compared to that for heterodyne detection. The two optical
fibers that couple at the y-splitter (laser beam and scattered
light signal) are connected to a case containing the laser and
detector electronics.
4
5.1.1.1 Nanoparticles that have a radius less than approxi-
mately one tenth the wavelength of the radiation undergo
Rayleigh scattering. In the Rayleigh scattering range, the
homodyne-detected signal level drops off by diameter, d, at
10
6
. The heterodyne-detected signal power drops off by only
10
3
. The pairs of scattered light detected in the homodyne
mode are the sum of two low optical power signals (i
scat
)
2
. See
Fig. 2 for diagrams of both the homodyne and heterodyne
detection methods. The signal detected in the heterodyne mode
is the sum of the mixed scattered light signal and the
high-power reference signal, (i
ref0
)×(i
scat
). The signal level for
the controlled reference mode can, thus, be made to be orders
of magnitude larger than the homodyne mode by providing a
high level of reference signal intensity and signal-to-noise
ratio.
5.1.1.2 The probe can be attached to the outside of the case
at the end of a rotational clamp, which is also adjustable up and
down. A variety of sample containers, such as beakers,
cuvettes, and test tubes, can be placed below the probe, and the
probe can be lowered into the sample, handheld, or attached to
the clamp. The clamp can also be rotated 180° so that the
sensing end of the probe is pointed upwards, and a drop of
sample as small as 2 µL can be deposited on the sapphire
window to make the measurement. This is the desired orien-
tation when the samples are small or expensive, or both. The
high power of the laser amplified detection signal allows
measurement of particle concentrations up to 40 mg ⁄mL,
which includes samples that are obtained already prepared,
such as inks. paints, pigments, and others, without diluting the
samples.
5
5.1.2 Platform B—The optical bench/probe described and
shown in Fig. 2 is mounted horizontally in a case with the
sensing end of the detector placed against a disposable or
permanent cuvette containing the sample. This platform mea-
sures all parameters mentioned previously except zeta poten-
tial.
5.1.3 Platform C—Two probes are placed such that the
windows of the probes are within the walls, on opposite sides,
of a permanent sample cell. This platform measures all
parameters in 5.1. It is the only platform that also measures
zeta potential (Fig. 3).
5.1.3.1 Zeta Potential Measurement—When measuring size,
only one of the probes is activated and functions as it does in
the other platforms. When measuring zeta potential, both
probes are activated. The measurement of zeta potential takes
advantage of the same power spectrum analysis (PSA) used for
measuring size. The backscatter and laser-amplified (hetero-
dyne) detection signals are collected as in the size
measurement, and the rapid sequencing of applied electric
fields prevents electroosmosis. The optical probe surfaces are
coated to provide electrical contact with the sample. One probe
determines the polarity of the particle charge and the other
measures the mobility of the particles in an electric field.
Polarity is measured in a pulsed electric field, while mobility is
measured in a high-frequency sine wave electric field excita-
tion. From the power spectrum distribution (PSD), the loading
index (LI) is calculated as the sum of the amplitudes of all
logarithmic frequency channels (refer to 5.2, Power Spectrum
Analysis) and depends on the particle concentration. The LI is
proportional to the number of particles in each channel. LI
values provide a single number for total scattering that can be
used to determine particle mobility in microns/seconds/volt/
centimetre and particle polarity as 6, positive or negative, and
volt/centimetre and particle polarity as 6, positive or negative.
Measuring mobility and zeta potential begins by measuring the
PSD and determining the LI with the excitation off. Then, the
PSD is measured with the high-frequency sine wave on, and a
ratio is taken. Polarity is determined by measuring the LI
before and after pulsed DC excitation. A ratio of LI after the
excitation divided by LI before excitation of less than one is a
positive polarity (concentration decreasing) and a ratio greater
than one is negative (concentration increasing) for a positively
charged probe surface (refer to Fig. 3).
5.1.3.2 Zeta potential dependence on concentration is an
important factor in getting correct measurements. The formu-
lation of complex and commercial mixtures highly loaded with
solids, such as paints, inks, pharmaceuticals, herbicides, and
composite plastics should not be diluted when measuring their
zeta potential. These samples are prepared with the proper
ionic or non-ionic surfactants to achieve an ionic strength that
provides excellent stability and prevents agglomeration. Dilu-
tion with any liquid other than that used in their preparation can
lower the ionic strength of the suspension, causing agglomera-
tion and changing the zeta potential of the original suspension.
4
The (U.S. Patent No. 5094532, Mar. 1992) is covered by a patent. Interested
parties are invited to submit information regarding the identification of an
alternative(s) to this patented item to the ASTM International Headquarters. Your
comments will receive careful consideration at a meeting of the responsible
technical committee, which you may attend.
5
Trainer, M. N., Freud, P. J., and Leonardo, E. M., “High-concentration
submicron particle size distribution by dynamic light scattering,” American
Laboratory, Vol 24, July 1992, p. 34.
FIG. 3 Schematic of Zeta Potential Cell/Probes Setup
E3427 − 24
4