The Zetasizer Nano range of instruments allows you to measure three characteristics of particles or molecules in a liquid medium.
These three fundamental parameters are particle size, zeta potential, and molecular weight. Unique technology within the Zetasizer system allows these parameters to be measured over a wide range of concentrations. Depending upon the options and accessories used, the Zetasizer system also has the ability to perform:
- Microrheology measurements
- Autotitration measurements if an MPT-2 accessory is connected,
- Trend measurements, including the determination of the Aggregation point.
The Zetasizer range features pre-aligned optics and programmable measurement position for the measurement of size and zeta potential over a wide concentration range and precise temperature control necessary for reproducible, repeatable and accurate measurements. In addition, other key parameters, such as conductivity, and with the MPT-2 Titrator, and pH can be measured.
The Zetasizer Nano range has been designed so that a minimal amount of user interaction is necessary to achieve excellent results. The use of Standard Operating Procedures (SOPs) and features such as the Folded capillary cell alleviate the need for constant attention.
Laser type
The laser fitted is identified by the colour on the oval badge on the cover.
- Instruments with a black and red badge fitted to the instrument cover either have a 4 mW 632.8nm ‘red’ laser or a 10 mW 632.8nm ‘red’ laser (Nano ZSP only) fitted.
- Instruments with a Black and green badge have a 532nm ‘green’ laser fitted.
Generally, red lasers are least suitable for blue samples, while green lasers are least suitable for red samples.
Particle Size?
The particle size measured in a Dynamic Light Scattering (DLS) instrument is the diameter of the sphere that diffuses at the same speed as the particle being measured.
The Zetasizer system determines the size by first measuring the Brownian motion of the particles in a sample using DLS and then interpreting a size from this using established theories.
Brownian motion is defined as:
“The random movement of particles in a liquid due to the bombardment by the molecules that surround them”.
The particles in a liquid move about randomly, and their speed of movement is used to determine the size of the particle.
It is known that small particles move or diffuse more quickly in a liquid than larger particles. This movement is carrying on all the time. So if we take two ‘pictures’ of the sample separated by a short interval of time we can see how much the particles have moved and therefore calculate their size.
Zeta potential and Electrophoresis?
Most liquids contain Ions; these are negatively and positively charged atoms called Cations and Anions, respectively. When a charged particle is suspended in a liquid ions of the opposite charge will be attracted to the surface of the suspended particle.
A negatively charged sample attracts positive ions from the liquid, and conversely, a positive charged sample attracts negative ions from the liquid.
Ions close to the surface of the particle will be strongly bound, while ions that are further away will be loosely bound forming what is called a Diffuse layer. Within the diffuse layer there is a notional boundary, and any ions within this boundary will move with the particle when it moves in the liquid; but any ions outside the boundary will stay where they are – this boundary is called the Slipping plane.
A potential exists between the particle surface and the dispersing liquid which varies according to the distance from the particle surface – this potential at the slipping plane is called the zeta potential.
Zeta potential is measured using a combination of the measurement techniques:
Electrophoresis and Laser Doppler Velocimetry, sometimes called Laser Doppler Electrophoresis. This method measures how fast a particle moves in a liquid when an electrical field is applied – i.e. its velocity.
Once we know the velocity of the particle and the electrical field applied we can, by using two other known constants of the sample – viscosity and dielectric constant – work out the zeta potential.
Molecular weight?
The molecular weight of a substance is the weight in atomic mass units (amu) of all the atoms in one molecule of that substance. Mathematically the molecular weight can be calculated from the molecular formula of the substance, being the sum of the atomic weights of all the atoms making up the molecule.
If we take as an example the molecular formula H2O (water) we can work out the molecular weight.
- In each molecule of water there are two atoms of Hydrogen (H2) and one atom
of Oxygen (O).
The atomic weight of hydrogen is 1.008 amu and that of oxygen is 15.999.
Therefore the molecular weight of water is 18.015 i.e.(1.008 x 2)+15.999.
This is a calculation using a known molecular formula and applying the values from the periodic table.
With the Zetasizer Nano series of instruments the molecular weight can now be determined by use of Static Light Scattering (SLS) measurement techniques.