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7 Analyzer
Ion source
Drawout plate and cylinder
The drawout plate forms another wall of the ionization chamber. The ion beam
passes through the hole in the drawout plate and into the drawout cylinder. The
drawout cylinder is slotted. The slots correspond to slots in the source body.
These slots allow carrier gas and un-ionized sample molecules or fragments to be
pulled away by the vacuum system. The drawout plate and drawout cylinder are
both at ground potential.
Ion focus
The voltage on the ion focus lens can be varied from 0 to -127 V dc. A typical volt-
age is between -70 and -90 V dc. In general:
• Increasing the ion focus voltage improves sensitivity at lower masses.
• Decreasing the ion focus voltage improves sensitivity at higher masses.
• Incorrect ion focus adjustment results in poor high mass response.
Entrance lens
The entrance lens is located at the entrance to the quadrupole mass filter.
This lens minimizes the fringing fields of the quadrupole which discriminate
against high-mass ions. There is a permanent +4.4 volt voltage added to the
entrance lens. The total voltage applied to the entrance lens is the sum of the
entrance lens offset and entrance lens gain and the +4.4 volt permanent offset.
Entrance lens voltage = +4.4 V dc + offset + (gain × mass)
Entrance lens offset
The entrance lens offset (
EntOff) controls the fixed voltage applied to the entrance
lens. It can be varied from 0 to -64 V dc (-20 V is typical). Increasing the entrance
lens offset generally increases the abundance of ions at low masses without sub-
stantially decreasing the abundance of high mass ions.
Entrance lens gain
Entrance lens gain (
EntLens) controls the variable voltage applied to the entrance
lens. It determines how many volts are applied for each amu. It can be varied
from
0 to -128 mV/amu. A typical range is 0 to -40 mV/amu.
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