1. Dominance of 1+ peptide ions.
Try the following: Spray a peptide solution into a triple quad and an ion
trap after tuning both with the same standard solution. The TQ will show a
a predominance of 2+ ions compared to the ion trap. What is the difference?
Both are fed by the same inlet/ionisation system. The difference lies in
the operating pressure of the instruments (10-6 for a triple quad against
10-3 for an ion trap). The higher bath gas pressure tends to strip the
protons away from a low mass ion (compare the spectra one obtains from
a TQ and an ion trap for a large molelcule such as a protein, there one
does not observe such a drastic shift due to the larger masses involved).
2. Isolation width and number of ions.
This is a mathematical problem and is best explained by a discussion of the
stability parameters of an ion in an rf field.
If one looks at an isotopically resolved ion with a triple quad, one
can get an increase of over 1,000 fold intensity by increasing the
resolution
to accept a window of 3 amu, by increasing the window to 10 the sensitivity
jumps again. This is the basis of all of Don Hunts work. If you want a
full discussion of this, go to a physics book and look up the Matthieu
equation. There is a good discussion of this in a J. Chem. Edu article that
I have forgotten. If there is interest in this aspect of MS and you Email me
I will try and dig the reference out.
Peter James ETH-Zentrum
peter.james@bc.biol.ethz.ch Protein Chemistry
++41 1 632 2919 (Phone) Universitaetstrasse 16
++41 1 632 1591 (Fax) 8092 Zurich,Switzerland