Technical Description: System
Ion Detection
B-10
Prima PRO & Sentinel PRO Mass Spectrometers User Guide
Thermo Fisher Scientific
voltages applied to the two pairs of quadrupole rods are equal and
opposite.
The quad lens DAC voltage can be varied from -25 V to +25 V but,
in practice, a variation of a few volts positive or negative is usually
sufficient to achieve a symmetrical, flat-topped, peak that provides
good, long-term precision from the analyser.
The deflector lens is used to divert the ion beam through one of two
resolving slits. The size of the slit is generally chosen so the
instrument can resolve adjacent peaks at the high mass end of the
scale. The higher the mass, the wider the peaks become. A
disadvantage of using a single slot is that low mass peaks are, in
comparison, very narrow making these peaks more susceptible to
drift in mass scale. On the Prima PRO instrument, two resolving slits
are fitted – one for low mass peaks (e.g. typically below mass 20 for
the Prima PRO and below mass 100 for the Sentinel PRO) and one
for high mass peaks. The normal resolving powers used are 20 and
60 for the Prima PRO and 85 and 140 for the Sentinel PRO.
The Faraday / multiplier deflectors divert the ion beam onto either
the Faraday or optional MCP detector. With a positive voltage
applied to the Faraday deflector (F-def) and the multiplier deflector
(M-def) grounded, the beam is diverted into the Faraday detector.
When a positive voltage is applied to (M-def) and (F-def) is
grounded, the beam diverts on to the MCP detector.
The resolving stack and detector assemblies are illustrated in Figure
B–7.
Figure B
–7.
Resolving stack and detector assemblies
The output of the MCP detector assembly is an electron current with
a magnitude 103 or 104 times that of the ion current striking the plate
Ion Detection
MCP Detector
Assembly
Quad lens
MCP assembly
Faraday assembly
Detector
connections
Deflector
electrodes
