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Functional Description
MS Detector
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______________________ Finnigan LTQ Hardware Manual _______________________
Summary of Mass Analyzer Operation
The processes that occur in the mass analyzer can be broken down into four
steps:
•
Ion storage
•
Ion isolation (SIM, SRM, CRM, or MS
n
(n > 1) full scan only)
•
Collision induced dissociation (SRM, CRM, or MS
n
(n > 1) full scan
only)
•
Ion scan out (the ion detection step)
For SRM and MS/MS full scan applications the ion isolation and collision
induced dissociation steps are performed once. For CRM and MS
n
(n > 1) full
scan applications the ion isolation and collision induced dissociation steps are
performed n-1 times.
Before ion storage, the following conditions are established:
•
Helium is present in the mass analyzer cavity at a partial pressure of about
0.1 Pa (10
-3
Torr).
•
Main RF voltage is set to the storage voltage.
•
Dc axial trapping voltages are set to the storage voltages.
•
Ion isolation waveform voltage, resonance excitation RF voltage, and
resonance ejection RF voltage on the exit rods are off.
With these conditions achieved, sample ions formed in the API source are
trapped in the mass analyzer if the ions have mass-to-charge ratios greater
than the minimum storage mass-to-charge ratio.
Next, for SIM, SRM, CRM, or MS
n
(n > 1) full scan, the ion isolation
waveform voltage is applied to the exit rods, in combination with a ramp of
the main RF voltage to a new storage voltage, to eject all ions except those of
the selected mass-to-charge ratio.
Then, for SRM, CRM, or MS
n
(n > 1) full scan analyses, the resonance
excitation RF voltage is applied to the exit rods to cause collision induced
dissociation. Product ions with mass-to-charge ratio greater than the
minimum storage mass-to-charge ratio are stored. (The minimum storage
mass during collision induced dissociation is typically set to one quarter of the
parent ion mass-to-charge ratio.)
For SRM and MS/MS full scan applications the ion isolation and collision
induced dissociation steps are performed once. For CRM and MS
n
(n > 1) full
scan applications the ion isolation and collision induced dissociation steps are
performed n-1 times.
Finally, the sample ions or product ions are scanned out: The main RF voltage
is ramped from low voltage to high voltage, and simultaneously the resonance
ejection RF voltage is applied to the exit rods to facilitate ejection. As the
main RF voltage is increased, ions of greater and greater mass-to-charge