Doubts about filament life-time with dirty samples

[martendal]

Hello, the instrument we use in our lab is a Shimadzu GC-MS QP 2010. We have been analysing leather samples in order to quantify phenols and chrorophenols. The first runs we carried out were performed in Scan mode, but too many extraneous peaks appeard, many of which saturated the detector, turning it off after the third time it happened. In order to overcome this problem, we decided to carry out the acquisition in SIM mode, gaining in sensitivity and selectivity in the detection. My doubt is: Although only a few m/z are detected, too many other non-target compounds passes through the filament but are not detected. This can decrease the life-time of the filament? What is the problem: a high sample mass reaching the filament or a high ionic current reaching the detector?

Best regards

[Don_Hilton]

The stuff in the ion source has no idea about what is going on in the analyzer section and detector. If filaments are on and there is "significant" gas pressure in the source, the filaments will sputter off material. Using SIM mode will, however, help with the life of the detector, as fewer ions strike the detector. To maximize filament life, keep the instrument leak free, do good sample prep, and turn on the filaments to acquire data only when you need to acquire data. This is true for all EI instruments, regardless of manufacturer.

[martendal]

Ok, thank you very much for your clarifying comments. Let me explain a little better what is going on. When I use the scan mode, a lot of peaks from the matrix co-elute with the analytes. Also, the intensities of these peaks are very high, many times saturating and turning off the detector. When I use the SIM mode, the detection becomes selective to the target analytes, but of course I know the same matrix peaks are coeluting together. This fact that a lot of material passes through the filament can decrease its life-time?

Best regards

[AICMM]

martendal,

Yes, lots of material hitting the filaments will shorten the life of the filament. It will also speed the need for cleaning the source. What I recall (having been a while) is that most sources don't really give you much flexibility in settings that can help you so look to Don's comment about only turning on the filament when needed. You might also talk to SIS to see if they have an extended life filament for your particular instrument.

REgarding the EM, since you are running SIM, you can probably afford to lower your EM voltage while getting the same sensitivity that you used to with Scan mode. That would also extend the life of your EM.

You also might have options in the sample prep which will minimize the amount of extraneous material hitting the source other than your target analytes. This could be worth exploring if this is a significant issue for you.

Best regards,

AICMM

[AICMM]

martendal,

One other thing, sorry. Since you are moving to SIM, are you moving to split injection? In this scenario, you maintain the EM voltage and decrease the amount injected to get back to the original Scan detection limits. But you put lots less crap on column and on detector.

Just a thought.

AICMM

[JI2002]

I wouldn't worry about filament life time, just clean the ion source and change the filaments more often. However, there is ion suppression issue when you have target compounds coeluting with huge matrix peaks, switch to SIM and dilute the samples 10X to 100X before analysis.

[tangaloomaflyer]

I'm interested in what the matrix peaks might be - hav you got an idea of what they might be? It definitely sounds like a clean up step would be helpful. Perhaps the acidic nature of the phenols might be helpful?

[martendal]

Thank you very much all you guys for helping me. We are now using a split injection in order to minimize the amount of material hitting the filaments and EM. By now I do not remember which peaks have been coeluting with the target analytes, but I can verify.

Best regards