Agilent GC/MS ghost peaks and tuning issues.

[pdostie]

Hi All,

I'm hoping with some help for our GC/MS it's an Agilent 6890N/5973N we've been seeing some some odd results for the past month or so now but have been unable to pin down the cause. We're finding inconsistencies running the same sample back to back using the same method.

We seem to be getting "ghost peaks" of mass 32 at various retention times along with some variability in peak size of of a given sample from run to run. I've included a few images of what we're seeing.

Auto tuning looks good, we don't have a significant air or water leak, but when we ramp the source lenses we see a decidedly non gaussian curve for the emissions spectra. The auto tune for the emissions values picks a point right in the trough of the curve.

Tec support believes that the problems lies in the inlet, but when I decouple the GC from the MS we still see the crazy emissions lens ramp.

Since seeing this activity we've

1. replaced the syringe, septum, liner, gold seal, gc-to-inlet ferrule, and cut approximately 3/4 of a meter off of the front of the column.

2. cleaned the inlet twice with the "gun brush" method using subsequent rinses of DCM, Acetone, and MeOH.

3. cleaned the source, replaced the filaments, and replaced the ceramic insulator around the repeller.

My main questions are why is the emissions lens looking so non-gaussian, and if this is not an electronics issue, why are we still seeing this non-gaussian curve for the emissions spectrum even after the gc has been decoupled for the ms?

oh, and how do I fix it?

Many thanks to all.

PDostie

Link to images: https://drive.google.com/folderview?id= ... sp=sharing

[Consumer Products Guy]

Are you able to start collecting data at m/z 35?

[pdostie]

Do you mean to say ignore everything below 35? I'm not very familiar with these instruments.

[Yama001]

I do not believe ramping the emission current is telling you anything. It is usually fixed at 35, other useful values range from 20 to 60.

[pdostie]

yama,

Is that to say the shape of the ramp curve does not matter? I don't have an idea of an ideal emissions ramp curve, but I'd imagine it's not supposed to look like what's posted.

[James_Ball]

yama,

Is that to say the shape of the ramp curve does not matter? I don't have an idea of an ideal emissions ramp curve, but I'd imagine it's not supposed to look like what's posted.

I never ramp my emissions unless I limit it to say 15-40 range. What you are ramping there is how hot the filament runs, and it definitely does not give a Gaussian curve when ramped. Repeller, Ion Focus, Entrance lens, and Entrance Lens Offset are the only things you need to ramp when tuning the instrument. I normally run my Emission at the 27-34 range depending if I have too much sensitivity at low or high masses. If you increase emission you get more breakdown of your analytes into smaller fragments so increasing emission increases low mass abundance and decreasing it can increase high mass abundance. It takes very little adjustment to make a big difference. Ramping it up to maximum will only shorten the life of the filament.