Bruker Daltonik GmbH Instrument Layout
3.1.4.1 Pulsed Ion Extraction
MALDI mass spectrometry is restricted by the energy distribution of the formed ions,
which is caused by the ionization process. Because not all the ions are desorbed and
ionized at the same time and at the same place inaccuracies occur related to energy,
location, and time. Also repulsive electrical forces cause an initial energy distribution of
the ions. Summarizing all these inaccuracies which happen during the ion formation
process cause that ions of the same mass do not have the same kinetic energy after
passing the acceleration field, but leave the source with a certain energy spread
resulting in an arrival at slightly different times on the detector. This effects in
broadening of the peak width and thus reducing the resolving power.
Pulsed Ion Extraction (PIE) is a technique to enhance resolution and sensitivity as well
of a TOF mass spectrometer.
After desorption ions are delayed extracted (phase 2 and 3, discussed beneath). This
method benefits in:
1. A softer acceleration process, because ions do not pass the dense plume of
matrix molecules and
2.
achieving a time focusing effect provided that the correct parameters are
applied.
Involved in the PIE process are three components of the ion source, i.e.
1. the target plate(P1) where the analyte is dropped on,
2. the second voltage plate (P2) which is an electrode mounted some mm apart
opposite to the sample position, and
3. the following grounded acceleration electrode (Figure 3-1, Figure 3-4).
Using PIE this arrangement works as follows:
Phase 1:
The target plate P1 with the analyte is always connected to potential IS/1. At first the
same potential is applied on plate P2. So the analyte is not yet exposed to any exterior
effects until to the laser shot. This event is the transition to
Phase 2:
where molecules and ions are set into motion by laser ionization/desorption with a
typical velocity of 700 m/s from P1 towards P2. Meanwhile the analyte is ionized.
There is still no electrical potential the ions are exposed to. Only the MALDI process
causes their kinetic energy at this moment, where through the evaporation of the
analyte surface particles are explosively pushed to that velocity. During the next few
hundred nanoseconds the analyte moves further towards P2.
ultraflex III User Manual, Version 1.0 26
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