For some mass analyzers (at least FT and TOF), resolution decreases with increasing mass. That's not
really the same thing as mass accuracy, but for low intensity peaks (which are often of interest)
will result in less accuracy. It's an easy phenomenon to observe: just look at peak from a profile
FT/TOF spectrum at 200m/z vs. 2000 m/z. Or if you're looking at proteins, at 20,000m/z you can
really tell!
-Matt
On 5/23/2011 9:19 AM, Bio X2Y wrote:
Hi,
I have recently posted this question to the BioStar forum
(http://biostar.stackexchange.com/questions/8476/why-is-mass-accuracy-of-mass-spectrometers-typically-expressed-in-ppm),
but perhaps this group is more suitable.
My first thought is that since mass (or rather /m/z/) is the thing being
measured, accuracy should
be measured in absolute units of /m/z/, i.e. thompsons (/Th/).
However, in practice, the relative unit /ppm/ seems to be used instead. I find
this confusing, since
/ppm/ will mean different things at different /m/z/ values.
e.g. (taken from
http://courses.chem.indiana.edu/c613/documents/AccurateMassSpectrometryLablecture.ppt)
* 5 ppm @ m/z 300 = ą0.0015 Th
* 5 ppm @ m/z 3000 = ą0.015 Th
When describing the latest-and-greatest new machines, the literature seems to
stick with /ppm/, e.g.
"Parts per million mass accuracy on an Orbitrap mass spectometer via lock mass
injection in a C-trap".
According to Gross in /Mass spectrometry: a textbook/: "As mass spectrometers
tend to have similar
absolute mass accuracies over a comparatively wide range, absolute mass
accuracy represents a more
meaningful way of stating mass accuracies than the more trendy use of ppm."
So, can someone perhaps shed light on why /ppm/ seems to be preferred?
Even statistical treatments tend to use /ppm/ where I might naively expect to
see /Th/, e.g. Fig. 1
from the Mann lab paper [http://www.ncbi.nlm.nih.gov/pubmed/17164402] graphs
the distribution of
mass deviations in terms of /ppm/.
Thanks for your time.
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