Trouble with the GC/MS loosing response.

[Chrom80]

Hello everybody,

I run EPA 8260 with four internal STD's. Lately my 4th internal STD (1,4-Dichlorobenzene-d4) is losing responses very rapidly. Also, i just noticed that the whole tail end compounds after that internal is losing responses as well. I keep doing calibrations to get by. However, it keeps dropping the responses. I cleaned the source a month ago. Also, tuned the MS again but the instrument keeps doing it again. Any help or guidance is appreciated.

Sincerely,
CP

[Don_Hilton]

When is the last time you did inlet maintanance - in particular changed the septum?

With a GC/system, there are many things to check. So if you check the MS by running a tune check it will tell you if the detection remains in the same condition. You did not indicate if tuning the instrument made a significant change in sensitivity.

On frequency of source cleaning - with some methods you can go for months with no issues. In other cases, you may be only able to go a few weeks. I have one technique that seems to foul a source in just two or three days (it will not be a production method). This is where the tune check information is important.

An important bit of diagnostic information: Are retention times shifting and are peak shapes changing?

[BMU_VMW]

if peak shapes are getting worse, you might want to check your injection path. septum , liner , gold plated seal, perhaps even have a look at the needle.

[James_Ball]

EPA 8260 would be using Purge and Trap, so inlet maintenance is pretty much not needed unless you are foaming a bunch of samples.

What Purge and Trap are you using?

Does it have a moisture control unit?

If so, when is the last time you cleaned the moisture trap?

What split ratio are you running and what are your inlet and transfer line temperatures?

If high boiling compounds build up in the moisture trap(they will condense on the walls just as the water will) when you bake the trap at high temperatures you can possibly be pyrolizing those into a thin carbon film on the walls of the moisture trap. This then acts similar to a chromatographic column, where the lighter compounds pass through while the heavier ones get trapped.

Also if you have the EM Voltage too high it can cause similar problems. Check out this app note:

http://www.chem.agilent.com/Library/app ... 0603EN.pdf

Page 11 describes the fluctuation of internal standard areas versus concentration of standards. It also has quite a few good tips on running purge and trap.

Higher boiling compounds can also be affected if your analytical trap becomes contaminated and is holding on to them more strongly than before.

This sounds more like a purge and trap problem than a mass spec problem, though it can be mass spec related too.

[Chrom80]

Thanks a lot. The peak shapes are just fine (width and shape) obviously the height is lower. In December I did the GC (Liner, O-ring, Septa & Gold seal) & MS (Source cleanup and changed both lamps) maintenance.

I'm using Tekmar's Velocity XPT, I changed the Analyte trap & Cleaned the moisture trap.
I'm running 40:1 split on a Agilent 6890, The inlet temp. is 200.

EM voltage is running at 2200.