Chromatography | 色谱法

📚 Chromatography | 色谱法

Chromatography is a powerful analytical technique used to separate, identify, and purify components of a mixture. It is one of the core practical techniques in AQA A-Level Chemistry, appearing in the organic analysis section (3.16) and forming the basis for instrumental analysis in modern laboratories.

色谱法是一种强大的分析技术，用于分离、鉴定和纯化混合物中的各组分。它是 AQA A-Level 化学中的核心实验技术之一，出现在有机分析章节（3.16），是现代实验室仪器分析的基础。

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1. The Principles of Chromatography | 色谱法的基本原理

All chromatographic techniques rely on the same fundamental principle: a mixture is dissolved in a mobile phase, which is then carried through a stationary phase. Components of the mixture interact differently with the two phases, causing them to travel at different rates and thus become separated.

所有色谱技术都依赖于相同的基本原理：混合物溶解在流动相中，然后随流动相通过固定相。混合物中的各组分与两相之间的相互作用不同，导致它们以不同的速率移动，从而实现分离。

The two phases can be described as follows:

两相可以描述如下：

• Stationary phase | 固定相: The phase that does not move. It may be a solid (e.g., silica gel) or a liquid supported on a solid surface.
• Mobile phase | 流动相: The phase that moves. It may be a liquid (in TLC and column chromatography) or a gas (in gas chromatography).

The separation occurs because components with a stronger affinity for the stationary phase move more slowly, while those with a stronger affinity for the mobile phase move more quickly. This differential migration is the essence of chromatography.

分离的发生是因为对固定相亲和力较强的组分移动较慢，而对流动相亲和力较强的组分移动较快。这种差异性迁移是色谱法的本质。

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2. Thin-Layer Chromatography (TLC) | 薄层色谱法

Thin-layer chromatography is a simple, rapid, and inexpensive technique widely used in organic chemistry. A small spot of the sample solution is applied to a plate coated with a thin layer of adsorbent — typically silica gel (SiO₂) or alumina (Al₂O₃) — which acts as the stationary phase. The plate is then placed upright in a developing chamber containing a small volume of solvent (the mobile phase).

薄层色谱法是一种简单、快速且廉价的有机化学技术。将少量样品溶液点在涂有薄层吸附剂——通常是硅胶（SiO₂）或氧化铝（Al₂O₃）——的薄层板上，该吸附剂作为固定相。然后将薄层板直立放入盛有少量溶剂（流动相）的展开缸中。

The solvent rises up the plate by capillary action, carrying the sample components with it. Once the solvent front has travelled most of the way up the plate, the plate is removed, and the solvent front is marked. Components that are coloured can be seen directly; colourless components must be visualised using a UV lamp or by staining with a suitable reagent such as iodine vapour.

溶剂通过毛细作用沿薄层板上升，并携带样品组分一起移动。当溶剂前沿移动到薄层板的大部分位置时，取出薄层板并标记溶剂前沿。有色组分可以直接观察；无色组分则需要用紫外灯或合适的显色剂（如碘蒸气）进行显色。

Separation in TLC depends on the balance between solubility in the mobile phase and adsorption to the stationary phase | 薄层色谱的分离取决于组分在流动相中的溶解度与对固定相的吸附之间的平衡

Non-polar compounds have a greater affinity for the non-polar mobile phase and therefore travel further up the plate. Polar compounds adsorb more strongly to the polar silica stationary phase and thus travel less far.

非极性化合物对非极性流动相具有更强的亲和力，因此在薄层板上移动得更远。极性化合物对极性硅胶固定相吸附更强，因此移动的距离较短。

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3. Retention Factor (Rf) Values | 保留因子（Rf）值

The retention factor, denoted Rf, is a quantitative measure of how far a component travels relative to the solvent front. It is calculated using the following equation:

保留因子（记作 Rf）是衡量组分相对于溶剂前沿移动距离的定量指标，计算公式如下：

Rf = distance moved by the component ÷ distance moved by the solvent front

Rf = 组分移动的距离 ÷ 溶剂前沿移动的距离

Both distances are measured from the origin (the point where the sample was originally spotted). Rf values are always between 0 and 1. A component that does not move at all has an Rf of 0, while a component that travels exactly with the solvent front has an Rf of 1.

两个距离都从原点（样品最初点样位置）开始测量。Rf 值始终在 0 到 1 之间。完全不移动的组分 Rf 值为 0，而恰好随溶剂前沿移动的组分 Rf 值为 1。

Rf values can be used to identify substances by comparing them with reference standards run on the same plate under identical conditions. It is important to note that Rf values are affected by several factors, including the solvent system, the adsorbent, temperature, and plate thickness. Therefore, Rf values are only reliably comparable when obtained under identical experimental conditions.

Rf 值可以通过与在相同条件下在同一块板上运行的参考标准品进行比较来鉴定物质。需要注意的是，Rf 值受多种因素影响，包括溶剂系统、吸附剂、温度和薄层板厚度。因此，只有在相同实验条件下获得的 Rf 值才具有可靠的比较意义。

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4. The Application of TLC in Purity Analysis | 薄层色谱法在纯度分析中的应用

TLC is widely used to monitor the progress of organic reactions and to check the purity of products. If a compound is pure, it will produce only a single spot on the TLC plate. The presence of additional spots indicates impurities or unreacted starting materials.

薄层色谱法广泛用于监测有机反应的进程和检验产品的纯度。如果化合物是纯净的，在薄层板上只会产生一个斑点。出现额外斑点则表明存在杂质或未反应的反应物。

In the AQA practical assessment, students are often required to use TLC to confirm the identity of a product by comparing its Rf value with that of a known reference compound. When running a TLC plate, the following techniques are essential:

在 AQA 实验考核中，学生通常需要使用薄层色谱法，通过比较产物与已知参考化合物的 Rf 值来确认产物身份。在运行薄层板时，以下操作技术至关重要：

• The origin (baseline) must be drawn in pencil, not ink, as ink would dissolve and run with the solvent. | 原点（基线）必须用铅笔绘制，不能用墨水笔，因为墨水会溶解并随溶剂迁移。
• The spotting capillary must be changed between different samples to avoid contamination. | 不同样品之间必须更换点样毛细管以避免交叉污染。
• The solvent level in the developing chamber must be below the origin line, otherwise the sample spots would dissolve directly into the solvent. | 展开缸中溶剂液面必须低于原点线，否则样品斑点会直接溶解到溶剂中。
• The developing chamber should be covered to create a saturated atmosphere, ensuring consistent solvent evaporation. | 展开缸应加盖以形成饱和气氛，确保溶剂蒸发的一致性。

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5. Column Chromatography | 柱色谱法

Column chromatography is a preparative technique used to separate larger quantities of a mixture. The stationary phase, typically silica gel or alumina, is packed into a glass column. The mixture is applied to the top of the column, and a solvent (the eluent) is continuously passed through the column, carrying the components down at different rates.

柱色谱法是一种制备性技术，用于分离较大量的混合物。固定相（通常为硅胶或氧化铝）填充在玻璃柱中。混合物加在色谱柱顶部，溶剂（洗脱剂）连续通过色谱柱，将各组分以不同速率携带向下移动。

As the components travel down the column, they separate into distinct bands. The bands are collected separately as they emerge from the bottom of the column. Components that interact weakly with the stationary phase elute first, while those that interact more strongly elute later.

当各组分沿色谱柱向下移动时，它们会分离成不同的色带。当色带从柱底部流出时，分别收集。与固定相相互作用较弱的组分先被洗脱出来，而相互作用较强的组分则较晚流出。

Elution order in column chromatography: weakly adsorbed components elute first; strongly adsorbed components elute last | 柱色谱洗脱顺序：弱吸附组分先被洗脱；强吸附组分最后被洗脱

The choice of eluent can be adjusted to optimise the separation. A less polar solvent elutes all components more quickly but may give poorer separation; a more polar solvent improves resolution but slows the overall process. Gradient elution — gradually changing the solvent polarity — is often used for complex mixtures.

洗脱剂的选择可以调整以优化分离效果。极性较小的溶剂洗脱所有组分更快，但分离效果可能较差；极性较大的溶剂改善分辨率，但会减慢整个过程。梯度洗脱——逐渐改变溶剂极性——常用于复杂混合物。

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6. Gas Chromatography (GC) | 气相色谱法

Gas chromatography is a highly sensitive analytical technique used to separate and analyse volatile organic compounds. In GC, the mobile phase is an inert carrier gas — typically nitrogen, helium, or hydrogen — which is referred to as the carrier gas. The stationary phase is a thin layer of a non-volatile liquid coated onto the inside of a long, narrow capillary column, or packed onto a solid support.

气相色谱法是一种高灵敏度的分析技术，用于分离和分析挥发性有机化合物。在气相色谱中，流动相是惰性载气——通常是氮气、氦气或氢气——称为载气。固定相是涂覆在细长毛细管柱内壁上的非挥发性液体的薄膜，或负载在固体载体上。

Key components of a gas chromatograph include:

气相色谱仪的关键组成部分包括：

• Carrier gas supply | 载气供应: Provides the mobile phase that sweeps the sample through the column.
• Injector | 进样器: Heated port through which the sample is introduced; it must be hot enough to vaporise the sample instantly.
• Column | 色谱柱: Contains the stationary phase; typically 30-60 m long for capillary columns.
• Detector | 检测器: Produces an electrical signal proportional to the amount of each component as it emerges from the column.

载气供应：提供流动相，将样品扫过色谱柱。进样器：加热的进样口，通过它引入样品；温度必须足够高以使样品瞬间气化。色谱柱：含有固定相；毛细管柱通常长 30-60 米。检测器：当各组分从色谱柱中流出时产生与组分量成正比的电信号。

The sample must be volatile, meaning it can be vaporised without decomposing. Non-volatile or thermally unstable compounds cannot be analysed directly by GC without prior chemical derivatisation.

样品必须是挥发性的，即能在不分解的情况下被气化。非挥发性或热不稳定的化合物不经预先化学衍生化就无法直接用气相色谱分析。

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7. Retention Time and Qualitative Analysis | 保留时间与定性分析

The retention time (t_R) is the time taken for a particular component to travel from the injection point through the column to the detector. Each component has a characteristic retention time under a given set of conditions, allowing tentative identification by comparison with known standards.

保留时间（t_R）是某一特定组分从进样点通过色谱柱到达检测器所需的时间。在给定条件下，每个组分都有其特征保留时间，可通过与已知标准品进行比较进行初步鉴定。

Retention time is affected by several variables:

保留时间受以下几个变量影响：

• Boiling point | 沸点: Lower-boiling (more volatile) compounds spend more time in the gas phase and elute more quickly.
• Polarity | 极性: Non-polar compounds interact weakly with a non-polar stationary phase and elute quickly; polar compounds interact more strongly and elute more slowly.
• Column temperature | 柱温: Higher temperatures reduce retention times for all components but may reduce resolution.
• Carrier gas flow rate | 载气流速: Higher flow rates decrease retention times.

沸点：低沸点（更易挥发）化合物在气相中停留时间更长，洗脱更快。极性：非极性化合物与非极性固定相相互作用弱，洗脱快；极性化合物相互作用更强，洗脱更慢。柱温：较高温度缩短所有组分的保留时间，但可能降低分辨率。载气流速：较高的流速缩短保留时间。

In the AQA specification, you should understand that identification by retention time alone is not conclusive — two different compounds can have identical or very similar retention times under the same conditions. For definitive identification, GC is coupled with mass spectrometry.

在 AQA 考纲中，你需要理解仅凭保留时间进行鉴定并非确定性的——两种不同化合物在相同条件下可能具有相同或非常相似的保留时间。为了确定性地鉴定，气相色谱需要与质谱联用。

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8. Gas Chromatography-Mass Spectrometry (GC-MS) | 气相色谱-质谱联用技术

Gas chromatography-mass spectrometry (GC-MS) is a powerful hyphenated technique that combines the separating power of gas chromatography with the identifying power of mass spectrometry. The GC separates the mixture into individual components, and the mass spectrometer then produces a mass spectrum for each component as it elutes from the column.

气相色谱-质谱联用（GC-MS）是一种强大的联用技术，结合了气相色谱的分离能力和质谱的鉴定能力。气相色谱将混合物分离为各单独组分，质谱仪随后对从色谱柱中流出的每个组分产生质谱图。

The mass spectrum provides information about the molecular mass and fragmentation pattern of each component. The molecular ion peak (M⁺) gives the relative molecular mass, while the fragmentation pattern serves as a unique “fingerprint” that can be matched against spectral libraries for definitive identification.

质谱图提供有关每个组分的分子质量和碎片化模式的信息。分子离子峰（M⁺）给出相对分子质量，而碎片化模式则作为独特的”指纹”，可与谱库进行匹配以实现确定性鉴定。

GC-MS is widely used in various applications:

GC-MS 在许多领域中得到广泛应用：

• Forensic science | 法医学: Detection of drugs, poisons, and explosives in biological samples.
• Environmental analysis | 环境分析: Monitoring pesticide residues, pollutants, and contaminants in water and soil.
• Food safety | 食品安全: Identifying additives, flavour compounds, and adulterants in food products.
• Drug testing | 药物检测: Screening athletes and workplace samples for banned substances.

法医学：检测生物样品中的药物、毒物和爆炸物。环境分析：监测水和土壤中的农药残留、污染物和有害物质。食品安全：鉴定食品中的添加剂、风味化合物和掺假物。药物检测：筛查运动员和工作场所样品中的违禁物质。

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9. Interpreting Gas Chromatograms | 气相色谱图的解读

A gas chromatogram is a plot of detector response (y-axis) against retention time (x-axis). Each peak in the chromatogram corresponds to a different component of the mixture. Interpretation involves two key aspects:

气相色谱图是检测器响应（y 轴）对保留时间（x 轴）的图谱。色谱图中的每个峰对应混合物中的不同组分。解读涉及两个关键方面：

Qualitative analysis: The retention time of each peak identifies the component (by comparison with standards).

定性分析：每个峰的保留时间用于鉴定组分（通过与标准品比较）。

Quantitative analysis: The area under each peak is proportional to the amount of that component present in the mixture. Therefore, peak areas can be used to determine the relative concentrations of components. This is achieved by constructing a calibration curve using known concentrations of standard solutions.

定量分析：每个峰的面积与混合物中该组分的量成正比。因此，峰面积可用于确定各组分的相对浓度。这通过使用已知浓度的标准溶液构建校准曲线来实现。

Peak area ∝ concentration of component | 峰面积 ∝ 组分的浓度

For quantitative analysis, an internal standard is often added to both the calibration standards and the unknown sample. This compensates for variations in injection volume and detector response, improving the accuracy of the quantification.

对于定量分析，通常将内标同时添加到校准标准品和未知样品中。这可以补偿进样量和检测器响应的变化，从而提高定量的准确性。

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10. Comparing Chromatographic Techniques | 色谱技术的比较

It is helpful to be able to compare the main chromatographic techniques covered in the AQA specification. The table below summarises their key features:

对比 AQA 考纲中涉及的主要色谱技术是很有帮助的。下表总结了它们的主要特征：

Technique | 技术	Mobile Phase | 流动相	Stationary Phase | 固定相	Purpose | 用途
TLC | 薄层色谱	Liquid solvent | 液体溶剂	Silica gel or alumina on a plate | 薄板上的硅胶或氧化铝	Qualitative analysis, reaction monitoring | 定性分析、反应监测
Column | 柱色谱	Liquid solvent | 液体溶剂	Silica gel or alumina packed in a column | 填充在柱中的硅胶或氧化铝	Preparative separation of larger quantities | 较大量的制备性分离
GC | 气相色谱	Inert carrier gas (N₂, He, H₂) | 惰性载气（N₂、He、H₂）	Non-volatile liquid coated on a capillary column | 涂覆在毛细管柱上的非挥发性液体	Qualitative and quantitative analysis of volatile compounds | 挥发性化合物的定性和定量分析

When choosing a technique, consider the nature of the sample (volatile or non-volatile), the quantity available, and whether qualitative or quantitative information is required.

在选择技术时，需要考虑样品的性质（挥发性或非挥发性）、可用量以及需要的是定性还是定量信息。

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11. Common Exam Pitfalls | 常见考试误区

Students frequently lose marks in chromatography questions due to several common errors. Being aware of these pitfalls can significantly improve your exam performance:

学生在色谱相关题目中常因几个常见错误而失分。了解这些误区可以显著提高你的考试成绩：

• Confusing the phases | 混淆两相: Remember that the mobile phase moves, while the stationary phase does not. In TLC, the solvent is the mobile phase; the silica gel is the stationary phase.
• Misinterpreting Rf values | 错误解读 Rf 值: Rf = distance moved by component ÷ distance moved by solvent front. Both distances are measured from the origin, not from each other.
• Writing the origin in ink | 用墨水笔画原点: The origin on a TLC plate must be drawn in pencil, since ink components would dissolve and run, contaminating the chromatogram.
• Forgetting why the solvent level matters | 忘记溶剂液面高度的重要性: The solvent level must be below the origin line, or the sample spots would be washed off into the solvent reservoir.
• Stating that Rf identifies a substance conclusively | 声称 Rf 值可以确定性鉴定物质: Rf values are only a preliminary indication. Identification requires comparison with known standards under identical conditions, and ideally confirmation by another technique such as mass spectrometry.
• Confusing retention time and peak area | 混淆保留时间和峰面积: Retention time identifies the component; peak area quantifies the amount.

许多学生在色谱相关题目中常因几个常见错误而失分。了解这些误区可以显著提高你的考试成绩：混淆两相：记住流动相是移动的，固定相不移动。在薄层色谱中，溶剂是流动相；硅胶是固定相。错误解读 Rf 值：Rf = 组分移动的距离 ÷ 溶剂前沿移动的距离。两个距离都从原点测量，而不是彼此之间。用墨水笔画原点：薄层板上的原点必须用铅笔绘制，因为墨水组分溶解后会迁移，污染色谱图。忘记溶剂液面高度的重要性：溶剂液面必须低于原点线，否则样品斑点会被冲入溶剂储槽中。声称 Rf 值可以确定性鉴定物质：Rf 值只是初步指示。鉴定需要在相同条件下与已知标准品进行比较，理想情况下还应通过另一种技术（如质谱）进行确认。混淆保留时间和峰面积：保留时间用于鉴定组分；峰面积用于定量组分的量。

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12. Worked Example | 例题解析

A student performs a TLC analysis of a reaction product. The product spot travels 3.2 cm, and the solvent front travels 8.0 cm. A reference sample of the expected compound travels 4.0 cm under the same conditions. Calculate the Rf value of the product and determine whether the product is pure.

一名学生对反应产物进行薄层色谱分析。产物斑点移动了 3.2 cm，溶剂前沿移动了 8.0 cm。在相同条件下，预期化合物的参考样品移动了 4.0 cm。计算产物的 Rf 值并判断产物是否纯净。

Solution | 解答:

Rf(product) = 3.2 ÷ 8.0 = 0.40

Rf(reference) = 4.0 ÷ 8.0 = 0.50

The Rf value of the product (0.40) does not match the Rf value of the reference compound (0.50). Therefore, the product spot is not the expected compound — the reaction may not have produced the expected product, or the product may be impure. To confirm purity, the TLC plate should be examined for additional spots under UV light or after staining.

产物的 Rf 值（0.40）与参考化合物的 Rf 值（0.50）不匹配。因此，产物斑点不是预期化合物——反应可能没有生成预期产物，或者产物可能不纯。为确认纯度，应在紫外灯下或显色后检查薄层板是否有额外斑点。

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