Beckman Coulter LS 13 320, User Manual

Page: 223 / 254

PN B05577AC B-5
Sample Handling
Liquid Sample Dispersion
B
Liquid Sample Dispersion
Liquid sample dispersion is process that attempts to suspend a dry powder in a liquid medium
before analysis. This medium can be aqueous or non-aqueous. Characteristics of the dry powder
dictate what medium is to be utilized as a diluent. Factors such as solubility, reactivity,
suspendibility and end use govern the choice of the media. For example, a non-aqueous medium
such as isopropanol is chosen over water as a diluent when determining particle size distribution
for a sample of fertilizer, which is soluble in water.
Many pharmaceutical powders, such as niacin, experience this solubility problem and require the
use of non-aqueous media. Powders can also be highly reactive. For instance, lithium hydride
explodes when in contact with water and cement cures in water. Next, suspendibility of large or
heavy powders in a medium may prompt you to use a highly viscous solvent such as glycerol. Finally,
pigments are often dispersed in toluene because toluene is the major component of the end
product. In the preceding examples, Beckman Coulter illustrates why sometimes it is beneficial to
select a non-aqueous medium over an aqueous one. Nevertheless, aqueous systems have fewer
precautions in the handling and disposal of waste, and water is far less costly than any organic
solvent.
CAUTION
The safe handling and disposal of hazardous organic materials have to be
practiced according to government regulations such as Occupational Safety and
Health Administration (OSHA) and Environment Protection Agency (EPA)
regulations.
The techniques that are used in the dispersion of powders in both aqueous and non-aqueous media
are quite similar. Dispersants aid wetting and stabilization of a suspension and can be ionic (anionic
or cationic) or non-ionic. However, concentrations of dispersants must be low. Otherwise, their
properties are reduced or even inverted (i.e., cause aggregation or flocculation). In dispersing a
powder in a nonaqueous medium, it is critical to select an organic solvent that is a good diluent as
well as dispersant, although there are many organic dispersant aids. Aqueous surfactants are
primarily responsible for reducing surface tension, hence aiding in the dispersion of powders in
aqueous systems. The first task in dispersing a dry powder is the wetting of the sample with diluent
or dispersant, if needed. Gentle spatulation of the resulting paste prior to addition of diluent further
aids the dispersion process. Once the diluent is added, a sample of the slurry can be placed on a slide
for a microscopic evaluation of the effectiveness of dispersion. Providing that the sample is well
dispersed, the use of a magnetic stir bar is suggested to keep the sample fully dispersed, so a
representative sample can be drawn for analysis while stirring is in progress. For example, in some
cases it is necessary to use sonication. This is the case with strongly aggregated pigments of earth
elements. Ultrasonics must be used with caution since it can cause aggregation, shattering and
heating if the duration is too long or the ultrasonic power is too strong.
Physical Methods of Liquid Dispersion
• Spatulation: use spatula to break up large clumps but not individual particles.
•
Sonication: use sound waves to break up aggregates; use either ultrasonic bath or probe.
• Dilution: high dilution can aid in dispersion by allowing more space between particles.
•
Degaussing coil or heat: can aid in the dispersion of magnetic particles.