离心Centrifugation

离心力常用相对离心力（RCF）表示：RCF = 1.118 × 10⁻⁵ × r × (rpm)²，单位 ×g，r 为旋转半径（cm）。沉降速率与颗粒质量、尺寸和密度差有关，用沉降系数表示，单位为斯韦德贝里（S，10⁻¹³ s），例如核糖体 70S、80S。差速离心逐级提高转速分离不同大小的颗粒；密度梯度离心（如蔗糖或氯化铯梯度）按浮力密度分离。

Centrifugal force is usually given as relative centrifugal force, RCF = 1.118 × 10⁻⁵ × r × (rpm)², in multiples of g, with r the radius in cm. Sedimentation rate depends on particle mass, size and density difference and is expressed as a sedimentation coefficient in svedbergs (S, 10⁻¹³ s), as in 70S and 80S ribosomes. Differential centrifugation raises the speed step by step to separate particles by size; density-gradient centrifugation in sucrose or caesium chloride separates them by buoyant density.

1864 年普朗特尔发明奶油离心分离器。1920 年代瑞典化学家斯韦德贝里发明超速离心机，用于测定蛋白质分子量，证明蛋白质是大分子，获 1926 年诺贝尔化学奖。1958 年梅塞尔森和斯塔尔用氯化铯密度梯度离心证明了 DNA 的半保留复制。

Prandtl invented the centrifugal cream separator in 1864. In the 1920s the Swedish chemist The Svedberg built the ultracentrifuge, used it to measure protein molecular weights and showed proteins are macromolecules, receiving the 1926 Nobel Prize in Chemistry. In 1958 Meselson and Stahl used caesium chloride density-gradient centrifugation to prove that DNA replicates semi-conservatively.

离心用于实验室少量沉淀分离（代替过滤）、血液分离血浆和血清、细胞器分离、病毒纯化、核燃料铀同位素浓缩（气体离心机分离 ²³⁵UF₆ 与 ²³⁸UF₆）、牛奶脱脂和污泥脱水。使用时离心管必须对称配平，转子不得超过额定转速，否则可能造成严重事故。

Centrifuges separate small precipitates in place of filtration, separate plasma and serum from blood, fractionate organelles, purify viruses, enrich uranium in gas centrifuges (separating ²³⁵UF₆ from ²³⁸UF₆), skim milk and dewater sludge. Tubes must be balanced symmetrically and rotors never run above their rated speed, or serious accidents can result.