📚 Analytical Chemistry: Instrumental Techniques and Qualitative Analysis | 分析化学：仪器技术与定性分析
Analytical chemistry is the branch of chemistry that deals with the identification, separation and quantification of chemical compounds. In modern A‑Level studies, instrumental techniques such as infrared spectroscopy, mass spectrometry, nuclear magnetic resonance and chromatography have largely replaced classical ‘wet‑chemistry’ methods for structure determination and purity analysis. This article provides a systematic revision of these techniques, including how spectra are interpreted and how chromatographic data are used in qualitative and quantitative analysis.
分析化学是化学中涉及化合物鉴定、分离和定量的分支。在现代 A‑Level 学习中，红外光谱、质谱、核磁共振和色谱等仪器技术已经在结构测定和纯度分析中基本取代了传统的“湿化学”方法。本文对这些技术进行了系统复习，包括如何解析谱图以及如何利用色谱数据进行定性和定量分析。
1. The Scope of Analytical Chemistry | 分析化学的范围
Analytical chemistry can be divided into qualitative analysis — what substances are present — and quantitative analysis — how much of each substance is present. Traditional methods relied on colour changes, precipitation and titration, but today instrumental methods offer greater speed, sensitivity and specificity. In Cambridge A‑Level Chemistry, you are expected to understand how data from different techniques are combined to identify organic molecules, assess purity and determine molecular structures.
分析化学可分为定性分析——存在哪些物质——和定量分析——每种物质有多少。传统方法依赖颜色变化、沉淀和滴定，但如今仪器方法提供了更高的速度、灵敏度和专一性。在剑桥 A‑Level 化学中，你需要理解如何组合不同技术的数据来鉴定有机分子、评估纯度并确定分子结构。
2. Infrared Spectroscopy (IR) | 红外光谱
d vibrations (stretching and bending). An IR spectrum plots percentage transmittance against wavenumber (cm⁻¹). The ‘ plays absorptions that correspond to specific bonds. For example, a broad peak around 2500–3300 cm⁻¹ indicates an O–H bond in carboxylic acids; a sharp peak near 1700 cm⁻¹ corresponds to C=O stretching.
红外光谱利用分子中的共价键在特征频率处吸收红外辐射，引起键的振动（伸缩和弯曲）。IR 谱图是透光率对波数 (cm⁻¹) 的图。1500 cm⁻¹ 以下的指纹区对每种化合物都是独特的，而官能团区（1500 cm⁻¹ 以上）显示与特定键对应的吸收。例如，2500–3300 cm⁻¹ 附近的宽峰指示羧酸中的 O–H 键；1700 cm⁻¹ 附近的尖峰对应 C=O 伸缩振动。
| Bond | Wavenumber range (cm⁻¹) | Intensity/shape |
|---|---|---|
| O–H (alcohols, free) | 3580–3650 | Sharp |
| O–H (carboxylic acids, H‑bonded) | 2500–3300 | Very broad |
| C=O (carbonyl) | 1680–1750 | Strong, sharp |
| C–O (esters, ethers) | 1000–1300 | Medium |
look for the absence of certain peaks as well as their presence. For instance, no broad O– k suggests the compound is not an aldehyde, ketone or carboxylic acid. The fingerprint region can be compared with a spectral library to confirm identity.
在分析 IR 谱图时，既要寻找峰的存在也要注意峰的缺失。例如，没有宽的 O–H 吸收可排除醇和羧酸；没有 C=O 峰表明该化合物不是醛、酮或羧酸。指纹区可与谱库比对以确证身份。
3. Mass Spectrometry (MS) | 质谱
A mass spectrometer ionises molecules and then separates the resulting ions according to their mass‑to‑charge ratio (m/z). For organic molecules, electron‑impact ionisation typically produces a molecular ion peak (M⁺) and a characteristic fragmentation pattern. The molecular ion peak gives the relative molecular mass (Mr) of the compound if the peak is sufficiently stable to be detected. The base peak is the most intense peak, assigned an abundance of 100%.
质谱仪将分子离子化，然后根据质荷比 (m/z) 分离生成的离子。对于有机分子，电子轰击离子化通常会产生分子离子峰 (M⁺) 和特征碎片峰。如果分子离子峰足够稳定可被检测到，则该峰给出化合物的相对分子质量 (Mr)。基峰是最强峰，丰度定为 100%。
H₂O (18), CO (28), and alkyl chains. For example, a primary alcohol often shows an M⁺ peak and a fragment at M⁻18 due to loss of water, and a peak at m/z 31 due to CH₂=OH⁺. By examining the fragmentation pattern, a chemist can deduce structural features.
碎裂的发生是因为分子离子具有过剩内能而断裂。常见的碎片丢失包括 CH₃ (m/z 15)、OH (17)、H₂O (18)、CO (28) 和烷基链。例如，伯醇常显示 M⁺ 峰以及因失水产生的 M⁻18 的碎片峰，以及 m/z 31 处来自 CH₂=OH⁺ 的峰。通过考察碎裂模式，化学家可以推断结构特征。
Example: Mass spectrum of butan‑2‑ at m/z 72; base peak at m/z 43 due to CH₃CO⁺; peaks at m/z 57 (M⁻CH₃) and m/z 29 (C₂H₅⁺).
示例：丁‑2‑酮 (CH₃COCH₂CH₃, Mr = 72) 的质谱图。M⁺ 在 m/z 72；基峰在 m/z 43 来自 CH₃CO⁺；m/z 57 (M⁻CH₃) 和 m/z 29 (C₂H₅⁺) 的峰。
4. Combining IR and MS for Structure Elucidation | 联合 IR 与 MS 解析结构
rmula. The approach is to use the molecular ion peak to confirm Mr, the IR spectrum to identify functional groups, and the fragmentation pattern to piece together the carbon skeleton. For example, a compound with M⁺ = 88, a strong IR peak at 1735 cm⁻¹ (C=O of an ester) and a major fragment at m/z 43 (CH₃CO⁺) suggests an ester like ethyl ethanoate.
在考试题中，通常会给出未知物的 IR 谱和质谱，以及其实验式或分子式。方法是使用分子离子峰确认 Mr，用 IR 谱确定官能团，再通过碎片模式拼接碳骨架。例如，一个化合物 M⁺ = 88，在 1735 cm⁻¹ 有强吸收（酯的 C=O），主要碎片峰 m/z 43 (CH₃CO⁺)，表明该化合物可能是乙酸乙酯之类的酯。
5. Nuclear Magnetic Resonance Spectroscopy: Basic Principles | 核磁共振波谱：基本原理
³C) in a strong magnetic field. These nuclei behave like tiny magnets and can absorb radio‑ vironment of the nucleus, which is reported as the chemical shift (δ) in parts per million (ppm). Tetramethylsilane (TMS) is used as the reference standard (δ = 0).
核磁共振波谱利用某些原子核（如 ¹H 和 ¹³C）在强磁场中的行为。这些核像小磁体，当磁场强度满足共振条件时能够吸收射频辐射。吸收的确切频率取决于该核所处的化学环境，用化学位移 (δ) 表示，单位为百万分之一 (ppm)。四甲基硅烷 (TMS) 用作参考标准 (δ = 0)。
A nucleus is shielded by the electron cloud around it; electrons generate a small opposing magnetic field that reduces the effective field experienced by the nucleus. Electronegative atoms (e.g. O, Cl) withdraw electrons, deshielding the proton and shifting its signal to higher δ values (downfield). Conversely, electron‑donating alkyl groups increase shielding, shifting signals upfield (lower δ).
原子核受到周围电子云的屏蔽；电子产生一个微小的反向磁场，削弱核所感受到的有效磁场。电负性原子（如 O、Cl）会吸电子，使质子去屏蔽，其信号向高 δ 值（低场）移动。相反，给电子的烷基增强屏蔽，使信号向高场（低 δ 值）移动。
6. Proton NMR (¹H NMR) | 质子核磁共振
A ¹H NMR spectrum provides three key pieces of information: the number of signals tells you how many different proton environments exist; the integration trace or relative area of each signal gives the ratio of protons in each environment; and the splitting pattern (multiplicity) tells you about neighbouring non‑equivalent protons, following the n+1 rule.
¹H NMR 谱图提供三条关键信息：信号的数量表明存在多少种不同的质子环境；每个信号的积分曲线或相对面积给出各环境中质子的比例；裂分模式（多重性）遵循 n+1 规则，告知相邻非等价质子的数目。
The n+1 rule: a proton signal is split into n+1 peaks, where n is the number of non‑ their meanings are: singlet (no adjacent H); doublet (one neighbouring H); triplet (two neighbouring H); quartet (three neighbouring H). The relative intensities in a multiplet follow Pascal’s triangle.
n+1 规则：一个质子的信号裂分成 n+1 个峰，其中 n 是相邻碳原子上非等价质子的数目。因此，有两个邻接质子的信号表现为三重峰（2+1 = 3）。常见的裂分模式及其含义为：单峰（无邻接 H）；双重峰（一个邻接 H）；三重峰（两个邻接 H）；四重峰（三个邻接 H）。多重峰中各峰的相对强度遵循帕斯卡三角形。
Table of typical chemical shift ranges for common proton environments:
- Alkane –CH₃, –CH₂– : δ 0.8 – 1.7
- H–C–C=O : δ 2.1 – 2.6
- H–C–O (alcohol, ether) : δ 3.3 – 4.5
- H–C–Cl/Br : δ 3.0 – 4.2
- Alkene H–C=C : δ 4.5 – 6.0
- Aromatic H : δ 6.5 – 8.5
- Aldehyde H–C=O : δ 9.5 – 10.0
- Carboxylic acid O–H : δ 10.0 – 12.0 (broad)
- Alcohol O–H : δ 1.0 – 5.5 (variable, broad, exchanges with D₂O)
Identifying the O–H or N–H protons can be confirmed by adding D₂O: the signal disappears because of deuterium exchange. This is an important examination point.
通过加入 D₂O 可以确认 O–H 或 N–H 质子：因氘交换，该信号消失。这是一个重要的考试知识点。
7. Carbon‑13 NMR (¹³C NMR) | 碳‑13 核磁共振
¹³C NMR spectroscopy tells us about the number of non‑equivalent carbon environments in a molecule. Only ¹³C nuclei (1.1% natural abundance) are detected, but modern spectrometers are sensitive enough to obtain spectra. Each signal corresponds to a chemically distinct carbon. The chemical shift ranges are much wider than for ¹H (0 – 220 ppm), and the carbonyl carbons (>160 ppm) are easily identified. Unlike ¹H NMR, ¹³C spectra are usually proton‑decoupled, so splitting is not observed; each signal appears as a singlet.
¹³C NMR 波谱可告知分子中非等价碳环境的数目。只有 ¹³C 核（天然丰度 1.1%）被检测到，但现代波谱仪的灵敏度足以获得谱图。每个信号对应一个化学上独特的碳。¹³C 的化学位移范围比 ¹H 宽得多（0 – 220 ppm），羰基碳 (>160 ppm) 容易识别。与 ¹H NMR 不同，¹³C 谱通常进行质子去耦，因此不观测到裂分；每个信号都是单峰。
Example: the symmetrical molecule ethane‑1,2‑diol (HOCH₂CH₂OH) has two carbon environments, so two ¹³C signals: one for the –CH₂OH carbons (δ ~63) and one for the –OH attached carbon if any? Actually symmetrical, only one type. Propanone (CH₃COCH₃) has two signals: carbonyl carbon at δ ~207 and methyl carbons at δ ~30.
示例：对称分子乙‑1,2‑二醇 (HOCH₂CH₂OH) 有一个碳环境（对称），因此一个 ¹³C 信号：–CH₂OH 碳在 δ ~63。丙酮 (CH₃COCH₃) 有两个信号：羰基碳在 δ ~207，甲基碳在 δ ~30。
8. Principles of Chromatography | 色谱法原理
Chromatography is a separation technique in which the components of a mixture are distributed between a stationary phase and a mobile phase. In thin‑layer chromatography (TLC), a glass or plastic plate coated with a thin layer of silica gel (SiO₂) or alumina (Al₂O₃) acts as the stationary phase, and a solvent moves up the plate by capillary action. In columnar techniques such as gas chromatography (GC) and high‑performance liquid chromatography (HPLC), the mobile phase is a gas or liquid pushed through a column packed with the stationary phase.
色谱是一种分离技术，混合物中的组分在固定相和流动相之间进行分配。在薄层色谱 (TLC) 中，涂有硅胶 (SiO₂) 或氧化铝 (Al₂O₃) 薄层的玻璃或塑料板作为固定相，溶剂通过毛细作用沿板上升。在气相色谱 (GC) 和高效液相色谱 (HPLC) 等柱色谱技术中，流动相是气体或液体，被推过装有固定相的色谱柱。
Each component has a characteristic retention factor (Rf in planar chromatography) or retention time (rt in GC and HPLC). Separation relies on the differential affinities of the components for the stationary and mobile phases, usually through adsorption or partition. More polar components interact more strongly with a polar stationary phase like silica and travel more slowly.
每个组分都有特征的比移值（平面色谱中的 Rf）或保留时间（GC 和 HPLC 中的 rt）。分离依赖于各组分对固定相和流动相的亲和力差异，通常通过吸附或分配。极性越强的组分与如硅胶这样的极性固定相相互作用越强，移动越慢。
9. Thin‑Layer Chromatography (TLC) | 薄层色谱
TLC is used for monitoring the progress of a reaction, checking the purity of a compound and identifying substances by comparison with known standards. A small spot of the sample is placed near the bottom of a TLC plate, and the plate is placed in a developing chamber containing a small depth of solvent. Once the solvent front has travelled an appropriate distance, the plate is removed and dried. Colourless spots can be visualised under UV light or by using a locating agent such as iodine vapour or ninhydrin (for amino acids).
TLC 用于监测反应进程、检查化合物纯度以及通过与已知标准品对比来鉴定物质。取少量样品在 TLC 板下端点上小点，将板放入含有少量展开剂的展开槽中。当溶剂前沿移动到适当距离后，取出板并干燥。无色斑点可在紫外光下观察，或使用显色剂如碘蒸气或茚三酮（用于氨基酸）使之可见。
The Rf value is calculated as: Rf = distance travelled by the spot / distance travelled by the solvent front. Rf values are dimensionless and depend on the stationary phase, solvent system and temperature. In exam questions, comparing Rf values of an unknown with a known standard run on the same plate confirms identity.
Rf 值的计算公式为：Rf = 斑点移动的距离 / 溶剂前沿移动的距离。Rf 值无量纲，且取决于固定相、溶剂系统和温度。在考试题中，通过比较在同一板上展开的未知物与已知标准品的 Rf 值可确证身份。
10. Gas Chromatography (GC) | 气相色谱
Gas chromatography is used to separate volatile compounds. The sample is vaporised and carried by an inert gas (the mobile phase, e.g. helium) through a long, narrow column containing a stationary phase — either a non‑volatile liquid coated on an inert solid support or the inner wall of a capillary column. A detector (often a flame ionisation detector) records components as they elute from the column, producing a chromatogram.
气相色谱用于分离挥发性化合物。样品气化后由惰性气体（流动相，如氦气）运载，通过一根含有固定相的长而窄的色谱柱——固定相可以是涂覆在惰性固体载体上的非挥发性液体，也可以是毛细管柱的内壁。检测器（常为火焰离子化检测器）在各组分从柱中洗脱时记录信号，产生色谱图。
The retention time (rt) is the time taken for a component to pass through the column. Under fixed conditions, a given compound has a reproducible retention time, allowing qualitative identification. The area under a peak is proportional to the amount of that component, enabling quantitative analysis if calibration is performed with standard solutions.
保留时间 (rt) 是某一组分通过色谱柱所需的时间。在固定条件下，给定化合物具有可重现的保留时间，可实现定性鉴定。峰面积与该组分的含量成正比，如果用标准溶液进行了校准，就可以进行定量分析。
11. High‑Performance Liquid Chromatography (HPLC) | 高效液相色谱
HPLC is a high‑pressure adaptation of column chromatography, useful for separating non‑volatile or thermally labile compounds. The stationary phase is packed into a column under high pressure, and the mobile phase is a solvent mixture that is pumped through. Common detectors are UV‑visible absorbance detectors or mass spectrometers. In reverse‑phase HPLC, the stationary phase is non‑polar (e.g. C₁₈ hydrocarbon chains) and the mobile phase is polar – the opposite of normal‑phase. Polar analytes elute first in reverse‑phase HPLC.
高效液相色谱是柱色谱的高压版本，适用于分离非挥发性或受热易分解的化合物。固定相在高压下填充于色谱柱中，流动相是被泵入的混合溶剂。常用检测器为紫外‑可见吸收检测器或质谱仪。在反相 HPLC 中，固定相是非极性的（如 C₁₈ 烃链），流动相是极性的，与正相相反。反相 HPLC 中极性分析物首先洗脱。
In forensic and pharmaceutical analyses, HPLC is routinely used to quantify active ingredients. The retention time and peak area from an HPLC chromatogram serve the same identification and quantitation roles as in GC, but the applications are far broader because the sample does not need to be volatile.
在法医和药物分析中，HPLC 通常用于定量活性成分。HPLC 色谱图中的保留时间和峰面积与 GC 一样发挥鉴定和定量的作用，但由于样品无需挥发，其应用范围要广泛得多。
12. Qualitative Chemical Tests in Organic Analysis | 有机分析中的定性化学检测
While instrumental techniques are paramount, A‑ olourise bromine water (electrophilic addition); aldehydes produce a silver mirror with Tollens’ reagent or a brick‑ ) solution; methyl ketones and secondary alcohols with a methyl group next to the C=O or C– odium carbonate (effervescence) or neutral iron(III) chloride (violet colour for phenols).
虽然仪器技术至关重要，但 A‑Level 大纲仍然要求掌握一些简单的台式试验，以便快速识别官能团。例如，烯烃能使溴水褪色（亲电加成）；醛类与托伦试剂产生银镜，或与费林（或本尼迪克特）溶液产生砖红色沉淀；甲基酮和邻羰基或邻羟基带甲基的二级醇与碘的碱溶液生成黄色的三碘甲烷沉淀（碘仿反应）。酸和酚可通过与碳酸钠的反应（产生气泡）或中性氯化铁试验（酚呈紫色）加以区分。
These tests remain useful in school laboratories and for quick preliminary analysis. However, spectroscopic and chromatographic methods provide unambiguous identification and are the focus of modern analytical chemistry. By integrating all these techniques, a chemist can confidently determine the structure and purity of an unknown compound.
这些检测在学校实验室和快速初步分析中仍然有用。然而，光谱和色谱方法可提供确切的鉴定，是现代分析化学的重点。通过整合所有这些技术，化学家可以可靠地确定未知化合物的结构与纯度。
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