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Microelectrophoresis Apparatus Mk II

The Rank Brothers Microelectrophoresis Apparatus Mk II can determine the electrophoretic mobility of a wide range of suspended particles. It has a wide and established user base in all fields where flocculation, dispersion, and surface adhesion are important parameters to control.

Microelectrophoresis Apparatus Mk II set up for use with Pyrex cell
Picture of Microelectrophoresis Apparatus Mk II set up for electrophoretic mobility measurements using Pyrex cylindrical thin walled cell, especially suitable for:
  • Very small particles (ultra-microscope conventional or laser illumination).
  • High electrolyte concentration (small cross section, giving small currents and little polarisation).
  • Rapid thermostatting and freedom from convection currents.

Physical and Biological Applications of Electrophoresis

The term electrophoresis refers to the motion of a charged particle when an electric field is applied. When the velocity of the particle is measured, by timing over a known distance, it is possible to calculate the electrical potential at the surface of shear between the particle and the surrounding medium and from this the electric charge contained within the surface of shear. When the particle is in a relative charge free medium, such as air or a low dielectric constant liquid, it becomes convenient to calculate the charge directly. Careful consideration must be given to whether the charge is an equilibrium property of the particle or whether it is an accidental property determined by the history of the particle and what it happens to have collided with. However in aqueous solution (or in liquids of all but the lowest dielectric constant) the surface potential and charge of a particle is a reproducible quantity that gives valuable information both on the way the particle can interact with other particles or surfaces and on the nature of the particle surface. In the latter respect it is noteworthy that only a few particles and therefore a minute surface area, are necessary for the measurement.

So far as inter-particle behaviour is concerned the electric charge is one of the most important factors leading to stability of a dispersion against flocculation, coagulation or adhesion to surfaces. In all technologies involving dispersions or suspensions, whether these are required to be flocculated or de-flocculated, a knowledge of the particle charge and/or potential is a necessary pre-requisite to prediction of the behaviour of the system, either alone or in the presence of additives.

Increasing use is being made of measurements of the electrophoretic mobility of the various particles present in blood to help predict and prevent clotting of the blood and its adhesion to the walls of both natural and artificial blood vessels.

Quite apart from using electrophoretic mobilities to explain or predict the behaviour of particles or surface coatings, such measurements can be used as an aid to identifying the chemical groupings present at the particle surface. For example, the variation of particle surface charge with variations of pH of aqueous solutions gives important information on the dissociation of surface acidic and basic groups and can be used to identify them. Alternatively, in complex biological systems, the mobilities of the various particles present (which to some extent can be obtained without separation) can be used to identify these particles and may be characteristic of pathological conditions. In this connection it should be remembered that electrophoretic mobilities are most susceptible to change in conditions and therefore most characteristic of the particles, when the mobilities are small. Conditions such as pH can often be adjusted to achieve this.

Capabilities of the Microelectrophoresis Apparatus Mk II

The Microelectrophoresis Apparatus Mk II can be used whenever the particles can be made visible relative to the suspending medium. Such visibility depends (apart from the intensity of the illumination and efficiency of the viewing optics) on the size of the particles and on the ratio of the particle refractive index to that of the surrounding medium. Using the quartz-iodine illumination unit of the standard instrument, aqueous dispersion of polystyrene particles (which have a rather unfavourable refractive ratio, about 1.1), shown to be visible down to a diameter of about 0.2 µm. Particles with a more favourable refractive index ratio, e.g. carbon particles, can be seen down to much smaller sizes but it is difficult to give any precise limit in cases where monodisperse suspensions cannot be separated.

Lower Limit of Particle Size

One effective means of lowering the limit of particle visibility is use of a laser illuminator. Even a 3 mW He:Ne continuous laser can concentrate more illumination into the observed volume than the 100 watt conventional illuminator and polystyrene particles of diameter only 0.09 µm become visible. It must be remembered however that the necessary illumination power goes up very sharply as the size of the particle decreases below the figures quoted. Rank Brothers are pleased to perform free trials of particle visibility on samples sent to us by prospective purchasers of the apparatus.

The cylindrical cell, with extremely thin walls and ultra microscope illumination, is especially suitable when very small particles are involved.

Upper Limit of Particle Size

There is obviously no upper limit of particle size so far as visibility is concerned, the effective limit being set by the rate of gravitational fall or rise in the dispersion medium concerned. The flat cell is especially useful here because sedimenting particles fall neither out of view nor out of the stationary level. Moreover provided the rectangular cross section of the flat cell has its major dimension vertical, the sediment collects on an electro-osmotically unimportant surface so that measurements at the stationary level remain valid. The vertical gravitational component of the particles' motion can quite properly be ignored while the horizontal electrophoretic component is measured. Since the field of view is about 500 µm across it follows that if times of vertical transit in excess of 20 seconds are accepted, the upper useful limit of particle diameter in aqueous solution at 25°C with particle specific gravity 2.0, is about 20 µm. Particles with density nearer to that of water can be used up to much larger sizes.

Microelectrophoresis Apparatus Mk II set up for use with flat cell
Picture of Microelectrophoresis Apparatus Mk II set up for use with flat cell, especially suitable for:
  • Large particles (which remain in field of view and stationary layer while sedimenting).
  • Low conductivity solvents (silica walls of high resistivity).
  • Applications requiring bright field or phase contrast illumination.

Instrument Details

The standard Microelectrophoresis Apparatus Mk II comes with the following:

  • Thermostatted water bath suitable for using cylindrical cell.
  • Thermostatted water bath suitable for using flat cell.
  • 100 watt Quartz iodide lamp illumination.
  • Constant voltage and constant current electrode supply (0–100 V, 0–5 mA approx.).
  • Circulating pump for water baths.
  • Temperature controller for water baths (ambient to 50°C).
  • A flat cell and holder.
  • A cylindrical cell and holder.
  • A pair of platinum electrodes.
  • A handset incorporating a digital voltmeter, ammeter (to monitor electrode voltage and current) and stopwatch.
  • Binocular microscope.

Rotating Prism System

The Rotating Prism System is an optional accessory for the Microelectrophoresis Apparatus Mk II, enabling faster and easier determination of particle speed. The system consists of a control unit, with a simple keyboard and display and a prism unit. The prism unit can be fitted by removing the binocular head from the Mk II, putting the prism unit on in its place, then putting the binocular head onto the prism unit.

The prism gives everything viewed through the microscope an apparent motion. The prism is rotated so that the apparent motion is in the opposite direction to the actual motion of the particles. The speed of the prism is then adjusted so that the particles appear stationary. The display then gives a direct reading of particle speed (after the unit has been calibrated initially).

The advantages of the Rotating Prism System are as follows:

  • A large number of particles are observed at once. The human eye can easily detect when (say) five particles are all stationary or if four are stationary and one is not, even though the eye is unwilling to time more than one moving particle at once.
  • The tendency, when timing individual particles, to pick out the big bright one is removed. Such large particles may not be typical of the total population.
  • A shorter time is needed to get an average based on a given number of particles.
  • If the particle mobilities are very heterogeneous then this is immediately obvious and steps can be taken.
  • If the dispersion is a mixture of two types of particle with different mobilities then not only is this immediately evident but the two types can be separately brought to the stationary state and their particle speed measured, in the presence of each other.

The prism can be stopped and measurements taken on individual particles in the usual manner.

Other Optional Accessories

  • A 3 mW laser can be supplied complete with mounting brackets to enable smaller particles to be measured.
  • A closed-circuit monochrome television system can be supplied to be used instead of the binocular microscope (not available to USA).


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