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Transpector XPR 3+ Operating Manual
When the MCP is grounded, the ions exiting the quadrupole through the exit lens
are collected on the Faraday Cup. The resulting current is conducted through the
signal output to the detection amplifier. When -1275 V is applied to the front of the
MCP, and between -500 and -50 V is applied to the back of the MCP, the ions
impinge on the front side of the MCP. The resulting electron current is collected by
the same Faraday electrode.
The front of the MCP is fixed at -1275 V in the EM mode for two reasons. First, the
ion beam exiting the quadrupole can be strongly divergent, -1275 V ensures that
the entire ion beam is deflected into the MCP. Second, if the ion’s kinetic energy as
it strikes the entrance of the EM is too low, severe mass discrimination effects can
occur. The -1275 V avoids both issues.
Use the minimum MCP voltage required to obtain the necessary peak amplitudes
and/or signal-to-noise ratio—a gain of 300 is recommended. Operating at higher
voltages than necessary will result in premature aging of the Electron Multiplier,
requiring early replacement. As the MCP ages, the voltage needed to get a specific
EM gain will increase.
3.5 Scanning Characteristics
As described above, the quadrupole acts as a mass filter for a mixed beam of
ions, rejecting those of both high and low mass, while passing those of an
intermediate mass. The selectivity of the mass filter is expressed in terms of
resolution, R, which is numerically given by the ratio of the center mass, M, to the
width, M (both in AMU), of the pass band. Since the number of the ions passed
by the filter falls off gradually as the edge of the pass band is approached, the
width is defined at the point where the ion current falls to some specified fraction
(usually 1/2 or 1/10) of the maximum value. The width of the pass band is
determined by the DC-to-RF ratio.
While the quadrupole drive circuits can be designed so that R varies in any desired
manner with M, it is usually most convenient to keep M constant at a value, which
ensures adequate separation of masses that are 1 AMU apart. This mode of
scanning is called Constant M. As a result, R is proportional to M, and therefore
the efficiency with which ions of mass M are transmitted through the quadrupole
decreases with M. Thus, the sensitivity of the sensor decreases as M increases.
3.6 The Zero Blast
When the mass filter is tuned to very low masses, the RF and DC voltages applied
to the rods approach zero. The quadrupole then ceases to act as a filter, and a
large current of unseparated ions is detected. This current is called the “zero blast."
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