Biology Reference
In-Depth Information
measured by an appropriate combination of magnetic field and
accelerating voltage. Figure
1
displays an example of derivation of
the
m
/
z
ratio of an ion in a magnetic sector mass spectrometer.
The nonmagnetic transmission quadrupole mass filter, which
laid the foundation for what we now call the 3D ion trap
(“Ionenkäfig”) and the linear quadrupole ion traps, was developed
in the 1950s at the University of Bonn by another mass spectrom-
etry pioneer, the German physicist Wolfgang Paul [
9
,
10
]. In 1959
Hans Georg Dehmelt, University of Washington, had built a high
vacuum magnetron trap, which he called the Penning Trap [
11
].
This trap employed a homogeneous magnetic field and a weak
electric quadrupole field produced by hyperbolic electrodes. The
Penning Trap has advantages over the radio frequency (Paul) trap
for precision measurements of properties of ions and stable
subatomic particles which have a nonzero electric charge. With this
device, Dehmelt was able to trap electrons for about 10 s and to
Fig.
1 Scheme of the separation of ions of different
m
/
q
by a magnetic sector illustrating how to derive the
mass (in unified mass units) of an ion detected at a magnetic field of 1.3 T after acceleration to 9,000 V in the
flight tube of a magnetic sector instrument of radius 0.4 m. In a magnetic field of strength
B
= 1.3 T, an ion of
charge
q
and velocity
v
experiences a force given by:
F
B
=
Bqv
, which is perpendicular to the direction of
B
and
to the direction of the motion of the ion. Hence, the ion travels in a curve path that has a radius (
r
= 0.4 m) in
a plane perpendicular to the direction of
B
. Since
F
B
is counterbalanced by the centrifugal force,
Bqv
=
mv
2
/
r
.
Substituting the value of
v
from the equation of the kinetic energy, where
m
is the mass of the ions in kg
(1u = 1 Da = 1.66 × 10
−27
kg),
v
is the velocity of the ion in m/s,
q
is the charge of the ion (
q
=
ze
;
z
is the integral
number of charge and
e
is the fundamental unit of charge = 1.602 × 10
−19
C), and
V
, is the ion acceleration
voltage,
m
(kg) =
zeB
2
r
2
/2
V
=
m
(
u
) = [(1)(1.602 × 10
−19
C)(1.3 T)
2
(0.4 m)
2
]/[(1.66 × 10
−27
kg)(2)(9,000
V
)] =
1,455.36 Da. Note that all units will cancel out and the mass of the ion will be calculated in Unified Mass Units
(1u = 1 Da) because T = kg/s C; and CV =
J
= kg m
2
/s
2
