比移值Retention factor

R_f 取决于化合物结构、固定相、展开剂极性、温度、薄层厚度和展开缸饱和程度。在硅胶板上：极性越大的化合物 R_f 越小；增大展开剂极性（如增加乙酸乙酯比例）使所有化合物 R_f 增大。常见官能团极性顺序（硅胶上保留由弱到强）：烷烃 < 卤代烃 < 醚 < 酯 < 酮 < 醛 < 胺 < 醇 < 羧酸。

R_f depends on structure, stationary phase, eluent polarity, temperature, layer thickness and tank saturation. On silica, more polar compounds have smaller R_f, and a more polar eluent (more ethyl acetate, say) raises every R_f. Retention on silica increases roughly alkanes < alkyl halides < ethers < esters < ketones < aldehydes < amines < alcohols < carboxylic acids.

R_f 用于比较和初步鉴定化合物，但受条件影响大，鉴定时应与标准品在同一块板上平行展开对照。理想分离时目标化合物 R_f 宜在 0.2–0.6 之间。二维薄层色谱在两个方向用不同展开剂展开，可分离复杂混合物。

R_f is used to compare and tentatively identify compounds, but it depends strongly on conditions, so identification should use an authentic standard run on the same plate. For good separation the target's R_f should lie between about 0.2 and 0.6. Two-dimensional TLC, developed in two directions with different eluents, separates complex mixtures.

R_f 概念随 1940 年代马丁和辛格的纸色谱一起提出，他们用纸色谱分离氨基酸，R_f 一词源于 retardation factor（阻滞因子）或 ratio of fronts（前沿比）。

The R_f concept arose with Martin and Synge's paper chromatography of amino acids in the 1940s; the term derives from 'retardation factor' or 'ratio to front'.