📚 A-Level CCEA Chemistry: Mass Spectrometry Key Points | A-Level CCEA 化学：质谱 考点精讲
Mass spectrometry (MS) is a cornerstone analytical technique in the CCEA A-Level Chemistry specification. It allows chemists to determine relative atomic and molecular masses, identify isotopic compositions, and uncover structural details of organic molecules. In the exam, you need to explain how a mass spectrometer works, interpret mass spectra, recognise the molecular ion peak and its isotope satellites, and use fragmentation patterns to deduce molecular structure. This article walks you through every key concept, common pitfalls, and typical CCEA-style questions.
质谱法是 CCEA A-Level 化学大纲中的核心分析技术。它帮助化学家测定相对原子质量和分子质量、识别同位素组成，并揭示有机分子的结构细节。考试中，你需要解释质谱仪的工作原理，解读质谱图，识别分子离子峰及其同位素卫星峰，并利用碎片离子峰推断分子结构。本文带你逐一梳理关键概念、常见易错点和典型 CCEA 题型。
1. What Is Mass Spectrometry? | 质谱法是什么？
Mass spectrometry is an analytical method that measures the mass-to-charge ratio (m/z) of ions. A sample is first vaporised, then ionised to produce positively charged species. These ions are separated in an electric or magnetic field according to their m/z values and detected. The output is a mass spectrum, a plot of relative intensity (abundance) against m/z.
质谱法是一种测量离子质荷比（m/z）的分析方法。样品先气化，然后电离生成带正电的物种。这些离子在电场或磁场中按其质荷比分离并被检测。输出的是质谱图，即相对强度（丰度）对 m/z 的图。
The technique can be used to find the relative atomic mass of an element from its isotopic abundance, to determine the relative molecular mass of a compound, and to identify structural fragments. In CCEA exams, you must be comfortable with both the underlying principles and the interpretation of data.
该技术可用于从同位素丰度求算元素的相对原子质量，测定化合物的相对分子质量，以及鉴定结构碎片。在 CCEA 考试中，你必须同时熟练掌握基本原理和数据解读。
2. Ionisation: Forming Positive Ions | 电离：形成正离子
The most common ionisation method in the CCEA syllabus is electron impact (EI). The sample is injected as a vapour and bombarded with a beam of high-energy electrons, typically with an energy of 70 eV. When a high-speed electron collides with a molecule M, it knocks out one of the molecule’s outer electrons, leaving a radical cation called the molecular ion:
CCEA 大纲中最常见的电离方法是电子轰击（EI）。样品以蒸气形式注入，并被高能电子束（通常为 70 eV）轰击。当一个高速电子与分子 M 碰撞时，它会打掉分子外层的一个电子，留下一个称为分子离子的自由基阳离子：
M + e⁻ → M⁺• + 2e⁻
M + e⁻ → M⁺• + 2e⁻
M⁺• is a radical cation because it has one unpaired electron and carries a +1 charge. This species is essential because its m/z value gives the relative molecular mass of the compound (provided it does not fragment immediately).
M⁺• 是自由基阳离子，因为它有一个未成对电子并带 +1 电荷。该物种至关重要，因为它的 m/z 值给出了化合物的相对分子质量（前提是它未立即碎裂）。
3. Acceleration and Deflection | 加速与偏转
After ionisation, the positive ions are accelerated by a strong electric field, giving them all the same kinetic energy (½mv²). They then enter a magnetic field (or a quadrupole analyser) where they are deflected. The degree of deflection depends on the m/z ratio: lighter ions (small m) and ions with a higher charge (larger z) are deflected more than heavier, singly charged ions.
电离后，正离子被强电场加速，使它们具有相同的动能（½mv²）。然后离子进入磁场（或四极杆分析器）发生偏转。偏转的程度取决于质荷比：质量小（m 小）和电荷高（z 大）的离子比质量大、带单电荷的离子偏转更显著。
In a magnetic sector instrument, by gradually changing the strength of the magnetic field or the accelerating voltage, ions of different m/z values are brought into focus on the detector one after another. Modern instruments often use time-of-flight (TOF) or quadrupole analysers, but the CCEA specification focuses on the principle of separation by m/z.
在磁扇形仪器中，通过逐渐改变磁场强度或加速电压，不同 m/z 值的离子依次聚焦到检测器上。现代仪器多采用飞行时间或四极杆分析器，但 CCEA 考纲主要关注按 m/z 分离的原理。
4. Detection and the Mass Spectrum | 检测与质谱图
When the sorted ions hit the detector, they generate a small electric current. The magnitude of the current is proportional to the number of ions arriving, i.e. the relative abundance of that particular ion. A computer processes these signals and displays a mass spectrum.
当排序后的离子撞击检测器时，会产生微小的电流。电流的大小与到达的离子数量成正比，即该离子的相对丰度。计算机处理这些信号并显示质谱图。
The mass spectrum plots relative abundance (often normalised so the tallest peak is 100%) on the vertical axis against the mass-to-charge ratio (m/z) on the horizontal axis. Since most ions carry a single charge (z=1), the m/z value is numerically equal to the mass of the ion in atomic mass units (u). This is why we often label peaks simply with mass numbers.
质谱图以相对丰度（通常归一化使最高峰为 100%）为纵坐标，以质荷比（m/z）为横坐标。由于大多数离子带单个电荷（z=1），m/z 数值上等于离子以原子质量单位（u）计的质量。这就是我们常直接用质量数标注峰的原因。
5. The Molecular Ion Peak (M⁺•) | 分子离子峰（M⁺•）
The molecular ion peak is, in principle, the peak at the highest m/z value in the spectrum (ignoring small isotopic peaks just beyond it). It corresponds to the unfragmented radical cation of the sample molecule. The m/z of the M⁺• peak gives the relative molecular mass (Mr) of the compound.
分子离子峰原则上是指质谱图中 m/z 值最大的峰（忽略紧邻的微小同位素峰）。它对应样品分子未碎裂的自由基阳离子。M⁺• 峰的 m/z 给出了化合物的相对分子质量（Mr）。
However, in some organic compounds (e.g., alcohols, branched alkanes), the molecular ion is unstable and may fragment so rapidly that the M⁺• peak is very small or even absent. In such cases, you must use other clues—such as fragmentation patterns and isotopic peaks—to determine the Mr. The CCEA exam expects you to recognise that the M⁺• peak may not always be the base peak (the tallest peak).
但在某些有机化合物（如醇、支链烷烃）中，分子离子不稳定，可能快速碎裂，导致 M⁺• 峰很小甚至消失。此时必须借助其他线索（如碎片模式、同位素峰）来确定 Mr。CCEA 考试要求你认识到 M⁺• 峰不一定是基峰（最高峰）。
6. M+1 and M+2 Peaks | M+1 和 M+2 峰
Slightly to the right of the M⁺• peak, small peaks at [M+1]⁺• and [M+2]⁺• often appear. These arise from the natural occurrence of heavier isotopes. The most important contributor to the M+1 peak is ¹³C, which has a natural abundance of ~1.1%. Thus, for a molecule containing n carbon atoms, the intensity of the M+1 peak relative to M⁺• is approximately 1.1 × n %.
在 M⁺• 峰的右侧，常出现微小的 [M+1]⁺• 和 [M+2]⁺• 峰。它们源自天然存在的较重同位素。M+1 峰最重要的贡献者是 ¹³C，其天然丰度约为 1.1%。因此，对于含 n 个碳原子的分子，M+1 峰相对于 M⁺• 的强度大约为 1.1 × n %。
The M+2 peak can reveal the presence of elements like chlorine (³⁷Cl, ~24%), bromine (⁸¹Br, ~49%), or sulfur (³⁴S, ~4.2%). If a compound contains one chlorine atom, the M+2 peak is about one‑third the height of the M peak. For one bromine atom, the M and M+2 peaks are almost equal in height.
M+2 峰可揭示氯（³⁷Cl，丰度约 24%）、溴（⁸¹Br，约 49%）或硫（³⁴S，约 4.2%）等元素的存在。若化合物含一个氯原子，M+2 峰高约为 M 峰的三分之一。含一个溴原子时，M 与 M+2 峰几乎等高。
7. Fragmentation of Molecular Ions | 分子离子的碎片化
The electron impact process imparts excess energy to the molecular ion, causing it to break apart. The fragmentation follows predictable pathways that produce a positively charged fragment and a neutral radical. Only the positively charged fragment is detected because the neutral species is not deflected by the magnetic field.
电子轰击过程将多余能量传递给分子离子，使其碎裂。碎裂遵循可预测的路径，产生一个带正电荷的碎片和一个中性自由基。只有正电荷碎片被检测，因为中性物种不被磁场偏转。
For example, in the mass spectrum of propane (CH₃CH₂CH₃), the molecular ion at m/z 44 may lose a methyl radical (•CH₃, mass 15) to give the ethyl cation C₂H₅⁺ at m/z 29. Further loss of H₂ can produce C₂H₃⁺ at m/z 27. The appearance of these fragments helps identify the parent structure.
例如，丙烷（CH₃CH₂CH₃）的质谱中，分子离子 m/z 44 可能失去一个甲基自由基（•CH₃，质量 15），生成乙基阳离子 C₂H₅⁺（m/z 29）。进一步失去 H₂ 可产生 C₂H₃⁺（m/z 27）。这些碎片峰的出现有助于鉴定母体结构。
8. Fragmentation Patterns of Alkanes and Alkyl Halides | 烷烃和卤代烷的碎片模式
Straight-chain alkanes typically show a series of peaks separated by 14 mass units, corresponding to the successive loss of CH₂ groups. The most abundant alkyl cations are those that form the most stable carbocations, e.g. C₃H₇⁺ (m/z 43) and C₄H₉⁺ (m/z 57). Branched alkanes show enhanced peaks at branching points because the resulting secondary or tertiary carbocations are more stable.
直链烷烃通常显示一系列间隔 14 个质量单位的峰，对应逐一失去 CH₂ 基团。最丰富的烷基阳离子是那些能形成最稳定碳正离子的碎片，如 C₃H₇⁺（m/z 43）和 C₄H₉⁺（m/z 57）。支链烷烃在支化点处的峰增强，因为产生的仲或叔碳正离子更稳定。
For alkyl halides (R–X), cleavage of the C–X bond yields an alkyl cation R⁺ and a halogen radical X•. For example, 1‑chloropropane (C₃H₇Cl) often exhibits a peak at m/z 43 (C₃H₇⁺) after losing Cl•. The molecular ion region also shows the characteristic isotope pattern of chlorine or bromine, which is a powerful diagnostic tool.
对于卤代烷（R–X），C–X 键断裂产生烷基阳离子 R⁺ 和卤素自由基 X•。例如，1‑氯丙烷（C₃H₇Cl）在丢失 Cl• 后常出现 m/z 43（C₃H₇⁺）的峰。分子离子区域还显示氯或溴的特征同位素模式，这是强有力的诊断工具。
9. Using Mass Spectra to Deduce Structure | 利用质谱推断结构
To identify an unknown compound, first locate the molecular ion peak and record its m/z. Check the M+1 and M+2 peaks to estimate the number of carbon atoms and detect halogens. Then examine the major fragment ions: the difference between M⁺• and a fragment peak represents a neutral radical lost (e.g., 15 for •CH₃, 29 for •C₂H₅, 35/37 for Cl•).
要鉴定未知化合物，首先确定分子离子峰并记下其 m/z。检查 M+1 和 M+2 峰以估算碳原子数目并检测卤素。然后检查主要碎片离子：M⁺• 与某碎片峰之间的差值代表丢失的中性自由基（例如，差值 15 为 •CH₃，29 为 •C₂H₅，35/37 为 Cl•）。
In CCEA exams, you may be given a mass spectrum alongside other data (e.g., empirical formula, IR spectrum). Look for a peak at m/z 77 (C₆H₅⁺) for an aromatic ring, or a peak at m/z 31 (CH₂OH⁺) for a primary alcohol. Always verify your proposed structure accounts for the major peaks and the isotope pattern.
在 CCEA 考试中，你可能会得到质谱图及其他数据（如最简式、红外光谱）。注意 m/z 77（C₆H₅⁺）指示芳环，m/z 31（CH₂OH⁺）指示伯醇。务必验证你提出的结构能够解释主要峰和同位素模式。
10. Calculating Relative Atomic Mass from Mass Spectra | 计算相对原子质量
The mass spectrum of an element shows peaks for each isotope, with abundances directly proportional to peak heights. The relative atomic mass
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