📚 Chromatography: Key Concepts | 色谱法考点精讲
Chromatography is an essential experimental technique in IGCSE WJEC Chemistry, used to separate and identify components in a mixture. Mastering its principles, practical setup, Rf value calculations, and applications will not only help you excel in the exam but also deepen your understanding of purity, molecular interactions, and analytical chemistry. This article covers every key point you need for the syllabus, with clear explanations and helpful tips.
色谱法是IGCSE WJEC化学中一项关键的实验技术，用于分离和鉴定混合物中的各组分。掌握其原理、实验装置、Rf值计算以及实际应用，不仅能帮助你在考试中脱颖而出，还能加深你对纯度、分子相互作用以及分析化学的理解。本文全面覆盖大纲要求的每个核心考点，并提供清晰的解释和实用技巧。
1. What is Chromatography? | 色谱法是什么？
Chromatography is a physical separation technique that distributes components of a mixture between two phases: a stationary phase and a mobile phase. The components move at different rates, allowing them to be separated. In IGCSE chemistry, the most common types are paper chromatography and thin‑layer chromatography (TLC).
色谱法是一种物理分离技术，它将混合物中的各组分分配在固定相与流动相两相之间。不同组分移动的速率不同，因此得以分离。在IGCSE化学中，最常见的类型是纸色谱法和薄层色谱法（TLC）。
The name “chromatography” comes from the Greek words for colour writing, because it was first used to separate coloured plant pigments. Today it is used for colourless substances as well, by applying locating agents.
“色谱法”一词源于希腊语的“颜色书写”，因为它最初被用来分离有色植物色素。如今，通过使用定位剂，它也可用于无色物质。
2. The Principle of Separation | 分离原理
Separation relies on the relative affinity of each component for the stationary phase and the mobile phase. A component that adsorbs more strongly onto the stationary phase will move more slowly, while a component that dissolves better in the mobile phase will travel further up the chromatogram.
分离基于各组分对固定相和流动相相对亲和力的差异。对固定相吸附力更强的组分移动得较慢，而在流动相中溶解度更高的组分则在色谱图上迁移得更远。
The balance between adsorption (attraction to the stationary phase) and solubility (attraction to the mobile phase) determines the distance travelled. This dynamic equilibrium is the heart of all chromatographic techniques.
吸附（对固定相的吸引）与溶解（对流动相的吸引）之间的平衡决定了迁移距离。这种动态平衡是所有色谱技术的核心。
3. Stationary and Mobile Phases | 固定相与流动相
In paper chromatography, the stationary phase is the water molecules adsorbed onto the paper’s cellulose fibres. The paper itself is not the stationary phase; it is the support. The mobile phase is a liquid solvent, such as water or ethanol, which moves up the paper by capillary action.
在纸色谱中，固定相是吸附在纸纤维素纤维上的水分子。纸本身不是固定相，它只是载体。流动相是一种液体溶剂，如水或乙醇，它通过毛细作用沿纸上行。
For thin‑layer chromatography, the stationary phase is usually a thin layer of silica gel (SiO₂) or alumina (Al₂O₃) coated on a glass or plastic plate. The mobile phase is again a suitable liquid solvent. A non‑polar stationary phase can be used for reverse‑phase chromatography, but at IGCSE level we focus on normal‑phase: a polar stationary phase with a liquid mobile phase.
对于薄层色谱，固定相通常是涂覆在玻璃板或塑料板上的薄层硅胶（SiO₂）或氧化铝（Al₂O₃）。流动相同样是一种合适的液体溶剂。虽然反相色谱会使用非极性固定相，但IGCSE阶段我们重点关注正相色谱：极性固定相配以液体流动相。
4. Paper Chromatography Experiment | 纸色谱实验
A typical experiment starts by drawing a pencil base line about 2 cm from the bottom of the chromatography paper. Never use ink for this line, as ink would also separate and contaminate the results. Small spots of the sample mixture and reference known substances are placed on the line using a capillary tube.
典型的实验始于在色谱纸底端约2 cm处用铅笔画一条基线。切勿用墨水画此线，因为墨水本身也会分离并污染结果。用毛细管将少量待测混合样和已知对照物的斑点加在基线上。
The paper is then placed in a beaker containing a small volume of solvent, ensuring the spots are above the solvent level. The beaker is covered to maintain a saturated atmosphere and to prevent evaporation. As the solvent rises, it carries the components upwards; when the solvent front approaches the top, the paper is removed and the solvent front is marked immediately.
然后将纸放入装有少量溶剂的烧杯中，确保斑点位于溶剂液面之上。用盖子盖好烧杯，以保持饱和气体氛围并防止蒸发。随着溶剂上行，它将组分带向上方；当溶剂前沿接近上端时，取出纸张并立即标记溶剂前沿的位置。
5. Interpreting a Chromatogram | 解读色谱图
A chromatogram is the visual result after the separation. Each separated component appears as a spot at a certain height. If the sample is pure, you will see only one spot. Two or more spots indicate a mixture. The identity of a substance can be tentatively confirmed by comparing its spot’s position with that of a known reference run under identical conditions.
色谱图是分离后的可视结果。每个分离出的组分表现某个高度上的一个斑点。如果样品是纯净物，只会出现一个斑点。出现两个或多个斑点则表明是混合物。通过将某物质斑点的位置与相同条件下运行的已知对照物进行比较，可以初步确认其身份。
Colourless spots can be visualised under a UV lamp if they fluoresce, or by spraying with a locating agent (e.g., ninhydrin for amino acids) that reacts to produce coloured compounds.
无色斑点若具有荧光性，可置于紫外灯下观察；或通过喷洒定位剂（如氨基酸可喷洒茚三酮），使其发生显色反应，从而生成有色化合物。
6. Calculating Rf Values | 计算Rf值
The retention factor (Rf) is a dimensionless number used to compare substances. It is calculated as:
保留因子（Rf）是一个无量纲的数值，用于比较不同物质。其计算公式如下：
Rf = distance travelled by the substance ÷ distance travelled by the solvent front
Both distances are measured from the pencil base line to the centre of the spot and to the solvent front respectively. Rf is always less than 1 and has no units. Under the same conditions (stationary phase, mobile phase, temperature), a given compound always has the same Rf value.
两个距离均从铅笔基线量起，分别到斑点中心和溶剂前沿。Rf 总是小于1且无单位。在相同条件下（固定相、流动相、温度），特定化合物的Rf值保持不变。
7. Factors Affecting Rf | 影响Rf的因素
Several factors can alter Rf values: the nature of the stationary phase, the polarity of the solvent, temperature, and the saturation of the developing chamber. A more polar solvent tends to move polar substances further, increasing their Rf in normal‑phase chromatography.
多种因素会改变Rf值：固定相的性质、溶剂的极性、温度以及展开槽的饱和程度。在正相色谱中，极性更强的溶剂会推动极性物质迁移得更远，从而增大其Rf值。
Conversely, a change in the stationary phase—such as using silica gel instead of paper—affects adsorption strength and thus the distance travelled. This is why Rf values are only valid when the experimental conditions are kept constant.
相反，固定相的改变（比如使用硅胶代替纸）会影响吸附强度，从而改变迁移距离。这就是为什么只有在实验条件保持恒定时Rf值才有参考意义。
8. Locating Agents for Colourless Substances | 无色物质的定位剂
Many organic compounds do not show visible colour after separation. To detect them, you can use UV light (if the substance absorbs UV) or locating agents. Common locating agents include iodine vapour (forms brown complexes with unsaturated organics), ninhydrin (turns purple with amino acids), and 2,4‑dinitrophenylhydrazine (for aldehydes and ketones).
许多有机化合物在分离后不会显现可见颜色。为了检测它们，可以使用紫外灯（如果物质能吸收紫外光）或定位剂。常见的定位剂有碘蒸气（与不饱和有机物形成棕色络合物）、茚三酮（与氨基酸反应呈紫色）以及2,4-二硝基苯肼（针对醛和酮）。
After spraying or exposing to the locating agent, the spots must be circled with a pencil immediately, as some colours may fade. These spots are then used to calculate Rf values.
喷洒或暴露于定位剂后，必须立即用铅笔圈出斑点，因为有些颜色可能会消退。然后用这些斑点来计算Rf值。
9. Purity and Composition Analysis | 纯度与成分分析
Chromatography provides a straightforward purity test: a pure substance yields a single, well‑defined spot, whereas an impure sample produces multiple spots or a streak. The technique can also indicate the relative proportions of components in a mixture by comparing spot intensities or areas.
色谱法提供了一种简便的纯度检测方法：纯物质产生单一清晰的斑点，而不纯的样品则会产生多个斑点或拖尾。通过比较斑点的强度或面积，该技术还可指示混合物中各组分的相对含量。
For quantitative work, instruments such as gas chromatography are used, but at IGCSE level, paper and thin‑layer chromatography give qualitative and semi‑quantitative information.
在定量分析中会用到气相色谱等仪器，但在IGCSE阶段，纸色谱和薄层色谱主要提供定性和半定量信息。
10. Thin‑Layer Chromatography (TLC) | 薄层色谱
TLC operates on the same principle as paper chromatography but uses a glass or plastic plate coated with a thin adsorbent layer. It offers better resolution, requires smaller sample sizes, and gives faster separations. The plates can also withstand higher temperatures and aggressive solvents.
薄层色谱的原理与纸色谱相同，但使用涂有薄层吸附剂的玻璃板或塑料板。它具有更好的分离度，所需样品量更少，分离速度更快。此外，TLC板还能耐受较高温度和腐蚀性溶剂。
In the WJEC specification, you should be able to compare TLC with paper chromatography and recognise that both are used for separating small amounts of non‑volatile mixtures.
根据WJEC大纲，你应能比较TLC与纸色谱，并认识到两者都用于分离少量非挥发性混合物。
11. Applications of Chromatography | 色谱的应用
Chromatography is widely used in forensic science (ink analysis, drug testing), pharmaceutical industry (purity checking), food science (detecting additives and contaminants), and environmental monitoring (pesticide residues). It is an indispensable tool in quality control and research.
色谱法广泛应用于法医学（墨水分析、药物测试）、制药工业（纯度检测）、食品科学（检测添加剂和污染物）以及环境监测（农药残留）。它是质量控制和科学研究中不可或缺的工具。
In the laboratory, you may use chromatography to separate coloured inks from felt‑tip pens or to identify amino acids in a mixture. Understanding these real‑world connections helps you answer application‑based questions in the exam.
在实验室中，你可能会用色谱法分离荧光笔中的彩色墨水，或鉴定混合物中的氨基酸。理解这些实际联系有助于在考试中回答基于应用场景的问题。
12. Exam Tips and Common Pitfalls | 考试技巧与常见陷阱
Always remember to use a pencil for the base line, never ink. Keep spots small and concentrated to avoid overlapping. Ensure the solvent level is below the spots, otherwise they will dissolve into the solvent reservoir instead of travelling up the paper. When measuring distances, use a ruler with millimetre precision.
永远记住用铅笔画基线，而不是墨水。保持斑点小而浓，避免重叠。确保溶剂液面低于斑点，否则斑点会直接溶解到溶剂池中，而不会沿纸上行。测量距离时，使用毫米刻度的直尺。
Avoid common calculation errors: always measure from the base line to the centre of the spot, not to its edge. If a spot is smeared, take the average position. Rf values must be reported to two decimal places and always less than 1. Never give a unit.
避免常见计算错误：始终从基线量至斑点中心，而非边缘。若斑点拖尾，取平均位置。Rf值须报告至小数点后两位，且始终小于1。切勿添加单位。
When explaining separation, use the terms ‘greater solubility in the mobile phase’ and ‘stronger adsorption to the stationary phase’ rather than vague references to ‘affinity’. Linking your answers to molecular polarity often earns full marks.
在解释分离过程时，应使用“在流动相中溶解度更大”和“对固定相吸附更强”，而不要笼统地提“亲和力”。将答案与分子极性联系起来往往能拿到满分。
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