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Photon Cross-correlation Spectroscopy
for particle size and stability analysis of opaque
suspensions and emulsions from 1 nm to 10 µm

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Photon Crosscorrelation Spectroscopy (PCCS) is a novel technique allowing for the simultaneous measurement of particle size and stability of opaque suspension or emulsions of nanoparticles in the size range of about 1 nm to some µm. It is a powerful extension of the well established Photon Correlation Spectroscopy (PCS), which is restricted to highly diluted suspensions, only.

Sympatec's Solution is PCCS with NANOPHOX

Photon Cross-correlation Spectroscopy extends PCS. The key principle of PCCS is a 3D cross correlation technique. In a special scattering geometry, the cross correlation of the scattered light allows for the precise separation of the single and the multiple scattered fractions.

PCCS set-upwith two incident laser beams PCCS measuring signals Cross correlation function for different amounts of single scattered light

PCCS is using two separate beams for the illumination of the same measuring volume creating two separate speckle patterns.

The intensity fluctuations are observed by two detectors positioned in a way, that identical scattering vectors q are used.

Now the cross correlation function of the two signals is used to monitor the dissolution of the near-order. The slope represents the particle size as for PCS. The amplitude represents the amount of single-scattered intensity.

PCCS has several advantages:

1. With PCCS the multiple scattering is completely suppressed. This enables PCCS to extend the evaluation method of PCS to opaque suspensions and emulsion.
2. High particle concentrations can be used, resulting in high count rates and short measuring times.
3. In addition the amplitude of the cross correlation function represents the amount of the single scattered light. This information can be used for the analysis of the stability of the sample. It is extremely sensitive as an increase in size by a factor of 10 raises the intensity of the scattered light by 106.
4. As multiple scattering is eliminated, the results are usually independent of the concentration.
Possible remaining changes with increasing concentration are due to changes of the particle-to-particle interactions. Now they can be directly investigated with a dilution series as any contribution of multiple scattering is avoided.

Measurements can be performed at any concentrations. If the concentration is too high only multiple scattered light remains and single scattered light cannot be detected. A result is not presented under these conditions.

5. The results are independent of the position of the measuring volume within the sample vial.
The position of the measuring volume can be chosen to get the highest count rates and thus the shortest measuring times.
6. High particle concentrations reduce the sensitivity of this method to impurities. So standard liquids and laboratory environments can be used, which simplifies the application.
7. The PCCS set-up can be simply switched by software into the PCS mode, by using one detector only and calculating the auto-correlation function. This enables a direct use of former experiences with PCS.

NANOPHOX

A novel instrument has been developed and optimised for industrial use. Housed in a small table top unit NANOPHOX contains light source, detectors, the correlator, and the sample containing vial and is ready for routine measurements in the laboratory.

Set-up:

Set-up of the optical path of NANOPHOX

 

Conditions for the scattering vectors:

Scattering geometry for PCCS

Technological challenge for PCCS:

  • the scattering vectors q have to be identical
    (in direction and value)
  • the scattering volume has to be identical

 

Fig. 1: Set-up of Sympatec's PCCS instrument, using two incident beams, two detectors, two photo-multiplier and a correlator.

Fig. 2: The scattering geometry has to be aligned in this special way. Then multiple scattering is suppressed completely.

Scattering geometry for PCCS

Scattering volume:

The scattering volume is defined be the intersection of the two focussed laser beams A and B, as shown in Fig.3. It can be approximated e.g. by two adjacent cones with radius r and height h to about
5.5E-4 µl or 5.5E-13 m³.

Fig. 3: Definition of the scattering volume
(marked in blue colour).

 

The operation is much simpler than with a conventional PCS system, because the samples need not be diluted. Costly cleaning and sample preparation effort is avoided.

symbol: wet dispersion for suspensions or emulsions

Dynamic Light Scattering (DLS)

is nowadays used on a routine basis for the analysis of particle sizes in the sub micrometer range. It provides an estimation of the average size and its distribution within a measuring time of a few minutes.

With DLS the motion of the particles in a suspension fluid is monitored optically. This movement is known as Brownian Molecular Movement and was correctly suggested by W. Ramsay in 1876 and confirmed by A. Einstein and M. Smoluchowski in 1905/06.

In the Stokes-Einstein theory the Brownian motion is depending on the viscosity of the suspending fluid, the temperature and the size of the particles. The particle size (i.e. the hydrodynamic diameter) can be evaluated from a measurement of the particle motion, if viscosity and temperature are known.

Monodisperse particles illuminated by a laser beam

The illumination of the particles by laser light results in a diffraction pattern showing a ring structure created by diffraction on the particles. This information is used by laser diffraction.

Diffraction pattern of the particle ensemble above

It also shows a fine structure from the diffraction between the particles, i.e. its near-order.

As the particles are moving from impacts of the thermal movement of the molecules of the medium, the particle positions change with time t.

The change of the position of the particles results in changes of the phases and thus in changes of the fine structure of the diffraction pattern. So the intensity in a certain point in the diffraction pattern fluctuates with time (for laser diffraction, these fluctuations are usually averaged out).

The fluctuations can be analysed in the time domain by a correlation function analysis or in the frequency domain by frequency analysis. Both methods are linked by Fourier transformation.

For further details please refer to the coming standard ISO 22412 (Dynamic Light Scattering).