low sensitivity for 6890N/5975

[zimanli]

Greetings,
We have an Agilent GC/MS 6890N/5975 system. I guess because our system is dirty. The sensitivity is really bad now. I used OFN 1pg/ul to check sensitivity. It is getting worse and worse. S/N is not that bad. I got 162, 66 and 56. But Signal-Avg nois=1443, 617, 423, which was 10000 a year ago. So my LOD is bad now.

I did air/water check
18/69=0.9%
28/69=0.25%
32/69=0.15%
44/69=0.09%
28/18=27.55%

If I compare tune history files, 502 relative abundance is lower (was 19 now 9); EMVolts is higher (was 1247 now 1718); IonFcus is lower (was 90.2 now 76.9), Entlens is lower (was 25.5 now19), ENtoffs is lower (was 20 now18) and Repeller is lower (was 27 now25.77).

I used Aero duster (peak 33) to check air leak, and did not find any.

In manual tune mode, the abundance for 18 is large (40000). The abundance for 28 (3466) and 32 (1629) are fine.

Recently we injected water solutions and large amount of dynamite samples.

What can I do to improve my sensitivity? Do I need to clean the ion source?


Thanks a lot!

[Peter Apps]

Yes, if it has not been cleaned for a year it is definitely going to be seriously dirty.

Peter

[zimanli]

But EMVolts is not very bad yet, right? Is it complicated to clean the ion source? Do I need to check anything else before I go ahead and clean the ion source, because I never did that before and am afraid that I can make things worse.

Peter, thanks a lot for the reply.

ziman

[AICMM]

Zimanli,

I am not familiar with 75's but very familiar with 73's. Before I cleaned, I would suggest a couple of things to think about. 1) How many other dirty samples have you run? If not that many, sources can go for a year without cleaning (I have seen it on VOA instruments.) 2) The 9% 502/69 suggests that the source may not be all that dirty. If you did not break about 5% I would say sure, but 9% is not bad. 3) You did not say what the abundance for 69 which is important to know. 4) You stated you have run a bunch of water recently (assuming you injected water and not water extracts) which tells me you have lots of it stuck in your MS. Notorioulsy hard to get rid of that silly stuff. Before cleaning source I would suggest elevating temperatures (source, transfer line, GC oven) for a while (several hours minimum) and see if this does not help things out.

Finally, if you have to clean I would not be too worried. The 73 and 75 sources are very forgiving and relatively easy to clean. Just make sure you squeeze down on the manifold when you are pumping the system down again to get a good seal. Good luck.