274
P. Madhusudhan et al.
Fig. 17 Schematic diagram of the TOF mass spectrometer
forms a single-field TOF MS. The single or double field mass spectrometers are used
on the basis of the resolution required in the experiment.
The ions formed in the interaction region are repelled by the repeller and then
accelerated by the electric field(s) in the extraction region. In the extraction region
itself, the ions of different mass-to-charge ratios are separated from one another and
form different peaks in the TOF spectrum. In the drift region, the ions travel with
constant velocity. A detector (microchannel plate (MCP)) is situated at the end of
the drift tube which detects the ions impinging on it.
In order to determine the dependence of the TOF on the mass-to-charge ratio of
the ions, let us consider the case of a single-field TOF MS. The distance s travelled
by an ion of mass m and charge q in the extraction region E s is given by
s = ut s +
1
2
at
2
s =
p z
m
t s +
1
2
qE s
m
t
2
s ,
(11)
where p z is the initial momentum of the ion along z-axis (TOF-axis is considered to
be the z-axis). Solving the quadratic equation for t s , we obtain
t s =
−
p z
m
±
p z
m
2
+ 2.s.
qE s
m
qE s
m
.
(12)
Velocity ‘v’of the ion at the end of the extraction region is given by v
2
= u
2
+ 2as.
v =
p z
m
2
+ 2.
qE s
m
.s.
(13)
Time spent by the ion in the drift tube t D =
D
v
. Thus, the total time of flight of the
ion is given by