Mass Spectrometry for IGCSE AQA Chemistry | IGCSE AQA 化学：质谱考点精讲

📚 Mass Spectrometry for IGCSE AQA Chemistry | IGCSE AQA 化学：质谱考点精讲

Mass spectrometry is a powerful technique used to determine the relative atomic mass of elements and identify isotopes. In IGCSE AQA Chemistry, understanding how to interpret mass spectra and calculate Ar values is a key skill that frequently appears in examinations. This article breaks down the essential theory, instrument steps, and worked examples to help you master the topic.

质谱法是一种用于测定元素相对原子质量和识别同位素的重要技术。在IGCSE AQA化学中，理解如何解读质谱图并计算Ar值是经常考察的关键技能。本文分解了基本理论、仪器步骤和例题，帮助你掌握这一主题。

1. What Is Mass Spectrometry? | 什么是质谱法？

Mass spectrometry is an analytical method that measures the mass-to-charge ratio (m/z) of ions. For an element, the mass spectrometer separates its isotopes and records their relative abundances. This allows chemists to calculate the average relative atomic mass you find on the periodic table.

质谱法是一种测量离子质荷比（m/z）的分析方法。对于一种元素，质谱仪可以分离它的各种同位素并记录它们的相对丰度。这使得化学家能够计算出你在元素周期表上看到的平均相对原子质量。

The technique relies on producing positive ions from a sample, accelerating them, and then deflecting them by a magnetic field. Lighter ions deflect more, while heavier ions deflect less. By detecting the number of ions hitting a specific point, a mass spectrum is generated.

该技术依靠从样品产生正离子，加速它们，然后通过磁场使它们偏转。较轻的离子偏转更多，较重的离子偏转较少。通过检测撞击特定点的离子数量，生成质谱图。

2. Basic Principles of a Mass Spectrometer | 质谱仪的基本原理

A mass spectrometer operates under vacuum to prevent air molecules from interfering with the ions. The main stages are: vaporisation (if the sample is not already a gas), ionisation, acceleration, deflection, and detection. Each stage plays a critical role in producing a clear mass spectrum.

质谱仪在真空下操作，以防止空气分子干扰离子。主要阶段包括：气化（如果样品不是气体）、电离、加速、偏转和检测。每个阶段在生成清晰的质谱图中都起着关键作用。

It is important to remember that only positively charged ions are produced in the ionisation chamber. This is because the electric field used for acceleration can only attract positive ions towards the negative plate.

重要的是要记住，在电离室中只产生带正电荷的离子。这是因为用于加速的电场只能将正离子吸引向负极板。

3. Stage 1 – Ionisation | 第一阶段 – 电离

Inside the ionisation chamber, a heated filament emits high-energy electrons. These electrons collide with the sample atoms (or molecules), knocking out an outer electron to form positive ions. For example: Mg(g) + e⁻ → Mg⁺(g) + 2e⁻. The resulting species is a 1+ charged ion which has lost one electron.

在电离室内，一根加热的灯丝发射出高能电子。这些电子与样品原子（或分子）碰撞，击出一个外层电子，形成正离子。例如：Mg(g) + e⁻ → Mg⁺(g) + 2e⁻。产生的粒子是带1+电荷的离子，失去一个电子。

Most ions produced carry a single positive charge, so their m/z value is numerically equal to their relative isotopic mass. However, some ions may carry a 2+ charge, leading to peaks at half the m/z value; these are rarely tested at IGCSE level.

大多数产生的离子带单个正电荷，因此它们的m/z值在数值上等于相对同位素质量。然而，一些离子可能带2+电荷，导致在m/z值一半处出现峰；这在IGCSE阶段很少考查。

4. Stage 2 – Acceleration | 第二阶段 – 加速

The positive ions are attracted towards a negative plate and accelerated through an electric field. All ions with the same charge gain the same kinetic energy. However, their velocity depends on their mass – lighter ions move faster than heavier ions.

正离子被吸引向负极板，并通过电场加速。所有带相同电荷的离子获得相同的动能。然而，它们的速度取决于质量——轻离子比重离子移动得更快。

This difference in speed is crucial for separation in the magnetic field to follow. Without acceleration, the ions would not reach the necessary velocity for deflection and detection.

这种速度差异对于接下来在磁场中的分离至关重要。如果没有加速，离子将无法达到偏转和检测所需的速度。

5. Stage 3 – Deflection | 第三阶段 – 偏转

The accelerated ions pass through a magnetic field, which deflects them according to their m/z ratio. The strength of the magnetic field can be varied to bring ions of different masses onto the detector. Ions with lower mass (or higher charge) are deflected more, while heavier ions are deflected less.

加速后的离子通过一个磁场，该磁场根据它们的质荷比使它们偏转。磁场的强度可以变化，以使不同质量的离子到达检测器。质量较小（或电荷较高）的离子偏转更多，而较重的离子偏转较少。

By carefully adjusting the magnetic field, the mass spectrometer scans a range of m/z values sequentially, building up the mass spectrum peak by peak.

通过仔细调节磁场，质谱仪按顺序扫描一系列m/z值，逐个峰地构建质谱图。

6. Stage 4 – Detection | 第四阶段 – 检测

When an ion hits the detector, it creates an electrical signal proportional to the number of ions arriving at that moment. This signal is recorded and displayed as a peak on the mass spectrum. The x-axis shows the m/z ratio, and the y-axis shows relative abundance (often as a percentage).

当离子撞击检测器时，它会产生一个与此刻到达的离子数量成正比的电信号。该信号被记录并在质谱图上显示为一个峰。x轴显示m/z比值，y轴显示相对丰度（通常为百分比）。

The tallest peak is often assigned an abundance of 100% and is called the base peak. Other peaks are measured relative to this peak. In element spectra, the base peak usually belongs to the most abundant isotope.

最高的峰通常被赋予100%的丰度，称为基峰。其他峰都是相对于此峰测量的。在元素谱图中，基峰通常属于最丰富的同位素。

7. Interpreting Mass Spectra of Elements | 元素质谱图的解读

For monatomic elements, each peak in the mass spectrum corresponds to a particular isotope. The m/z value at the peak centre indicates the isotopic mass number (since charge = +1). The height of the peak shows the relative abundance of that isotope.

对于单原子元素，质谱图中的每个峰对应一种特定的同位素。峰中心的m/z值表示同位素质量数（因为电荷 = +1）。峰的高度显示该同位素的相对丰度。

If you know the mass of each isotope and its relative abundance, you can calculate the mean mass of an atom taking into account the proportions in which the isotopes occur. This is the relative atomic mass, Ar.

如果你知道每种同位素的质量及其相对丰度，就可以在考虑各同位素所占比例的情况下计算出原子的平均质量。这就是相对原子质量Ar。

8. Calculating Relative Atomic Mass | 计算相对原子质量

The formula for relative atomic mass from mass spectrum data is:

从质谱数据计算相对原子质量的公式是：

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

When abundances are given as percentages, this simplifies to the sum of (mass × percentage) divided by 100. If abundances are relative heights, simply divide by their total sum.

当丰度以百分比给出时，简化为（质量 × 百分比）之和除以100。如果丰度是相对高度，则只需除以它们的总和。

Always show your working clearly. Multiply each isotopic mass by its percentage abundance, add the products together, then divide by 100 (or the total abundance). Present the final Ar to an appropriate number of significant figures.

始终清晰地展示你的计算步骤。将每种同位素的质量乘以其丰度百分比，相加后再除以100（或总丰度）。将最终Ar值保留到适当的有效数字位数。

9. Worked Example: Chlorine | 例题：氯

A mass spectrum of chlorine shows two peaks: one at m/z = 35 with 75% abundance, and another at m/z = 37 with 25% abundance. Calculate the relative atomic mass of chlorine.

氯的质谱图显示两个峰：一个在 m/z = 35，丰度75%；另一个在 m/z = 37，丰度25%。计算氯的相对原子质量。

Solution: Ar = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5. This matches the value on the periodic table. In reality, chlorine’s spectrum also includes small peaks at m/z = 70, 72, 74 from Cl2+ ions but those are ignored for Ar calculation.

解法：Ar = (35 × 75 + 37 × 25) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5。这与元素周期表上的数值吻合。实际上，氯的谱图还包括来自Cl2+离子的m/z = 70、72、74等小峰，但在计算Ar时忽略这些。

10. Worked Example: Boron | 例题：硼

The mass spectrum of boron has peaks at m/z = 10 (relative abundance 20) and m/z = 11 (relative abundance 80). Determine the Ar of boron.

硼的质谱图在 m/z = 10（相对丰度20）和 m/z = 11（相对丰度80）处有峰。计算硼的Ar。

Here the total abundance is 100, so Ar = (10 × 20 + 11 × 80) / 100 = (200 + 880) / 100 = 1080 / 100 = 10.8. The periodic table value is about 10.8, confirming the calculation.

这里的总丰度为100，所以Ar = (10 × 20 + 11 × 80) / 100 = (200 + 880) / 100 = 1080 / 100 = 10.8。元素周期表值约为10.8，计算无误。

11. Common Mistakes and Tips | 常见错误与技巧

Many students forget to include the charge when writing the m/z label; remember that m/z stands for mass-to-charge ratio. On an element spectrum, the x-axis is labelled m/z (or sometimes mass/charge), and the y-axis is relative abundance. Do not confuse it with mass alone.

许多学生在写m/z标签时忘记包含电荷；记住m/z代表质荷比。在元素谱图上，x轴标记为m/z（有时为质量/电荷），y轴为相对丰度。不要将其与单独的质量混淆。

When calculating Ar, always multiply the mass by the abundance first and then sum. Never average the masses directly. Also check your answer against the periodic table – a large discrepancy indicates a mistake.

在计算Ar时，始终先将质量乘以丰度，然后求和。千万不要直接对质量求平均。还要对照元素周期表检查你的答案——较大的偏差表明计算有误。

Another pitfall is misreading the abundance scale: sometimes it is given as relative heights, not percentages. In such cases, the same formula applies but you divide by the total sum of the heights.

另一个易错点是误读丰度标度：有时它以相对高度给出，而不是百分比。在这种情况下，相同的公式适用，但你要除以高度的总和。

12. Summary and Key Takeaways | 总结与重要知识点

The mass spectrometer determines the relative atomic mass of an element by separating its isotopes. The key processes are ionisation (by electron impact), acceleration by an electric field, deflection in a magnetic field, and detection to produce a mass spectrum.

质谱仪通过分离元素的同位素来测定其相对原子质量。关键过程包括电离（通过电子轰击）、电场加速、磁场偏转以及检测生成质谱图。

From a mass spectrum, you can identify the isotopic masses and their percentages, then calculate the weighted average Ar. This skill is examined regularly in the IGCSE AQA Chemistry papers and reinforces your understanding of isotopic abundance.

从质谱图中，你可以识别同位素质量及其百分比，然后计算加权平均Ar。这一技能在IGCSE AQA化学试卷中经常考查，并能加深你对同位素丰度的理解。

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Mass Spectrometry for IGCSE AQA Chemistry | IGCSE AQA 化学：质谱 考点精讲