ppm in mass spectrometry

[Brian Gau]

Hi 

Parts-per-million:

The exact mass/charge of an ion is calculated from the mono-isotopic mass,
not the average, for each element in its elemental composition.  An electron
mass (or several) must be added or taken away to get the exact ion
mass/charge (and divide by the charge if doubly, triply, etc. charged).

The ppm difference between an observed ion mass/charge and exact ion
mass/charge is simply:

(observed - exact)/exact X 1000000

In general a |ppm| difference smaller than 5 is quite good agreement, though
the best Fourier transform ion cyclotron resonance and Orbitrap
spectrometers deliver sub ppm accuracy.

This all supposes you know what you're looking at.  If you're looking at an
unknown, then you take confidence in its measured mass when the calibrated
spectrometer verifies the calibrant mass/charge values to within 5 or better
ppm.

Good Friday,

Brian

-----Original Message-----
From: methods-bounces from oat.bio.indiana.edu
[mailto:methods-bounces from oat.bio.indiana.edu] On Behalf Of Esmaeil Sadroddiny
Sent: Thursday, October 23, 2008 11:40 AM
To: methods from net.bio.net
Subject: ppm in mass spectrometry

Can somebody explain simple definition from ppm in mass spectrometry

Regards
Esmaeil 


      
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