IB Chemistry: Mass Spectrometry Essentials | IB 化学：质谱考点精讲

📚 IB Chemistry: Mass Spectrometry Essentials | IB 化学：质谱考点精讲

Mass spectrometry (MS) is a powerful analytical technique used to determine the relative atomic and molecular masses of atoms and molecules, deduce molecular structures, and identify unknown compounds. By ionising a sample and then separating the resulting ions according to their mass-to-charge ratio (m/z), a mass spectrometer produces a characteristic spectrum that reveals the mass of the molecular ion, isotopic composition, and fragmentation pattern. For IB Chemistry, mastering how to interpret mass spectra, identify the molecular ion peak, explain isotope peaks, and calculate average relative atomic masses is essential.

质谱法 (MS) 是一种强大的分析技术，用于测定原子和分子的相对原子质量及相对分子质量、推断分子结构并鉴定未知化合物。通过将样品离子化，再根据离子的质荷比 (m/z) 进行分离，质谱仪会生成一张特征谱图，揭示分子离子的质量、同位素组成以及碎片化模式。对 IB 化学而言，掌握如何解读质谱图、识别分子离子峰、解释同位素峰以及计算平均相对原子质量至关重要。

1. What Is a Mass Spectrometer? | 质谱仪是什么？

A mass spectrometer is an instrument that analyses substances by producing gas-phase ions from the sample, separating these ions in electric and magnetic fields, and then detecting them. The entire process occurs under high vacuum to prevent collisions between ions and air molecules. The output is a mass spectrum, a plot of relative abundance against mass-to-charge ratio.

质谱仪是通过将样品转化为气态离子，在电场和磁场中对离子进行分离，再进行检测来对物质进行分析的仪器。整个过程在高真空下进行，以防止离子与空气分子发生碰撞。其输出结果是一张质谱图，即相对丰度对质荷比所作的图。

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

In electron impact (EI) ionisation, the sample is vaporised and bombarded with high-energy electrons (typically 70 eV). This knocks an electron out of a molecule, forming a positively charged radical cation called the molecular ion:

M + e⁻ → M⁺• + 2e⁻

The molecular ion often has enough internal energy to break apart into smaller fragment ions, providing structural information. EI is used mainly for volatile organic compounds and gives extensive fragmentation.

在电子轰击 (EI) 电离中，样品被气化，然后用高能电子（通常为 70 eV）轰击。电子会将一个电子从分子中打出去，形成一个带正电的自由基阳离子，称为分子离子：

M + e⁻ → M⁺• + 2e⁻

分子离子通常具有足够的内能，可进一步断裂成较小的碎片离子，从而提供结构信息。EI 主要用于挥发性有机化合物，并且会产生丰富的碎片。

3. Ionisation Methods: Electrospray Ionisation | 离子化方法：电喷雾电离

Electrospray ionisation (ESI) is a softer technique often used for larger biomolecules and polar compounds. The sample is dissolved in a volatile solvent and sprayed through a fine capillary at high voltage, producing charged droplets. As the solvent evaporates, sample molecules gain protons (or other cations), generating [M+H]⁺ ions without extensive fragmentation.

电喷雾电离 (ESI) 是一种较软的电离技术，常用于较大的生物分子和极性化合物。样品溶解在挥发性溶剂中，通过一根施加高电压的细针喷出，形成带电液滴。随着溶剂蒸发，样品分子获得质子（或其他阳离子），生成 [M+H]⁺ 离子，且几乎不发生碎片化。

4. Acceleration, Deflection and Detection | 加速、偏转与检测

Once ions are formed, they are accelerated by an electric field so that all ions with the same charge acquire the same kinetic energy. They then enter a magnetic (or electric) field where they are deflected: lighter ions and ions with higher charge are deflected more. By varying the field strength, ions of different m/z values are focused onto the detector in turn.

一旦离子形成，它们便被电场加速，使得具有相同电荷的所有离子获得相同的动能。接着，它们进入磁场（或电场）发生偏转：质量越轻、电荷越高的离子偏转越大。通过改变场强，不同 m/z 值的离子依次聚焦到检测器上。

The detector records the ion current and converts it into a signal. The intensity of each signal is plotted as relative abundance, with the tallest peak – the base peak – set at 100%.

检测器记录下离子流并将其转化为信号。每个信号的强度以相对丰度的形式作图，其中最高的峰——基峰——设定为 100%。

5. Understanding m/z and the Mass Spectrum | 理解 m/z 与质谱图

The mass-to-charge ratio, written as m/z, is a dimensionless quantity because the charge z is often +1 in organic MS. Therefore, the m/z value is effectively equal to the mass of the ion in unified atomic mass units. The spectrum shows m/z on the x‑axis and relative abundance on the y‑axis.

质荷比写作 m/z，是一个无量纲量，因为在有机质谱中电荷 z 通常为 +1。因此，m/z 值实际上等于离子的质量（以统一原子质量单位计）。质谱图的 x 轴为 m/z，y 轴为相对丰度。

Molecular ion peak (M⁺): the peak corresponding to the intact molecular ion. It gives the relative molecular mass (Mᵣ) of the compound.
分子离子峰 (M⁺)：对应完整分子离子的峰，它给出了化合物的相对分子质量 (Mᵣ)。
Base peak: the most intense peak, assigned a relative abundance of 100%. All other peaks are measured relative to it.
基峰：强度最大的峰，相对丰度定为 100%，其他所有峰均相对于它进行量度。
Fragment peaks: peaks at lower m/z arising from the break‑up of the molecular ion.
碎片峰：出现在较低 m/z 处的峰，由分子离子断裂产生。

6. Identifying the Molecular Ion Peak | 识别分子离子峰

The molecular ion peak is the highest m/z peak in a simple spectrum, but not always the tallest. To identify it, check that the peak obeys the nitrogen rule: an organic compound containing an even number of nitrogen atoms (including zero) will have an even‑numbered molecular mass; an odd number of nitrogens gives an odd mass. Also, the M⁺ peak must be capable of losing logical neutral fragments (e.g., –CH₃, –OH, –Cl) to produce the next highest m/z peaks, following the ‘rule of losses’.

分子离子峰是简单谱图中 m/z 最高的峰，但不一定是最高的峰。要识别它，需检查该峰是否符合氮规则：含偶数个氮原子（包括零）的有机化合物，其分子量为偶数；含奇数个氮原子则为奇数。此外，M⁺ 峰必须能够失去合理的中性碎片（如 –CH₃, –OH, –Cl），生成次高的 m/z 峰，这称为“丢失规则”。

7. Fragmentation: Why It Happens and What It Tells Us | 碎片化：发生原因及其信息

When a molecular ion is formed with excess internal energy, it can break covalent bonds to form a positively charged fragment ion and a neutral radical. The fragmentation pattern is characteristic of a molecule’s structure. For example, alkanes often generate a series of peaks 14 mass units apart (CH₂ groups), with abundance maxima at branched points where carbocations are more stable.

当分子离子形成时若具有过剩的内能，它会断裂共价键，生成一个带正电荷的碎片离子和一个中性自由基。碎片化模式是分子结构的特征。例如，烷烃通常会生成一系列相差 14 个质量单位（CH₂ 基团）的峰，丰度最大值出现在分支点，因为那里的碳正离子更稳定。

Common fragment ions: m/z 15 (CH₃⁺), 29 (C₂H₅⁺), 43 (C₃H₇⁺), 57 (C₄H₉⁺). The stability of carbocations (3° > 2° > 1°) explains why certain fragments dominate.

常见碎片离子：m/z 15 (CH₃⁺), 29 (C₂H₅⁺), 43 (C₃H₇⁺), 57 (C₄H₉⁺)。碳正离子的稳定性（3° > 2° > 1°）解释了为何某些碎片占主导地位。

8. Isotope Peaks: The M+1 and M+2 Signals | 同位素峰：M+1 和 M+2 信号

Because many elements exist as a mixture of isotopes, the mass spectrum shows small peaks one or two mass units above the molecular ion peak. The relative heights of these isotope peaks provide a fingerprint for certain atoms.

由于许多元素以同位素混合物的形式存在，质谱图会在分子离子峰之上 1 或 2 个质量单位处出现小峰。这些同位素峰的相对高度为某些原子提供了指纹信息。

¹³C contributes to the M+1 peak: about 1.1% per carbon atom. A molecule with n carbons has an M+1 peak that is roughly n × 1.1% of M⁺ height.
¹³C 对 M+1 峰有贡献：每个碳原子约占 1.1%。含有 n 个碳的分子，其 M+1 峰高度大约为 M⁺ 峰的 n × 1.1%。
³⁷Cl and ⁸¹Br produce characteristic M+2 peaks: chlorine has two isotopes ³⁵Cl (75%) and ³⁷Cl (25%), giving a 3:1 ratio for M : M+2. Bromine has ⁷⁹Br (50%) and ⁸¹Br (50%), giving a 1:1 ratio.
³⁷Cl 和 ⁸¹Br 产生特征 M+2 峰：氯有两种同位素 ³⁵Cl (75%) 和 ³⁷Cl (25%)，得出 M : M+2 约为 3:1。溴有 ⁷⁹Br (50%) 和 ⁸¹Br (50%)，得出 M : M+2 约为 1:1。
For two chlorine atoms, the pattern becomes 9:6:1 for M : M+2 : M+4. For two bromines, the pattern is 1:2:1.
对于两个氯原子，模式变为 M : M+2 : M+4 = 9:6:1；对于两个溴原子，模式为 1:2:1。

9. Calculating Relative Atomic Mass from a Mass Spectrum | 由质谱图计算相对原子质量

The average relative atomic mass (Aᵣ) of an element can be calculated directly from the mass spectrum of its atomic ions. For each isotopic peak, multiply the isotopic mass by its relative abundance (as a percentage), sum the products, and divide by the total abundance (usually 100).

元素的平均相对原子质量 (Aᵣ) 可直接由其原子离子的质谱图计算得到。对每个同位素峰，用同位素质量乘以其相对丰度（百分比），求和后除以总丰度（通常为 100）。

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

Example: Boron has ¹⁰B (relative abundance 20%) and ¹¹B (80%).

Aᵣ = (10 × 20 + 11 × 80) / 100 = (200 + 880) / 100 = 10.8

示例：硼有 ¹⁰B（相对丰度 20%）和 ¹¹B（80%）。

Aᵣ = (10 × 20 + 11 × 80) / 100 = (200 + 880) / 100 = 10.8

For molecules, the molecular ion peak gives the Mᵣ based on the most abundant isotopes (e.g., ¹²C, ¹H, ¹⁶O) and is often called the monoisotopic mass.

对于分子，分子离子峰给出的是基于最丰富同位素（如 ¹²C、¹H、¹⁶O）的相对分子质量，通常称为单一同位素质量。

10. Using Mass Spectra to Determine Molecular Formula | 利用质谱确定分子式

Combined with elemental analysis data, the molecular ion peak and isotope peaks can give the molecular formula. The M+1 peak indicates the number of carbon atoms. The presence of M+2 peaks reveals halogens like Cl and Br. High‑resolution mass spectrometry (HRMS) can measure the exact mass of the molecular ion to several decimal places, allowing direct determination of the elemental composition.

结合元素分析数据，分子离子峰和同位素峰可以给出分子式。M+1 峰指示碳原子数目，M+2 峰的存在揭示 Cl、Br 等卤素。高分辨质谱 (HRMS) 能够将分子离子的精确质量测量到小数点后多位，从而直接确定元素组成。

For example, an ion with exact mass 28.0061 must be N₂⁺ (calculated 28.0061), not CO⁺ (27.9949). This precision distinguishes between molecules with the same nominal mass.

例如，精确质量为 28.0061 的离子必定是 N₂⁺（计算值 28.0061），而不是 CO⁺（27.9949）。这种精度能区分具有相同名义质量的不同分子。

11. Interpreting Extended Fragmentation Patterns | 解读复杂的碎片化模式

When spectra show a cluster of peaks, correlate the differences in m/z with common neutral losses:

Loss of (u)	Possible fragment
15	•CH₃
17	•OH
18	H₂O
28	C₂H₄ or CO
29	•C₂H₅ or •CHO
31	•OCH₃
35/37	•Cl
43	•C₃H₇ or CH₃CO•

By recognising these differences, you can piece together the structure. For a carbonyl compound, an α‑cleavage often yields an acylium ion R–C≡O⁺, giving a prominent peak at m/z = Mᵣ – R′.

通过识别这些差异，你可以拼凑出结构。对于羰基化合物，α‑断裂常产生酰基正离子 R–C≡O⁺，在 m/z = Mᵣ – R′ 处给出明显的峰。

12. Exam Tips and Common IB Pitfalls | IB 考试技巧与常见误区

Always check the m/z axis scale – the molecular ion peak may be small but is the highest m/z peak (ignoring insignificant isotope peaks).
一定要检查 m/z 轴的刻度——分子离子峰可能很小，但它仍是 m/z 值最大的峰（忽略极小的同位素峰）。
Do not confuse the base peak with the molecular ion peak; the base peak is simply the tallest.
不要将基峰与分子离子峰混淆；基峰只是最高的峰。
When calculating Aᵣ, remember to divide by 100 if using percentage abundances, or by total fractional abundance. Write the formula clearly.
计算 Aᵣ 时，如果用的是百分比丰度，请记得除以 100；如果用的是小数丰度，则除以总丰度。请清晰写出计算公式。
For fragments, only positively charged species are detected; neutral radicals are lost and do not appear in the spectrum.
碎片方面，只有带正电荷的物种能被检测到；中性自由基丢失后不会出现在谱图中。
If a question provides high‑resolution mass data, use the exact masses to deduce the molecular formula directly.
如果题目提供了高分辨质谱数据，要使用精确质量直接推断分子式。
Be ready to draw a simple fragmentation mechanism to account for the base peak.
准备好画出简单的碎片化机理来解释基峰的形成。

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IB Chemistry: Mass Spectrometry Essentials | IB 化学：质谱 考点精讲