Basic Principles of Mass Spectrometry
13
The chosen ions of the resolved ion beam (RB) reconverge on and
pass through a defining slit on the analyzer exit plate (A) and then fall on
the ion collector (IC), on which they are neutralized. In the following we
discuss some questions related to the measurement of isotope ra~ios.
Special arrangements for the inlet system are necessary because the
instability of the ions produced and the mass separation require a high
vacuum. If the mean free path length (flight without collision with other
molecules) of molecules is large compared with the dimensions of the
tubing through which the gas is flowing, then we refer to this as molecular flow. During molecular flow the gas particles do not influence each
other. Therefore, the gas flow velocity ofthe lighter component is greater
than the heavier component, and this means that the heavier isotope is
preferentially enriched in the gas reservoir. To avoid such a mass discrimination, normally the isotope abundance measurements of gaseous
substances are carried out utilizing viscous gas flow. During the viscous
gas flow the free path length of molecules is small, indicating that the gas
particles influence each other. The normal gas pressure is around
100 torr. At the end of the inlet system through which we have viscous
gas flow, there is a "leak", an orifice or a constriction in the flow line.
In the ion source, where the gas pressure is between 10- 4 to 10- 6 torr,
the gas flow is always molecular. Because ions are massive and move
with a lower velocity in relation to electrons, the region through which
the particle passes must be under very low gas pressure.
In general, ions are produced thermally or by electron impact. Positive ions of gaseous samples are provided most reliably by electron
bombardment. The ribbon-shaped electron beam that ionizes the gas
sample when it is leaked into the source is usually twisted by a coaxial
magnetic field to increase the efficiency of ionization.
There is a minimum threshold energy below which ionization does
not occur. The production of an excessively intense electron beam results
in a large spread of velocity, and leads to poor resolution. In practice it is
not sufficient for the mass analyzer to have only a dispersive property;
the mass analyzer must also possess a directional focusing property. Ions
emerging from the source within a small aperture angle must be refocused.
In 1940 NIER introduced the sector magnetic analyzer. In this type of
analyzer, deflection takes place in a wedge-shaped magnetic field. The
ion beam enters and leaves the field at right angles to the boundary, so
the deflection angle is equal to the wedge angle, namely 60°.
The sector instrument has the advantage of its source and detector
being comparatively free from the mass-discriminating influence of the
analyzer field.
13
The chosen ions of the resolved ion beam (RB) reconverge on and
pass through a defining slit on the analyzer exit plate (A) and then fall on
the ion collector (IC), on which they are neutralized. In the following we
discuss some questions related to the measurement of isotope ra~ios.
Special arrangements for the inlet system are necessary because the
instability of the ions produced and the mass separation require a high
vacuum. If the mean free path length (flight without collision with other
molecules) of molecules is large compared with the dimensions of the
tubing through which the gas is flowing, then we refer to this as molecular flow. During molecular flow the gas particles do not influence each
other. Therefore, the gas flow velocity ofthe lighter component is greater
than the heavier component, and this means that the heavier isotope is
preferentially enriched in the gas reservoir. To avoid such a mass discrimination, normally the isotope abundance measurements of gaseous
substances are carried out utilizing viscous gas flow. During the viscous
gas flow the free path length of molecules is small, indicating that the gas
particles influence each other. The normal gas pressure is around
100 torr. At the end of the inlet system through which we have viscous
gas flow, there is a "leak", an orifice or a constriction in the flow line.
In the ion source, where the gas pressure is between 10- 4 to 10- 6 torr,
the gas flow is always molecular. Because ions are massive and move
with a lower velocity in relation to electrons, the region through which
the particle passes must be under very low gas pressure.
In general, ions are produced thermally or by electron impact. Positive ions of gaseous samples are provided most reliably by electron
bombardment. The ribbon-shaped electron beam that ionizes the gas
sample when it is leaked into the source is usually twisted by a coaxial
magnetic field to increase the efficiency of ionization.
There is a minimum threshold energy below which ionization does
not occur. The production of an excessively intense electron beam results
in a large spread of velocity, and leads to poor resolution. In practice it is
not sufficient for the mass analyzer to have only a dispersive property;
the mass analyzer must also possess a directional focusing property. Ions
emerging from the source within a small aperture angle must be refocused.
In 1940 NIER introduced the sector magnetic analyzer. In this type of
analyzer, deflection takes place in a wedge-shaped magnetic field. The
ion beam enters and leaves the field at right angles to the boundary, so
the deflection angle is equal to the wedge angle, namely 60°.
The sector instrument has the advantage of its source and detector
being comparatively free from the mass-discriminating influence of the
analyzer field.