PARTICLE ANALYSIS PROGRAM AT LRL
153
sedimentation-rate groups by gravitational settling. Small particles
require centrifugation. In practice, particle samples fall into one of
two classes: those in which many milligrams of particulate material
are present, most particles being greater than ly in size, and those
in which less than a milligram of material is present, with the particles being smaller than a few microns in size. Clean separations require that the particle volume be kept low relative to the fluid volume
(less than 0.5%) and that a density gradient be maintained in the sedimentation fluid. For gravitational settling the particle sample is added
to the top of a 30-cm-long, 1-in.-diameter glass column (see Fig. 1).
The density gradient is maintained
by cooling the system from the bottom, The top of the column is sealed,
and samples are collected at the
base of the column. The collecting
vessel is changed periodically to
obtain about ten equal-mass particle
fractions. During the changing process, the liquid column is held by a
'
16-mesh wire screen sealed across
the bottom of the glass column. Each
fraction is filtered through a weighed
Millipore
filter, weighed, and
mounted for counting. Following a
sedimentation period of about one
day, the column is drained in sections, and the particles remaining in
each section are treated as individual
particle fractions. For centrifugal
Separation, centrifuge tubes of the
type shown in Fig. 2 are used. A
density
mixing
gradient is established by
fluids of unequal density.
Centrifugation speeds and times are
[f
selected to provide 10 to 12 particle
fractions of approximately equal ac-
tivity. Again, the column charge is
maintained by a 16-mesh screen at
the base of the centrifuge column. A
Small aliquot of each particle fraction
Fig, 1—Gravitational sedimentafion column.
is used to prepare a microscope slide. The remaining solution is
filtered through a weighed Millipore filter with pore diameters of
0,014 and the filter is mounted for counting. A flow diagram of the
analysis is shown in Fig. 3.