Biology Reference
In-Depth Information
2
Pre-proteomics Mass Analyzers: Basic Concepts and Operating Principles
In 1897, experiments by the British physicist Joseph John Thomson
at the Cavendish Laboratory at Cambridge, UK, deflecting a beam
of cathode rays (discovered by Eugen Goldstein in 1886) by elec-
tric and magnetic fields, led to the discovery of the electron [
3
].
The electric deflection is given by Θ =
Fqd
/
mv
2
, where Θ is the
angular electric deflection,
F
is applied electric intensity,
q
is the
charge of the cathode ray particles,
d
is the length of the electric
plates,
m
is the mass of the cathode ray particles, and
v
is the veloc-
ity of the cathode ray particles.
F
,
d
, and
θ
were measurable, and
Thomson had already calculated the ray's velocity. Therefore by
measuring the deflection Thomson could calculate the charge-to-
mass ratio (
q
/
m
) of the cathode ray particles (the electron). Almost
at the same time as Thomson, Wilhem Wien, in Berlin (1899) con-
structed a device with parallel electric and magnetic fields that sep-
arated the positive rays according to their charge-to-mass ratio
(
q
/
m
) and found, as Thomson did, that they are about 2,000
times lighter than the atoms of hydrogen [
4
]. Thomson later
improved on the work of Wien by reducing the pressure in the
discharge tube to create the mass spectrograph. In 1912, Thomson
and his research assistant, the Canadian-American physicist Arthur
Jeffrey Dempster, channeled a stream of ionized neon through a
magnetic and an electric field and measured its deflection by plac-
ing a photographic plate in its path [
5
]. The separation of neon
isotopes by their mass was the first example of mass spectrometry.
Subsequently, A.J. Dempster (1918) [
6
] and the British chemist
and physicist Francis William Aston (1919) [
7
,
8
] established the
basic theory and design of mass spectrometers that is still used to
this day. The first sector field mass spectrometer was the result of
these fundamental physics breakthroughs. Thomson and Aston
were honored for their achievements, which laid the foundation of
mass spectroscopy, and received Nobel Prizes in Physics and
Chemistry in 1906 and 1922, respectively. Wilhem Wien had been
awarded the 1911 Nobel Prize for his work on heat radiation.
The fundamental equation of all mass spectrometric techniques
that governs the dynamics of charged particles in electric and mag-
netic fields in vacuum, as operating in a sector instrument, is the
Lorentz force law:
F
=
q
(
E
+
vB
), where
E
is the electric field strength,
B
is the magnetic field induction,
q
is the charge of the particle
(
q
=
ze
,
z
= charge number and
e
= elementary charge; −1 for the elec-
tron, +1 for the proton), and
v
is its current velocity (expressed as a
vector). Equating the Lorentz force with the centripetal force
(
F
=
mv
2
/
r
), which is perpendicular to both
B
and
v
and keeps the
object moving in a circle, gives
qvB
=
mv
2
/
r
, where
m
is the mass of
the ion and
r
the radius of the ion trajectory. Since the speed of the
ion is related to its accelerating voltage
V
by 1/2
mv
2
=
qV
, the mass-
to-charge ratio (
m
/
q
) equals
B
2
r
2
/2
V
. Therefore, for a fixed radius
of curvature, an ion with a particular
m
/
q
can be isolated and
