📚 IGCSE AQA Chemistry: Chromatography Exam Revision | IGCSE AQA 化学：色谱 考点精讲
Chromatography is a powerful separation technique used to analyse and identify substances in a mixture. In IGCSE AQA Chemistry, you must understand the principles behind paper and thin-layer chromatography, be able to interpret chromatograms, calculate Rf values, and evaluate the purity of a sample. This article covers all essential concepts, practical steps, key equations, and common exam pitfalls to help you secure full marks on chromatography questions.
色谱是一种强大的分离技术，用于分析和鉴定混合物中的成分。在 IGCSE AQA 化学中，你必须理解纸色谱和薄层色谱的原理，能够解释色谱图，计算 Rf 值，并判断样品的纯度。本文涵盖所有核心概念、实验步骤、关键公式以及常见考试失误，帮助你在色谱相关题目中稳拿满分。
1. What Is Chromatography? | 什么是色谱法？
Chromatography is a physical method of separation that distributes components of a mixture between two phases: a stationary phase and a mobile phase. The mobile phase moves through the stationary phase, carrying the sample with it. Different substances travel at different rates because they have different affinities for the two phases.
色谱是一种物理分离方法，它将混合物中的组分分布在两相之间：固定相和流动相。流动相穿过固定相，带着样品一起移动。不同物质因对两相亲和力不同，而具有不同的移动速率，从而实现分离。
The name comes from the Greek words 'chroma' (colour) and 'graphein' (to write), as early applications separated coloured plant pigments. Today, the technique works equally well for colourless substances, often detected using UV light or chemical locating agents.
"色谱"一词来源于希腊语中的"颜色"和"书写"，因为早期的应用是分离有色植物色素。如今，该技术同样适用于无色物质，通常用紫外光或化学显色剂进行检测。
2. The Two Phases in Chromatography | 色谱的两相体系
Every chromatographic system has a stationary phase and a mobile phase. In paper chromatography, the stationary phase is the water molecules bound to the cellulose fibres of the paper. In thin-layer chromatography (TLC), the stationary phase is a thin layer of an adsorbent such as silica gel or alumina coated on a glass or plastic plate.
每个色谱系统都有固定相和流动相。在纸色谱中，固定相是吸附在纸张纤维素上的水分子。在薄层色谱（TLC）中，固定相是涂覆在玻璃或塑料板上的薄层吸附剂，如硅胶或氧化铝。
The mobile phase is a liquid solvent (or mixture of solvents) that travels up the paper or plate by capillary action. The choice of solvent, or eluent, determines how well components separate. A more polar solvent will carry polar substances further if the stationary phase is non-polar, and vice versa.
流动相是一种液体溶剂（或混合溶剂），通过毛细作用沿纸或板向上移动。溶剂（洗脱液）的选择决定了组分的分离效果。如果固定相是非极性的，极性较强的溶剂会将极性物质带得更远，反之亦然。
- Stationary phase: paper (cellulose) in paper chromatography; silica gel or Al₂O₃ in TLC.
- 固定相：纸色谱中为纸（纤维素）；TLC中为硅胶或 Al₂O₃。
- Mobile phase: a suitable solvent, e.g. water, ethanol, or a mixed solvent system.
- 流动相：合适的溶剂，如水、乙醇或混合溶剂体系。
- Separation depends on the relative attraction of each component for the two phases.
- 分离取决于每个组分对两相的相对吸引力。
3. Paper Chromatography: Step by Step | 纸色谱法：逐步操作
Accurate practical technique is essential for a valid chromatogram. Follow these steps for a typical IGCSE paper chromatography experiment:
正确的实验操作是获得有效色谱图的关键。典型 IGCSE 纸色谱实验步骤如下：
- Draw a pencil line (baseline) about 1–2 cm from the bottom edge of the chromatography paper. Never use ink because ink itself is a mixture and will separate with the solvent, contaminating the baseline.
- 在色谱纸底边上方约1–2厘米处画一条铅笔线（基线）。切勿使用墨水笔，因为墨水本身就是混合物，会随溶剂分离，污染基线。
- Using a fine capillary tube, place a small spot of each sample on the pencil line. Keep spots small and concentrated to avoid smearing and overlapping.
- 用细毛细管在铅笔线上点上每种样品的微小斑点。斑点要小而浓，避免扩散和重叠。
- Pour a suitable solvent into a chromatography chamber (beaker) to a depth no higher than the pencil line. The baseline must remain above the solvent surface so the spots do not dissolve into the solvent reservoir.
- 向色谱缸（烧杯）中倒入适量溶剂，液面不得高于铅笔线。基线必须保持在溶剂液面之上，以免斑点直接溶入溶剂池。
- Suspend the paper so its bottom edge dips into the solvent. Cover the chamber with a lid to ensure the atmosphere is saturated with solvent vapour.
- 悬挂纸张，使其底边浸入溶剂中。盖上缸盖，确保缸内充满溶剂蒸气。
- Allow the solvent to rise up the paper by capillary action. Remove the paper when the solvent front has moved near the top edge.
- 让溶剂通过毛细作用沿纸上升。当溶剂前沿接近顶端时取出纸张。
- Immediately mark the solvent front with a pencil, then let the paper dry.
- 立即用铅笔标出溶剂前沿，然后晾干纸张。
- If the spots are colourless, view under UV light or spray with a locating agent (e.g. ninhydrin for amino acids) to make them visible.
- 若斑点为无色，需在紫外光下观察或喷洒显色剂（如茚三酮用于氨基酸）使之显色。
This process yields a chromatogram—a visual record of the separated components.
这一过程产生色谱图——分离组分的可视化记录。
4. Thin-Layer Chromatography (TLC) | 薄层色谱法
Thin-layer chromatography uses a glass, plastic, or aluminium sheet coated with a thin layer of adsorbent, usually silica gel (SiO₂) or aluminium oxide (Al₂O₃). TLC generally gives faster runs and better resolution than paper chromatography.
薄层色谱使用涂有薄层吸附剂（通常为硅胶 SiO₂ 或氧化铝 Al₂O₃）的玻璃、塑料或铝板。TLC 通常比纸色谱运行更快，分辨率更高。
The stationary phase is polar because silica gel has –OH groups on its surface. A polar mobile phase will compete with the sample components for adsorption sites, carrying polar substances further. In non-polar solvents, polar substances adhere more strongly to the stationary phase and move less.
固定相是极性的，因为硅胶表面带有 –OH 基团。极性的流动相会与样品组分竞争吸附位点，将极性物质带得更远。在非极性溶剂中，极性物质更牢固地吸附在固定相上，移动距离较短。
Key points for IGCSE:
IGCSE 要点：
- TLC plates are handled carefully to avoid contaminating the adsorbent layer.
- TLC 板需小心拿取，避免污染吸附层。
- Spots are applied in the same way as paper chromatography.
- 点样方式与纸色谱相同。
- The plate is placed in a developing chamber with the solvent; the chamber is covered.
- 将薄层板放入盛有溶剂的展开缸中，盖上盖子。
- After development, the solvent front is marked and the plate dried before visualising.
- 展开后标记溶剂前沿，晾干薄层板，然后显色。
5. Rf Values: Calculation and Meaning | Rf 值的计算与含义
The retention factor (Rf) is a ratio used to identify substances under identical chromatographic conditions. It is calculated using the formula:
保留因子（Rf）是一个比值，用于在相同色谱条件下鉴定物质。计算公式为：
Rf = distance moved by substance ÷ distance moved by solvent front
Rf = 物质移动距离 ÷ 溶剂前沿移动距离
- Both distances are measured from the pencil baseline to the centre of the spot and to the solvent front, respectively.
- 两个距离均从铅笔基线量起，分别量至斑点中心及溶剂前沿。
- Rf values have no units and are always less than 1 (since the substance cannot travel further than the solvent front).
- Rf 值无单位，且始终小于 1（因为物质移动距离不可能超过溶剂前沿）。
- Rf is often quoted to two decimal places (e.g. 0.67).
- Rf 值通常保留两位小数（如 0.67）。
- A substance has a fixed Rf value under constant conditions (same stationary phase, mobile phase, temperature).
- 在恒定条件下（相同固定相、流动相、温度），一种物质的 Rf 值固定不变。
Example calculation:
示例计算：
ce moved by spot = 4.5 cm, distance moved by solvent = 6.0 cm. Then Rf = 4.5 ÷ 6.0 = 0.75.
斑点移动距离 = 4.5 cm，溶剂移动距离 = 6.0 cm。则 Rf = 4.5 ÷ 6.0 = 0.75。
6. Interpreting Chromatograms | 解析色谱图
A chromatogram tells you how many components are in a mixture and allows tentative identification by comparing Rf values with known standards run on the same chromatogram.
色谱图能告诉你混合物中含有多少种组分，并可通过与在同一图上运行的标准物比对 Rf 值进行初步鉴定。
If a sample produces only one spot on the chromatogram, it is likely pure. However, a single spot does not guarantee purity—another substance with the same Rf in that particular system would appear at the same location. To confirm purity, you must run the sample in at least two different solvent systems. If it consistently gives a single spot, it is considered pure.
如果样品在色谱图上只产生一个斑点，则它很可能是纯物质。但单个斑点并不能保证纯度——在该特定体系中具有相同 Rf 值的另一种物质也会出现在同一位置。要确认纯度，你必须在至少两种不同溶剂体系中运行样品。如果始终只显示一个斑点，才可认为它是纯的。
Mixtures separate into multiple spots. The number of spots equals the minimum number of components present (some may overlap). By comparing spot positions and colours, you can identify components against known reference substances.
混合物则会分离出多个斑点。斑点数量等于混合物中至少含有的组分数（某些组分可能重叠）。通过比较斑点的位置和颜色，对照已知参照物即可鉴定组分。
7. Factors Affecting Separation | 影响分离的因素
Several factors influence how well components separate and how distinct the spots appear:
以下几个因素会影响组分的分离程度和斑点的清晰度：
- Solvent polarity: A more polar mobile phase will move polar substances further. Adjusting solvent polarity can improve separation.
- 溶剂极性：极性越强的流动相会将极性物质带得越远。调整溶剂极性可改善分离效果。
- Stationary phase: The nature of the stationary phase (adsorbent activity, water content in paper) affects retention.
- 固定相：固定相的性质（吸附剂活性、纸的水含量）影响保留行为。
- Temperature: Higher temperatures increase solvent evaporation and may speed up the run but also alter Rf values. The chromatography chamber should be kept at a constant temperature where possible.
- 温度：温度升高加剧溶剂蒸发，可能加快运行，但也会改变 Rf 值。色谱缸应尽量保持恒温。
- Spot size: Overly large spots cause streaking and poor resolution. Spots should be as small and symmetrical as possible.
- 斑点大小：斑点过大会导致拖尾和分辨率差。斑点应尽可能小且对称。
- Chamber saturation: A lid maintains a saturated atmosphere, preventing the solvent from evaporating unevenly. An unsaturated chamber can cause the solvent front to travel unevenly.
- 缸内饱和度：盖子保持饱和蒸气，防止溶剂蒸发不均。非饱和的展开缸会导致溶剂前沿移动不均匀。
8. Choosing the Appropriate Solvent | 选择合适的溶剂
Chromatography often requires selecting a solvent that best separates the components of interest. If the solvent is too strong, all components will move close to the solvent front, giving high Rf values with little separation. If the solvent is too weak, the components will remain near the baseline.
色谱通常需要选择能最佳分离目标组分的溶剂。若溶剂太强，所有组分都会移动至溶剂前沿附近，Rf值偏高，分离效果差。若溶剂太弱，组分则滞留于基线附近。
A good solvent produces well-spaced spots with Rf values between about 0.2 and 0.8. Sometimes a mixture of solvents, such as ethanol and water in different proportions, is used to tune the polarity.
合适的溶剂应产生分布均匀的斑点，Rf 值大约在 0.2 至 0.8 之间。有时会使用混合溶剂，如不同比例的乙醇和水，来调节极性。
Water is a polar solvent; ethanol and propanone (acetone) are less polar; hexane is non-polar. For food dyes, water or water–ethanol mixtures are common. For leaf pigments, a mixture of propanone and petroleum ether works well.
水是极性溶剂；乙醇和丙酮极性较弱；己烷是非极性溶剂。对于食用色素，水或水-乙醇混合物常用。对于叶绿素，丙酮与石油醚的混合物效果好。
9. Advantages and Limitations of Chromatography | 色谱的优点与局限性
Chromatography is widely used because it is simple, rapid, requires very small amounts of sample, and can separate complex mixtures. However, it also has limitations that exam questions often ask you to evaluate.
色谱因操作简便、快速、所需样品量极少且能分离复杂混合物而被广泛应用。但它也有局限性，考试常要求你进行评价。
Advantages / 优点：
- Minimal sample required (a few microlitres) – non-destructive for paper chromatography if spots are not chemically altered.
- 所需样品极少（几微升）——若斑点未经化学显色处理，纸色谱为非破坏性检测。
- Simple apparatus and inexpensive.
- 仪器简单，成本低廉。
- Ability to separate very similar compounds (e.g. amino acids, dyes).
- 能分离极其相似的化合物（如氨基酸、染料）。
Limitations / 局限：
- Qualitative or semi-quantitative only; it does not give the exact mass of each component without additional detection.
- 仅为定性或半定量分析；若无额外检测手段，无法精确给出各组分质量。
- Substances with very similar Rf values may not separate fully.
- Rf 值非常接近的物质可能无法完全分离。
- It cannot handle very volatile or thermally unstable compounds well unless special techniques are used.
- 除非使用特殊技术，否则难以处理极易挥发或热不稳定的化合物。
- Identification is not definitive—coincident Rf values can occur.
- 鉴定不是确定的——可能出现 Rf 值巧合的情况。
10. Practical Applications of Chromatography | 色谱的实际应用
IGCSE AQA Chemistry highlights several real-world uses. You should be able to link the principle of chromatography to these contexts:
IGCSE AQA 化学强调若干实际应用。你应能将色谱原理与以下情境联系起来：
- Food colourings: Analysing artificial dyes in sweets and drinks to check compliance with regulations.
- 食用色素：分析糖果和饮料中的人工合成色素，检查是否符合法规。
- Forensic science: Analysing ink samples from forged documents; comparing dyes in fibres at a crime scene.
- 法医学：分析伪造文件上的墨水样本；比对犯罪现场的纤维染料。
- Pharmaceuticals: Purity testing of drug compounds; monitoring reaction progress.
- 制药：药物化合物的纯度检测；监测反应进程。
- Amino acid analysis: Separating and identifying amino acids in protein hydrolysates.
- 氨基酸分析：分离并鉴定蛋白质水解产物中的氨基酸。
- Environmental monitoring: Detecting pollutants in water and soil samples.
- 环境监测：检测水和土壤样品中的污染物。
11. Required Practical: Chromatography of Inks or Food Dyes | 必做实验：墨水或食用色素的色谱分离
The AQA IGCSE required practical involves separating coloured substances. You must know the apparatus, method, safety precautions, and expected outcome in detail.
AQA IGCSE 必做实验涉及分离有色物质。你必须详细了解仪器、方法、安全措施及预期结果。
Typical setup / 典型装置：
| Apparatus / 仪器 | Purpose / 用途 |
| Chromatography paper / 色谱纸 | Stationary phase / 固定相 |
| Capillary tube / 毛细管 | Applying tiny, concentrated spots / 点上微小浓缩斑点 |
| Beaker with lid / 带盖烧杯 | Chromatography chamber / 色谱缸 |
| Solvent (e.g. water) / 溶剂（如水） | Mobile phase / 流动相 |
| Pencil and ruler / 铅笔和尺 | Marking baseline and measuring / 画基线并测量 |
Safety / 安全：
- Wear eye protection. Many organic solvents are flammable and may be harmful by inhalation.
- 佩戴护目镜。许多有机溶剂易燃，吸入可能有害。
- Work in a well-ventilated laboratory. Keep solvents away from open flames.
- 在通风良好的实验室操作。溶剂远离明火。
- Avoid skin contact with solvent and locating agents.
- 避免皮肤接触溶剂和显色剂。
12. Common Mistakes and Exam Tips | 常见错误与考试技巧
Many students lose marks on chromatography questions due to simple errors. Avoid these pitfalls:
许多学生在色谱题目上因小错误而失分。避开以下陷阱：
- Using a pen instead of a pencil for the baseline: This is a classic error. Ink dissolves in the solvent and produces extra spots.
- 用钢笔而非铅笔画基线：这是经典错误。墨水溶于溶剂，产生额外斑点，无法复原。
- Letting the spots sit below the solvent level: The sample would wash off the paper into the solvent reservoir, producing no chromatogram.
- 让斑点浸没在溶剂液面之下：样品会被洗脱进入溶剂池，得不到色谱图。
- Making the spots too large: This causes overlapping and tailing, making Rf measurement inaccurate.
- 斑点过大：导致重叠和拖尾，Rf 值测量不准。
- Not covering the beaker: Solvent evaporates from the paper surface, causing uneven solvent front movement.
- 未覆盖烧杯：溶剂从纸面蒸发，导致溶剂前沿移动不均。
- Measuring distance from the spot's edge: Always measure to the centre of the spot.
- 从斑点边缘开始测量距离：务必量至斑点中心。
- Confusing Rf ratio order: Rf = substance ÷ solvent, not the reverse.
- 弄反 Rf 比值次序：Rf = 物质 ÷ 溶剂，切勿颠倒。
- Claiming a substance is pure based on one spot in one solvent: You need evidence from two different solvents.
- 凭一种溶剂中得到一个斑点就声称物质纯净：你需要两种不同溶剂中的证据。
When asked to explain why a substance has a higher Rf value, link your answer directly to the relative attraction for the stationary and mobile phases.
当要求解释为何某种物质 Rf 值较高时，答案应直接联系到该物质对固定相和流动相的相对吸引力。
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