IGCSE OCR Chemistry: Mass Spectrometry Key Points | IGCSE OCR 化学：质谱考点精讲

📚 IGCSE OCR Chemistry: Mass Spectrometry Key Points | IGCSE OCR 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 unknown compounds. In IGCSE OCR Chemistry, you need to understand how a mass spectrometer works, how to interpret simple mass spectra, and how to calculate relative atomic masses from isotopic abundance data.

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和鉴定未知化合物。在 IGCSE OCR 化学中，你需要理解质谱仪的工作原理、学会解读简单的质谱图，并能够利用同位素丰度数据计算相对原子质量。

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

Mass spectrometry is an instrumental method that separates charged particles (ions) according to their mass-to-charge ratio (m/z). It can provide accurate information about the isotopic composition of an element and the relative molecular mass of a compound.

质谱是一种根据离子的质荷比 (m/z) 分离带电粒子的仪器分析方法。它能提供元素同位素组成的准确信息以及化合物的相对分子质量。

In a mass spectrum, the x-axis represents the mass-to-charge ratio (m/z), and the y-axis shows the relative abundance (or relative intensity) of each ion detected.

在质谱图中，x 轴代表质荷比 (m/z)，y 轴显示每种被检测到的离子的相对丰度（或相对强度）。

Since the charge on most ions formed is +1, the m/z value directly gives the mass of the ion in atomic mass units (u).

由于大多数形成的离子带 +1 电荷，因此 m/z 值直接以原子质量单位 (u) 给出离子的质量。

2. Main Stages of a Mass Spectrometer | 质谱仪的主要工作阶段

A mass spectrometer operates under a high vacuum and involves four key stages: ionisation, acceleration, deflection, and detection. The vacuum is essential to prevent ions from colliding with air molecules, which would alter their paths.

质谱仪在高真空条件下运行，包含四个关键阶段：电离、加速、偏转和检测。真空环境是必要的，以防止离子与空气分子碰撞而改变其路径。

Understanding the purpose and conditions of each stage is a common OCR exam requirement.

理解每个阶段的目的和条件是 OCR 考试中常见的要求。

Ionisation: sample is converted into positive ions.
Acceleration: ions are speeded up by an electric field.
Deflection: ions are bent by a magnetic field according to their m/z.
Detection: ions hit a detector, creating a current proportional to abundance.

电离：样品转化为正离子。
加速：离子通过电场加速。
偏转：离子在磁场中根据其 m/z 发生偏转。
检测：离子撞击检测器，产生与丰度成正比的电流。

3. Ionisation – Electron Impact | 电离——电子轰击法

In the ionisation chamber, a vaporised sample is bombarded with high-energy electrons emitted from a heated filament. An electron is knocked out of a sample atom or molecule, forming a positive ion (M⁺·).

在电离室中，气化的样品被热灯丝发射的高能电子轰击。样品原子或分子被打出一个电子，形成正离子 (M⁺·)。

For example, magnesium atoms become Mg⁺ ions: Mg(g) + e⁻ → Mg⁺(g) + 2e⁻. Usually only one electron is lost, giving a singly charged ion with m/z equal to the isotopic mass.

例如，镁原子变成 Mg⁺ 离子：Mg(g) + e⁻ → Mg⁺(g) + 2e⁻。通常只失去一个电子，产生带单电荷的离子，其 m/z 等于同位素质量。

The ionised particles may also break into fragments; however, at IGCSE level, we mainly focus on molecular ion peaks or the intact atomic ions for elements.

电离的粒子也可能碎裂成碎片；但在 IGCSE 阶段，我们主要关注分子离子峰或元素的完整原子离子。

Key fact: the positive ions are then attracted out of the ionisation chamber by a negatively charged plate.

关键事实：然后正离子被带负电的极板吸引，离开电离室。

4. Acceleration | 加速

The positive ions pass through an electric field with a high potential difference (often several thousand volts). This accelerates all ions to a high, constant kinetic energy.

正离子通过具有高电势差（通常几千伏）的电场。这使得所有离子加速到高且恒定的动能。

Because ions have the same kinetic energy, lighter ions travel faster than heavier ions. This is crucial for the subsequent separation by the magnetic field.

由于离子具有相同的动能，轻离子比重离子运动得更快。这对于后续的磁场分离至关重要。

The relationship can be expressed as: KE = ½mv². With constant KE, velocity v is inversely proportional to the square root of mass m.

这一关系可表述为：KE = ½mv²。在 KE 恒定的情况下，速度 v 与质量 m 的平方根成反比。

5. Deflection by a Magnetic Field | 磁场偏转

The accelerated ions enter a strong magnetic field applied at right angles to their path. The magnetic field exerts a force on moving charged particles, causing them to follow a curved trajectory.

加速后的离子进入与其运动方向垂直的强磁场。磁场对运动带电粒子施加力，使其沿曲线轨迹运动。

The radius of curvature depends on the mass-to-charge ratio (m/z). Lighter ions or those with a higher charge are deflected more. For singly charged ions, the lighter ones are deflected more easily.

曲率半径取决于质荷比 (m/z)。较轻的离子或带电荷较多的离子偏转更大。对于单电荷离子，质量越轻偏转越大。

By varying the magnetic field strength, ions of different m/z values are brought to focus on the detector one after another, producing the mass spectrum.

通过改变磁场强度，不同 m/z 值的离子被依次聚焦到检测器上，产生质谱图。

6. Detection and Data Output | 检测与数据输出

When ions hit the detector plate, they gain electrons and generate a small electric current. The size of this current is proportional to the number of ions striking the detector per unit time – that is, the relative abundance.

当离子撞击检测器板时，它们获得电子并产生微小的电流。该电流的大小与单位时间内撞击检测器的离子数量成正比——即相对丰度。

A computer processes the signal and displays the mass spectrum as a bar graph of relative abundance against m/z. The peak with the highest intensity is called the base peak, assigned an abundance of 100 %.

计算机处理信号，并将质谱图显示为相对丰度对 m/z 的条形图。强度最高的峰称为基峰，其丰度定为 100%。

The detector needs to be very sensitive because the ion currents are extremely small (in the picoampere range).

检测器需要非常灵敏，因为离子电流极小（在皮安培范围内）。

7. Molecular Ion Peak and Base Peak | 分子离子峰与基峰

For a molecular compound, the peak at the highest m/z in the mass spectrum usually corresponds to the molecular ion, M⁺. This ion is formed by the loss of one electron from the intact molecule, so its m/z is equal to the relative molecular mass (Mr).

对于分子化合物，质谱图中最高 m/z 处的峰通常对应于分子离子 M⁺。该离子由完整分子失去一个电子形成，因此其 m/z 等于相对分子质量 (Mr)。

For example, a simple alcohol like ethanol (C₂H₅OH) shows a molecular ion peak at m/z = 46. The base peak is the tallest peak, often arising from the most stable fragment; in ethanol it often appears at m/z = 31 due to CH₂OH⁺.

例如，乙醇 (C₂H₅OH) 这样简单的醇在 m/z = 46 处显示分子离子峰。基峰是最高的峰，通常来自最稳定的碎片；在乙醇中基峰常出现在 m/z = 31，对应于 CH₂OH⁺。

In IGCSE OCR papers, you are more likely to see atomic mass spectra for elements (isotopic peaks) rather than complex fragmentation patterns.

在 IGCSE OCR 试卷中，你更可能看到元素的原子质谱（同位素峰），而不是复杂的碎片化模式。

8. Isotopes and Mass Spectra of Elements | 同位素与元素质谱图

Many elements exist as a mixture of isotopes. A mass spectrum of an element shows separate peaks for each isotope, with heights proportional to their natural abundance.

许多元素以同位素混合物的形式存在。元素的质谱图显示各同位素的独立峰，峰高与其天然丰度成正比。

For instance, magnesium has three stable isotopes: ²⁴Mg (79 %), ²⁵Mg (10 %) and ²⁶Mg (11 %). Its mass spectrum displays three peaks at m/z 24, 25, and 26, with intensities in the ratio approximately 79:10:11.

例如，镁有三种稳定同位素：²⁴Mg (79%)、²⁵Mg (10%) 和 ²⁶Mg (11%)。其质谱图在 m/z 24、25 和 26 处显示三个峰，强度比大约为 79:10:11。

Exam questions often ask you to calculate the relative atomic mass (Ar) using such data, or to sketch the mass spectrum from given isotopic abundances.

考试题目通常要求你利用这些数据计算相对原子质量 (Ar)，或根据给定的同位素丰度画出质谱示意图。

9. Calculating Relative Atomic Mass from Mass Spectra | 从质谱图计算相对原子质量

The relative atomic mass (Ar) is the weighted mean mass of an atom of an element relative to 1/12th the mass of an atom of carbon-12. Using mass spectral data, it is calculated with the formula:

相对原子质量 (Ar) 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 之比。利用质谱数据，计算公式为：

Ar = Σ (isotopic mass × % abundance) ÷ 100

If abundances are given as proportions (decimals), simply multiply each isotopic mass by its fractional abundance and sum the results. Remember, the sum of all isotopic abundances must be 100 % or 1.

如果丰度以比例（小数）给出，只需将各同位素质量乘以各自的小数丰度并求和。切记，所有同位素丰度之和必须为 100% 或 1。

Worked example: Natural boron consists of 20.0 % ¹⁰B (mass = 10.0 u) and 80.0 % ¹¹B (mass = 11.0 u).
Ar(B) = (10.0 × 20.0 + 11.0 × 80.0) ÷ 100 = (200 + 880) ÷ 100 = 10.8.

计算实例：天然硼由 20.0% ¹⁰B (质量 = 10.0 u) 和 80.0% ¹¹B (质量 = 11.0 u) 组成。
Ar(B) = (10.0×20.0 + 11.0×80.0) ÷ 100 = (200 + 880) ÷ 100 = 10.8。

OCR questions may also present the mass spectrum as a table; always check the m/z values and their corresponding relative intensities carefully.

OCR 考题也可能将质谱数据以表格形式呈现；务必仔细核对 m/z 值及其对应的相对强度。

10. Identifying Diatomic Elements: The Case of Chlorine | 识别双原子元素：以氯为例

Chlorine exists as diatomic Cl₂ molecules. Its mass spectrum is more interesting because it displays a pattern of peaks arising from the combinations of the two isotopes, ³⁵Cl and ³⁷Cl, in a 3:1 abundance ratio.

氯以 Cl₂ 双原子分子存在。其质谱图更有趣，因为它显示由两种同位素 ³⁵Cl 和 ³⁷Cl（丰度比约为 3:1）组合产生的峰形模式。

Possible Cl₂ ions and their m/z:

[³⁵Cl–³⁵Cl]⁺ m/z = 70; probability = (3/4)² = 9/16 ≈ 56.3 %
[³⁵Cl–³⁷Cl]⁺ m/z = 72; probability = 2 × (3/4 × 1/4) = 6/16 ≈ 37.5 %
[³⁷Cl–³⁷Cl]⁺ m/z = 74; probability = (1/4)² = 1/16 ≈ 6.25 %

可能的 Cl₂ 离子及其 m/z：

[³⁵Cl–³⁵Cl]⁺ m/z = 70；概率 = (3/4)² = 9/16 ≈ 56.3%
[³⁵Cl–³⁷Cl]⁺ m/z = 72；概率 = 2×(3/4×1/4) = 6/16 ≈ 37.5%
[³⁷Cl–³⁷Cl]⁺ m/z = 74；概率 = (1/4)² = 1/16 ≈ 6.25%

The mass spectrum thus shows three peaks in a characteristic 9:6:1 ratio. Recognising this pattern is a very common IGCSE OCR exam skill for identifying chlorine.

因此质谱图显示三个特征峰，比例约为 9:6:1。识别此模式是 IGCSE OCR 考试中鉴别氯的一项常见技能。

11. Bromine Mass Spectrum – A Similar Logic | 溴的质谱图——类似逻辑

Bromine also consists of two isotopes, ⁷⁹Br and ⁸¹Br, in nearly equal abundance (approximately 50.5 % and 49.5 %). Its diatomic mass spectrum shows a triplet of peaks at m/z 158, 160 and 162 in the ratio roughly 1:2:1.

溴也由两种丰度几乎相等的同位素 ⁷⁹Br 和 ⁸¹Br（约 50.5% 和 49.5%）组成。其双原子质谱图在 m/z 158、160 和 162 处显示三峰，比例大致为 1:2:1。

The peak at m/z 160 is the tallest because the ⁷⁹Br–⁸¹Br combination is twice as likely as either homonuclear pair. This simple 1:2:1 pattern is a strong indicator for the presence of bromine.

m/z = 160 的峰最高，因为 ⁷⁹Br–⁸¹Br 组合的概率是任一纯核素组合的两倍。这个简单的 1:2:1 模式是溴存在的强有力指标。

For a monatomic bromine spectrum (Br atoms), you would simply observe two peaks at m/z 79 and 81 with nearly equal heights.

对于单原子溴的质谱 (Br 原子)，你只会观察到 m/z 79 和 81 的两个峰，高度几乎相等。

12. Common Pitfalls and Exam Tips | 常见误区与应试技巧

Many students confuse the base peak with the molecular ion peak. Remember: the molecular ion peak is the one with the highest m/z for the intact molecule, while the base peak is simply the most intense peak (height = 100 %).

许多学生混淆基峰和分子离子峰。请记住：分子离子峰是完整分子对应最高 m/z 的峰，而基峰只是强度最大的峰（高度 = 100%）。

When calculating relative atomic mass, always check that you have used the correct m/z values and that your abundances are converted to the correct units (percent or decimal). A common mistake is to divide by 100 twice or to misuse the percentage.

计算相对原子质量时，务必检查是否使用了正确的 m/z 值，并将丰度转换为正确的单位（百分比或小数）。一个常见错误是除以 100 两次或误用百分比。

For diatomic isotopes, remember the statistical combination probabilities and practise spotting the patterns (9:6:1 for Cl₂, 1:2:1 for Br₂). Do not forget that the base peak might not always be the molecular ion.

对于双原子同位素，记住统计组合概率，并练习识别模式（Cl₂ 的 9:6:1，Br₂ 的 1:2:1）。不要忘记基峰并不总是分子离子峰。

Finally, be prepared to label the axes of a mass spectrum correctly: ‘relative abundance’ on the y-axis and ‘m/z’ (mass/charge) on the x-axis. Sometimes ‘mass/charge’ is given as ‘m/e’ in older resources, but m/z is standard.

最后，准备好正确标注质谱图的坐标轴：y 轴为 ‘relative abundance’，x 轴为 ‘m/z’（质量/电荷）。在较旧的资料中有时表示为 ‘m/e’，但标准是 m/z。

The mass spectrometer is a high-vacuum device; state that the vacuum prevents collisions between ions and air molecules, ensuring accurate deflection.

质谱仪是高真空设备；要指出真空防止离子与空气分子碰撞，确保偏转的准确性。

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IGCSE OCR Chemistry: Mass Spectrometry Key Points | IGCSE OCR 化学：质谱 考点精讲