC oven to isolate the MS from the GC. We will try and monitor the methane through the "generate report" method you have suggested.

Thank you very much

(+) 5977 Chemical Ionization - 21Apr2021 - Files - GC/MS - Agilent Community

The vacuum should be great after pumpdown. It should be good with the methane flowing. It should be great if you shut down the methane flow in Manual Tune and the vacuum should go quite quickly back to nearly the value it was before the methane turned on.  That is normal operation.

How long are you waiting to test after venting, cleaning, and pumping down again? The source needs to be, for PCI, at 300° C for a few hours before testing as thermal stability should make the tunes more reproducible.

The tunes with the column out will not be normal. The column flow is necessary for the ion source, EI or CI, to function normally. It will give you some idea of the system.  The column flow plus reagent gas flow with the CI source pushes the ions out of the ionization chamber through the 1mm hole in the drawout plate and the 0.5mm hole at the filament.  If the column flow is missing, the CI Reagent Gas Flow may be increased to compensate as the number of ions coming out of the source is not typical.

Please share a PCI tune from when you were happy with it in the past and a tune or two now for comparison.

How old is the rough pump?  If the rough pump's pumping speed is not great weird things happen...

Yes in the past we have had a good vacuum at all points of operation. However, as the MSD ramps the methane flow rate the vacuum gets worse and we are assuming the bad vacuum (or at least worse than what we have had before) are due to the increase methane flow rate.

After pumping down we bake the source at 300C for 12 hours everytime before running an autotune.

Thank you for the note on the column- we are trying anything at this point as we have ran out of ideas but we will re-connect the column.

I will send another message with some tune reports thank you!