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Transpector XPR 3+ Operating Manual
The zero blast, present in all quadrupole-based sensors, can interfere with the
observation of masses 1 and 2 when significant quantities of higher-mass ions are
present. In some instruments, the magnitude of the zero blast is concealed by
preventing the voltages from reaching zero.
3.7 High Pressure Effects
Since Transpector XPR 3+ is designed to operate at pressures in the milliTorr
range, it has some special operating features. The principal difference is that the
interaction of ions with the neutral gas molecules in the sensor cannot be
neglected.
The interaction of ions with ambient gas molecules is described by the
mean-free-path property of the gas environment. The mean-free-path is the
average distance that an ion travels before interacting with a gas molecule. The
numerical value of the mean-free-path is dependent on the type of ion, the type of
gas atmosphere and the gas pressure (i.e. the concentration of gas molecules).
[1]
where:
is the mean-free-path,
K
is a constant depending on the ion and gas species,
P
is the pressure of the gas
The mean-free-path grows proportionally shorter as the gas pressure in the
sensor increases. The effect of collisions of ions with the gas molecules is to
prevent the ions from reaching the collector and being measured. Thus the
sensor output is no longer directly proportional to the concentration of the gas
species being measured.
K
P
----
=
