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Astons Mass Spectrograph
The apparatus used by Aston is shown in the figure
The stream of positive ions from a discharge tube after passing
through two narrow slits
S1 and S2 enters the electric field between metal plates P1 and
P2. Due to electric field E, allpositive ions the same q m value
are not only deviated by an angle but are dispersed by an
angle d due to their different velocities.
The beam is then allowed to pass through a magnetic field M
acting at right angle to the
electric field which deviates the particles by an angle and
reconverges them by d .
Direction and magnitude of the field so adjusted that it
produces a deviation of the beam in
opposite direction and brings all ions having same q m value
(but different velocities) to a focus
at one point F. ions having different q m values are brought to
focus at different points on the
photographic plate.
Condition of focussing:
Consider a group of ions having the same q m value but moving
with different velocities.
I) Let E be strength of electric field, l1length of the electric
field and let q, m and v be
charge,
mass and velocity of the ion.
Then, linear displacement of ion from its path due to electric
field
2
1
1 2
1
2
lqEd
m v=
Angular deviation of the beam
1 1
2
1
1
2
d lqE
l m v = =
And angular dispersion of the beam
1
3
qEl dvd
m v =
2d dv
v
= 1
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II) Similarly, if B is strength of magnetic field and l2 is
length of path in the magnetic
field,
Then, linear displacement of ion from its path due to magnetic
field
2
22 21
2lBqvd
m v=
Deviation produced due to magnetic field2
2
2
1
2
d Bql
l mv = =
And angular dispersion
2
2
1
2
Bql dvd
m v =
d dvv
= ..2
From eqns. 1 and 2, we get
2d d
=
or,2
d
d
= 3
Bainbridges Mass Spectrograph:
The beam of positive ions from a discharge tube is collimated by
two slits S 1 and S2 and enters a velocity
selector as shown in figure.In velocity selevtor, magnetic field
B is perpendicular to electric field E and
the ion beam.If the two fields are so adjusted that their
effects nullified each other, we can have
qE Bqv=
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Or,E
vB
=
All the ions having this velocity do not suffer any deviation in
the velocity selector and thus
enter the analyzing chamber A through the slit S3 where they are
subjecte3d to a strong uniform
magnetic field B` perpendicular to plane of the paper and
outwards. Ions having same q m values
traverse a semi-circular path of radius r given by
mvr
B q=
2mvB qv
r
=
Q
Since B and v are constants, for all ions of the same charge, we
can write
r m
So, we get a linear mass scale on the photographic plate. Ions
of different masses strike the
photographic plate at different points, thus giving a typical
mass spectrum.
