Re: Accurate Mass Measurement

Hi "Jimmy"

} There have been extensive discussions here on accurate mass
} measurements by QToF and FTMS. How about Finnigan TSQ Quantum with
} high resolution software (FWHH 0.1 u)? 
} Is it good enough for small molecule accurate mass determination?

Well ... there is a significant difference between "high resolution"
and "accurate mass". Unfortunately, both are often confused or taken
as synonyms for each other (very often by marketing people ;-)

"High resolution" only means that you are capable of resolving two
closely neighboring peaks from each other. However, this does not say
_anything_ about the accuracy of the mass assignment. In other words,
you may have very high resolution, but be completely off scale.

(a footnote ... traditionally "high resolution" was in excess of
10'000, measured at 10% valley definition. Anything below that would
be "medium resolution" according to those terms - scnr).

In contrast to this, "accurate mass" means that you are capable of
determining the exact, "true" mass of a peak, which in turn lets you
calculate elemental composition etc. Provided that you have
reasonable ion statistics (read: peak shape), accurate mass does not
necessarily require high resolution (but it does not hurt either ;-).

} What about ion traps? What resolution can we expect compared with
} QToF and FTMS?

As for the ion trap or the triple quad (Quantum), indeed I would love
to see a comparative study of those vs. FTMS and/or ToF. While the
elevated resolution should not be a problem to achieve, I would
expect the accurate mass issue to be somewhat more difficult ... in
particular for the ion traps (space charging, influence of external
fields, temperature, etc.). 

It's not accidently that FTMS and magnetic sectors still are "the"
machines for accurate mass work ... TOF are getting better, but are
still not at a stage where you can do real routine accurate-mass work
in the mid-mass range, where results would be comparable to FTMS. 

} can we get accurate mass with 5 ppm

Why "5 ppm"? Depending on the compound, that may be overkill, or by
far not enough ... take for example some low-mass pharmaceutical
compound with a few F in there, and calculate the possible masses.
You may well end up with *dozens* of reasonable combinations in a
window of less than 5 ppm. 

Thus, the error limit depends on the problem that you are going to
resiolve; there is no "generic" answer.

Cheers + HTH,
 
- Joerg
 
 
-- 
joerg.hau(at)swissonline.ch * Lausanne, Switzerland
http://www.mysunrise.ch/users/joerg.hau/
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