Chromatography Forum

We use Agilent GC/MS 5975. The repeller value after cleaning in around 34.I have put anew one in and the initial value was around 21 but after being left overnight it went up to 34
i have checked :
PFTBA present
Filaments are practically new
EMVolts 1247
peak widths of 69, 219, and 502 are within 0.59 - 0.60 ( with good abundance)
Good vacuum


The repeller is coming in at 34.81 (good gaussian curve during autotune; it's smooth)
Ion source cleaned (alumina slurry + sonication with DI water, MEOH, acetone,DCM)
Ceramic insulator sleeves for repeller are new & uncracked
Is there a way to set the repeller value in the autotune?

Tune eval is passing but I cannot get the repeller back down to the 20s range. Any ideas / advice is much appreciated

I, too use a 5975c with inert source. When you say it "went up to 34" do you mean when you re-ran the autotune? Is there a reason you're trying to get the repeller value back to the 20s? There is a way to turn it down manually. In the Tune screen, first make sure you have the right tune file loaded, choose parameters screen, manual tune. and change the repeller to whatever value you're comfortable with. Just be sure to save the tune file when you're done. In my (limited) experience I've always left the repeller as high as possible, as long as that point maximizes 69, 219, and 502. Ramp your repeller for those three ions. I'll bet all three maximize at 34, if that's what autotune chose. I would leave it there if so. You might even get to turn down the EM a notch or two and get the same sensitivity you had before.

I always choose a value that gives the maximum signal possible for all three, giving me a balanced sensitivity across all ions. Of course if you want to suppress the signal on the high end, then lower it. If you want to suppress the signal at the low end then raise it up.

I have always used pesticide grade hexane as my final cleaning solvent. I've been advised in the past to avoid methylene chloride whenever possible because of HCl residue left behind from the manufacturing process. We had a lot of autosampler syringes seize up on us because we used DCM as our rinse.

34v may be a little higher than what you would normally see but it isn't excessive. The only way to move the repeller maximum is by changing the emission current, but that will alter the balance between high and low mass. If you move emission lower you will have more high mass abundance and if you move it higher you will have more low in mass. I normally run mine around 28-32mA emission current, if I am having trouble getting enough mass 50 I will bump it up, if too much I will lower it. Run it as low as you can and your filaments will last longer.

Years ago I think I remember hearing from someone that the higher your repeller, the dirtier the source. But I've run mine on the higher side and lower side for 8260 and haven't seen any direct correlation between repeller value and cleanliness of the source. Not sure if that's what you were thinking about.