GC/MS varying the EM volts

[STC]

Hello all,

We have a

GC: HP 6890
MSD: HP 5973
Chemstation software

And we were wondering how can we adjust/vary the voltages in the software to yield different mass spectra (TIC). We are trying to get results on the mass spectra that yields peaks for bigger molecules. Would we have to increase/decrease certain ones?

The other question is about standardizing the mass spectra by adding in a known compound to the sample and having the software adjust to the appropriate peak that we expect to see. Can our machine do that with out having to add a known compound? We would prefer if we didn't contaminate our sample at all and still achieve a standard/corrected mass spectra. If so, what method on the software can we use and/or hardware settings we can tune to achieve a standard result?


Thanks in advance!

[carlo.annaratone]

There was a nice primer on Chromacademy, but can't find it now

[CE Instruments]

Hopefully all you are asking for is a basics of GC/MS course, Chromacademy just ran one. You need to tune your MS which sets the masses up and tunes the quad. More advanced is to vary the tune manually to get better transition of higher mass ions.
If you have seen things like varying the eV on the source to get CI spectra from an EI source or to use a lock mass compound to get more accurate spectra you have the wrong machine it won't work.

[James_Ball]

If you just want to standardize the mass analyzer to give you a spectra that will match up with standardized library spectra then you should start with a standard Autotune on the instrument and possibly use the DFTPP target tune if you are working with heavier molecules. The Autotune though should give you decent spectral matches when compared to library spectra.

[carlo.annaratone]

I must correct James Ball, because the tune that gives you the standard ratios for libray search is called "stune.u" or "standard spectra tune" on the Agilent hardware. Default autotune or "atune.u" will try to maximize the intensity of all fragments without checking for their ratios.

[James_Ball]

I must correct James Ball, because the tune that gives you the standard ratios for libray search is called "stune.u" or "standard spectra tune" on the Agilent hardware. Default autotune or "atune.u" will try to maximize the intensity of all fragments without checking for their ratios.

That is correct. I keep forgetting they changed the name since I first began running them 20+ years ago. It is standard spectra autotune not standard autotune as I mentioned, good catch!

[mckrause]

@Carlos: It depends on the Chemstation revision. Rev B will run the 5973.

What you want to do is try to maximize 502. This will give you your best sensitivity for heavier molecules. None of the standard autotuning routines provided by Agilent will really do this. You can gain sensitivity by losing resolution (set peaks widths around 0.7 amu). You'll have to manually adjust the lenses to get the maximum sensitivity for 502. This may mean that your DFTPP spectra looks quite strange (219 may be your base peak). As long as you're not trying to to library searches you'll be fine. Just remember that when you tune the system in this manner it is not stable so you have to check your tune frequently.