Sensitivity loss in gc-ms

[Dekker]

Hi
My gc-ms instrument sensitivy droped seriously (more than 1000 times) two days ago:

The lowest concentration that now shows a good signal is about 10 ppm in SIM mode whereas I had very good signal at 0.1 ppm. There is similar loss in Scan mode.

I did the followings:

1. Ion source cleaning and installing new filaments
2. Changing inlet liner and septa
3. Cleaning gold seal in inlet
4. Reinstalling the column and cutting 20 cm ftom the head.

Tune data (sensitivity and peak shape) is good as previous. The only strange case is when I select "extractor" mode, there is no enhancement in ions counts (in tune) compared with standard mode. I don't know is it related to sensitivity loss or not.

My instrument: agilent 7890- 5977A ( with extractor EI ion source).

Any comment?

[Rndirk]

Based on your description, I would suspect that the analytes are simply not reaching the detector.

A number of things i'd check:

- Use test solutions of which you are sure what the concentration, solvent,.. is (= make them yourself)
- Is it possible the method was changed? For instance, the split flow could be increased. Double check the parameters or use a standard splitless method.
- Check for leaks with an electronic leak detector along the column.

Did something noticeably happen 2 days ago when the sensitivity drop occurred?

[Dekker]

Thanks Rndirk

My check solution and instrument parameters are Ok.
A new column was reinstalled and all fittings (not along the column) was checked with leak-check solution.

Unfortunately I have no electronic leak detector, but there is good flow at the end of column and changed with changing set point (the end of column was immersed in methanol for bubble check).

There was no noticeably event when the sensitivity dropped.

[Rndirk]

One thing that could explain the symptoms is that the syringe plunger is not attached to the autosampler. Check it and also look if there's a normal amount of solution being injected when you start it.

For more possible fixes I think we could use some more clues. What kind of compounds are you analyzing?

Try to put the MS in full scan mode and manually inject something, to see if it reaches the detector.

[Don_Hilton]

And try a different syringe. I've seen sensitivity return to an instrument with a change of syringe...

[suman_k11]

as per your description, I think that the analytes are simply not reaching the detector.
below check
- Use test solutions of actual concentration, solvent,0.1ppm
- Is it possible the method was changed?along with manually injection do not use autosampler For instance, the split flow could be increased. Double check the parameters or use a standard split less method.


Thanx
Suman Jha

[tkubowicz]

Hello

It could be problem with ionization in ion source.
Make sure that column is properly installed in MSD interface - just cut a bit, take new MSD nut and ferrule and reinstal column (it should be 1-2 mm from the tip of transer line)

Regards

Tomasz Kubowicz