📚 Mass Spectrometry in IGCSE WJEC Chemistry | IGCSE WJEC 化学：质谱考点精讲
Mass spectrometry is a powerful analytical technique used to determine the relative atomic mass of elements and to identify the structure of molecules by measuring the mass-to-charge ratio (m/z) of ions. In the WJEC IGCSE Chemistry syllabus, students are expected to understand the principles of the mass spectrometer, interpret simple mass spectra, and perform calculations of relative atomic mass from isotopic abundance data.
质谱是一种强大的分析技术，通过测量离子的质荷比 (m/z) 来测定元素的相对原子质量并鉴定分子结构。在 WJEC IGCSE 化学课程中，学生需要理解质谱仪的工作原理，解读简单的质谱图，并利用同位素丰度数据计算相对原子质量。
1. Introduction to Mass Spectrometry | 质谱简介
Mass spectrometry allows chemists to determine the mass of individual atoms or molecules by converting the sample into gaseous positive ions and then separating these ions according to their mass-to-charge ratio. It is essential for obtaining accurate atomic masses listed in the periodic table and for identifying unknown compounds via fragmentation patterns.
质谱技术通过将样品转化为气态正离子，然后按质荷比将这些离子分离，使化学家能够测定单个原子或分子的质量。它对于获取元素周期表中精确的原子量以及通过碎片化规律鉴定未知化合物至关重要。
2. Principle of Mass Spectrometry | 质谱原理
The fundamental principle is that charged particles moving in a magnetic field are deflected by an amount that depends on their mass and charge. Lighter ions with higher charges are deflected more, while heavier ions with lower charges are deflected less. By measuring the deflection, the instrument plots an abundance against m/z spectrum.
基本原理是：在磁场中运动的带电粒子会发生偏转，偏转程度取决于其质量和电荷。质量较轻、电荷较高的离子偏转更大，而质量较重、电荷较低的离子偏转较小。通过测量偏转，仪器可绘制出丰度对质荷比（m/z）的谱图。
3. The Mass Spectrometer – Instrumentation | 质谱仪组成
A mass spectrometer typically operates in four main stages, all under high vacuum to prevent collisions with air molecules: ionisation, acceleration, deflection, and detection. Modern instruments may vary in design, but the core principles remain as required by WJEC.
质谱仪通常在高真空条件下分四个主要阶段运行，以防止与空气分子碰撞：离子化、加速、偏转和检测。现代仪器在设计中可能有所不同，但核心原理与 WJEC 大纲要求一致。
4. Step 1: Ionisation | 第一步：离子化
The sample is injected as a vapour and bombarded with high-energy electrons from an electron gun. This knocks out an electron from each atom or molecule, forming positively charged ions (cations): X(g) + e⁻ → X⁺(g) + 2e⁻. These ions are often referred to as molecular ions, M⁺, if the sample is a molecule.
样品以蒸气形式进入并被来自电子枪的高能电子轰击。这会使每个原子或分子失去一个电子，形成带正电荷的离子（阳离子）：X(g) + e⁻ → X⁺(g) + 2e⁻。如果样品是分子，这些离子通常被称为分子离子 M⁺。
In electron impact ionisation, the high energy can also cause the molecular ion to break apart into smaller fragments – a process called fragmentation – which provides structural information.
在电子轰击离子化中，高能量还可能使分子离子分解成更小的碎片——这一过程称为碎片化——从而提供结构信息。
5. Step 2: Acceleration | 第二步：加速
The positive ions are attracted towards negatively charged plates and are accelerated by a strong electric field. All ions gain the same kinetic energy (½mv²) but because their masses differ, lighter ions move faster than heavier ones.
正离子被带负电的极板吸引，并在强电场中加速。所有离子获得相同的动能（½mv²），但由于质量不同，较轻的离子比较重的离子运动得更快。
Kinetic energy = ½mv², therefore velocity v = √(2KE/m)
动能 = ½mv²，因此速度 v = √(2KE/m)
6. Step 3: Deflection | 第三步：偏转
The high-speed ions enter a magnetic field at right angles to their path. The magnetic force acts as a centripetal force, causing the ions to move in a curved path. The radius of curvature depends on the m/z ratio: ions with a low m/z are deflected more than those with a high m/z.
高速离子进入与其运动方向垂直的磁场。磁力充当向心力，使离子沿曲线路径运动。曲率半径取决于 m/z 比：m/z 低的离子比 m/z 高的离子偏转更大。
By varying the strength of the magnetic field or the accelerating voltage, ions of different m/z can be brought to the detector one after another, producing a mass spectrum.
通过改变磁场强度或加速电压，不同 m/z 的离子可以依次到达检测器，从而产生质谱图。
7. Step 4: Detection | 第四步：检测
When ions hit the detector, they generate a small electric current proportional to their abundance. The signal is amplified and recorded, and a computer plots a graph of relative abundance (y-axis) against the mass-to-charge ratio, m/z (x-axis). Since the charge is usually +1, m/z is numerically equal to the ion’s mass.
当离子撞击检测器时，会产生与其丰度成正比的小电流。信号被放大并记录，计算机绘制出相对丰度（y轴）对质荷比 m/z（x轴）的图谱。由于电荷通常为 +1，m/z 数值上等于离子的质量。
8. Interpreting a Mass Spectrum | 质谱图解读
A mass spectrum consists of a series of vertical lines (peaks) at different m/z values. Each peak corresponds to a specific ion. The peak with the greatest intensity is called the base peak and is assigned a relative abundance of 100%, with all other peaks scaled accordingly.
质谱图由一系列位于不同 m/z 值的垂直线（峰）组成。每个峰对应一种特定的离子。强度最高的峰称为基峰，被赋予相对丰度 100%，其他所有峰均据此按比例缩放。
The mass spectrum of an element shows peaks for its isotopes, while that of a compound shows a molecular ion peak and various fragment ion peaks.
元素的质谱图展示其同位素峰，而化合物的质谱图则显示分子离子峰和各种碎片离子峰。
9. Molecular Ion Peak (M⁺) | 分子离子峰
The molecular ion peak is the peak corresponding to the unfragmented parent ion, M⁺, formed by loss of one electron from the original molecule. It appears at the highest m/z value in the spectrum (ignoring isotope peaks) and gives the relative molecular mass (Mᵣ) of the compound.
分子离子峰对应于未碎裂的母体离子 M⁺，由原始分子失去一个电子形成。它出现在质谱图的最高 m/z 值处（忽略同位素峰），提供化合物的相对分子质量 (Mᵣ)。
In some cases the molecular ion peak may be very small or absent if the ion is particularly unstable and fragments completely.
在某些情况下，如果分子离子特别不稳定并完全碎片化，分子离子峰可能很小或缺失。
10. Fragment Ions and Base Peak | 碎片离子与基峰
Fragment ions are produced when the molecular ion breaks down. They appear at lower m/z values and their pattern acts like a fingerprint for the molecule. The most abundant fragment (the base peak) often corresponds to the most stable carbocation or other ion formed during fragmentation.
碎片离子由分子离子分解产生。它们出现在较低的 m/z 值，其分布模式如同分子的指纹。最丰富的碎片（基峰）通常对应于碎裂过程中形成的最稳定的碳正离子或其他离子。
By examining the differences in m/z between peaks, chemists can deduce which parts of the molecule have been lost, e.g., loss of 15 u suggests a CH₃ group, loss of 29 u could be C₂H₅ or CHO.
通过检查峰之间的 m/z 差值，化学家可以推断分子中丢失了哪些基团，例如丢失 15 u 提示 CH₃ 基团，丢失 29 u 可能是 C₂H₅ 或 CHO。
11. Isotopic Peaks and Relative Atomic Mass | 同位素峰与相对原子质量
Elements that have naturally occurring isotopes give multiple peaks in their mass spectrum. For example, chlorine exists as ³⁵Cl and ³⁷Cl with relative abundances of 75% and 25%, giving a 3:1 ratio of peaks at m/z 35 and 37. The mass spectrum of Cl₂ shows peaks at m/z 70, 72 and 74 due to combinations of isotopes.
具有天然同位素的元素在其质谱图中会出现多个峰。例如，氯以 ³⁵Cl 和 ³⁷Cl 存在，相对丰度分别为 75% 和 25%，在 m/z 35 和 37 处出现 3:1 的峰比例。Cl₂ 的质谱图由于同位素组合，在 m/z 70、72 和 74 处出现峰。
The relative atomic mass (Aᵣ) of an element can be calculated from the mass spectrum by taking the weighted average of the masses of its isotopes, using the relative abundances as weights.
元素的相对原子质量 (Aᵣ) 可以从质谱图计算，利用同位素质量的加权平均值，以相对丰度作为权重。
12. Calculation of Relative Atomic Mass | 相对原子质量计算
To calculate Aᵣ from a mass spectrum, use the formula: Aᵣ = Σ (isotopic mass × relative abundance) / Σ (relative abundances). The following table shows a simple example for magnesium.
要根据质谱图计算 Aᵣ，使用公式：Aᵣ = Σ（同位素质量 × 相对丰度）/ Σ（相对丰度）。下表为镁的简单示例。
| Isotope 同位素 | Mass 质量 | Relative Abundance (%) 相对丰度 (%) |
|---|---|---|
| ²⁴Mg | 24 | 79 |
| ²⁵Mg | 25 | 10 |
| ²⁶Mg | 26 | 11 |
Aᵣ = (24×79 + 25×10 + 26×11) / (79+10+11) = (1896 + 250 + 286) / 100 = 2432/100 = 24.32
WJEC exam questions commonly provide a mass spectrum or abundance data and require students to perform such a calculation, rounding the answer to an appropriate number of decimal places.
WJEC 考试题目通常提供质谱图或丰度数据，要求学生进行此类计算，并将答案修约至适当的小数位数。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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