GC-MS: Solvent peak in SIM. Why?

[Graf Chromato]

Hello everybody,

I am new to GC-MS and without having any background in that field I was asked to develop a method for detecting 3-MCPD with the equipment we own at my company.

I was told when new methods are developed the acquisitions should start from the beginning of the run and the solvent peaks should be recorded just to see that the injections were successful.
I know that detecting the solvent peaks influences the lifetime of the filament negatively, therefore I am trying to stop that procedure as soon as possible.

Anyways there is one thing regarding the solvent peaks I don't understand.
While scanning in full mode I get a huge solvent peak in the NL range of around 10^9. Performing the library search I can identify the solvent I use, in my case iso-octane with a mass of 114.23.
Switching to SIM I still get a solvent peak in the range of around 10^5.
What I don't understand is why I get that peak. I am looking for a mass of 147 and always thought that every other mass was not detected and displayed.

Is somebody here that can explain me why I get such a peak?


Best regards
Graf Chromato

[James_Ball]

The amount of solvent is so great that it overwhelms the ability of the mass filter to remove all of the ions from the path. So even though the mass filter is rejecting all other masses, some ions of other masses are managing to make it through the filter to the detector.

If the normal scan gives a peak of 10^9 and the SIM is giving a peak of 10^5 then it is being reduced by 10^4. In other words you are getting 100,000 making it through out of 1,000,000,000 ions being produced in the source which is 0.01% passing through. The mass filter is doing pretty good to filter out 99.99% of all interfering ions that are not at the m/z you are looking for.

You will also see a similar effect if you inject water or methanol and begin your scan range at about m/z 45 because even though they have masses lighter than your scan range you will still have a response that looks like a high background of every mass in the scan range.

The more you run mass spec, the more you will see strange things happen, it is an ever learning experience

[Graf Chromato]

Ah ok, I understand.

Thank you very much James for this very good explanation.