Microtrac S3500, Operation And Maintenance Manual, Page: 25 / 38

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9/03 Microtrac S3500 Particle Size Analyzer Operation and Maintenance 19
Aspiration systems are subject to stratification to a much lesser extent. The sample may stratify somewhat
in the process of depositing it and distributing it in the delivery channel. Again, consuming the entire
sample during a measurement will eliminate any possible detrimental effects from stratification.
3.3 Dispersion
In general, a valid measurement depends on a sample in which individual particles are viewed by the
S3500. Some materials are easy to disperse into discrete individual particles and may be added to the
system with no prior preparation. Other materials tend to agglomerate, or form larger groups of particles
that need to be broken apart. In some applications the agglomerated state is what is being measured, and
different sample handling techniques are needed.
The dispersion techniques presented here are general in nature. They are gathered from a wide range of
user experience. There are no universal solutions - establishing parameters for proper dispersion is an
empirical process, and each application is different. Proper dispersion is critical to accurate results, so
experimentation to determine proper dispersion techniques will be necessary. Please contact Microtrac
Technical Support with any questions regarding dispersion of your samples.
3.3.1 Wet Sample Dispersion
Wet samples may be dispersed prior to introducing them to the S3500 analyzer system. (Some Microtrac
recirculators have an ultrasonic option built-in.) Once dispersed, flow through the recirculation system
should prevent agglomeration from occurring while a measurement is made.
Wet sample dispersion consists of two steps: adding wetting agents to reduce surface tension and promote
mixing in the carrier fluid, and applying ultrasonic energy to break up the agglomerated sample into
discrete particles.
Wetting Agents
Wetting agents include water, surfactants, dispersants, and solvents. Please refer to the appropriate
recirculator manual for further guidance on wetting agent selection and any restrictions on what chemicals
are permissible in your system. The S3500 analyzer is compatible with all fluids permissible in any
Microtrac recirculator.
Surfactants or dispersants used in excess can cause formation of bubbles which may affect the
measurement. Excess surfactant can also lead to re-agglomeration when reaching the surfactant critical
micelle concentration.
Ultrasonic Energy
Ultrasonic energy has been found to be the most reliable and controllable means to disperse a wet sample.
High shear devices such as tissue homogenizers should not be used because they tend to produce artificial
distributions and a non-representative sample.
Ultrasonic energy can applied via a bath or a probe. It can be applied in a container with concentrated
sample, which is then introduced into the recirculator. Some Microtrac recirculators have a built-in
ultrasonic probe to simplify this process.
Note that it is possible to apply too much ultrasonic energy, resulting in attrition of the sample and a shift
of the size distribution to smaller sizes. Evaluation of the effects of ultrasonic energy should be conducted
with microscopy and/or S3500 measurements.
Please refer to the appropriate recirculator manual for further guidance on the application of ultrasonic
energy. Contact Microtrac Technical Support with any questions specific to your application.