12
A. Theoretical and Instrumental Background
RIECK (1956), DUCKWORTH (1958), McDoWELL (1963), BRUNEE and
VOSHAGE (1964), and HINTENBERGER (1966).
In principle, a mass spectrometer may be divided into four different
parts: 1) the inlet system, 2) the ion source, 3) the ion deflector or mass
separator, and 4) the ion detector. A diagram of the essential parts of a
Nier-type 60-degree mass spectrometer is presented in Fig. 5.
A beam of electrons (EB) is emitted by a heated metal ribbon filament (F), usually tungsten or rhenium, and is directed to pass between
two parallel plates (PI and P2). The beam is collimated by means of a
weak magnetic field. Positive ions are formed as a result of collisions
between the gas molecules and the electrons passing between PI and P 2 •
The ions are drawn out of the electron beam through the slit in P2 by the
action of an electric field, and are further accelerated when passing between P2 and a third plate P3 • The potential difference between P2 and
P3 (acceleration potential) is continuously adjustable from zero to several kilovolts. The ions entering the magnetic field are essentially monoenergetic, i.e., they have a constant energy acquired in falling through the
potential difference between the electron beam and the grounded plate
P3•
The positive ions with a charge e' will acquire energy equal to e'
volts (eV) when passing through the electric field. After passing through
the electric field, all the singly charged ions will possess the same kinetic
energy:
1/2 m'v 2 = e' V .
(1)
The light ions will move with a high velocity, the heavy ones with a low
velocity.
When the ions enter the magnetic field in a direction perpendicular
to the magnetic lines of force, they will be subjected to a force
perpendicular to both the direction of the field and the direction of their
motion. The magnitude of this force depends on the field strength and
on the charge and velocity of the ions. This force is represented by the
vector equation
e'v'B
F=-c
(2)
where B is the strength of the magnetic field, e' is the charge, v' is the
velocity ofthe moving particle, and c is the velocity oflight. If we combine
Eqs. (1) and (2), it can be shown that the path the ion follows in the
magnetic field is a function of the mass of that ion. This fact is the basis
of mass spectroscopy. The ions are forced to travel in an arc, the radius
of which depends on their mass and energy. Heavy ions will have greater
radius values than the light ions.
A. Theoretical and Instrumental Background
RIECK (1956), DUCKWORTH (1958), McDoWELL (1963), BRUNEE and
VOSHAGE (1964), and HINTENBERGER (1966).
In principle, a mass spectrometer may be divided into four different
parts: 1) the inlet system, 2) the ion source, 3) the ion deflector or mass
separator, and 4) the ion detector. A diagram of the essential parts of a
Nier-type 60-degree mass spectrometer is presented in Fig. 5.
A beam of electrons (EB) is emitted by a heated metal ribbon filament (F), usually tungsten or rhenium, and is directed to pass between
two parallel plates (PI and P2). The beam is collimated by means of a
weak magnetic field. Positive ions are formed as a result of collisions
between the gas molecules and the electrons passing between PI and P 2 •
The ions are drawn out of the electron beam through the slit in P2 by the
action of an electric field, and are further accelerated when passing between P2 and a third plate P3 • The potential difference between P2 and
P3 (acceleration potential) is continuously adjustable from zero to several kilovolts. The ions entering the magnetic field are essentially monoenergetic, i.e., they have a constant energy acquired in falling through the
potential difference between the electron beam and the grounded plate
P3•
The positive ions with a charge e' will acquire energy equal to e'
volts (eV) when passing through the electric field. After passing through
the electric field, all the singly charged ions will possess the same kinetic
energy:
1/2 m'v 2 = e' V .
(1)
The light ions will move with a high velocity, the heavy ones with a low
velocity.
When the ions enter the magnetic field in a direction perpendicular
to the magnetic lines of force, they will be subjected to a force
perpendicular to both the direction of the field and the direction of their
motion. The magnitude of this force depends on the field strength and
on the charge and velocity of the ions. This force is represented by the
vector equation
e'v'B
F=-c
(2)
where B is the strength of the magnetic field, e' is the charge, v' is the
velocity ofthe moving particle, and c is the velocity oflight. If we combine
Eqs. (1) and (2), it can be shown that the path the ion follows in the
magnetic field is a function of the mass of that ion. This fact is the basis
of mass spectroscopy. The ions are forced to travel in an arc, the radius
of which depends on their mass and energy. Heavy ions will have greater
radius values than the light ions.