High-resolution, submicron particle size distribution analysis using gravitational-sweep sedimentation

Sedimentation velocity is a powerful tool for the analysis of complex solut ions of macromolecules. However, sample turbidity imposes an upper limit to the size of molecular complexes currently amenable to such analysis. Furth ermore, the breadth of the particle size distribution, combined with possib le Variations in the density of different particles, makes it difficult to analyze extremely complex mixtures. These same problems are faced in the po lymer industry. where dispersions of latices, pigments, lacquers, and emuls ions must be characterized, There is a rich history of methods developed fo r the polymer industry finding use in the biochemical sciences. Two such me thods are presented. These use analytical ultracentrifugation to determine the density and size distributions for submicron-sized particles. Both meth ods rely on Stokes' equations to estimate particle size and density, wherea s turbidity, corrected using Mie's theory, provides the concentration measu rement. The first method uses the sedimentation time in dispersion media of different densities to evaluate the particle density and size distribution . This method works provided the sample is chemically homogeneous, The seco nd method splices together data gathered at different sample concentrations , thus permitting the high-resolution determination of the size distributio n of particle diameters ranging from 10 to 3000 nm. By increasing the rotor speed exponentially from 0 to 40,000 rpm over a 1-h period, size distribut ions may be measured for extremely broadly distributed dispersions. Present ed here is a short history of particle size distribution analysis using the ultracentrifuge, along with a description of the newest experimental metho ds. Several applications of the methods are provided that demonstrate the b readth of its utility, including extensions to samples containing nonspheri cal and chromophoric particles.