📚 GCSE CIE Chemistry: Mass Spectrometry Essentials | GCSE CIE 化学：质谱 考点精讲
Mass spectrometry is a powerful analytical technique used to determine the relative atomic mass of elements, identify molecular structures, and detect isotopes. For GCSE CIE Chemistry, you need to grasp how a mass spectrometer works, interpret mass spectra, and calculate relative atomic mass from isotopic abundances. This article breaks down every essential concept with clear explanations and examples, ensuring you are fully prepared for exam questions on this topic.
质谱是一种强大的分析技术，用于测定元素的相对原子质量、鉴定分子结构以及探测同位素。对于 GCSE CIE 化学，你需要掌握质谱仪的工作原理、解读质谱图，并根据同位素丰度计算相对原子质量。本文通过清晰的解释与示例，逐一分解所有核心概念，确保你为涉及该专题的考试问题做好充分准备。
1. What Does a Mass Spectrometer Do? | 质谱仪的作用
A mass spectrometer measures the mass-to-charge ratio (m/z) of ions. Since the charge on the ions produced is typically +1, the m/z value directly reflects the mass of the ion. This allows chemists to find the masses of atoms and molecules, and the relative amounts of different isotopes present in a sample.
质谱仪测量离子的质荷比 (m/z)。由于产生的离子通常带 +1 电荷，m/z 值直接反映了离子的质量。这使得化学家可以找出原子和分子的质量，以及样品中不同同位素的相对含量。
2. Key Stages of the Mass Spectrometer | 质谱仪的关键阶段
The instrument operates through four main stages: vaporisation and ionisation, acceleration, deflection, and detection. Each stage must occur under a high vacuum to prevent ions from colliding with air molecules, which would scatter the beam and ruin the results.
该仪器通过四个主要阶段运行：汽化与电离、加速、偏转和检测。每个阶段都必须在高真空下进行，以防止离子与空气分子碰撞，否则会使离子束散射并破坏结果。
3. Vaporisation and Ionisation | 汽化与电离
The sample is first vaporised if it is not already a gas. Then it is bombarded with high-energy electrons from an electron gun. These electrons knock out an outer electron from the sample atoms or molecules, forming positive ions (cations). For an atom X, the process is: X(g) → X⁺(g) + e⁻. This is called electron impact ionisation.
如果样品不是气体，首先将其汽化。然后用来自电子枪的高能电子轰击它。这些电子从样品原子或分子中击出一个外层电子，形成正离子（阳离子）。对于原子 X，过程为：X(g) → X⁺(g) + e⁻。这称为电子轰击电离。
4. Acceleration | 加速
The positively charged ions are attracted towards negatively charged plates and pass through slits that create a narrow beam. An electric field accelerates them so that all ions with the same charge end up with the same kinetic energy. Lighter ions will therefore travel at higher speeds than heavier ions carrying the same charge.
带正电的离子被带负电的极板吸引，穿过产生窄束的狭缝。电场使它们加速，从而使所有带相同电荷的离子最终具有相同的动能。因此，较轻的离子比较重的离子（带相同电荷）行进速度更快。
5. Deflection in a Magnetic Field | 磁场中的偏转
The accelerated ions enter a magnetic field applied at right angles to their path. The magnetic field exerts a force that deflects the ions into a curved trajectory. For ions with the same charge, the amount of deflection depends on their mass: lighter ions are deflected more, heavier ions are deflected less. Also, ions with a higher charge are deflected more. By varying the magnetic field strength, ions of different m/z values can be brought to the detector.
加速后的离子进入与其路径成直角的磁场。磁场施加一个力，使离子偏转成弯曲的轨道。对于带相同电荷的离子，偏转程度取决于它们的质量：较轻的离子偏转更多，较重的离子偏转更少。此外，电荷越高的离子偏转越多。通过改变磁场强度，可以将不同 m/z 值的离子依次引入检测器。
6. Detection and Data Output | 检测与数据输出
When ions strike the detector, an electric current is produced. The size of the current is proportional to the number of ions arriving at that moment. A computer processes these signals and produces a mass spectrum, which plots relative abundance (or relative intensity) on the y-axis against mass-to-charge ratio (m/z) on the x-axis.
当离子撞击检测器时，会产生电流。电流的大小正比于到达的离子数量。计算机处理这些信号并生成质谱图，图上纵轴为相对丰度（或相对强度），横轴为质荷比 (m/z)。
7. Understanding the Mass Spectrum of an Element | 理解元素的质谱图
For a pure element, the mass spectrum shows peaks corresponding to its isotopes. Since each ion carries a 1+ charge, the m/z value gives the mass number of the isotope. The height of each peak represents the relative abundance of that isotope. For example, the mass spectrum of chlorine shows two major peaks at m/z 35 and 37, with heights in a ratio of about 3:1, corresponding to ³⁵Cl and ³⁷Cl.
对于纯元素，质谱图显示出与其同位素对应的峰。由于每个离子带 1+ 电荷，m/z 值即为该同位素的质量数。每个峰的高度代表该同位素的相对丰度。例如，氯的质谱图在 m/z 35 和 37 处有两个主要峰，峰高比约为 3:1，分别对应 ³⁵Cl 和 ³⁷Cl。
8. Calculating Relative Atomic Mass (Ar) | 计算相对原子质量 (Ar)
Relative atomic mass is the weighted average mass of an atom of an element compared to 1/12 the mass of a carbon-12 atom. From a mass spectrum, you can calculate Ar using the formula:
Ar = Σ (isotopic mass × relative abundance) / Σ (relative abundances)
Sometimes abundances are given as percentages. Make sure to convert percentages to numbers (e.g., 75% means 75, and you divide by 100 if expressing as a fraction). Let’s work through an example with magnesium.
相对原子质量是元素一个原子的加权平均质量与一个碳-12 原子质量的 1/12 的比值。根据质谱图，你可以使用公式计算 Ar：
Ar = Σ (同位素质量 × 相对丰度) / Σ (相对丰度之和)
有时丰度以百分比给出。确保将百分比转换为数字（例如，75% 意味着 75，若作为分数则除以 100）。我们以镁为例计算。
9. Worked Example: Magnesium Isotopes | 实例：镁的同位素
The mass spectrum of magnesium shows three peaks:
| m/z (isotopic mass) | Relative abundance (%) |
|---|---|
| 24 | 78.99 |
| 25 | 10.00 |
| 26 | 11.01 |
Step 1: Multiply each isotopic mass by its relative abundance.
24 × 78.99 = 1895.76
25 × 10.00 = 250.00
26 × 11.01 = 286.26
Step 2: Sum these products: 1895.76 + 250.00 + 286.26 = 2432.02
Step 3: Sum the relative abundances: 78.99 + 10.00 + 11.01 = 100.00
Step 4: Divide: Ar = 2432.02 / 100.00 = 24.32 (to 2 decimal places).
This matches the value in the Periodic Table.
质谱图显示镁有三种同位素峰：（表格略，见上）
步骤 1：用每个同位素质量乘以其相对丰度。
24 × 78.99 = 1895.76
25 × 10.00 = 250.00
26 × 11.01 = 286.26
步骤 2：将这些乘积相加：1895.76 + 250.00 + 286.26 = 2432.02
步骤 3：将相对丰度相加：78.99 + 10.00 + 11.01 = 100.00
步骤 4：相除：Ar = 2432.02 / 100.00 = 24.32（保留两位小数）。这与元素周期表中的数值一致。
10. Molecular Mass Spectra and Fragmentation | 分子质谱与碎片化
For molecules, the mass spectrum is more complex. The peak with the highest m/z value (ignoring tiny isotopic peaks) is usually the molecular ion peak, M⁺, which gives the relative molecular mass (Mr) of the compound. However, the molecular ion often breaks apart (fragments) inside the spectrometer. These fragment ions produce peaks at lower m/z values, and the pattern of fragments can act as a ‘fingerprint’ to help identify the compound.
对于分子，质谱图更加复杂。具有最高 m/z 值的峰（忽略微小的同位素峰）通常是分子离子峰 M⁺，它给出了化合物的相对分子质量 (Mr)。然而，分子离子常常在质谱仪内部裂解（碎片化）。这些碎片离子在较低的 m/z 值处产生峰，碎片图谱可以作为“指纹”来帮助鉴定化合物。
11. Diatomic Molecules and Multiple Peaks | 双原子分子与多重峰
When a diatomic element such as chlorine (Cl₂) is analysed, the mass spectrum shows peaks not only for Cl⁺ ions (m/z 35, 37) but also for Cl₂⁺ molecular ions. Because chlorine has two isotopes, Cl₂ molecules can exist in three combinations: ³⁵Cl–³⁵Cl (m/z 70), ³⁵Cl–³⁷Cl (m/z 72), and ³⁷Cl–³⁷Cl (m/z 74). The relative heights of these molecular peaks can be predicted from the isotopic abundances, and they follow simple statistical probabilities.
当双原子元素如氯 (Cl₂) 被分析时，质谱图不仅显示 Cl⁺ 离子（m/z 35、37）的峰，还显示 Cl₂⁺ 分子离子的峰。由于氯有两种同位素，Cl₂ 分子可以存在三种组合：³⁵Cl–³⁵Cl (m/z 70)、³⁵Cl–³⁷Cl (m/z 72) 和 ³⁷Cl–³⁷Cl (m/z 74)。这些分子峰的高度可以根据同位素丰度预测，它们遵循简单的统计概率。
12. Common Exam Tips and Pitfalls | 常见考试技巧与易错点
Always check the charge on the ion. In GCSE CIE, almost all ions are 1+, so m/z equals the mass number. When calculating Ar, use the mass numbers from the spectrum, not the exact isotopic masses. Ensure you use the correct abundances: if given as percentages, the denominator is 100. Do not confuse the molecular ion peak with the base peak (the tallest peak set to 100% relative abundance). The base peak is often a fragment, not the M⁺ peak. Also, remember that the y-axis is relative abundance, not absolute number of ions. If a question asks for the number of isotopes, count the significant peaks in the atomic ion region, ignoring fragments for molecules.
始终检查离子所带电荷。在 GCSE CIE 中，几乎所有离子都是 1+，因此 m/z 等于质量数。计算 Ar 时，使用谱图中的质量数，而不是精确的同位素质量。确保使用正确的丰度：如果以百分比给出，分母为 100。不要将分子离子峰与基峰（设为 100% 相对丰度的最高峰）混淆。基峰常常是碎片，而不是 M⁺ 峰。另外，记住纵轴是相对丰度，而不是离子的绝对数量。如果问题是求同位素数目，则计算原子离子区域中的显著峰，对于分子则忽略碎片峰。
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