Agilent gc-ms tune

[alcorn]

...

[James_Ball]

I found a strange thing with my 7000 and tuning, which is if you ramp the collision cell parameters the sensitivity afterwards seems to drop 10 fold. If I clean the source and run a normal tune and only tweak the parameters for the ion source all is ok, but once I try to test the collision cell parameters the response for m/z69 will drop from 1x10^7 to about 2x10^6. This is still sensitive enough but is odd how it happens.

Are you trying to tune for EPA methods or just maximizing across the full spectrum?

[alcorn]

I think the problem I have is related only to MS1 and I didn't check the collision cell yet.
Actually in a normal day the response for 69 is about 2x10^6 which now drops to about 1x10^5. This is not sensitive enough for determination of food pollutions.

I'm not trying to tune for EPA methods and only just maximizing across the full spectrum.

The remark of my problem is high voltage for repeller. I think, if I could find the reason of this high level of voltage, the sensitivity problem will be solved.

Thanks.

[James_Ball]

Do you have "EI high sensitivity mode" checked in the Ion Source settings? I know this allows for a lower Repeller setting to be used. I am tuning for EPA 8270 method currently and to achieve the low mass sensitivity I have my repeller set to 16v, but it would give better sensitivity at higher masses if I increased it. The optimum repeller setting across all of my instruments (5973 5975 and 7000) vary between 15v and 40v and any time I clean a source it setting will change somewhat. I just adjust to whatever voltage gives me the best results and is stable.

For doing not EPA work I usually just run Autotune and run with the settings it gives as those are the most sensitive across the mass ranges. Though setting a tune and checking it each day with something like BFB or DFTPP instead of entering the tune program is what I have always preferred because if I autotune each day then I must recalibrate each day because I have made changes to the instrument that can effect the results.

[alcorn]

What I want is achieving high sensitivity, actually sensitivity as previous days. I've tuned my instrument in autotune mode since I bought it and everytime there was a problem about repeller voltage and so sensitivity, it was solved by cleaning and sensitivity was backed to it's normal situation, but it seems it doesn't work this time and I should do something else. I think there was a problem which leads to decrease sensitivity.

[James_Ball]

On older single quad instruments I have had repellers get to the point they either had to be aggressively cleaned with the green sand paper, then repolished or even just replaced. It usually took years to get them that dirty, but it can happen. Have you tried a new repeller in it yet?

[alcorn]

I didn't replace repeller yet, but it is cleaned monthly by alumina powder. I have no access to green sand paper, can I use something else to polish the surface of repeller? How much polishing is needed and is it possible using other type of sand papers?

[James_Ball]

I guess Agilent stopped sending the green paper with their tool kits, that is where I got mine on older instruments. Scientific Instrument Service (SIS) used to sell a pack of sanding cloth in grits from 1200 to 20,000 I believe they called it Shark Skin polishing cloths. I used those to go from a very rough surface all the way to a mirror finish when cleaning my sources for the 5970 and 5971 instruments. It can take an hour to do a whole source but it always seemed when I did that it would work better and stay clean longer. Alumina powder works good for light cleaning, but if contaminates get really deep into the metal it takes a little more to get it out.

How do you use the alumina powder? I have always mixed it with Cirtanox soap dipped cotton swabs in it and rub really hard until they no longer turn black quickly. I have also used a Dremel Tool with a soft brush tip or cotton swab and either Jeweler's Rouge or Chrome Polish. Both work really well but require more cleaning to remove the waxy residue. Even though the parts now ship with a rather mat finish, I usually polish them to a mirror surface when I clean them, just seems they stay cleaner longer.

[alcorn]

I have always mixed alumina powder with methanol and by a cotton swabs as included in Agilent tool kits to polish the repeller.
Yesterday I changed repeller with a new polished one. Unfortunately sensitivity is still low (without significant change) with a repeller voltage of 22 volts.
Someone told me the problem may related to quadruple and it should be cleaned. Is quadruple can affect on the sensitivity whereas all other tune parameters are passed?

[James_Ball]

Quadrupole can cause low sensitivity, but usually you will begin to have drift in the peak widths which causes the loss in sensitivity, or a gain if the widths get wider. If you tune one day then manually check the tune the next before doing autotune and the peak widths in profile change by more than a few hundredths then they quads can be dirty.

Have your EM voltages been going up also with each tune? Could be the EM is getting dirty or wearing out. Also the HED can get dirty and cause loss of sensitivity. But for a quad to become dirty enough to cause problems you would have to be analyzing really dirty samples.

[alcorn]

Thanks
So, there was no problem for quadrupole as you explanation: peak widths don't change and there is no peak drift significantly.

EM voltage is 2000 volts and is constant (+/- 50 volts) during tunes.
What am I supposed to to do? What sequence should be done? Is it good starting with changing EM or cleaning the quadruple?

[James_Ball]

If the EM is running at 2000v then I would begin there. You should be able to get high counts even at about 1000-1200 on a new EM. If the HED is dirty then that could also lower the sensitivity.

If you ramp the repeller, does it reach a maximum at high voltage or is that where the autotune is setting it?

[alcorn]

I tuned in both auto and manual modes. In autotune, repeller voltage is set 20-30 volts automatically. When I increase this voltage in manual tune, counts increase but resolution and mass is deteriorated.

[James_Ball]

It is a balancing act when manual tuning. If increasing the repeller gives more abundance but throws off the mass and width then you have to run the width and axis parts of the autotune to bring them back into spec. Usually after you manual tune, the last thing to do is run the width and axis to make sure those are where they should be.

After you get the tune looking how you want it for mass balance then run the Gain Curve setting in the Detector tab to set the EM volts.

[alcorn]

Thanks James-Ball

I cleaned HED and the problem is solved. But I was wondering why quad is not contaminated but HED is.

Thanks for your precious advise.