= Isopycnic centrifugation
= Equilibrium density-gradient centrifugation
{synonym}
In <isopycnic centrifugation>, a species concentrates near the position where the liquid's <density> matches its <buoyant density>. In a <density gradient> increasing outward, material displaced inward encounters less-dense liquid and tends to move outward; material displaced outward encounters denser liquid and tends to move inward. <Diffusion> broadens the resulting band. If the <density gradient> never reaches the species' <buoyant density>, a matching equilibrium position need not exist within the vessel.https://www.beckman.com/resources/technologies/centrifugation/ultracentrifugation/[An Introduction to Ultracentrifugation, Isopycnic Centrifugation: matching buoyant density and diffusion in equilibrium gradients]{ref}
The 1958 paper <The replication of DNA in Escherichia coli by Matthew Meselson and Franklin W. Stahl> (pp. 672–676, Figures 1–4) makes the distinction concrete. Its separation apparatus was a Spinco Model E ultracentrifuge with an <Escherichia coli> lysate in a <cesium chloride density gradient>. Ultraviolet <wave absorption> photographs of the rotating sample were scanned with a recording microdensitometer: peak position reported <buoyant density>, while signal above baseline reported <DNA> <concentration>.<The replication of DNA in Escherichia coli by Matthew Meselson and Franklin W. Stahl>[p. 676, Experimental: Spinco Model E and recording microdensitometer; p. 675, Figure 4: peak position and baseline]{ref}
Mixed lysates labeled fully with the <nitrogen> <isotope> $^{15}\mathrm{N}$ and unlabeled lysates supplied reference bands; a mixture including intermediate material checked its position relative to those references. Figure 1 followed band formation over time, helping distinguish a migrating distribution from an approximately stationary one. These controls address resolution, calibration and approach to equilibrium. They do not make every ultraviolet-absorbing component identifiable as <DNA>, or turn a band into a direct image of individual strands. The <DNA replication> article develops the copying inference; here the key result is a separation by <buoyant density>, not a claim that heavier-labeled <DNA> is physically larger.<The replication of DNA in Escherichia coli by Matthew Meselson and Franklin W. Stahl>[pp. 672–673, Figures 1–2; p. 675, Figure 4 density references and microdensitometer baseline; p. 676, Experimental: Spinco Model E and ultraviolet photographs]{ref}
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