Chromatography | 色谱考点精讲

📚 Chromatography | 色谱考点精讲

Chromatography is a powerful technique used to separate, identify and analyse the components within a mixture. It is widely used in forensic science, food testing and pharmaceutical research. Understanding the underlying principles and being able to interpret chromatograms is an essential skill for IGCSE Chemistry, particularly for the CCEA specification where practical application and calculations are frequently examined.

色谱是一项强大的技术，能对混合物中的成分进行分离、鉴定与分析。它广泛应用于法医学、食品检测及药物研发。理解其基本原理并能够解读色谱图是 IGCSE 化学的一项关键技能，尤其在 CCEA 考试中，常会考察实际应用及相关计算。

1. What is Chromatography? | 什么是色谱？

Chromatography is a physical separation method that distributes the components of a mixture between two phases: a stationary phase and a mobile phase. The separation occurs because different substances travel at different rates through the stationary phase when carried by the mobile phase.

色谱是一种物理分离方法，它将混合物的组分分布在两相之间：固定相和流动相。分离得以发生，是因为不同物质在被流动相携带通过固定相时，移动的速率各异。

In paper chromatography, which is the most common type studied at IGCSE level, the stationary phase is a special type of absorbent paper, and the mobile phase is a liquid solvent or mixture of solvents. As the solvent soaks up the paper, it carries the components of the mixture with it, causing them to separate out into distinct spots.

在 IGCSE 阶段最常见的是纸色谱，其固定相是一种特殊的吸收纸，流动相则是液体溶剂或混合溶剂。随着溶剂沿纸上升，它会带着混合物的组分一起移动，使它们分离成一个个清晰的斑点。

2. Mobile Phase and Stationary Phase | 流动相与固定相

All chromatographic techniques depend on a dynamic equilibrium between a mobile phase and a stationary phase. The mobile phase is the phase that moves, such as a liquid solvent or a carrier gas. The stationary phase is a solid, or a liquid supported on a solid, that does not move.

所有的色谱技术都依赖于流动相和固定相之间的动态平衡。流动相是移动的一相，比如液体溶剂或载气。固定相则是保持不动的固体，或是附着于固体上的液体。

In paper chromatography, the cellulose fibres of the paper act as the stationary phase. Very often, water molecules bound to the cellulose also participate in the separation process. The mobile phase is a carefully selected solvent that can dissolve the mixture and travel up the paper by capillary action.

在纸色谱中，纸张的纤维素纤维充当固定相。通常，与纤维素结合的水分子也会参与分离过程。流动相则是经过精心选择的溶剂，它能溶解混合物并通过毛细作用沿纸上行。

3. Setting Up Paper Chromatography | 搭建纸色谱装置

A pencil line (the baseline) is drawn near the bottom of the chromatography paper. The mixture is spotted onto this line using a fine capillary tube. The spot must be small and concentrated to avoid spreading. The pencil line is essential because pencil graphite is insoluble and will not interfere with the separation.

在色谱纸的底端附近画一条铅笔线（基线）。用细毛细管将混合物点在基线上。斑点必须小而浓，以防止扩散。使用铅笔线至关重要，因为铅笔石墨不溶于溶剂，不会干扰分离过程。

After the spot has dried, the paper is placed upright in a chromatography tank containing a shallow layer of solvent. The baseline must be above the level of the solvent to prevent the spot from dissolving directly into the bulk solvent. The tank is covered with a lid to maintain a saturated atmosphere of solvent vapour.

待斑点干燥后，将纸垂直放入盛有薄层溶剂的色谱缸中。基线须在溶剂液面之上，以防斑点直接溶解到大块溶剂里。色谱缸需盖上盖子，以维持溶剂蒸气的饱和氛围。

4. Running the Chromatogram | 展开色谱图

As the solvent front rises up the paper by capillary action, it dissolves the components of the mixture at the baseline. Each component partitions itself between the mobile solvent phase and the stationary paper phase. Substances that have a stronger affinity for the mobile phase travel further up the paper, while those with a stronger affinity for the stationary phase move more slowly.

当溶剂前沿通过毛细作用沿纸上行时，它会溶解基线处的混合物组分。每一个组分都会在流动的溶剂相和固定的纸相之间进行分配。对流动相亲和力较强的物质随纸上升得更远，而对固定相亲和力较强的物质则移动较慢。

Once the solvent front has moved an appropriate distance, the paper is removed and the position of the solvent front is immediately marked with a pencil. The paper is then dried. If the separated components are coloured, they will be visible as spots above the baseline. If they are colourless, a locating agent is required.

一旦溶剂前沿上升至适当的高度，便将纸取出，并立即用铅笔标出溶剂前沿的位置。接着将纸干燥。如果分离出的组分带有颜色，它们就会在基线上方以斑点形式显现。若组分无色，则需使用显色剂。

5. Using Locating Agents | 使用显色剂

Many organic compounds, such as amino acids and sugars, are colourless after separation. A locating agent is used to make these spots visible. Common locating agents include ninhydrin (which reacts with amino acids to produce purple spots) and iodine vapour (which can be absorbed by many organic substances to give brownish spots).

许多有机化合物，如氨基酸和糖类，在分离后是无色的。需用显色剂使这些斑点可见。常见的显色剂有茚三酮（与氨基酸反应产生紫色斑点）和碘蒸气（可被许多有机物吸收而显现出棕色斑点）。

The dried chromatogram is sprayed with or dipped in the locating agent and then heated gently in an oven. Under ultraviolet (UV) light, some substances fluoresce and can also be located without chemical treatment. In all cases, the spots should be circled carefully with a pencil to preserve a permanent record.

将干燥的色谱图喷洒或浸入显色剂中，然后在烘箱中温和加热。在紫外灯下，某些物质会发出荧光，无需化学处理即可定位。无论哪种方式，都需用铅笔小心圈出斑点，以便永久记录。

6. Calculating Rf Values | 计算 Rf 值

The retention factor, or Rf value, is a numerical measure of how far a substance travels relative to the solvent front. It is unique for a given compound under specific conditions (same stationary phase, same solvent, constant temperature).

比移值，即 Rf 值，是衡量物质相对于溶剂前沿移动距离的数值指标。在特定条件下（相同的固定相、相同的溶剂、恒定的温度），某一化合物的 Rf 值是固定的。

Rf = distance travelled by substance / distance travelled by solvent front

Rf = 物质移动距离 / 溶剂前沿移动距离

Both distances are measured from the origin (the baseline) to the centre of the spot and to the solvent front, respectively. Rf is a ratio and has no units; its value is always less than 1. For example, if a red dye travels 2.8 cm and the solvent front travels 5.6 cm, its Rf = 2.8 ÷ 5.6 = 0.50.

两段距离都是从原点（基线）分别量至斑点中心和溶剂前沿。Rf 是一个比值，没有单位，其数值总是小于 1。例如，若一红色染料移动 2.8 cm，溶剂前沿移动 5.6 cm，则其 Rf = 2.8 ÷ 5.6 = 0.50。

7. Interpreting Chromatograms | 解读色谱图

A chromatogram shows the number of components in a mixture: a pure substance produces only one spot, whereas a mixture produces multiple spots. By comparing the Rf values of unknown spots with those of known reference samples run on the same chromatogram, the identity of a substance can be confirmed.

色谱图可显示混合物中的组分数目：纯净物仅产生一个斑点，而混合物则会产生多个斑点。将未知斑点的 Rf 值与同一色谱图上已知参照样品的 Rf 值进行比较，便可确认该物质的身份。

In theory, if two spots have the same Rf value under identical conditions, they are likely to be the same substance. However, for unambiguous identification, the sample should be run in at least two different solvents. If the Rf values match in both systems, the conclusion is much more reliable.

理论上，在相同条件下，若两个斑点的 Rf 值相同，它们很可能是同一种物质。但为了确凿鉴定，样品至少应在两种不同的溶剂中展开。若在两种溶剂体系下 Rf 值均匹配，结论将更为可靠。

Sometimes, a two-way chromatography technique is used to separate complex mixtures. After the first run, the paper is dried, turned by 90 degrees and run again in a second solvent. This provides a much clearer separation for substances that have very similar Rf values in the first solvent.

有时会使用双向色谱技术来分离复杂的混合物。第一次展开后，将纸干燥，旋转 90 度，再在第二种溶剂中展开。这样，对于在第一种溶剂中 Rf 值非常相近的物质，能实现更清晰的分离。

8. Factors Affecting Rf Values | 影响 Rf 值的因素

The Rf value is not an absolute physical constant; it is affected by several experimental factors. The composition of the mobile phase, the type of paper used, the temperature and the degree of saturation of the tank with solvent vapour can all cause slight variations.

Rf 值并非绝对的物理常数，它受若干实验因素影响。流动相的组成、所用纸张的类型、温度以及色谱缸内溶剂蒸气的饱和程度，都会引起细微变化。

For this reason, when reporting an Rf value, the exact conditions must be stated. In the laboratory, it is common practice to run known standards alongside the unknown mixture on the same paper, so that the Rf comparison is done under identical conditions, eliminating most sources of error.

因此，在报告 Rf 值时，必须注明具体条件。在实验室中，通常的做法是在同一张纸上与未知混合物并排点加已知标准品，这样 Rf 的比较就能在相同条件下进行，从而消除大部分误差来源。

9. Applications of Chromatography | 色谱的应用

Chromatography is indispensable in forensic science for analysing ink samples from forged documents and identifying drugs or dyes. In the food industry, it is used to check for artificial colours and to monitor the purity of food additives.

色谱在法医学中不可或缺，用于分析伪造文件上的墨水样本以及鉴定毒品或染料。在食品工业中，它被用来检测人工色素并监控食品添加剂的纯度。

Environmental scientists use chromatography to detect pollutants in water and air, while in the pharmaceutical industry it ensures that medicines contain the correct ingredients and are free from harmful impurities. Even in sports, chromatography is used for drug screening in athletes.

环境科学家用色谱检测水和空气中的污染物，制药工业则依靠它确保药物含有正确的成分且不含有害杂质。甚至在体育领域，色谱也被用于运动员的药物筛查。

10. Purity and Chromatography | 纯度与色谱

Chromatography provides a simple and effective way to check the purity of a sample. A pure compound will always yield a single, well-defined spot. If the substance is impure, additional spots will appear, each representing a different contaminant.

色谱为检验样品纯度提供了一种简单而有效的方法。纯净的化合物总会呈现一个清晰单一的斑点。如果物质不纯，则会出现额外的斑点，每个斑点代表一种不同的杂质。

The absence of extra spots, however, does not guarantee absolute purity, as some impurities may not be detectable under the conditions used. Nevertheless, when combined with the determination of a substance’s melting or boiling point, chromatography forms a cornerstone of purity analysis in CCEA practical examinations.

不过，没有额外斑点并不能保证绝对纯净，因为在所用条件下某些杂质可能无法检出。尽管如此，结合物质熔点或沸点的测定，色谱构成了 CCEA 实验考试中纯度分析的基石。

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Chromatography | 色谱 考点精讲