High mass sensitivety loss after GC-MS Source cleaning

[biotechno]

Hi,

We work with a HP 5963 Mass Detector.
When autotune reachs certain levels we procede with a source cleaning.

But sometimes source cleaning result in a worse sensitivity at higher masses (502 m/z autotune).

After cleaning, autotune results show:
- EMVolts decrease like expected.
- Absolute Abundance of low and medium mass increased like expected.
- But high mass abundance decreased significantly, leaving us with a relative abundance around 4%.

Does anyone knows what may cause this?
As far as we know, we follow the same cleaning protocol as HP technitians.

[Alexandre]

A lot comes with good mass spectrometry... From my observation, ~10 years of experience is needed to be on top technically wise. It is naïve to expect otherwise from some employers.

Cleaning. Cleaning is an art of its own. Glassware should be MS clean. It can not be washed with phosphorus or ionic aliphatic detergents. Sometimes after it was cleaned MS get worth, no matter what. Re-do it. One lab reported on a conference that his employee’s aftershave was the reason of MS get beserk.

Solvents, air to dry - all things are important. Don’t hurry too. The longer MS is pumped down the better for 502.

[biotechno]

Yes, usually with time (weeks) we see an increase in 502 m/z sensitivity before we start to see a decrease from being dirty.

Indeed, we could be more carefull with the tools and glass material that we use for open and cleaning the source. They are clean but not.. "surgical" clean.

I guess personal higiene items (perfumes, and stuff) can also be avoided on those days.

Thank you very much for your aswer.

[willnatalie]

yeah I usually have a cloth I lay everything on and I touch nothing with my hands, cloth gloves only. I take my time doing this and it usually take a few hours. After which I do not check the tuning till the next day. If you are scrubbing with the Al2O3 and water, make sure you soniqate for a while in DI water to remove all of it. I usually do this 2 to 3 times.

good luck.

[aldehyde]

I've read that the ions move through the quadrupole region with a coriolis effect such that higher mass ions tend to be on the outer edges of the 'cloud' of ions as they move through. As a result they tend to be the first masses effected when something is affecting the ion reaching the detector end of the MS (ie, bad ion focusing or low mean free path due to leak or other causes of poor vacuum.)

Do you have a gauge to determine your vacuum level? Check masses of N2, O2, H2O to leak check. Did your tuning voltages change dramatically after cleaning? It is possible if you used the alumina powder that a bit of powder is clinging to the source still.

edit: ^^ this guy has it right. I clean and then rinse all of the pieces, rubbing with clean lint free cloth of q-tip, then sonicate in hexane, then water, then methanol. At the water sonicating stage I usually rinse once or twice. At the end visually expect for any remaining powder. Definitely use good quality solvents and never anything from a plastic bottle.

[biotechno]

Thank you both for your replies.

Yes, we sonicate the pieces several times in diferent best "LC quality" solvents.

Visually, we see no powder what so ever, but it is the only explanation we also come up with, since the problem seem to disapear with time.

Although we are suspicious about the power, we can't understand why would the power give problems only at the higher mass (502 m/z) while everything else gets much better after cleaning.

We never checked for vacum performance vs 502 m/z.
Vaccum levels seem ok for us, with N2, O2, and H20 % under handbook limits.
But we can check to see if there is a relation. Thanks for the sugestion.

[biotechno]

We see no relation betwen air molecules % and 502 m/z performance.

We even had a severe leak problems (50% N2) for a few days some moths ago, with a good 502 (m/z) perfomance.

[AICMM]

biotechno,

1) 4% 502 used to be good in my world. We only cleaned down near 0.5% or so. But it has been a while.

2) Have you ever replaced your repeller and the insulators? As I recall this was a significant item in high mass ions and not one that could necessarily be resolved by cleaning.

Best regards,

AICMM

[biotechno]

When it comes to 502 m/z sensitivity, it all depends in what you need to do with your equipement. There is company I know where it is impossible to do their job with 502 m/z below 10%.

Here we also need to detect high masses at very low concentrations, so a decrease in 502 m/z means that we no longer can see our compounds at low levels.

We have changed insulators. The repeller, no.
But since the problem happends immediatly after cleaning, and seems to get better with time, it would be strange to be a part used up, or something. It droped from 8% to 4% after cleaning.

Although i believe a repleacement of some parts would solve the problem, we can't replace parts everytime we clean the source. Don't believe my boss would enjoy that.

I believe the problem is somehow in the way we clean. Powder contamination seems to us as the more likely problem so far. But wouldn´t be surprised if it would be something else that we still didnt think of, like... dificulty in cleaning the borders of "Draw out plate" and "Repeller", or something like that.

[JI2002]

I would recommend cleaning the ion source following the instructions in the manual. I sonicate the parts in methylene chloride, then acetone and methanol. I do not see why water is used.

When comparing the ion ratios, in my experience, do not do autotune. Instead, load the tune file you use before you cleaned the source, and then just manually adjust the tune parameters. Save it as a new tune file and compare the ion ratios to the previous tune.

[biotechno]

Thanks.

We have already tried reseting the values, and optimizing the detector manually. Didn't changed anything.

We are now using water for source cleaning due to HP technitions recomendation.