I had a recent discussion about orbitrap signal as a function of resolving power setting . My understanding of the orbitrap detection is that a fixed packet of ions, as determined by the automatic gain control (AGC), is measured as a waveform and is then converted to MS signal using Fourier transformation (or similar). Compared to an ion trap or time-of-flight (TOF), which gains resolution by spending more time on each mass window, I suspect the orbitrap would gain signal instead of lose signal with increased resolving power because the orbi spends more time collecting the waveform for all trapped oscillations.
The AGC setting is a target setting for the number if ions collected in the c-trap before being injected into the orbitrap for analysis. Increase in resolution does not necessarily mean an increase in sensitivity. Resolving power will increase if orbitrap scans for longer periods but there is a limit until the signal dies or the ions start colliding into each other. For AGC, there is a limit because if you pack to many ions into the c-trap it leads to space charge effects with the ions basically repelling each other and this causes you to lose signal. A TOF gains resolution via the length of it's flight tube. An ion trap or FT-ICR-ms gains resolution by trapping ions for longer periods of time but are generally lot slower than TOF or orbitrap for analysis.
Thanks but you haven't answered my question. Let me rephrase the question with a hypothetical situation:
Assume you are directly injecting a standard peptide at a constant rate into a Q-exactive. If you change the orbitrap resolution from 30,000 to 120,000, which spectra will have a greater total ion count? Has anyone done this for tuning?
The ion count is determined by the c-trap, increased resolution might allow you to better resolve peaks with better ppm. This might show up as ion counts for some ions(depending on ppm filter) but its because they're being resolved better not necessarily due to higher ion count.
Mr. Meyer,
The 'resolving power' in Orbitrap is proportional to square root of [(kq.m-1)], which is the kinetic energy of already formed ion along the axis of the analyzer. This 'frequency resolving power' is equal to twice the mass resolving power. The mass resolving power is determined by the FWHM of the analyte MS peak. When the FWHM decreases, i.e. increase the resolution, that the inaccuracy of the determination of the MS peak 'area' is reduced. This is especially important for overlaping MS peaks, i.e. very close m/z. Therefore with high-to-ultra-high resolving power which is in the capacity of Orbitrap is achieved not only accurate identification of analytes in a complex multicomponent mixture, but as well a highly accurate quantitation (i.e intensities determination of closely disposed MS peaks).
This matter is very well explained with corresponding equations and graphs in the following paper:
1. R. Perry, R. Cooks, R. Noll, Orbitrap mass spectrometry: instrumentation, ion motion and application, Mass Spectrometry Reviews, 2008, 27, 661– 699
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