Drifting Signal 5973

[cat4lyst]

Hello!

I am having trouble with a 6890/5973 which is running P&T being used for 8260. I am hoping to bounce some ideas around with all the expertise here. Problem is as follows; after source maintenance a target BFB tune is preformed and a calibration is run shortly there after. Calibration has excellent response in general. its almost as if the mass spec is too sensitive, low levels have excellent abundances, 4x that of a redundant system running next to it. The high cal points peaks look saturated, they are fat and rounded at the apex, not sharp. this is giving us many quadratic fits in which the high cal point are causing. moving forward with this calibration the MS response fluctuates almost run to run. It most apparent in and overlay of calibration checks (ccv) also in IS responses. The fluctuation is across the entire chormatorgram there is no bias to any specific peak. It can also be seen overlays looking at the TIC baseline.

I have covered many of the typical items troubleshooting this problem;
cleaned source, new filaments, new em (currently tuning 1.5kV), new column, full inlet maintenance, rinsed split lines, new epc, dipped quad per agilent's instruction's at 100 amu, concentrator is a stratum with low miles.
the HED assembly was replaced about 1 year ago, agilent tech support referred to as a noisy hed, I had mass present for every scanned amu. I am thinking there is something still going on... possibly the log amp??

as i am typing I ran the adjust rfpa at amu 800, over ten minutes voltage increased from 554mV to 567mV, is that typical?

any help please! banging my head on the wall right now . I am happy to elaborate on anything that was left unclear. tried to cover most of the basis.

[LindseyPyron]

Can you provide some additional information??

What is your cal range?
Column, flow rates and split ratio?

[cat4lyst]

we are running a .5ppm to 400ppm cal range

this mass spec is a diffusion pump model it is running a 1 ml/min flow rate. split ratio is 5:1.

[James_Ball]

we are running a .5ppm to 400ppm cal range

this mass spec is a diffusion pump model it is running a 1 ml/min flow rate. split ratio is 5:1.

That calibration range is pretty wide, but possible. You will almost certainly see a few compounds that over range while some are difficult to see on the low end.

Two things to consider. The split ratio is fairly low, which means your desorb flow is going to be low. If you can get a desorb flow of around 20ml/min total flow you will be better off and more stable run to run. Second is the diffusion pump. They are great and will be stable for the long haul with volatiles, but they take a while to become stable. After I vent ours I know that I can tune and run the next day, but that I will be recalibrating within a week as it drifts into what will be its final stable sensitivity until the next venting. The message that says wait 4 hours for stability is optimistic. Usually if I need to vent for maintenance I try to do it on Friday and let the instrument stabilize over the weekend, but even then you will be recalibrating after a few weeks when using the diffusion pump.

Something else I noticed is 1500V on the EM. If it is new I would expect a lower setting near 1100-1200. We also had trouble with IS drifting higher as calibration concentration increased then dropping back down to regular levels when running samples. The only way we could solve this was switching to the ETP ribbon style EMs. Some people do just fine with regular EMs, but we didn't have much luck until we switched.

Since you seem to have plenty of low end sensitivity the first thing I would try is increasing the split ratio up to at least 20:1, then adjust EM volts until you see all the targets in the low calibrator, you should then see less over ranging on the high calibrators.

[LindseyPyron]

I agree with James (as usual)...Split ratio and EM change should solve your problem.

[cat4lyst]

Thanks for the useful advice guys,

just a couple things. this instrument, as well as 7 other redundant setups, have been running this split rate and cal range for 10 plus years now. So while I do appreciate the expert advice, the split and cal range are not of particular importance considering the history using these parameters. If i was in doing development surly these suggestions would be examined. I know these parameters aren't perfect but we have had success using them for quite a while now. I need to bring this instrument back using the historical method, as we know it works.
I neglected to note the EM was changed several months back so I am not particularity worried about the tune requiring 1500v to meet the abundance criteria. Thanks for the advice on the ETP multipliers, I only hear good things about these may have to give one a whirl.

With all that being said, I am actually leaning towards a couple things now.

1: antsy operators: I believe the mass spec was tuned way too early (like as soon as it was "up to temp") and then calibrated, while I was out. I came back to some interesting phenomena. This is a constant battle for me...
2: the mass spec was out of use for a while. As you said James, diffusion pumps are slow to acclimate, especially when you add a bunch of water in the mix.

We ended up calibrating again after a few days had passed. Things are looking pretty good up to now. I am happy to say. I sincerely appreciate all the help, it's always good to hear some outside ideas.

[James_Ball]

Glad it is working better. I was definitely leaning towards the diff pump not having time to settle in, just wanted to throw out the other things I had seen.