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

📚 Mass Spectrometry in IGCSE CIE Chemistry: Key Points | IGCSE CIE 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 the structure of molecules. In the IGCSE CIE Chemistry syllabus, understanding how a mass spectrometer works and how to interpret mass spectra is essential for tackling related exam questions.

质谱法是一种强大的分析技术，用于确定元素的相对原子质量和分子结构。在 IGCSE CIE 化学大纲中，理解质谱仪的工作原理以及如何解读质谱图是应对相关考题的关键。

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

Mass spectrometry provides information about the masses of atoms and molecules. It can be used to find the relative atomic mass of an element from its isotopic composition, and it can also help identify unknown compounds by their fragmentation patterns.

质谱法提供有关原子和分子质量的信息。它可以利用同位素组成求算元素的相对原子质量，还可以通过碎裂图谱帮助鉴定未知化合物。

In this technique, a sample is vaporised and ionised, then the resulting positive ions are separated according to their mass-to-charge ratio (m/z). The instrument produces a mass spectrum, which is a plot of relative abundance against m/z.

在该技术中，样品先被气化并离子化，然后根据所产生的正离子的质荷比（m/z）进行分离。仪器生成质谱图，即以相对丰度对 m/z 所作的图谱。

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2. The Four Key Stages | 四个关键阶段

A mass spectrometer consists of four main operational stages: ionisation, acceleration, deflection and detection. The entire system is kept under a high vacuum to prevent the positive ions from colliding with air molecules, which would interfere with their flight path.

质谱仪主要由四个操作阶段构成：离子化、加速、偏转和检测。整个系统保持在高真空下，以防止正离子与空气分子碰撞而干扰其飞行路径。

First, the sample is converted into gaseous positive ions. Then these ions are accelerated by an electric field to a high velocity. A magnetic field deflects the ions, and the degree of deflection depends on their m/z. Finally, ions with a specific m/z reach the detector and generate an electrical signal.

首先，样品转化为气态正离子。然后这些离子被电场加速至高速。磁场使离子发生偏转，偏转程度取决于其 m/z。最后，具有特定 m/z 的离子到达检测器并产生电信号。

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3. Ionisation: Electron Impact | 离子化：电子轰击

The sample is bombarded with a beam of high-energy electrons. These electrons knock out an electron from the atoms or molecules, forming positive ions. For a molecule X, this can be represented as: X → X⁺ + e⁻.

样品受到一束高能电子轰击。这些电子从原子或分子中击出一个电子，形成正离子。对于分子 X，可表示为：X → X⁺ + e⁻。

The ion formed without fragmentation is the molecular ion, M⁺. However, the high-energy process often causes the molecule to break apart into smaller fragment ions, giving a characteristic fragmentation pattern.

未碎裂而形成的离子是分子离子 M⁺。但高能过程常常导致分子分解成较小的碎片离子，从而产生特征性的碎裂图谱。

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4. Acceleration and Deflection | 加速与偏转

The positive ions are accelerated by a strong electric field so that they all have roughly the same kinetic energy. They then enter a magnetic field, which causes them to move in a curved path.

正离子被强电场加速，因此它们获得大致相同的动能。接着它们进入磁场，磁场使离子沿弯曲路径运动。

Ions with a lower m/z value (lighter or more highly charged) are deflected more than ions with a higher m/z value. By varying the magnetic field strength or the accelerating voltage, ions of different m/z can be brought to the detector one after another.

质荷比 m/z 较小的离子（较轻或带较多电荷）比 m/z 较大的离子偏转得更多。通过改变磁场强度或加速电压，可以使不同 m/z 的离子依次到达检测器。

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

When an ion strikes the detector, it accepts an electron, causing a small electric current to flow. This current is amplified and recorded by a computer, which plots the relative abundance of each ion against its m/z value.

当离子撞击检测器时，它接受一个电子，产生微小的电流。该电流被放大并由计算机记录，计算机以相对丰度对 m/z 值作图。

The detector can measure very tiny currents, allowing the mass spectrometer to analyse extremely small samples with high precision. The resulting mass spectrum is a series of peaks.

检测器能测量极微弱的电流，使得质谱仪能以高精度分析极小量的样品。所得的质谱图是一系列峰。

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6. The Mass Spectrum: Axes and Peaks | 质谱图：坐标轴与峰

The horizontal axis (x-axis) of a mass spectrum is labelled ‘m/z’, which is the mass-to-charge ratio. Because most ions produced carry a single positive charge (+1), the m/z value is numerically equal to the mass of the ion in atomic mass units.

质谱图的横轴（x 轴）标为 ‘m/z’，即质荷比。由于产生的大部分离子带单个正电荷（+1），m/z 值在数值上等于离子的质量（以原子质量单位计）。

The vertical axis (y-axis) shows the relative abundance of each ion. The peak with the greatest abundance is called the base peak and is assigned an abundance of 100%. All other peaks are measured relative to the base peak.

纵轴（y 轴）显示各离子的相对丰度。丰度最高的峰称为基峰，其丰度被设定为 100%。所有其他峰的丰度均相对于基峰进行测量。

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7. Molecular Ion Peak (M⁺) | 分子离子峰（M⁺）

The molecular ion peak corresponds to the whole molecule that has lost just one electron, written as M⁺. It appears at the highest m/z value in the spectrum (not counting any tiny peaks from isotopic variants at slightly higher m/z).

分子离子峰对应于只失去一个电子的整个分子，记为 M⁺。它出现在谱图中 m/z 值最大的位置（不计同位素变体产生的稍高 m/z 值的微小峰）。

This peak gives the relative molecular mass of the compound. If the molecule is very fragile, the molecular ion peak may be very small or even absent, but for many stable molecules it is clearly visible.

此峰给出化合物的相对分子质量。如果分子非常脆弱，分子离子峰可能很小甚至不出现，但对于许多稳定分子，它清晰可见。

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8. Base Peak and Fragmentation | 基峰与碎裂

The base peak is the tallest peak in the spectrum. It represents the most stable fragment ion formed during ionisation. Fragmentation occurs because the bombarding electrons provide enough energy to break chemical bonds within the molecular ion.

基峰是谱图中最高的峰，代表离子化过程中形成的最稳定的碎片离子。碎裂的发生是因为轰击电子提供了足够的能量来断裂分子离子中的化学键。

Fragment ions appear at lower m/z values. The pattern of fragment peaks can act as a ‘fingerprint’ for a molecule and is used to deduce its structure, although IGCSE questions often focus simply on identifying the molecular ion and isotopic peaks.

碎片离子出现在较低的 m/z 值处。碎片峰的图谱可作为分子的“指纹”，用于推断其结构，不过 IGCSE 考题通常聚焦于识别分子离子和同位素峰。

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9. Isotopic Peaks: Cl, Br and Others | 同位素峰：氯、溴等

Elements that have more than one stable isotope, such as chlorine and bromine, give multiple peaks in the mass spectrum. For example, atomic chlorine shows two peaks at m/z 35 and 37 because of the isotopes ³⁵Cl and ³⁷Cl.

具有多种稳定同位素的元素，如氯和溴，在质谱图中会产生多重峰。例如，原子氯因存在同位素 ³⁵Cl 和 ³⁷Cl 而在 m/z 35 和 37 处出现两个峰。

The relative abundance ratio of ³⁵Cl : ³⁷Cl is approximately 3 : 1. For bromine, ⁷⁹Br and ⁸¹Br occur in almost equal amounts, giving a 1 : 1 ratio, so two peaks of similar height are observed at m/z 79 and 81.

³⁵Cl 与 ³⁷Cl 的相对丰度比约为 3 : 1。对于溴，⁷⁹Br 和 ⁸¹Br 的丰度几乎相等，形成 1 : 1 的比率，因此在 m/z 79 和 81 处观察到两个高度近似的峰。

These characteristic isotopic patterns help to identify the presence of Cl or Br in a compound. Organic molecules containing one chlorine atom show a molecular ion region with M⁺ and (M+2)⁺ peaks in a 3:1 ratio.

这些特征的同位素分布有助于识别化合物中是否含氯或溴。含一个氯原子的有机分子会在分子离子区域显示 M⁺ 和 (M+2)⁺ 峰，强度比为 3:1。

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

The relative atomic mass (Ar) of an element is the weighted average mass of all its naturally occurring isotopes, taking into account their relative abundances. The mass spectrum directly provides the m/z values of the isotopes and their relative intensities.

元素的相对原子质量（Ar）是其所有天然同位素质量的加权平均值，计及它们的相对丰度。质谱图直接提供各同位素的 m/z 值和它们的相对强度。

The general formula for the calculation is:

计算公式如下：

Ar = (abundance₁ × mass₁ + abundance₂ × mass₂ + …) / total abundance

相对原子质量 = (丰度₁ × 质量₁ + 丰度₂ × 质量₂ + …) / 总丰度

You can use either percentage abundance (then divide by 100) or the actual peak heights. Remember to multiply each isotopic mass by its relative abundance, sum them up, and then divide by the sum of the abundances.

你可以使用百分比丰度（然后除以 100）或实际峰高来计算。切记将每个同位素质量乘以其相对丰度，求和，再除以丰度之和。

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11. Worked Example: Chlorine Isotopes | 实例：氯的同位素

A mass spectrum of atomic chlorine shows two peaks: m/z 35 with a relative abundance of 75% and m/z 37 with a relative abundance of 25%.

原子氯的质谱图显示两个峰：m/z 35，相对丰度 75%；m/z 37，相对丰度 25%。

Isotope	Mass number	Relative abundance (%)
³⁵Cl	35	75
³⁷Cl	37	25

Applying the formula: Ar = (75 × 35 + 25 × 37) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5.

代入公式：Ar = (75 × 35 + 25 × 37) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5。

Thus, the relative atomic mass of chlorine is 35.5. This method can be applied to any element provided its mass spectrum and isotopic abundances are known.

因此，氯的相对原子质量为 35.5。只要给出质谱图和同位素丰度，这种方法可应用于任何元素。

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12. Tips for Exam Questions | 考试技巧

Always assume the charge on the ion is +1 unless told otherwise, so m/z equals the ion mass. When calculating relative atomic mass, use the correct isotopic masses and do not confuse the molecular ion peak with fragment peaks.

除非题目另有说明，始终假设离子带 +1 电荷，因此 m/z 等于离子质量。在计算相对原子质量时，使用正确的同位素质量，切勿将分子离子峰与碎片峰混淆。

If the spectrum shows a cluster of peaks for the molecular ion region (e.g. M⁺ and M+2 peaks in a 3:1 ratio), this is strong evidence for the presence of chlorine. A 1:1 pattern suggests bromine. Use these clues in structure determination questions.

如果谱图在分子离子区域显示一组峰（如 M⁺ 和 M+2 峰呈 3:1 比例），这强烈表明含氯。1:1 的图谱则提示含溴。在结构推断题中善用这些线索。

Practice interpreting mass spectra of common elements like chlorine, bromine, and simple organic molecules. Remember that the base peak is not always the molecular ion peak, and fragmentation peaks should be ignored when calculating relative atomic mass.

多练习解读常见元素（如氯、溴）及简单有机分子的质谱图。牢记基峰并不总是分子离子峰，计算相对原子质量时应忽略碎片峰。

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