Beckman Coulter LS 13 320, User Manual

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PN B05577AC B-1
APPENDIX B
Sample Handling
The probability of obtaining a sample, which perfectly represents the parent distribution, is remote.
For example, when powder is poured into a heap, size separation or partitioning occurs with the
fine particles located at the center of the heap. Also, when a container of powder is subjected to
vibration the fine particles percolate through the coarse particles. Therefore, it is not a good
practice to just scoop off a small sample and attempt to predict the properties of the bulk from an
examination of that sample. In addition, fine powders suspended in a liquid medium often tend to
partition, meaning that larger particles will migrate downward while smaller particles remain in
suspension. The viscosity of the liquid plays a key role in the rate that particles migrate through a
medium. Although sampling is only one step in the overall process of characterizing the size
distribution of the bulk, care in performing this step will produce a representative size distribution.
Material Handling
Liquid Sampling
The sampling of powders suspended in a liquid medium requires the use of devices such as rollers,
magnetic stirrers and tube rotators as well as manual inversion and aspiration with a pipette, etc.
These are generally used to keep particles in suspension while attempting to draw a measurement
sample. Even though these techniques are effective in obtaining a representative sample from
liquid, the overall sampling for powder samples will be representative only when the initial solid
sampling is performed correctly.
Solid Sampling
The sampling of powders involves a reduction process from bulk stage down to a measurement
stage, which must be representative of the bulk sample. Clearly before any sampling is performed
the bulk material must be well mixed.
Stationary non-flowing materials, such as fine cohesive powders, sticky materials, moist materials,
or fibrous solids, do not have a tendency to segregate but may not be uniform. Hence it is necessary
to pass these materials through a mixer before storage. Surface sampling with a scoop usually yields
a good representation of the bulk. If more than one sample is taken and analyzed separately, the
variations in the results are usually minimal.
Stored free-flowing materials will segregate by particle size. When poured into a heap, fines tend to
percolate to the center while coarse particles roll down to the outside. In vibrating containers,
coarse material will migrate to the surface even if larger particles are denser than the smaller
particles, for example, a steel ball placed in a beaker of sand. It is not recommended to remove a
sample from the surface due to the tendency to segregate.