Particle Size Analysis - Nano Range

Malvern Zetasizer Nano ZS Particle Size Analyser

The Malvern Zetasizer Nano ZS particle size analyser is an instrument can be used for the characterization of samples that contain colloids, nanoparticles and some macromolecules. The Zetasizer system uses Non-Invasive Backscatter (NIBS) optics in the characterization of samples. This instrument uses a variety of techniques that allow it to measure qualities of particles such as the particle size, zeta potential and molecular weight

Principle

A technique known as Dynamic Light Scattering (DLS) is used for the measurement of particle sizes for a given sample. The principle of DLS is that fine particles and molecules that are in constant random thermal motion (Brownian motion) travel at a speed relative to their size compared to the larger particles which travel slower. The Zetasizer Nano ZS uses a laser to illuminate the particles in a given sample to produce and observe the speckle patterns produced by the particles. A sensitive avalanche photodiode detector detects the scattering intensity at a specific angle which fluctuates with time. The changes in intensity as detected by the detector are analysed with a digital auto correlator which generates a correlation function to generate a curve. By using this curve, the size and the size distribution for particles in a given sample can be analysed. The speed of Brownian motion is affected by temperature; therefore, the precise maintenance of temperature is imperative in the accurate measurement of particle size.

The instrument utilizes laser Doppler electrophoresis in order to determine the zeta potential of a given sample. During electrophoresis, an electrical field is maintained across the solution through which the particles can move freely. The speed of a particle is proportional to the field strength and the zeta potential of a given sample. By measuring the speed of movement by laser Doppler electrophoresis, the instrument then uses established theories to calculate the zeta potential. The Zetasizer Nano uses Static Light Scattering (SLS) for the determination of the scattering intensity of a number of known concentrations of a given macromolecule in a sample solution. As a result of this measurement, a weight-average molecular weight could be obtained

Strengths

• Wide range of temperature adjustments are possible
• Ability to switch between different solvent systems
• Ability to measure both zeta potential and particle size for the exact sample

Limitations

• The particle size analysis is limited to particles within the range of 0.6 nm to 6 µm in diameter
• Zeta potential measurements are limited to particles within the range of 5nm to 10 µm
• Particle measurement is heavily affected by the density of the particles as the gravitational pull on the heavy particles reduce the measurement accuracy
• Measurement depend heavily on the sample preparation conditions
• The 633 nm laser is least suitable for blue coloured samples 

Applications

• Particle size measurements can be used in industries/fields such as inks and pigments, cement, ceramics, pharmaceuticals, cosmetics, soils and sediments, food and drink etc.
• Zeta potential measurements could be used in a variety of applications such as to reduce the development times for stable dispersions and protein solutions, to understand reasons for product stability or instability leading to the improvement of shelf life etc.

Technical Specifications

• Size Range: 0.6nm to 6 Microns
• Size range for Zeta Potential: 5nm to 10 microns
• Laser: Red laser with a wavelength of 633 nm
• Size measurement angle: 173° 
• Concentration Range for size measurement: 0.1ppm
• Mobility range: Nominally +/- mcm/Vs
• Conductivity range 0 to 20mS/cm
• Temperature range 2°C to 90°C (70°C maximum with folded capillary cell)