Chromatography | IB Edexcel Chemistry: Key Points on Chromatography

📚 Chromatography | IB Edexcel Chemistry: Key Points on Chromatography

Chromatography is a powerful analytical technique used to separate, identify, and quantify components within a mixture. It plays a critical role in both IB and Edexcel Chemistry syllabuses, linking core concepts of intermolecular forces, equilibrium, and quantitative analysis. Understanding the principles of mobile and stationary phases, retention mechanisms, and the calculation of Rf values is essential for interpreting chromatograms and solving examination problems.

色谱是一种强大的分析技术，用于分离、鉴定和量化混合物中的组分。它在IB和Edexcel化学课程中都占据重要地位，将分子间作用力、平衡和定量分析等核心概念联系起来。理解流动相与固定相的原理、保留机制以及Rf值的计算，是解读色谱图和解决考试问题的关键。

1. Introduction to Chromatography | 色谱概述

Chromatography derives from the Greek words ‘chroma’ (colour) and ‘graphein’ (to write). The technique separates components of a mixture based on their differential distribution between a mobile phase and a stationary phase. All chromatographic methods rely on dynamic equilibrium: as the mobile phase moves, components continuously partition between the two phases. Those with stronger affinity for the mobile phase travel faster, while those that interact more strongly with the stationary phase are retained longer.

色谱一词源于希腊语“chroma”（颜色）和“graphein”（书写）。该技术基于组分在流动相和固定相之间的分配差异来分离混合物。所有色谱方法都依赖于动态平衡：随着流动相移动，组分在两相之间不断分配。与流动相亲和力较强的组分移动更快，而与固定相作用更强的组分则保留时间更长。

Chromatography is essential in forensic science, pharmaceutical analysis, environmental monitoring, and food chemistry. In the IB and Edexcel specifications, students are expected to describe the operation of paper chromatography, thin-layer chromatography (TLC), column chromatography, gas chromatography (GC), and high-performance liquid chromatography (HPLC), along with their applications.

色谱在法医科学、药物分析、环境监测和食品化学中不可或缺。在IB和Edexcel大纲中，学生需要描述纸色谱、薄层色谱(TLC)、柱色谱、气相色谱(GC)和高效液相色谱(HPLC)的操作及其应用。

2. Basic Principles: Mobile Phase and Stationary Phase | 基本原理：流动相与固定相

Every chromatographic system consists of a stationary phase (a solid or a liquid supported on a solid) and a mobile phase (a liquid or a gas) that flows through or over the stationary phase. Separation is governed by the balance of intermolecular interactions such as hydrogen bonding, dipole-dipole forces, and London dispersion forces between sample molecules and the two phases. The choice of phases determines the separation mechanism, which may be adsorption, partition, ion-exchange, or size-exclusion.

每个色谱系统都由固定相（固体或负载在固体上的液体）和流动相（液体或气体）组成，流动相流过或穿过固定相。分离过程取决于样品分子与两相之间氢键、偶极-偶极力和伦敦色散力等分子间相互作用的平衡。相的选择决定了分离机理，可能是吸附、分配、离子交换或尺寸排阻。

In adsorption chromatography (e.g., TLC, column chromatography), the stationary phase is a finely divided solid onto which sample molecules adsorb. In partition chromatography (e.g., paper chromatography, GC with liquid stationary phase), separation relies on solubility differences between the mobile phase and the liquid stationary phase coated on an inert support. Understanding these fundamental interactions helps predict elution order.

在吸附色谱（如薄层色谱、柱色谱）中，固定相是细碎固体，样品分子吸附其上。在分配色谱（如纸色谱、使用液体固定相的气相色谱）中，分离依赖于样品在流动相和涂覆在惰性载体上的液体固定相之间的溶解度差异。理解这些基本相互作用有助于预测洗脱顺序。

The mobile phase can be a single solvent or a mixture. Adjusting the polarity of the mobile phase alters the relative affinity of components. In general, ‘like dissolves like’ applies: polar mobile phases elute polar compounds faster in normal-phase chromatography, while non-polar solvents favour non-polar analytes.

流动相可以是单一溶剂或混合物。调节流动相的极性会改变各组分的相对亲和力。一般来说，“相似相溶”原理适用：在正相色谱中，极性流动相更快洗脱极性化合物，而非极性溶剂更有利于非极性分析物。

3. Partition Coefficient (Kd) | 分配系数 (Kd)

The partition coefficient Kd (or distribution constant) quantifies the distribution of a solute between the stationary and mobile phases at equilibrium. It is defined as Kd = concentration of solute in stationary phase / concentration of solute in mobile phase. A larger Kd value indicates stronger retention in the stationary phase, resulting in slower migration. In partition chromatography, Kd is relatively constant under fixed conditions, which allows reproducible retention times and Rf values.

分配系数 Kd（或分布常数）用于量化平衡时溶质在固定相与流动相之间的分配情况。其定义为 Kd = 固定相中溶质浓度 / 流动相中溶质浓度。Kd 值越大，表明溶质在固定相中保留越强，迁移越慢。在分配色谱中，在固定条件下 Kd 相对恒定，这使得保留时间和 Rf 值具有可重复性。

In a chromatographic column, the retention factor k’ is a related expression: k’ = (t_R – t_M)/t_M, where t_R is the retention time of the analyte and t_M is the hold-up time (time for unretained species). While IB and Edexcel do not require extensive calculation of k’, understanding the concept of partition explains why different substances separate. The fundamental equation linking separation efficiency to Kd is ingrained in the plate theory.

在色谱柱中，保留因子 k’ 是相关的表达式：k’ = (t_R – t_M)/t_M，其中 t_R 是分析物的保留时间，t_M 是死时间（未被保留物质通过的时间）。虽然IB和Edexcel不要求大量计算 k’，但理解分配的概念可以解释为什么不同物质会分离。将分离效率与 Kd 联系起来的基本方程植根于塔板理论。

Examination questions often ask students to relate retention behaviour to intermolecular forces. For example, a more polar compound in a polar stationary phase will have a larger Kd and thus a smaller Rf or longer retention time, due to stronger dipole-dipole and hydrogen-bonding interactions.

考试题目常要求学生将保留行为与分子间作用力联系起来。例如，在极性固定相中，极性更强的化合物由于更强的偶极-偶极和氢键作用，Kd 较大，因此 Rf 较小或保留时间更长。

4. Paper Chromatography | 纸色谱

Paper chromatography is a simple partition technique where the stationary phase is water molecules adsorbed onto cellulose fibres of chromatography paper. The mobile phase is a suitable solvent or solvent mixture that travels up the paper by capillary action. A small spot of the sample mixture is placed near the bottom of the paper (the baseline), and the paper is suspended in a developing chamber containing the solvent. As the solvent front ascends, components separate according to their solubility in the mobile phase and their adherence to the water-cellulose stationary phase.

纸色谱是一种简单的分配技术，固定相是吸附在色谱纸纤维素纤维上的水分子。流动相是适宜的溶剂或混合溶剂，通过毛细作用沿纸上升。将少量样品点样在纸的底部附近（基线），将纸悬挂在含有溶剂的展开缸中。随着溶剂前沿上升，各组分根据其在流动相中的溶解度以及对水-纤维素固定相的附着程度不同而分离。

This method is commonly used for separating coloured compounds such as plant pigments and ink dyes, but colourless substances can be visualised using UV light or chemical locating agents (e.g., ninhydrin for amino acids). Paper chromatography is often the first technique taught because it illustrates the core concepts of mobile phase, stationary phase, and Rf values without needing sophisticated equipment. However, it has lower resolution and slower separation than TLC.

该方法常用于分离有色化合物，如植物色素和墨水染料，但无色物质可通过紫外光或化学显色剂（例如氨基酸的茚三酮）显色。纸色谱通常是教学中最先介绍的技术，因为它无需复杂设备即可说明流动相、固定相和 Rf 值的核心概念。然而，与 TLC 相比，其分离度较低且速度较慢。

Experimental details that examiners emphasise include: drawing the baseline in pencil (not pen, to avoid ink running), ensuring the baseline is above the solvent level, using a closed chamber to maintain a saturated atmosphere, and allowing the solvent to run until it nears the top of the paper before marking the solvent front immediately.

考官强调的实验细节包括：用铅笔（不能用钢笔，以免墨水扩散）画基线、确保基线高于溶剂液面、使用密闭缸体保持饱和气氛，以及让溶剂行进到接近纸的顶端后立即标记溶剂前沿。

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

Thin-layer chromatography uses a stationary phase consisting of a thin layer of adsorbent material (typically silica gel SiO₂ or alumina Al₂O₃) coated on a glass, plastic, or aluminium plate. The mobile phase is a solvent or solvent mixture that rises by capillary action. TLC is primarily an adsorption chromatography technique, although some partition may occur depending on the stationary phase and binder. It offers better resolution, faster runs, and greater reproducibility than paper chromatography.

薄层色谱使用的固定相是涂布在玻璃、塑料或铝板上的薄层吸附剂（通常为硅胶 SiO₂ 或氧化铝 Al₂O₃）。流动相是依靠毛细作用上升的溶剂或混合溶剂。TLC 主要是一种吸附色谱技术，但根据固定相和粘合剂的不同，也可能发生一定的分配作用。与纸色谱相比，TLC 具有更好的分辨率、更快的运行速度和更高的重现性。

The polar silica gel surface contains silanol (Si–OH) groups that strongly interact with polar functional groups of analytes via hydrogen bonding and dipole-dipole forces. Thus, in normal-phase TLC, more polar compounds are retained more strongly and have lower Rf values. Students should be able to predict the order of Rf values for a given set of compounds based on polarity and functional groups.

极性硅胶表面含有硅羟基(Si–OH)，通过氢键和偶极-偶极力与分析物的极性官能团发生强烈相互作用。因此，在正相 TLC 中，极性更强的化合物保留得更牢固，Rf 值更低。学生应能根据极性和官能团预测给定化合物的 Rf 值大小顺序。

After development, the plate is dried and spots are visualised. Coloured spots can be seen directly; colourless compounds are detected using a UV lamp (if the plate contains a fluorescent indicator) or by spraying with locating agents such as iodine vapour or potassium permanganate. In examination answers, reference to these visualisation techniques is often rewarded. Two-dimensional TLC, where the plate is developed in a second direction with a different solvent, can further separate complex mixtures.

展开后，板干燥并进行斑点检视。有色斑点可直接观察；无色化合物则使用紫外灯（若板含荧光指示剂）或喷洒显色剂（如碘蒸气或高锰酸钾）来检测。在考试答案中，提及这些显色技术通常可以得分。二维 TLC 是在与第一次垂直的方向上用不同的溶剂再次展开，可进一步分离复杂混合物。

6. Rf Value: Calculation and Factors | Rf 值：计算与影响因素

The retention factor Rf (retardation factor) is a dimensionless quantity used in planar chromatography (paper and TLC) to characterise the position of a separated component. It is calculated as:

Rf = distance travelled by the component / distance travelled by the solvent front

保留因子 Rf（比移值）是平面色谱（纸色谱和 TLC）中用于表征分离组分位置的无量纲量。其计算公式为：

Rf = 组分移动的距离 / 溶剂前沿移动的距离

Both distances are measured from the origin (baseline) to the centre of the spot and to the solvent front, respectively. Rf values range between 0 (component remains at origin) and 1 (component moves with the solvent front). In practice, values typically fall between 0.2 and 0.8 for optimal separation. An Rf value is constant under identical experimental conditions (same stationary phase, mobile phase, temperature, and chamber saturation) and can be used for qualitative identification by comparison with standards run on the same plate.

这两个距离均是从原点（基线）分别测量到斑点中心和溶剂前沿。Rf 值范围在 0（组分留在原点）到 1（组分随溶剂前沿移动）之间。实际中，最佳分离的 Rf 值通常落在 0.2 到 0.8。在相同实验条件下（相同固定相、流动相、温度和腔室饱和度），Rf 值是常数，可于同一板上与标准品对比，用于定性鉴定。

Several factors influence Rf values: the polarity of the mobile phase, the nature of the stationary phase, temperature, and the thickness of the adsorbent layer. In TLC, increasing the polarity of the mobile phase generally increases the Rf of polar analytes by competing more effectively for adsorption sites. Examiners may ask students to explain how varying the solvent composition affects separation or to calculate Rf values from a given chromatogram.

影响 Rf 值的因素有：流动相的极性、固定相的性质、温度以及吸附层厚度。在 TLC 中，增加流动相的极性通常会提高极性分析物的 Rf，因为溶剂能更有效地竞争吸附位点。考官可能要求学生解释改变溶剂组成如何影响分离，或从给定的色谱图计算 Rf 值。

Rf values have no units and are often reported to two decimal places.
Rf 值无单位，通常报告到小数点后两位。
The ratio of distances is invariant to the length of plate development, but spot diffusion can cause uncertainty.
距离的比值与板的展开长度无关，但斑点扩散会导致不确定性。

7. Column Chromatography | 柱色谱

Column chromatography is a preparative technique used to separate and collect individual components of a mixture. A vertical glass column is packed with a stationary phase (commonly silica gel or alumina) in the form of a slurry. The sample mixture is loaded onto the top of the column, and the mobile phase (eluent) is continuously added. Components travel down the column at different rates, separated by a process of repeated adsorption-desorption equilibria, and are collected in separate fractions as they elute from the bottom.

柱色谱是一种制备技术，用于分离和收集混合物的各个组分。垂直玻璃柱中以浆状装填固定相（通常为硅胶或氧化铝）。样品混合物加载到柱顶，并持续添加流动相（洗脱剂）。各组分以不同速率沿柱下行，通过重复的吸附-解吸平衡过程实现分离，并在从底部洗脱时分别收集。

The principle is essentially the same as TLC, but on a larger scale. More polar components interact more strongly with the polar stationary phase and move more slowly, requiring a larger volume of eluent or a more polar solvent to be eluted. A solvent gradient, where the polarity of the eluent is gradually increased, is often used to efficiently elute components of increasing polarity. This is called gradient elution.

其原理与 TLC 基本相同，只是规模更大。极性较强的组分与极性固定相的相互作用更强，移动更慢，需要更多的洗脱剂或极性更强的溶剂才能被洗脱。溶剂梯度，即逐渐增加洗脱剂的极性，常被用于高效洗脱极性递增的组分，这称为梯度洗脱。

In examination contexts, students may be asked to explain how a mixture of compounds with different polarities can be separated by column chromatography, identifying the order of elution. Typical questions involve a mixture of hydrocarbons, ketones, alcohols, and carboxylic acids on a silica gel column, with hexane as the initial mobile phase. The least polar hydrocarbon elutes first, followed by ketone, then alcohol, and finally the most polar carboxylic acid.

在考试中，可能要求学生解释如何利用柱色谱分离不同极性的化合物混合物，并指出洗脱顺序。典型题目涉及在硅胶柱上用己烷作初始流动相分离烃、酮、醇和羧酸的混合物。极性最小的烃首先被洗脱，其次是酮，然后是醇，最后是极性最强的羧酸。

8. Gas Chromatography (GC) | 气相色谱 (GC)

Gas chromatography is an instrumental technique used to separate and analyse volatile, thermally stable compounds. The mobile phase is an inert carrier gas (helium, nitrogen, or hydrogen) that flows through a long, thin capillary column coated with a liquid stationary phase or packed with a solid stationary phase. The sample is injected into a heated injection port where it vapourises and is swept onto the column. Components are separated based on their boiling points and their interaction with the stationary phase, and are detected as they exit the column, usually by a flame ionisation detector (FID) or a mass spectrometer (GC-MS).

气相色谱是一种用于分离和分析挥发性、热稳定化合物的仪器技术。流动相是惰性载气（氦气、氮气或氢气），流经涂有液体固定相或填充有固体固定相的长细毛细管柱。样品注入加热的进样口汽化，并被载气带入色谱柱。各组分基于其沸点以及它们与固定相的相互作用而分离，并在离开色谱柱时被检测，通常使用火焰离子化检测器(FID)或质谱仪(GC-MS)。

The output is a gas chromatogram, a plot of detector response versus time. Each component appears as a peak. The retention time (t_R) is the time from injection to the peak maximum; it is characteristic of a compound under specific conditions. The area under the peak is proportional to the amount of that component, allowing quantitative analysis. A small, sharp peak for unretained substances gives the dead time t_M. Resolution between adjacent peaks depends on the column efficiency and the selectivity of the stationary phase.

其输出是气相色谱图，即检测器响应随时间变化的图。每个组分表现为一个峰。保留时间 (t_R) 是从进样到峰顶的时间；在特定条件下，这是化合物的特征值。峰面积与该组分的量成正比，可进行定量分析。未被保留物质的窄尖峰给出死时间 t_M。相邻峰之间的分离度取决于柱效和固定相的选择性。

Temperature programming in GC (gradually raising the column temperature during a run) improves separation and reduces analysis time for mixtures with a wide boiling-point range. For high-boiling or thermally labile compounds, derivatisation may be required to increase volatility. Students should be familiar with interpreting chromatograms, identifying components by comparing retention times with standards, and calculating percentage composition from peak areas.

GC 中的程序升温（在运行过程中逐渐升高柱温）可改善分离并缩短宽沸程混合物的分析时间。对于高沸点或热不稳定化合物，可能需要衍生化以增加挥发性。学生应熟悉解读色谱图、通过与标准品比较保留时间来鉴定组分，以及根据峰面积计算百分含量。

9. High-Performance Liquid Chromatography (HPLC) | 高效液相色谱 (HPLC)

High-performance liquid chromatography is a versatile column chromatography technique that uses high pressure to force a liquid mobile phase through a tightly packed column of very small stationary phase particles (typically 3-10 µm). This allows for high-resolution, rapid separations of non-volatile and thermally sensitive compounds that cannot be analysed by GC. HPLC is widely used in pharmaceutical analysis, biomolecule separation, and food testing. The two common modes are normal-phase HPLC (polar stationary phase, non-polar mobile phase) and reverse-phase HPLC (non-polar stationary phase, polar mobile phase), with the latter being predominant in modern applications.

高效液相色谱是一种多功能的柱色谱技术，它利用高压迫使液体流动相通过紧密填充的极细固定相颗粒柱（通常为3-10 μm）。这使得分离不能用于GC的非挥发性和热敏性化合物成为可能，且具有高分辨率和快速分离的特点。HPLC 广泛用于药物分析、生物分子分离和食品检测。两种常见模式是正相 HPLC（极性固定相，非极性流动相）和反相 HPLC（非极性固定相，极性流动相），后者在现代应用中占主导地位。

In reverse-phase HPLC, the stationary phase is typically silica modified with long hydrocarbon chains (e.g., C18 octadecylsilane). The mobile phase is a mixture of water and a miscible organic solvent such as methanol or acetonitrile. Non-polar compounds interact strongly with the hydrophobic stationary phase and elute later, while polar compounds prefer the aqueous mobile phase and elute early. This is opposite to the elution order in normal-phase chromatography, which often confuses students; careful attention to the phase type is essential.

在反相 HPLC 中，固定相通常是用长烃链（例如 C18 十八烷基硅烷）改性的硅胶。流动相是水与可混溶的有机溶剂（如甲醇或乙腈）的混合物。非极性化合物与疏水固定相强烈相互作用，较晚洗脱；而极性化合物则倾向水相流动相，较早洗脱。这与正相色谱的洗脱顺序相反，常使学生混淆；仔细关注相的类型至关重要。

The HPLC system consists of a solvent reservoir, pump, injector, column, detector (often UV-visible absorbance), and data station. Retention time is used for qualitative identification, while peak area provides quantitative data when calibrated with standards. HPLC can also be coupled with mass spectrometry (LC-MS) for highly sensitive and specific analysis. Examination questions may ask candidates to explain why HPLC is preferred over GC for certain samples (e.g., aqueous biological samples, peptides) and to compare the two techniques.

HPLC 系统由溶剂储液器、泵、进样器、色谱柱、检测器（通常为紫外-可见吸收）和数据站组成。保留时间用于定性鉴定，而峰面积在校准后提供定量数据。HPLC 也可与质谱联用 (LC-MS)，进行高灵敏度和高特异性的分析。考试题目可能会要求考生解释为何某些样品（如水性生物样品、肽类）首选 HPLC 而非 GC，并比较这两种技术。

10. Interpretation of Chromatograms and Applications | 色谱图解读及应用

Interpreting chromatograms, whether from paper, TLC, GC, or HPLC, requires a systematic approach. In planar chromatography, the Rf values and spot colours (or response to locating agents) are compared with reference standards run on the same plate. The number of spots indicates the number of components, although overlapping spots may mask incomplete separation. In column-based techniques, the number of peaks generally corresponds to the number of components, but co-elution (two compounds having the same retention time) can occur. Peak symmetry and resolution are indicators of separation quality.

解读色谱图，无论是来自纸色谱、TLC、GC 还是 HPLC，都需要系统的方法。在平面色谱中，将 Rf 值和斑点颜色（或对显色剂的反应）与同板上运行的标准品进行比较。斑点数表示组分数，但重叠斑点可能掩盖未完全分离的情况。在基于柱的技术中，峰数一般对应于组分数，但共洗脱（两种化合物具有相同保留时间）也可能发生。峰对称性和分离度是分离质量的指标。

Quantitative analysis can be performed using calibration curves. For GC and HPLC, a series of standard solutions of known concentration are injected, and the peak areas are plotted against concentration. The concentration of an unknown sample is then determined from its peak area using the calibration graph. In TLC, semiquantitative comparison can be done by visual estimation of spot intensities.

定量分析可使用校准曲线进行。对于 GC 和 HPLC，注入一系列已知浓度的标准溶液，将峰面积对浓度作图。然后利用校准图，通过未知样品的峰面积确定其浓度。在 TLC 中，可通过视觉估计斑点强度进行半定量比较。

Chromatography has vast real-world applications: detecting drugs and metabolites in urine for forensic toxicology, monitoring pesticide residues in food, analysing hydrocarbons in petroleum, separating enantiomers using chiral HPLC, and purifying proteins in biotechnology. In the context of IB and Edexcel examinations, linking these applications to the appropriate technique demonstrates a deeper understanding. For example, GC-MS is the method of choice for volatile arson accelerants, while HPLC suits for non-volatile drugs.

色谱具有广泛的现实应用：在法医毒理学中检测尿液中的药物和代谢物，监测食品中的农药残留，分析石油中的碳氢化合物，使用手性 HPLC 分离对映异构体，以及在生物技术中纯化蛋白质。在 IB 和 Edexcel 考试中，将这些应用与适当的技术联系起来可以体现更深的理解。例如，GC-MS 是分析挥发性纵火助燃剂的首选方法，而 HPLC 则适用于非挥发性药物的分析。

When comparing techniques, students should consider the physical state of the sample, its thermal stability, volatility, polarity, and the required sensitivity and resolution. A common examination question is: ‘Suggest a chromatographic method suitable for separating a mixture of amino acids and explain your reasoning.’ The expected answer includes mentioning that amino acids are non-volatile and thermally labile, so HPLC (or TLC/paper chromatography) would be appropriate rather than GC.

在比较技术时，学生应考虑样品的物理状态、热稳定性、挥发性、极性以及所需的灵敏度和分离度。一个常见的考题是：“建议一种适用于分离氨基酸混合物的色谱方法并解释你的理由。”预期答案包括提到氨基酸是非挥发性和热不稳定的，因此 HPLC（或 TLC/纸色谱）比 GC 更为合适。

Published by TutorHao | IB Edexcel Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Chromatography | IB Edexcel Chemistry: Key Points on Chromatography