densitygradient

A density gradient is a spatial variation in density within a medium, typically extending from higher to lower density. Such gradients are used to separate mixtures of particles, cells, or molecules according to buoyant density or sedimentation behavior. In centrifugation, gradients are formed by layering solutions of increasing density or by self-generating diffusion, and they can be continuous or stepwise.

There are two principal modes of separation. Isopycnic (equilibrium) density gradient centrifugation relies on particles reaching a position in the gradient where their buoyant density equals that of the surrounding medium, resulting in bands at their isopycnic point. Rate-zonal (kinetic) gradient centrifugation separates components primarily by size or shape as they sediment through the gradient before reaching equilibrium, with the gradient providing a medium that slows diffusion and enhances resolution.

Common gradient media include sucrose, Percoll, Nycodenz, and cesium chloride. Gradients may be designed as continuous or discrete (stepwise) layers and are used in ultracentrifugation or preparative centrifugation. The choice of medium, gradient format, and centrifugation conditions determines the resolution and suitability for a given sample.

Applications span biology and materials science. They include purification of viruses, subcellular organelles (such as mitochondria and lysosomes), lipoproteins, nucleic acids, and protein complexes, as well as analytical separations of cells or macromolecules. Practical considerations encompass gradient stability, diffusion blurring, temperature control, sample loading, and compatibility of gradient media with the material of interest.