Topics Covered
Background
Equipment
Used
Sample
Preparation and Measurement
Results
Conclusion
Background
Absorbing samples
are a challenge for classical dynamic light scattering instruments. The
difficulty arises from the reduction in the intensity of scattered light using a
scattering angle of 90°. This is for two reasons, the absorption reduces the
power of the incident beam, and the amount of scattered light is further reduced
by having to pass through the sample before being
detected.
In this application
report, a highly absorbing sample, Phenolphthalein, at pH 9.5 is measured. At
this pH Phenolphthalein is red.
Although the
molecular weight is low at 318 Daltons, this report also
shows that even molecules this size can be measured.
Equipment
Used
Malvern HPPS Syringe and 0.2ìm syringe
filter.
Sample Preparation and
Measurement
Phenolphthalein was
prepared at saturation concentration in water adjusted to pH 9.5 with ammonia.
The sample was then filtered through a 0.2ìm syringe filter into a standard
disposable polystyrene cuvette. A detection position close to the inner cuvette
wall was selected. The measurement time was selected as 120s and analyzed using
the default size distribution analysis.
Results
The sample was
highly absorbing. The "0° monitor" detected that only 1% of the intensity of
incident laser beam remained after passing though 10mm of the solution. Due to
the measurement being done in the back-scatter configuration of the HPPS, little
light was absorbed as the incident beam and scattered light had to travel only a
small distance through the sample. This allowed the determination of both the
aggregates in the sample of. 0.5% at 150nm diameter and the size of the
Phenolphthalein molecule itself which was calculated as 0.6 nm. This is in
excellent agreement with the size calculated for the Cholesterol molecule
(387Da, hydrodynamic size 0.64nm, reported in another application
report.
Figure 1.
Particle size
analysis for Phenolphthalein determined using a Malvern
HPPS.
Conclusion
This
application report demonstrates that measurement of the particle size of
absorbing samples is possible using the Malvern HPPS. This is enabled by the backscattering optics
arrangement, which also contributes to the high sensitivity of the system. These
characteristics can also be used for measurements of dyes, pigments inks and
toner samples.
Source: "The Size Measurement Of Highly Absorbing
Samples - Phenolphthalein,
Ph9.5, Mw 318da", Application Note
by Malvern Instruments.
For more information on this source please
visit Malvern
Instruments Ltd (UK) or Malvern Instruments
(USA).