Differential centrifugal sedimentation is a very powerful nano particle size analysis tool where sedimentation of nanoparticles can be obtained in a reasonable time scale and with extremely high resolution. This is possible despite the fact that nanoparticles are expected to settle very slowly. Sedimentation by gravity is based on the principle that larger and heavier particles sediment faster than smaller and lighter particles. Knowing the particle density and centrifugal force, the Stokes equal sphere diameter can be calculated. Since very small particles such as nanoparticles are very light and will sediment very slowly, the sedimentation needs to be greatly accelerated and this can be achieved by the high rotational speed of a small disc centrifuge with speeds up to 24.000 rpm. The disc centrifuge can in this way cover a rather wide dynamic range of particle sizes from 2 nanometers (very high speed) up to 80 micrometers (very low speed) although the focuss is clearly on the nanoparticles size range. Despite the wide dynamic particle size range, the technique is still capable of analyzing particle size distributions with very high resolution.
In our laboratory we use the latest CPS DC24000 UHR disc centrifuge from CPS Instruments, which is the top model with high nanoparticle size resolution. By having the option to work with different viscous liquids and gradients, and besides the option of speed ramping the rotational speed of the disc, a clear focus can be set on a certain nano particle size range that is of particular interest. This enables an even more increased size resolution in a distinct nano size range.
Although traditionally differential sedimentation analysis of particles with a lower density than water could not be analyzed in water, the disc centrifugal system does have the option to analyze nanoparticles with a density lower than water. For such materials the inverse sedimentation principle is practised since the nanoparticles will finally float to the surface rather then settle to the bottom and therefore the particles will not be introduced at the upper part of the liquid but the nanoparticles will be transferred to the bottom of the liquid phase and are forced to go upwards. In this way many different types of nanoparticle suspicious materials can still be investigated successfully.
Differential centrifugal sedimenation is one of the various nano particle sizing techniques and can be used as a confirmatory method for nanoparticles analysis and nanomaterial classification. Although BET surface area analysis is typically used as a screening method for nanoparticle presence, besides the differential centrifugal sedimentation analysis in the disc centrifuge also electron microscopy is considered as a confirmatory technique for identifying the presence of nanoparticles in solid materials.
Differential Centrifugal Sedimentation FAQs
Differential centrifugal sedimentation, or DCS, is a particle size analysis technique that separates particles according to their sedimentation behaviour in a centrifugal field. The sedimentation time is related to particle size, density and the applied centrifugal force.
In DCS analysis, nanoparticles are subjected to high centrifugal forces in a rotating disc centrifuge. Larger or heavier particles sediment at a different rate than smaller or lighter particles. By measuring the sedimentation behaviour and accounting for particle density, an equivalent particle diameter can be calculated.
DCS can cover a broad particle size range, depending on the instrument settings and analysis conditions. The technique can measure particles from approximately 2 nanometres up to around 80 micrometres, with its main strength being high-resolution analysis in the nanoparticle size range.
DCS offers high resolution and can distinguish between particle populations with relatively small differences in size. The technique is particularly useful for analysing nanoparticle size distributions and detecting small changes or multiple particle populations in a sample.
Some disc centrifuge systems can analyse nanoparticles with a density lower than water by using an inverse sedimentation approach. In this case, the particles move upwards through the liquid rather than settling towards the bottom. This makes it possible to analyse a broader range of low-density nanoparticle materials.
