Page 177 - Instrumentation Reference Book 3E

Analysis methods that measure size directly 161
of light. At these very small sizes, however. inter-
ference and diffraction effects cause significant
errors, and below the wavelength of light the
particles are not resolvable. Microscope counting
is covered by British Standard 3406.
Smaller particles. down to 0.001 pm diameter,
can be examined using the electron microscope.
The two major disadvantages of microscopy
are: the restricted depth of focus which means
that examination of a sample with a wide size
range involves continual refocusing and the real
possibility of missing “out-of-focus” particles
during a scan; it depends more than most other
methods on good representativeness.
Several techniques are available for the prep-
aration of slides, the all-important factor being
that the sample is fully representative. The par-
ticles also need to be well separated from each
other. A common method is to place a small
fraction of the sample onto the slide with one Figure 11.7 Examplesof eyepiecegraticules.
drop of am organic fluid such as methanol or
propanol which disperses the particles. Subse-
quent evaporation leaves the particles suitably
positioned. The fluid obviously must not react The slide is scanned in lateral strips, each strip
with the particles, but it must have the ability to an order or so wider than the largest particles. the
“wet” them. Agitation with a soft brush can help. objective being to cover the whole of the slide
If the particles have a sticky coating on them, it area containing particles. Typically one edge of
may be necessary to remove this first by washing. a chosen reference particle will be aligned with a
One technique is to agitate, perhaps in water, to major graticule line using the longitudinal stage
allow ample time for all the particles to settle and adjustment. The slide will then be traversed lat-
then to pour off the fluid carefully, repeating as erally along that line and all particles to the right
necessary. Obviously the particles must not be of that line will be counted, and measured using
soluble in the fluid. Sometimes the material is first the eyepiece scale. The slide will then be traversed
agitated in a fluid and then one drop of the longitudinally to the right until the original par-
particle-laden fluid transferred to the slide. ticle is in the same relative position but. for example,
Representativeness can only be tested by the five major graticule lines further over and the
repeatability of results from a number of samples. counting process is repeated for particles within
Techniques have been devised for transferring the strip formed by the two lines. The process
samples from suspension onto films within the involves selecting new reference particles as
suspension. It is sometimes possible to collect necessary. To avoid duplication, if a particle lies
samples directly onto sticky slides coated with on one of the strip edge-lines, it is counted as if it
grease, gelatin, or even rubber solution. were in the strip to the right. Particles are allo-
Earlier methods of microscope counting cated to size bands suitably chosen to give. say, 10
involved the use of an optical micrometer by points on the distribution curve. The tedium is
which a cross-hair could be aligned with each side relieved if operators work in pairs, one observing,
of each particle in turn and the difference meas- one recording. alternately.
ured. As can be imagined this was slow and Some graticules have been designed containing
tedious and now the most commonly used systems of opaque and open circles, the sizes
methods involve calibrated field graticules. The arranged in various orders of progression. This
graticules are engraved with a scale (Figure 11.7) can assist the classification of particles by coni-
nominally divided, for example, into 20 pm, parison into size bands, each bounded by one of
100 pm, and 1 mm steps. Calibration is dependent the circles.
on the magnification and this is usually finely When sizing irregularly shaped particles, micro-
adjustable by moving the objective relative to scope counting introduces a bias because the
the eyepiece slightly, or by adjusting the zoom if particles tend to lie in their most stable orient-
available. Calibration is effected by comparing ation. By making measurement of a distribution
the field graticule with a stage graticule, similarly of randomly orientated particles on a slide along
and accurately engraved. When set, the stage a fixed direction, one obtains a two-dimensional
graticule is replaced by the sample slide. statistical mean diameter.