Topics Covered
Dynamic
Light Scattering and Particle Size Measurement
Factors Affecting Light
Scattering
Case Study -
Cholesterol
Equipment Used
Sample Preparation and
Measurement
Example and Results
Conclusion
Dynamic Light
Scattering and Particle Size Measurement
In principle, any 'small' particle, even
under 1nm, can be measured using the DLS method. All that is required is that
the particles scatter sufficient light, the particles diffuse freely and that
the number f particles in the measurement volume are sufficient to suppress
number fluctuations.
Factors Affecting
Light Scattering
The amount of light scattered is a function
of the relative refractive indices of the particle or molecule and the
dispersant. The number of particles in the detection volume is unlikely to be a
problem with sub 10nm particles.
However, for such small particles, the level
of the scattered light usually is very small, even at higher concentrations, and
only the most sensitive instruments are capable of performing DLS measurements
in this range. The Malvern HPPS has a claim to be the most sensitive DLS system
available and sizing of even sub-nanometer particles is within reach.
Case Study -
Cholesterol
The sample chosen to test the sensitivity
was Cholesterol at approximately 20mg/ml in Butanone.
Equipment Used
Malvern HPPS, Cholesterol, 1,2-Butanone, syringe / 0.2ìm
syringe filter, square glass cuvette, 375ìl sample volume.
Sample Preparation and
Measurement
100mg Cholesterol was dissolved in 5ml
degassed 1,2-Butanone and the resultant solution filtered through a 0.2ìm
syringe filter, into a clean and dust free glass cuvette.
The cuvette was inserted into the Malvern HPPS and 30 minutes was allowed to elapse to ensure
thermal equilibration of the sample at 50°C. After this period, a measurement of
120s duration was performed.
Example and
Results
The correlation function in (Fig. 1) clearly
shows at least two slopes indicating a short time fluctuation due to a small
component (A), and a slower component (B) indicating the presence of larger
material. These slower fluctuations could be seen in the butanone solvent when
measured on its own, while the faster component was absent.
Figure 1. Particle size distribution of
cholesterol as measured by a Malvern HPPS using dynamic light
scattering.
This demonstrated that the fast component is
due to the cholesterol, with the hydrodynamic diameter calculated as
0.64nm.
As expected, when the weight distribution is
calculated, the cholesterol is the dominating component. While literature values
for the hydrodynamic diameter of cholesterol in 1,2- Butanone are not easily
found, the particle size measured here comes close to the value expected when
calculated using a simple relationship between molecular weight and hydrodynamic
diameter.
Conclusion
The extremely high sensitivity of the Malvern HPPS allows size measurement of sub-nanometer
particles and molecules in solution, as demonstrated by the analysis of
Cholesterol. The count rate obtained from this experiment indicates measurement
at a concentration as low as 10mg/ml is feasible.
Source: "Measurement Of Sub-Nanometer Size Particles
Using Dynamic Light Scattering Cholesterol, Mw 387Da", Application Note
by Malvern Instruments.
For more information on this source please
visit Malvern
Instruments Ltd (UK) or Malvern Instruments
(USA).