Atomic Mass Spectrometry
TOF based Mass Spectrometry
Same kinetic energy
Different velocities for different masses
Different times of flight for different masses
1
Effect of Magnetic Field on Moving Charged
Particles
Effect of Magnetic Field on Moving Charged
Particles
• Ions accelerated by a potential V into a magnetic field B
• Trajectory in magnetic field is defined by the balance
between the force of the magnetic filed on ion and the
centrifugal force
• The relationship between mass, radius, accelerating
potential and magnetic field strength is
m eB 2 R 2
=
z
2V
2
Relationship between Mass, Radius of
Trajectory, Field and Voltage
force − of − magnetic − field − on − ion : FB = ezvB
mv 2
Centrifugal − force : Fc =
R
mv 2
KE =
= zeV
2
B : magnetic − field − strength
R : radius − of − trajectory
V : accelerating − voltage
(1)ezvB =
( 2)
mv 2
R
m eRB
=
z
v
mv 2
= zeV
(3) KE =
2
2 zeV
( 4) v =
m
m
eRB
(5) =
z
2 zeV
m
3
Obtaining a Mass Spectrum
• Magnetic scanning: change current in a
electromagnet
– Keep V constant
– Scan B
– Keep R constant
Quadrupole Analyser
• Four parallel rod-like metal
electrodes.
• Direct (dc) and alternating
voltages (ac) are applied to
rods
• Applied potentials of
opposite rods are
¾ (U+Vcos(ωt))
¾ -(U+Vcos(ωt))
• U is a dc voltage
• Vcos(ωt) is an ac voltage.
4
The Quadrupole is a “Mass Filter”.
– For given dc and ac voltages, only ions of a
certain mass-to-charge ratio pass through the
quadrupole filter and all other ions are thrown
out of their original path.
– A mass spectrum is obtained by monitoring
the ions passing through the quadrupole filter
as the voltages on the rods are varied.
– There are two methods: varying ω and
holding U and V constant, or varying U and V
(U/V) fixed for a constant ω.
The Quadrupole is a “Mass Filter”.
• Generally the frequency
of the ac field is constant
• The radius is constant
• Potential of direct current
and alternating current
are increased to allow all
ions through the
quadrupole sequentially
5
Quadrupole MS
6