Air/Water check, isotope ratio

[MichaelVW]

I'm used to seeing air & water check results for a 5973 where 18 and 28 are both at about 0.5-2%

Lately I'm getting results that are too good: both are less than 0.1%

I'm also seeing isotope ratios that can't be right. I've seen as high as a 40% ratio of 19:18.

If I run a Standard Tune, it shows 18 and 28 at about 0.5%.

Any idea what's going on? A dirty source maybe, or an issue with mass assignments on the low end?

[mathieu_stability]

Hi Michael

3 things to check :
1/ give the source a clean with green abrasive paper.
Agilent will tell you that is old school and not necessary, but years of polishing with the Al2O3 slurry can cause the issues you are observing.
A proper abrasive clean of the repeller and draw-out lens once a year is worth trying.
2/ Check/clean the orifices of the electron beam
3/ try a new EI filament

the stune and atune will give different tuning results.
Check the isotope ratios using the tune evaluation utility.

[James_Ball]

Also if your tune is using Dynamic Entrance Lens Offset, if the values are not optimized for masses below 50 it could be making the air and water less abundant.

I do have one instrument where my air and water are almost undetectable with a normal tune, I just wish they would all seal that well.