Mass Spectrometry in IGCSE Chemistry | IGCSE 化学质谱考点精讲

📚 Mass Spectrometry in IGCSE Chemistry | IGCSE 化学质谱考点精讲

Mass spectrometry is a powerful analytical technique used to determine the isotopic composition of elements and to calculate relative atomic masses. In IGCSE Chemistry, you need to understand the basic principles of how a mass spectrometer works, interpret simple mass spectra, and use relative abundance data to determine relative atomic masses. This revision guide covers all key points clearly with bilingual explanations.

质谱是一种强大的分析技术，用于确定元素的同位素组成并计算相对原子质量。在 IGCSE 化学中，你需要理解质谱仪的基本工作原理，解读简单的质谱图，并利用相对丰度数据计算相对原子质量。这篇考点精讲以双语清晰讲解所有要点。

1. What is Mass Spectrometry? | 什么是质谱？

Mass spectrometry is an instrumental method that separates gaseous ions according to their mass-to-charge ratio (m/z). It provides information about the isotopes present in a sample and their relative abundances. The technique is widely used to identify elements, determine molecular structures, and measure relative atomic masses.

质谱是一种仪器方法，根据气态离子的质荷比 (m/z) 对其进行分离。它能够提供样品中存在的同位素及其相对丰度的信息。该技术广泛用于鉴定元素、确定分子结构以及测量相对原子质量。

2. The Basic Principle: Ionisation, Acceleration, Deflection, Detection | 基本原理：离子化、加速、偏转、检测

A mass spectrometer operates under vacuum. The sample is first vaporised and then bombarded with high-energy electrons to produce positive ions. These ions are accelerated by an electric field, deflected by a magnetic field according to their m/z, and finally detected.

质谱仪在真空下运行。样品首先被气化，然后用高能电子轰击产生正离子。这些离子被电场加速，根据其 m/z 在磁场中偏转，并最终被检测。

Key steps: Vaporisation → Ionisation → Acceleration → Deflection → Detection.

关键步骤：气化 → 离子化 → 加速 → 偏转 → 检测。

3. The Mass Spectrometer Diagram | 质谱仪示意图

Although you are not required to draw a detailed diagram, you should be able to label the main components: sample inlet, ionisation chamber, accelerating plates, magnetic field region, and detector. The entire path is under a high vacuum to prevent collisions with air molecules.

虽然不要求画出详细示意图，但你应该能够标注主要部件：进样口、离子化室、加速板、磁场区域和检测器。整个路径处于高真空下，以防止与空气分子碰撞。

Sample inlet: introduces the gaseous sample. | 进样口：引入气态样品。
Ionisation chamber: where positive ions are formed. | 离子化室：生成正离子的地方。
Accelerating plates: apply a high voltage to accelerate the ions. | 加速板：施加高压加速离子。
Magnetic field: causes deflection of the ion beam. | 磁场：使离子束偏转。
Detector: records the ions, producing a mass spectrum. | 检测器：记录离子，生成质谱图。

4. Ionisation Methods: Electron Impact | 离子化方法：电子轰击

In electron impact ionisation, the vaporised sample is hit by a stream of high-energy electrons. This knocks out an electron from a sample atom or molecule, forming a positive ion (cation). For an element X: X(g) + e⁻ → X⁺(g) + 2e⁻. Often a single positive charge is formed, so m/z is numerically equal to the mass of the ion.

在电子轰击离子化中，气化后的样品被高能电子流轰击。这会从样品原子或分子中打出一个电子，形成正离子（阳离子）。对于元素 X：X(g) + e⁻ → X⁺(g) + 2e⁻。通常形成单电荷正离子，因此 m/z 数值上等于离子的质量。

For molecules, this can also cause fragmentation – the molecular ion can break into smaller positively charged fragments. In IGCSE, we mainly focus on atomic ions for relative atomic mass calculations.

对于分子，这也可能导致碎片化——分子离子可以断裂成更小的带正电的碎片。在 IGCSE 中，我们主要关注用于相对原子质量计算的原子离子。

5. Acceleration and Deflection by Magnetic Field | 加速与磁场偏转

The positive ions are attracted towards a negatively charged plate and pass through slits to form a beam. They gain kinetic energy: ½mv² = qV, where m is the mass of the ion, v its velocity, q its charge, and V the accelerating voltage. A magnetic field then applies a force perpendicular to the ion’s motion, causing it to move in a curved path.

正离子被负电板吸引，并通过狭缝形成离子束。它们获得动能：½mv² = qV，其中 m 是离子的质量，v 是速度，q 是电荷，V 是加速电压。然后磁场对离子运动施加垂直力，使其沿曲线路径运动。

The radius of curvature r is given by r = √(2mV/qB²) for a fixed magnetic field strength B. Ions with a larger m/z are deflected less (larger radius), while ions with a smaller m/z are deflected more. By varying the magnetic field or accelerating voltage, ions of different m/z are sequentially brought to the detector.

曲率半径 r 在固定磁场强度 B 下为 r = √(2mV/qB²)。m/z 较大的离子偏转较小（半径更大），m/z 较小的离子偏转较大。通过改变磁场或加速电压，不同 m/z 的离子依次到达检测器。

6. Detection and Mass Spectrum | 检测与质谱图

The detector counts the number of ions hitting it per unit time. The result is displayed as a mass spectrum – a graph with mass-to-charge ratio (m/z) on the x-axis and relative abundance on the y-axis. The tallest peak is assigned an abundance of 100, and other peaks are scaled relative to it.

检测器统计单位时间内撞击的离子数。结果以质谱图显示——x 轴为质荷比 (m/z)，y 轴为相对丰度。最高峰被赋予丰度 100，其他峰以此为标准按比例缩放。

For elements, each peak corresponds to an isotope. The peak height (or area) gives the relative abundance of that isotope in the sample.

对于元素，每个峰对应一种同位素。峰高（或峰面积）给出了样品中该同位素的相对丰度。

7. Relative Abundance and Isotopes | 相对丰度与同位素

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. In a mass spectrum, they appear as separate peaks. For example, chlorine has two major isotopes: ³⁵Cl (mass ~35 u) and ³⁷Cl (mass ~37 u). The spectrum shows peaks at m/z = 35 and m/z = 37 with relative abundances roughly in the ratio 3:1.

同位素是同一元素具有相同质子数但不同中子数的原子。在质谱图中，它们表现为分开的峰。例如，氯有两种主要同位素：³⁵Cl（质量约 35 u）和 ³⁷Cl（质量约 37 u）。谱图显示 m/z = 35 和 m/z = 37 处的峰，相对丰度比例约为 3:1。

The relative atomic mass (Aᵣ) takes into account the mass and abundance of each isotope. It is the weighted average of the isotopic masses.

相对原子质量 (Aᵣ) 考虑了每种同位素的质量和丰度。它是同位素质量的加权平均值。

8. Calculating Relative Atomic Mass from Mass Spectrum | 从质谱计算相对原子质量

To calculate Aᵣ, use the formula:

计算 Aᵣ 使用以下公式：

Aᵣ = Σ (isotopic mass × relative abundance) / Σ relative abundance

If the abundances are given as percentages, the denominator is 100:

如果丰度以百分比给出，分母为 100：

Aᵣ = (m₁ × a₁% + m₂ × a₂% + …) / 100

Example: Chlorine mass spectrum shows ³⁵Cl with 75% abundance and ³⁷Cl with 25% abundance.

示例：氯的质谱图显示 ³⁵Cl 丰度为 75%，³⁷Cl 丰度为 25%。

Isotope	Mass (u)	% Abundance
³⁵Cl	35	75
³⁷Cl	37	25

Aᵣ = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5

Thus, the relative atomic mass of chlorine is 35.5. Always show your working in exam questions.

因此，氯的相对原子质量为 35.5。在考试题中务必展示计算过程。

9. Molecular Ion Peak and Fragmentation | 分子离子峰与碎片峰

When a compound is analysed, the peak with the highest m/z (often the rightmost significant peak) usually corresponds to the molecular ion (M⁺). Its m/z value gives the relative molecular mass (Mᵣ) of the compound. For example, methane CH₄ gives a molecular ion peak at m/z = 16.

当分析化合物时，最高 m/z 的峰（通常是最右边的重要峰）通常对应于分子离子 (M⁺)。其 m/z 值给出了化合物的相对分子质量 (Mᵣ)。例如，甲烷 CH₄ 给出 m/z = 16 的分子离子峰。

Fragmentation occurs when the molecular ion breaks apart. Smaller fragment ions produce peaks at lower m/z. The pattern of fragment peaks can act as a ‘fingerprint’ to identify the molecule. In IGCSE, you may be asked to identify the molecular ion peak and deduce the Mᵣ, or to recognise simple fragments like loss of a methyl group (15 u).

当分子离子断裂时发生碎片化。较小的碎片离子在更低 m/z 处产生峰。碎片峰的分布模式可以作为识别分子的“指纹”。在 IGCSE 中，你可能需要确定分子离子峰并推导 Mᵣ，或识别简单的碎片，例如丢失一个甲基（15 u）。

10. Identifying Elements and Molecules | 鉴别元素与分子

A pure element shows only peaks corresponding to its isotopes. The number of peaks and their positions allow you to identify the element (e.g., two peaks for Br at m/z 79 and 81 in approximately equal abundance; three peaks for Mg at 24, 25, 26).

纯元素的质谱只显示其同位素的峰。峰的数量和位置使你能鉴别该元素（例如，溴在 m/z 79 和 81 处有两个丰度大致相等的峰；镁在 24、25、26 处有三个峰）。

For a compound, the molecular ion peak indicates the Mᵣ. Common diatomic molecules like Cl₂ show a characteristic pattern of peaks because of combinations of isotopes: ³⁵Cl-³⁵Cl (70), ³⁵Cl-³⁷Cl (72), ³⁷Cl-³⁷Cl (74) with abundance ratios 9:6:1. This is often not required in IGCSE but is useful for deeper understanding.

对于化合物，分子离子峰指示 Mᵣ。常见的双原子分子如 Cl₂ 因为同位素组合而显示出特征峰型：³⁵Cl-³⁵Cl (70)、³⁵Cl-³⁷Cl (72)、³⁷Cl-³⁷Cl (74) 丰度比为 9:6:1。这在 IGCSE 中通常不要求，但有助于深入理解。

11. Summary of Key Points | 考点总结

Vaporisation and ionisation: Sample is turned into positive ions, usually by electron impact. | 气化与离子化：样品通常通过电子轰击转化为正离子。
Acceleration and deflection: Ions are accelerated by an electric field and deflected by a magnetic field; deflection depends on m/z. | 加速与偏转：离子被电场加速并被磁场偏转；偏转取决于 m/z。
Detection: A mass spectrum plots relative abundance vs m/z. | 检测：质谱图绘制相对丰度对 m/z 的关系。
Isotope peaks: Each peak represents an isotope; peak height = relative abundance. | 同位素峰：每个峰代表一种同位素；峰高 = 相对丰度。
Aᵣ calculation: Weighted average of isotopic masses using relative abundances. | Aᵣ 计算：用相对丰度求同位素质量的加权平均值。
Molecular ion peak: Gives the relative molecular mass of a compound. | 分子离子峰：给出化合物的相对分子质量。
Fragmentation: Provides structural clues; the base peak is the most abundant fragment. | 碎片化：提供结构线索；基峰是丰度最高的碎片。

12. Common Exam Questions and Tips | 常见考题与技巧

Exam tip 1: Always check the axes of the mass spectrum. If the y-axis is labelled ‘relative abundance’, use those numbers directly. If percentages are given, divide any sum by 100. Never forget to state the unit (no unit for Aᵣ, as it is relative).

考试技巧 1：务必检查质谱图的坐标轴。如果 y 轴标记为“相对丰度”，直接使用那些数字。如果给出百分比，求和后除以 100。永远别忘了 Aᵣ 没有单位，因为它是相对值。

Exam tip 2: When calculating Aᵣ, show your multiplication and addition steps clearly. If you use the formula Aᵣ = (m₁ × %₁ + m₂ × %₂) / 100, make sure you’ve correctly identified the mass of each isotope from the m/z axis.

考试技巧 2：计算 Aᵣ 时，清晰地展示乘法和加法步骤。如果你使用公式 Aᵣ = (m₁ × %₁ + m₂ × %₂) / 100，确保你从 m/z 轴上正确识别了每个同位素的质量。

Exam tip 3: For molecular ions, the peak at the highest m/z is not always the base peak. The molecular ion peak might be quite small. Do not confuse the base peak (tallest peak, often a fragment) with the molecular ion peak. The molecular ion peak gives the Mᵣ.

考试技巧 3：对于分子离子，最高 m/z 处的峰不一定是基峰。分子离子峰可能非常小。不要将基峰（最高峰，通常是碎片）与分子离子峰混淆。分子离子峰给出 Mᵣ。

Exam tip 4: When explaining the deflection, remember that ions with larger m/z are deflected less. If you are asked how the instrument separates ions, mention that the magnetic field strength or accelerating voltage is varied to direct ions of a particular m/z onto the detector.

考试技巧 4：在解释偏转时，记住 m/z 较大的离子偏转较小。如果你被问及仪器如何分离离子，提到通过改变磁场强度或加速电压，将特定 m/z 的离子引导至检测器。

Exam tip 5: Some questions may ask you to predict the mass spectrum of a diatomic element like Cl₂ given the isotopic abundances. Use simple combinatorial probability: for Cl₂, the probability of ³⁵Cl-³⁵Cl = (0.75)² = 0.5625, ³⁵Cl-³⁷Cl = 2×0.75×0.25 = 0.375, and ³⁷Cl-³⁷Cl = (0.25)² = 0.0625. Multiply by 100 to get approximate relative abundances. This is a higher-level application, but good to know.

考试技巧 5：有些问题可能要求你预测双原子元素如 Cl₂ 的质谱图，给定同位素丰度。使用简单的组合概率：对于 Cl₂，³⁵Cl-³⁵Cl 的概率 = (0.75)² = 0.5625，³⁵Cl-³⁷Cl = 2×0.75×0.25 = 0.375，³⁷Cl-³⁷Cl = (0.25)² = 0.0625。乘以 100 得到近似的相对丰度。这是一个更高层次的应用，但了解会更好。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Mass Spectrometry in IGCSE Chemistry | IGCSE 化学质谱考点精讲