Agilent 5975C-7890A Problems-Help, Please!

[nnunes1411]

Hello To everyone!

First sorry for my rookie questions and my english but im not an expert on GC and in english. Im only a user for the last 6 years and only to quantify 2,4,6-Trichloroanisol (ld 0,3 ppt) in SPME. With my Thermo ECD no problems until now. With my agilent 5975C-7890A im having some problems since last 4 months. SIM mode. Main problems: a) RT shift up and down b) poor sensitivity and repeatability c) counts of ISTD peak big variation sample to sample d) on low concentrations (0,3 and 0,5 ppt) no peaks. I change ion (from 195 to 212) it was better at beginning but now its the same....no peaks at these concentrations.
The technical assistance in my country don't know so much to help me. He said it was a dirty source and he clean it. The problem persists. I put new filaments, new liner, septum...basic maintenance. Attached the last autotune and tune to see if you can try to help me. What can i tried to check or do to see if this goes better. Thanks in advance for any suggestions.
http://tinypic.com/view.php?pic=jgpqx4&s=9#.Wnx6dGa5yCQ
http://tinypic.com/view.php?pic=16lbhck&s=9#.Wnx6s2a5yCQ

[Gizmo]

As someone who has been a victim of cork taint I'd like to try to help with your endeavor and hopefully others with better ideas than me will chime in as well as cork taint must be fought against. Cork taint will not stand.....

Some thoughts - I've seen RT shift in some cases where the ethanol concentration of my sample was high. Some of the method abstracts I've seen on this rely on the use of salting and perhaps that allowed for a dilution of the original sample and mitigation of the alcohol absorption onto the fiber.

Which of course might have a negative effect on the already problematic sensitivity issue but like I said, these are just thoughts.

As for the sensitivity issue - assuming your LOD was based on ~3 to 5X noise - perhaps finding the exact ion with the highest response on your instrument rather than scanning for plus or minus 0.3 which I believe is the default. And based on the spectrum I would stick with 195 as it is the highest ion in that spectra and determine which specific fragment gives you the highest peak from 194.7 to 195.3 and limit your detection to that one. I have never seen large enough differences to have an effect doing this but in the struggle to achieve your S/N ratio perhaps it could help.

As for variation and repeatablity - your use of an internal standard might make that the lesser of your problems as long as your ratios are tight. But perhaps making sure your SPME technique in terms of temperature, spin rate, and fiber depth are consistent might be worth looking into.

[nnunes1411]

Thanks for your comments Gizmo. Any comments about tune image?

[Gizmo]

Thanks for your comments Gizmo. Any comments about tune image?

Nothing is really screaming at me on that output but I'll defer to others on parameters beyond the basics of abundances and EM volts - based on those I would say that the MSD itself is not your issue but hopefully others will take a quick look to make sure there is nothing obvious that can be associated with your current dilemma.

[James_Ball]

The tunes look good, nice clean baseline without much noise.

One thing that Agilent has presented in a document before that might seem odd is to actually lower the EM voltage instead of raising it for better results. If you lower the EM until you can just see the lowest amount you need to see you will also decrease the background noise which will actually give you better signal to noise ratio. If using SIM this works very well.

Are you injecting with split or splitless mode?

[nnunes1411]

The tunes look good, nice clean baseline without much noise.

One thing that Agilent has presented in a document before that might seem odd is to actually lower the EM voltage instead of raising it for better results. If you lower the EM until you can just see the lowest amount you need to see you will also decrease the background noise which will actually give you better signal to noise ratio. If using SIM this works very well.

Are you injecting with split or splitless mode?

Thanks for your comments. Im injecting in splitless mode. But one of the problems is the baseline with ion 195 so mucho noise, waves.... you think is not related with MSD? its a problem of column? thanks

[Gizmo]

Are the other ions you are checking as noisy as 195? What is your abundance level for the baselines on these ions?

[James_Ball]

The tunes look good, nice clean baseline without much noise.

One thing that Agilent has presented in a document before that might seem odd is to actually lower the EM voltage instead of raising it for better results. If you lower the EM until you can just see the lowest amount you need to see you will also decrease the background noise which will actually give you better signal to noise ratio. If using SIM this works very well.

Are you injecting with split or splitless mode?

Thanks for your comments. Im injecting in splitless mode. But one of the problems is the baseline with ion 195 so mucho noise, waves.... you think is not related with MSD? its a problem of column? thanks

If all other masses are behaving as they should, then I would suspect the column more than the MSD. If you remove the column and place a blank ferrule on the inlet to isolate the MSD and scan in manual tune, it should be very stable. Monitor for 195 there and see if you see any contamination, then put the column back in and monitor again. If you see the 195 with the column in but not without it, then you know it is the column.

[aldehyde]

Hi, your tune looks good. The MS is likely fine, the problem is more likely to be due to the inlet or SPME fiber.

I would do a blank injection in scan mode and check for contamination. You can go inlet the inlet/injection parameters and change it to valve/immediate start in order to not make an injection. Or set it to manual injection and then just trigger the start yourself.

If things look good in scan mode I would focus on the liner type, method injection parameters, equilibration time etc.