Measuring the relative concentration of particle populations using differential centrifugal sedimentation†
The factors that affect the accuracy and precision of differential centrifugal sedimentation (DCS) for the analysis of nanoparticle concentration are described. Particles are separated by their sedimentation rate and detected using light absorption. In principle, the relative concentration of particles in different populations can be found, but the uncertainty in such measurements is unclear. We show that the most appropriate measurement of particle concentration using this technique is the mass concentration, rather than the number concentration. The relative mass concentration of two discrete populations can be measured with reasonable precision, usually without resorting to complicated data analysis. We provide practical approaches to find the relative mass concentrations for two cases: spherical particles of different materials and agglomerated particles of the same material. For spherical particles made of different materials, naïve analysis of the results can provide relative mass concentrations that are many orders of magnitude in error. Correction factors can be calculated that reduce the error to less than 50%. In the case of agglomerated particles we show that errors of less than 20% are possible and demonstrate, in the case of gold particles, that a combination of UV-visible spectroscopy and DCS enable practical values of mass and number based particle concentrations to be obtained.