Why GC-MS spectrum of methanol does not match well Library

[thohry]

Hi all,
I have a very simple question:
When I inject some sample diluted in MeOH in GC/MS analysis, sometimes the solvent peak appears because the solvent delay is not long enough. The problem is that if I search the library for the solvent (MeOH), the matching quality is very low: about 2 or 3 and most of the time, MeOH is behind some other names. For other solvents, like hexane, the MQ is often 90 or higher.
Any explanation for this phenomenon?

Thanks

[Don_Hilton]

There are a number of possibilities - and without seeing the spectrum I can only guess what is going on. But, I will note that solvent peaks are large enough to overload the detector in the MS, resulting in distorted spectra, And if you have sufficient overload, you may even get some ion-molecule chemimstry going on.

[Cmdr Keen]

In my applications I also use methanol as solvent for standards. In a 30m 95er column it appears at 'bout 1.6 minutes of retention time.
What does your spectrum look like? As main mass I detect 32 (which would be the molecular mass)

[JTM]

You're using the standard spectra tune or the autotune?

That could make a slight difference.

[Rolandas Plausinaitis]

First factor - m/z from which you start aquire your spectrum. Might be, that some major peaks of methanol spectrum (like m/z 15) are not included in your scan interval. So library match can not be high if that MS peak is not present in your measured spectrum.

Second factor - noise/contamination at higher m/z values. If there are prominent MS peaks, these will be included in library search and will distort the result.

[thohry]

Thanks for all of your replying. In fact, when I use n-hexane or DCM as solvent, the library match is always higher than 90% or so. Probably it is because I cut the lower end of the spectrum rather high (20 amu).

[AICMM]

thohry,

Methanol (32), oxygen (32) does not help your cause at all. Having said that, I would actually second Don's overload comment as the most likely source. If interested, run it as a second component in something heavy like DMSO and turn off the filament after a couple of minutes... Or just check the skewed early or late scans.

Best regards.