Calibration of Extinction Spectrometer with Polystyrene
Latex Particles
In the first test, 240 nm spherical latex particles were used and five
different samples of increasing number concentrations were tested. In the second test, 80
and 100 nm latex spheres were tested. Experimentally measured extinction spectra for 240
nm (different particle concentrations) and 100 and 80 nm are shown in Figure 1 and Figure
2 below. The size distribution obtained by inverting the extinction data using our
proprietary code based on a modified Chahine iteration algorithm are also included in the
figures. It is observed from the figures that the results are in good agreement with the
sizes of latex particles used. Results from our inversion algorithm are highly
reproducible. A remarkable achievement here is that this technique can provide accurate
size even when the particle size is smaller than the wavelength, a region where existing
commercial static light scattering instruments fail. Our technique is continuously
improved to meet evolving industry needs.
The small peak seen next to the larger peak in Figure 1 is a numerical
artifact at low particle concentrations. The peak disappears at high particle
concentrations. A possible solution to overcome this error is to run measurements at
different particle loadings. We are also refining our numerical simulations to overcome
this problem.
Figure 1(a-b) - Experimental data and size after inversion for 240 nm latex
particles
Even though the particles used in the measurements were monodispersed,
the results in Figure 2 show a slight spread in the distribution. This is because the
spectrometer used was not designed for this purpose and there are variations of beam shape
with wavelength. The beam was not circular but changes its shape during propagation before
reaching the detector. As the particle size decreases the data spread increases i.e.,
these issues become important in determining the instrument accuracy for this application.
We are currently building a spectrometer with a well-defined beam size and shape
specifically for particle sizing to overcome this problem.
Figure 2(a-b) - Experimental data and size after inversion for 80 and 100 nm
particles
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