
Overview of PSD Analysis
Voyager
™
Biospectrometry
™
Workstation User’s Guide 7-3
7
PSD fragment
ions
At higher laser intensities, some molecular ions decompose
into PSD fragment ions in the flight tube after they leave the
ion source (the post-source decay process).
Before fragmentation, the intact molecular ion travels with a
kinetic energy of:
After fragmentation, the fragment ions continue travelling with
the same velocity as the original ion, but with reduced kinetic
energy. Fragment ions travel with the original (precursor) ion
until they reach the reflector where they separate from the
original ion and behave as though they received less initial
acceleration. Therefore, PSD fragment ions are not correctly
focused and appear at a mass higher than the expected mass.
After fragmentation, the fragment ion travels with a kinetic
energy of:
Focusing
fragment ions
with Mirror Ratio
setting
Consider an ion (M+H
+
) fragmenting during flight into two
fragments, A and B, of lower mass than the original ion. Both
of the following reactions occur:
If MH
+
= 1,000 Da, AH
+
= 700 Da, and BH
+
= 300 Da.
Figure 7-1 shows the projected flights of these ions within the
reflector portion of the mass spectrometer.
KE = 1/2 mv
2
where:
KE
=
kinetic energy (accelerating voltage)
m
=
mass
v
=
velocity
KE
(fragment ion)
= KE
(original ion)
where:
KE
=
kinetic energy (accelerating voltage)
m
=
mass
m
(fragment ion)
m
(original ion)
()
MH
+
MH
+
AH
+
+ B
A + BH
+
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