📚 Mass Spectrometry in IGCSE Edexcel Chemistry | IGCSE Edexcel 化学：质谱考点精讲
Mass spectrometry is a powerful analytical technique that you must understand for your IGCSE Edexcel Chemistry exam. It is used to measure the masses of atoms and molecules, determine isotopic abundances, and calculate relative atomic masses. This revision guide covers all the key concepts, from the working of the mass spectrometer to interpreting spectra and tackling typical exam questions.
质谱是一种你必须掌握的强大分析技术，用于IGCSE Edexcel化学考试。它用来测量原子和分子的质量，确定同位素丰度，并计算相对原子质量。本篇复习指南涵盖了所有关键概念，从质谱仪的工作原理到解析谱图，再到应对典型考题。
1. What is a Mass Spectrometer? | 什么是质谱仪？
A mass spectrometer is an instrument that separates gaseous ions according to their mass-to-charge ratio (m/z). It provides a mass spectrum, which is a plot of relative abundance against m/z. In IGCSE, we use it mainly to find relative atomic masses and to identify isotopes.
质谱仪是一种根据离子的质荷比（m/z）分离气态离子的仪器。它给出质谱图，即相对丰度对m/z的曲线图。在IGCSE中，我们主要用它来计算相对原子质量和识别同位素。
The machine works under high vacuum to prevent the ions from colliding with air molecules, which would alter their paths.
仪器在高真空下工作，以防止离子与空气分子碰撞，因为那会改变离子的轨迹。
2. Vapourisation and Sample Introduction | 汽化与进样
The sample is injected and heated strongly so that it turns into a gas. This is essential because the ions must be in the vapour phase to be accelerated and deflected by electric and magnetic fields.
样品被注入并剧烈加热，使其转变为气体。这至关重要，因为离子必须处于蒸气相，才能被电场和磁场加速和偏转。
A tiny amount of sample is used, and the entire inlet system is evacuated to maintain the vacuum.
使用的样品量极少，且整个进样系统被抽空以维持真空。
3. Ionisation | 离子化
During ionisation, the vaporised sample is bombarded with a stream of high-energy electrons from an electron gun. These electrons knock out an outer electron from the atoms or molecules, generating positive ions (cations).
在离子化过程中，气化后的样品被来自电子枪的高能电子流轰击。这些电子会打掉原子或分子的外层电子，生成正离子（阳离子）。
The equation for the ionisation of an atom X is: X(g) + e⁻ → X⁺(g) + 2e⁻
原子X的离子化方程式为：X(g) + e⁻ → X⁺(g) + 2e⁻
This process mainly produces singly-charged positive ions, which are then drawn towards the negative plate of the acceleration stage. Usually, molecules form the molecular ion M⁺.
这个过程主要产生单电荷正离子，然后它们被拉向加速阶段的负电板。通常，分子会形成分子离子M⁺。
4. Acceleration | 加速
The positive ions are accelerated by a strong electric field between two charged plates. All the ions gain the same kinetic energy, but their velocities differ because velocity depends on mass for a given kinetic energy.
正离子在两个带电板之间的强电场中被加速。所有离子获得相同的动能，但由于动能一定时速度取决于质量，因此离子的速度不同。
Lighter ions will move faster than heavier ones, which is crucial for the separation that follows.
较轻的离子比较重的离子移动得更快，这对后续的分离至关重要。
5. Deflection by Magnetic Field | 磁场偏转
The fast-moving ions enter a magnetic field at right angles to their path. The magnetic field causes the ions to follow a curved trajectory. The amount of deflection depends on the mass-to-charge ratio (m/z).
快速移动的离子垂直进入磁场。磁场使离子沿曲线轨迹运动。偏转的程度取决于质荷比（m/z）。
Ions with a smaller m/z (lighter or more highly charged) are deflected more, while ions with a larger m/z (heavier or less charged) are deflected less. By varying the magnetic field strength, ions of different m/z values can be brought to the detector one after another.
m/z 较小的离子（较轻或带电荷较多）偏转更多，而 m/z 较大的离子（较重或带电荷较少）偏转较少。通过改变磁场强度，可以使不同 m/z 值的离子依次到达检测器。
6. Detection and Data Output | 检测与数据输出
When ions strike the detector, they gain electrons and produce an electric current. The size of the current is directly proportional to the number of ions hitting the detector, which reflects the relative abundance of that ion in the sample.
当离子撞击检测器时，它们获得电子并产生电流。电流的大小与撞击检测器的离子数量成正比，这反映了样品中该离子的相对丰度。
The signal is amplified and recorded by a computer to produce a mass spectrum: a graph of relative abundance (y-axis) against mass-to-charge ratio, m/z (x-axis). The most abundant peak is called the base peak and is assigned a relative abundance of 100.
信号被放大并由计算机记录下来，生成质谱图：相对丰度（y轴）对质荷比 m/z（x轴）的图。丰度最高的峰称为基峰，其相对丰度定为100。
7. Understanding the Mass Spectrum | 理解质谱图
In a mass spectrum, each peak corresponds to an ion of a particular m/z value. For singly-charged ions, the m/z value directly gives the relative isotopic mass (since z = 1). The height of each peak tells us the relative abundance of that isotope or fragment.
在质谱图中，每个峰对应特定 m/z 值的离子。对于单电荷离子，m/z值直接给出相对同位素质量（因为z=1）。每个峰的高度告诉我们该同位素或碎片的相对丰度。
For atomic mass spectra, we see peaks representing the different isotopes of an element. For example, chlorine gives two principal peaks at m/z = 35 and 37, corresponding to ³⁵Cl⁺ and ³⁷Cl⁺.
对于原子质谱，我们看到代表元素不同同位素的峰。例如，氯在m/z = 35和37处给出两个主要峰，分别对应³⁵Cl⁺和³⁷Cl⁺。
8. Isotopes and Relative Atomic Mass | 同位素与相对原子质量
Isotopes are atoms of the same element with different numbers of neutrons. They have the same chemical properties but different masses. The relative atomic mass (Aᵣ) of an element is the weighted average mass of its isotopes relative to 1/12th the mass of a carbon-12 atom.
同位素是同一元素中中子数不同的原子。它们具有相同的化学性质，但质量不同。元素的相对原子质量（Aᵣ）是其同位素相对于碳-12原子质量的1/12的加权平均质量。
The mass spectrometer provides both the masses of the isotopes (m/z values) and their relative abundances (from peak heights). Using these data, we can calculate Aᵣ.
质谱仪同时提供同位素的质量（m/z值）及其相对丰度（根据峰高）。利用这些数据，我们可以计算Aᵣ。
9. Calculating Relative Atomic Mass from Mass Spectra | 由质谱计算相对原子质量
To calculate Aᵣ, use the formula:
要计算Aᵣ，使用公式：
Aᵣ = (m₁ × %₁ + m₂ × %₂ + …) ÷ 100
where m₁, m₂ are the isotopic masses and %₁, %₂ are the percentage abundances. The abundances can also be given as relative peak heights, in which case divide by the sum of the relative abundances instead of 100.
其中 m₁、m₂ 是同位素质量，%₁、%₂ 是百分比丰度。丰度也可以用相对峰高给出，这时除以相对丰度的总和而非100。
Worked example: Element Q has two isotopes. The mass spectrum shows:
示例： 元素Q有两种同位素。其质谱图显示：
| m/z | Relative abundance (%) |
|---|---|
| 50 | 80 |
| 52 | 20 |
Calculation:
计算过程：
Aᵣ = (50 × 80 + 52 × 20) ÷ 100 = (4000 + 1040) ÷ 100 = 50.4
Thus, the relative atomic mass of Q is 50.4. Remember to include units only if asked; Aᵣ itself is a ratio with no units.
因此，Q的相对原子质量为50.4。注意除非题目要求，否则不写单位；Aᵣ本身是一个比值，无单位。
10. Special Case: Diatomic Gases like Cl₂ | 特殊情况：双原子气体如 Cl₂
When a diatomic element such as chlorine is analysed, the mass spectrum shows peaks not only for the atomic ions (³⁵Cl⁺ and ³⁷Cl⁺) but also for molecular ions: Cl₂⁺. Because there are two isotopes of chlorine, the molecular ion region shows three peaks.
当分析象氯这样的双原子元素时，质谱不仅显示原子离子峰（³⁵Cl⁺和³⁷Cl⁺），还显示分子离子峰：Cl₂⁺。因为氯有两种同位素，分子离子区出现三个峰。
The combinations and their approximate relative probabilities (using ³⁵Cl = 75%, ³⁷Cl = 25%) are:
同位素组合及其近似的相对概率（设³⁵Cl占75%，³⁷Cl占25%）为：
- ³⁵Cl–³⁵Cl⁺ : m/z = 70 ; probability = 0.75 × 0.75 = 0.5625 (relative peak height ~ 9)
- ³⁵Cl–³⁷Cl⁺ : m/z = 72 ; probability = 2 × 0.75 × 0.25 = 0.375 (relative peak height ~ 6)
- ³⁷Cl–³⁷Cl
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