PARTICLE CHARGING AT LOW PRESSURES
267
where Z; = mobility of ions, cm?/statvolt-sec
x = width of drift space, cm
V = potential difference across drift space, statvolts
— t= transit time of ions, sec
The operation of the small ion-mobility analyzer shownin Fig. 2 is
briefly as follows: A square-wave voltage is applied to the ion shutter,
which regulates the flow of ions into the drift space. The ion-collecting
plate is maintained at a negative d-c potential (for positive ions), V,
with respect to the screen, which is held at ground potential. The
transit time of the ions in flowing across the drift space under the
potential gradient, V/x, is measured from the pattern obtained on the
oscilloscope. Figure 4a shows the transit time of the ions for various
applied voltages plotted against 1/V, and a straightline is obtained for
each pressure. The slopes of these straight lines are used to calculate
the ion mobilities that are shown in Fig. 4b for positive ions at various
pressures. The fact that the straight lines in Fig. 4a do not pass through
the origin of the coordinate axes as required by Eq. 4 indicates that
7
27.18 IN. Hg
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10.1 IN. Hg
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-
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-
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0.1
06
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07
(1/V) x 10° PER VOLT
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10
PRESSURE, IN. Hg
|
Pitiis
100
Fig. da—Transit time of positive tons for various applied voltages and
at various pressures.
Fig. 4b —Mobility of positive ions.
1.0