Photon correlation spectroscopy (PCS), also known as dynamic light scattering (DLS), measures particle size through analysis of intensity fluctuations in scattered light from a sample undergoing Brownian motion. PCS uses laser light, optical components like lenses and detectors, and a digital correlator to measure an autocorrelation function from which particle hydrodynamic size can be determined. It has advantages like requiring only a small sample amount and being non-destructive, but disadvantages include low resolution where particle sizes must differ significantly and potential masking of smaller particles by larger particles.

1. Photon correlation spectroscopy
D.Mahalaxmi
PA/2016/104

2. Contents :
• PRINCIPLE
• INSTRUMENTATION DESIGN
• STABILITY OF SAMPLE
• ADVANTAGES
• DISADVANTAGES

3. PRINCIPLE:
Other names for PCS –DLS,IFS,QES.
Principle : particle size – measuring the changes –intensity of
scattered light from a suspension or solution due to Brownian
motion.
Illuminates the
Sample
173
90

4. LIGHT SCATTERING FLUCTUATIONS DUE TO BROWNIAN MOTION VS. TIME
AUTOCORRELATION FUNCTION FROM PCS
For a sample where all of
the particles are the same
size.
The optical signal shows
random changes due to the
randomly changing relative
position of the particles
The signal can be interpreted using an
autocorrelation function. Incoming data is
processed in real time with a digital signal
processing device known as a correlator and
the autocorrelation function

5. Where: Dh = the hydrodynamic diameter
Dt = the translational diffusion coefficient
kB = Boltzmann’s constant
T = temperature
η = dynamic viscosity

6. INSTRUMENTATION DESIGN:
Light source : sufficiently intense light source
–narrow band ,polarized , monochromatic.

7. • Optical system : lens is enclosed in a
temperature controlled scattering cell
surrounded by refractive index matching
liquid. Typically all experiments -10-160
degrees angular range.
• Detectors: PMT,APD & SPCM.

8. Digital correlator:
ACF –formed by recording number of photons arriving in each sample time &
bits registered are used to maintain the records of photon counts.

9. Stability of sample:
• An unstable sample –inappropriate sample
preparation.
• Hydrophilic sample surfaces-treat with
stabilizing surfactant.
• Hydrophobic samples surfaces-treat with
methyl alcohol.
• Use appropriate surfactants-to remove dust,
use of deionised and filtered water is utmost
important in any serious size measurements.

11. ADVANTAGES:
• Mini. Information –sample.
• Mixtures of samples – viscosity of medium
must be known accurately.
• Mini. amount of sample.
• Fast , simple & no destructive –sample recover
is possible.

12. DISADVANTAGES:
• Extremely low resolution : particle size must
differ by 50% or more to reliably detect two
peaks.
• A small quantity of small size particles can
easily be ‘hidden’ in a much larger quantity of
larger size particles.