GCMS Check Standard Response Decreasing Issue

[HOUchem]

Using GCMS for CCL4 and DMCC concentration determination. Been having an issue lately where the check standards we use will fail so we run a calibration and remains good for several days until all of a sudden we get half the response factor we need. We recalibrate and it goes back to behaving until this happens again sometimes in the other direction.

Liner and septum changes happen very often and we've had people come and clean out the whole system for us several times. Any tips for troubleshooting?

Think it may be a mass drift or retention time drift problem. Maybe leak in the system?

[Rndirk]

Hi,

Leaks: Run an air/water check and see if there you have a significant amount of oxygen and/or water in your MS. Use an electronic leak detector to detect carrier gas at gas supply lines, in the oven and around the inlet.

How often do you perform an autotune? What about MS source cleaning?

For more suggestions we'll need some details like:

- What kind of injection (split, splitless,..)?
- Which solvent
- Matrix of the samples?
- ..

[dblux_]

...Think it may be a mass drift ...

No

or retention time drift problem?

Do they really drift ?

[HOUchem]

- What kind of injection (split, splitless,..)?
- Which solvent
- Matrix of the samples?
- ..

Split injection (40:1)
Same is in either Hexane or a Chloroformate (2-EHCF).

Tuning is done frequently and the last tune showed low water and oxygen content.

As far as MS cleaning I assume the maintenance specialist took care of this when they came through to do work on the instrument as their first step was to clean the whole thing.

[James_Ball]

How old is the electron multiplier?

What type MS is it?

Depending on how dirty samples are, source cleaning could need to be more frequent.

Does the response change after doing an autotune? Autotune usually changes the overall sensitivity a little each time you do it, so you normally need to recalibrate after each autotune. I prefer to tune once, then use something known like Bromofluorobenzene or DFTPP to check the tune on a daily basis as in the EPA methods.