
Mass
Spectrometer
PlasmaQuant MS Series
126
Figure 8-19 - The entrance lens and the entrance lens / plate removal tool
The negative DC potentials applied to the entrance lens and plate control the
geometry of the electrostatic field. As the ions enter the field they are accelerated past
the entrance lens and plate and forced into the fringe filter assembly. The potential
applied to the fringe rods produces an electromagnetic field, which depending on its
phase, can repel or attract the ion beam. The increased velocity provided to the ions by
the entrance lens and plate assist the ions in overcoming the initial effect of RF fringe
field.
As the entrance lens is exposed to the ion beam it tends to collect photons and
neutrals generated by the ion beam during the trip from the mirror to the entrance
lens/plate. Due to this bombardment, the lens surface becomes contaminated after
prolonged use. The proximity of the entrance lens and plate relative to each other and
the fringe rod assembly is very important in creating uniform electrostatic fields
favorable to efficient ion transfer.
The entrance lens is typically maintained at a potential of 0V however, the DC voltage
applied to it is variable via the Aspect MS software within a range of -50 to 0 V. The
entrance plate supply has a variable DC voltage range from -200 to 0 V. It is set to -50
V as a default.
The fringe bias controls the relative DC voltage applied to the fringe rods. The bias
voltage is applied equally to both pairs of rods.
Although the system is designed to operate with at a 0V DC potential for diagnostic
purposes, it is often useful to apply a bias voltage. The DC fringe bias voltage is
variable under Aspect MS software control in the range of -10 V to +10 V.
Fringe bias