薄层色谱：载体、固定相及比移值

1.薄层色谱的分离过程　薄层色谱的分离过程用英文描述如下：

Chromatography systems may involve liquid （LC）or gaseous （GC）moving phases.Either of these may be used with liquid （LLC or GLC）or with solid （LSC or GSC）stationary phases.The stationary phase may be retained within a tube and the moving phase passes through it.Alternatively， the bed may be in a flat layer， the edge of which is dipped in a pool of the liquid that is to serve as the moving phase.In this type of experiment， thin-layer chromatography （TLC）or paper chromatography， the liquid moves by capillary attraction.In any case， provision must be made to observe the results of the elution； the type of detection system used depends on the details of the chromatographic system employed.

The stationary phase most often used is silica gel.Alumina， diatomaceous earth， and cellulose are also used.These are commercially available compounds with a binder such as Plaster of Puris， which strengthens the dried layer.Plates are prepared by evaporating the solvent from a slurry of the stationary phase.One convenient method is to use microscope slides.Two slides held together can be dipped into the slurry， separated， and dried to provide two plates.The sample is placed in a small spot near one edge of the bed.

薄层色谱是将固定相涂布于玻璃、铝箔、塑料片等载板上形成均匀的薄层，将预分离的物质点加在薄层的一端，放置在展开缸中，选用适当的展开剂，利用毛细作用从薄层点样的一端展开到另一端，使性质不同的物质得以分离。

薄层色谱常用的载体有玻璃板、铝箔、塑料片等。可分离亲脂性化合物的固定相有硅胶、氧化铝、乙酰纤维素及聚酰胺，分离亲水性化合物常选用纤维素、硅藻土及聚酰胺。

2.薄层色谱的技术参数

（1）比移值Rf。比移值Rf指一个化合物在薄层板上升的高度与展开剂上升的高度之比。

（2）相对比移值Ri，s。它是指被分离物质（s）与参比物（i）的比移值Rf之比。(www.xing528.com)

比移值用英文描述如下。

For application， the sample is usually dissolved in a volatile solvent so that the area can be effectively minimized by introduction of small increments with evaporation of the solvent after each addition.This is the analog of on-column loading in column chromatography.The plate is elute or developed by immersing it in the carrier with the sample just above the surface.This is done in a closed container， the atmosphere of which is saturated with vapors of the carrier to avoid changes in composition caused by evaporation.When the carrier has moved up the plate， the chromatography is dried and given some treatment to render the fractionated sample visible.

Sample movement on TLC can be quantified by comparing the distance moved by the compound with that moved by the solvent front.This is expressed by the parameter RF， which is the ratio of the distance of sample to solvent movement.RF values are usually not very reproducible，so identification is most effectively made by chromatographying a valid sample on the same plate with the unknown.

A variation of this technique can be used to fractionate mixtures in sufficient amount to permit recovery of the fractions.This is called preparative TLC or sometimes thick-layer chromatography.To accommodate larger sample sizes， the thickness of the bed must be increased to 1 mm or larger.Devices can be purchased to use in casting carefully controlled layers of slurry on plates.

薄层板的制备用英文描述如下：

For application， the sample is usually dissolved in a volatile solvent so that the initial area can be effectively minimized by introduction of small increments with evaporation of the solvent after each addition.This is the analog of on-column loading in column chromatography.The plate is elute or developed by immersing it in the carrier with the sample just above the surface.This is done in a closed container， the atmosphere of which is saturated with vapors of the carrier to avoid changes in composition caused by evaporation.When the carrier has moved up the plate， the chromatography is dried and given some treatment to render the fractionated sample visible.

A simple alternative consists in placing several layers of masking tape along the edges of a plate.The slurry is poured on the plate and spread to the thickness defined by the tape with the aid of a rod.Sample is introduced in a thin， straight line along one edge of the plate， and it is developed in the same fashion as for small plates.The developed plate can be sectioned and the fractions extracted from the adsorbent material.As is generally true of chromatographic procedures， scaling small experiments upwards to the preparative scale requires care to avoid alternation of conditions that may reduce efficiency.