Particle size analysis has traditionally
been dominated by historical techniques
such as sieving or sedimentation
procedures. Over the past 10–15
years, laser diffraction (static light scattering) has taken over a large part
of the market because of its speed of
analysis, ease of use, reproducibility,
and efficiency. For most applications
in the 100-nm to 1000-μm range, laser
diffraction’s advantages far outweigh
the initially higher cost of an
advanced optical instrument.
As the technologies have matured,
the instruments offered by various
competitors have, to a great degree,
converged into a similar design
approach and similar performance.
Differences among various manufacturers
may be based on historical
developments or on the product being
optimized for a perceived target market.
As the technique becomes more
widely accepted, new applications
arise that have different requirements
and require a different approach. The
challenge to the manufacturers is to
identify these needs and design a
product that will satisfy a wide range
of customer requirements.
Figure 1 - Partica LA-950 particle size analyzer.
The Partica LA-950 (Figure 1) laser diffraction particle size analyzer (Horiba Instruments, Inc., Irvine, CA) was developed as a solution to
customer requirements for existing
markets as well as emerging ones. After
the company performed a thorough
market study and discussed requirements
with end users, the design targets
were formulated to provide the
greatest utility and to be a step forward
for users of particle size analyzers.
The new optical system design provides
a very wide particle size measurement
range (0.01–3000 μm), while
maintaining high performance in the
submicron range and increasing the
large size limit in order to extend the
use of the technique to new markets.
In addition, particular attention has
been paid to the user’s ability to generate
accurate, reliable data quickly
and easily. Sample-to-sample analysis
time has been reduced to 1 min
total, and the built-in automation of
the sampling systems and software
eliminates time-consuming manual
tasks such as filling with dispersant
fluid, selecting conditions, and initiating
each step of the analysis
sequence. Combined with the outstanding
precision of the analysis
system, reproducibility and repeatability
from site to site have been
improved dramatically.
The submicron measurement improvement
is made possible by development
of the company’s existing high-angle
scattering measurement technology.
Since the scattered signal from particles
smaller than the wavelength of the
incident light are at higher scattering
angles, low intensity and not highly
resolved like those from larger particles,
an increased number of more sensitive
high-angle detectors and a higher intensity
and shorter-wavelength solid state,
light-emitting diode (LED) second
light source has been added to
increase the intensity of the scattered
signal. This results in increased sensitivity,
accuracy, and resolution for smaller
particles, thus allowing measurement of
a wide range of new nanosized materials
that would normally require a different
analytical technique, such as photon
correlation spectroscopy (PCS). The
end user benefits because this analytical
capability is combined with the system’s
larger size capabilities, possibly avoiding
the need to obtain a second analyzer.
The larger size measurements are
made possible by a reverse-Fourier
optical design with a folded optical
path, which achieves the longer focal
lengths necessary while reducing the
footprint to almost half of that
required by systems with an equivalent
upper size limit. The optical bench itself is made of cast aluminum
for rigidity and thermal stability,
which contributes to the system’s precision compared to competitive optical
systems with components mounted
on sheet metal or on circuit boards.
Since market size does not often allow
for a range of models for specific applications,
the Partica LA-950 was
designed as a wide-measurement-range,
general-purpose analytical system.
In addition to the dramatic
improvement seen in measurement
technology, the main opportunities for
advancement in laser diffraction particle size analyzers include usability,
analysis speed, and the ability to tailor
the system for specific applications.
Different applications require alternative
sample dispersion systems for
optimum analytical performance. For
users with multiple types of samples,
the traditional approach has been to
make the change from one sampling
system to another as easy and fast as
possible. This usually requires
removing a measurement cell, disconnecting
the sampling system, and
replacing the components for the
alternate type.
Figure 2 - The Partica’s measurement chamber showing the multiple cell positions and ease of access for cleaning.
The Partica LA-950’s optical bench
mounts the optical components vertically,
allowing the measurement cell
to be mounted on a sliding cell tray
that passes through an opening in the
bench. This allows for up to three different
measurement cells to be
mounted simultaneously, and for the
sampling systems to remain in place.
Changing from one measurement
method to another is accomplished in
as little as 5 sec by moving the cell
tray from one position to the next
(Figure 2). The sampling system connections
remain in place at all times.
The instrument automatically recognizes
the sampler type and configures
the software for the sample operation.
This allows for a single instrument to
be used for multiple types of analysis
without lengthy reconfiguration or
operator involvement.
Customer requirements call for a
range of sample dispersion and measurement
systems for a vast array of
sample types. These include dry powder analysis, small-volume samplers
for precious or toxic samples or for dispersing
media, large-volume systems
for increased sample weights to
improve sampling, autosamplers for
maximum sample throughput, and
organic solvent compatible systems.
The standard fluid circulation system
includes a high-volume fill pump that
can fill the flow system in 3 sec, controlled
by a multilevel fill sensor. The
high-efficiency circulation pump is
powerful enough to suspend even the
largest dense particles, but can be
adjusted to be gentle enough for friable
particles or emulsions. A built-in
ultrasonic probe is included to allow
in-system dispersion of agglomerates,
thus avoiding the need for external
sample preparation.
The best return on investment is provided
when the greatest number of
samples can be run on a single system.
The analysis time for most laser
diffraction instruments is on the order
of 3–5 min. The Partica LA-950 has
been designed to accomplish a
sample-to-sample time of 1 min total,
including filling with dispersant, alignment, recording of background,
circulation of sample, analysis, and
drain and rinse cycles. The careful
design of the dispersant filling system,
alignment mechanism, electronics,
and optical system can provide high
precision, resolution, and accuracy
over the entire size range, thus making
the return on investment much
more attractive.
For applications requiring toxic dispersants
or precious samples, or in situations
where waste disposal is an issue,
small-volume sampling options are
available with as small as 15 mL total
volume. The measurement performance
is equivalent to full-volume systems,
while preserving precious samples.