📚 Mass Spectrometry for IB & CIE Chemistry | IB CIE 化学质谱考点精讲
Mass spectrometry is a powerful analytical technique used to determine the relative atomic mass, identify isotopes, and deduce the structure of organic compounds. For IB and CIE Chemistry students, mastering the interpretation of mass spectra is essential for both data-based questions and theoretical understanding. This article provides a comprehensive, exam-focused revision covering key principles, fragmentation patterns, and calculations you need to know.
质谱是一种强大的分析技术，用于测定相对原子质量、识别同位素以及推测有机化合物的结构。对于 IB 和 CIE 化学学生来说，掌握质谱图的解读对数据分析和理论题都至关重要。本文提供全面的考点精讲，覆盖核心原理、碎片模式以及你需要掌握的各类计算。
1. What is Mass Spectrometry? | 什么是质谱？
Mass spectrometry (MS) is an instrumental method that measures the mass-to-charge ratio (m/z) of ions. A sample is vaporised, ionised, and the resulting ions are separated according to their m/z values. The output is a mass spectrum showing relative abundance against m/z.
质谱（MS）是一种测量离子质荷比（m/z）的仪器方法。样品先气化、电离，产生离子再按 m/z 值分离，得到的是以相对丰度对 m/z 表示的质谱图。
This technique does not involve electromagnetic radiation; it is not a form of spectroscopy in the traditional sense. Instead, it is a physical separation method that provides precise molecular mass and structural information.
该技术不涉及电磁辐射，因此不属于传统光谱。它是一种物理分离方法，能够提供精确的分子质量和结构信息。
2. Basic Principles of Operation | 基本操作原理
A mass spectrometer works under high vacuum. The sample is introduced, ionised, accelerated by an electric field, deflected by a magnetic field, and finally detected. Ions with a lower m/z are deflected more than those with a higher m/z.
质谱仪在高度真空下工作。样品导入后电离，经电场加速，再由磁场偏转，最后被检测。m/z 越小的离子偏转角度越大。
The entire process can be summarised as: vaporisation → ionisation → acceleration → deflection → detection. The abundance of each ion is recorded, giving the mass spectrum.
整个过程可概括为：气化→ 电离→ 加速→ 偏转→ 检测。各类离子的丰度被记录，即得到质谱图。
3. Key Components of a Mass Spectrometer | 质谱仪的关键组件
The sample inlet introduces a tiny amount of gaseous sample. The ionisation chamber bombards molecules with high-energy electrons (in electron impact) or uses an electrospray to generate ions. Next, an electric field accelerates the positive ions to a constant kinetic energy.
进样系统导入微量气态样品。电离室用高能电子轰击（电子轰击法）或使用电喷雾产生离子。然后电场将正离子加速至恒定的动能。
A strong magnetic field deflects the ions into a curved path. The radius of curvature depends on m/z: lighter ions or those with higher charge are deflected more. The detector records the ion current, and a data system converts this into a mass spectrum.
强磁场使离子偏转成弯曲轨道，曲率半径取决于 m/z：较轻或带较高电荷的离子偏转更大。检测器记录离子流，数据系统转换成质谱图。
4. Ionisation Methods: Electron Impact and Electrospray | 电离方式：电子轰击与电喷雾
In electron impact (EI), a beam of high-energy electrons (typically 70 eV) knocks out an electron from a molecule M, forming a radical cation M⁺·. This M⁺· is the molecular ion. Excess energy often causes extensive fragmentation, making EI useful for structural fingerprinting.
电子轰击（EI）中，一束高能电子（通常 70 eV）从分子 M 上打出一个电子，形成自由基阳离子 M⁺·。此 M⁺· 就是分子离子。多余的能量常导致大量碎裂，使得 EI 适合结构指纹识别。
Electrospray ionisation (ESI) is a softer technique. The sample solution is sprayed from a charged needle, producing protonated or deprotonated molecules such as [M+H]⁺ or [M−H]⁻. ESI causes little fragmentation and is used for large biomolecules.
电喷雾离子化（ESI）是较软的技术。样品溶液从带电针头喷洒，产生加合质子或去质子的分子，如 [M+H]⁺ 或 [M−H]⁻。ESI 碎裂少，常用于生物大分子。
For CIE and IB, focus on electron impact and its role in generating M⁺· and fragment ions. Remember: M⁺· peak corresponds to the relative molecular mass Mᵣ of the compound if it appears.
CIE 和 IB 重点掌握电子轰击法及其产生 M⁺· 和碎片离子的作用。记住：若出现 M⁺· 峰，它对应化合物的相对分子质量 Mᵣ。
5. Mass-to-Charge Ratio (m/z) and Detection | 质荷比 (m/z) 与检测
The mass spectrometer separates ions by their m/z value, not simply by mass. For singly charged ions (z = 1), m/z equals the mass of the ion in atomic mass units. Most ions in basic MS are singly charged, so the m/z scale is effectively a mass scale.
质谱仪根据 m/z 而非单纯质量来分离离子。对于单电荷离子（z = 1），m/z 在数值上等于离子的质量（原子质量单位）。基础质谱中大部分离子带单电荷，因此 m/z 标尺就是质量标尺。
Detection produces signals proportional to the number of ions. The tallest peak is assigned a relative abundance of 100% and called the base peak; other peaks are scaled accordingly.
检测产生的信号与离子数量成正比。最高峰设定为相对丰度 100%，称为基峰；其他峰按比例标度。
6. Molecular Ion Peak and Base Peak | 分子离子峰与基峰
The molecular ion peak (M⁺·) represents the unfragmented molecule minus one electron. It gives the relative molecular mass Mᵣ of the compound, provided it is stable enough to reach the detector. In some compounds, the M⁺· peak is very small or absent.
分子离子峰（M⁺·）代表未碎裂的分子减去一个电子。如果该离子足够稳定到达检测器，它给出化合物的相对分子质量 Mᵣ。有些化合物的 M⁺· 峰极小甚至缺失。
The base peak is the most intense peak in the spectrum. It may or may not be the molecular ion. It corresponds to the most stable cation (or radical cation) formed, and its m/z value helps identify the compound.
基峰是谱图中最强的峰，它可能是也可能不是分子离子。它对应最稳定的阳离子（或自由基阳离子），其 m/z 值有助于鉴定化合物。
7. Interpreting Mass Spectra: Isotopic Abundance | 解读质谱：同位素丰度
Many elements exist as isotopes. Chlorine has ³⁵Cl and ³⁷Cl with ~3:1 abundance; bromine has ⁷⁹Br and ⁸¹Br ~1:1. Mass spectra of chlorine- or bromine-containing compounds show characteristic M, M+2, M+4 patterns.
很多元素有同位素。氯有 ³⁵Cl 和 ³⁷Cl，丰度比约 3:1；溴有 ⁷⁹Br 和 ⁸¹Br，约 1:1。含氯或溴化合物的质谱会出现特征的 M, M+2, M+4 峰簇。
For a compound with one Br atom, two molecular ion peaks of roughly equal intensity appear at M and M+2. For one Cl atom, the M+2 peak is about one-third the height of the M peak. These patterns are powerful evidence for halogen presence.
对于含一个 Br 原子的化合物，会出现强度几乎相等的 M 和 M+2 两个分子离子峰。含一个 Cl 原子时，M+2 峰高约为 M 峰的三分之一。这些模式是卤素存在的有力证据。
Carbon has ¹²C (~98.9%) and ¹³C (~1.1%). In large organic molecules, the small M+1 peak arises mainly from ¹³C isotopes. This helps students confirm the number of carbon atoms.
碳有 ¹²C（约 98.9%）和 ¹³C（约 1.1%）。在较大的有机分子中，微小的 M+1 峰主要来自 ¹³C，可帮助学生确认碳原子数目。
8. Calculating Relative Atomic Mass from Mass Spectra | 从质谱计算相对原子质量
The relative atomic mass Aᵣ of an element is calculated from the mass spectrum of its atomic ions. Multiply each m/z value by its relative abundance, sum these products, and divide by the total relative abundance.
元素的相对原子质量 Aᵣ 由其原子离子的质谱计算。将每个 m/z 值乘以对应的相对丰度，求和后除以总相对丰度。
Aᵣ = (Σ (m/z × relative abundance)) / Σ (relative abundance)
For example, the mass spectrum of magnesium shows peaks at m/z 24 (abundance 79), 25 (10), 26 (11). Aᵣ = (24×79 + 25×10 + 26×11) ÷ (79+10+11) ≈ 24.3.
例如镁的质谱显示 m/z 24（丰度 79）、25（10）、26（11），Aᵣ = (24×79 + 25×10 + 26×11) ÷ (79+10+11) ≈ 24.3。
Always show the division step and express the answer with appropriate significant figures. Exam questions often provide tabulated data for this calculation.
一定要展示除法步骤，并以适当的有效数字表达答案。考试常以表格形式提供数据要求计算。
9. Fragmentation Patterns in Organic Compounds | 有机化合物的碎片模式
When a molecular ion breaks apart, it produces fragment ions. The fragmentation pattern acts as a fingerprint for the molecule. Stable carbocations such as R₃C⁺ are often responsible for prominent peaks.
分子离子碎裂产生碎片离子。碎片模式好比分子的指纹，稳定的碳正离子如 R₃C⁺ 常对应突出的峰。
Alkanes give clusters of peaks 14 mass units apart (CH₂). Alcohols often show an M−18 peak from loss of H₂O, and an M−15 peak from loss of CH₃. Ketones undergo α‑cleavage to give acylium ions.
烷烃会产生间隔 14 质荷单位的峰簇（CH₂）。醇类常有 M−18 峰（失 H₂O）和 M−15 峰（失 CH₃）。酮发生 α‑裂解产生酰基正离子。
The spectrum of butanone includes peaks at m/z 72 (M⁺·), 57 (loss of CH₃), 43 (CH₃CO⁺, base peak), and 29 (C₂H₅⁺). Recognising these typical losses is tested frequently.
丁酮的质谱包括 m/z 72（M⁺·）、57（失 CH₃）、43（CH₃CO⁺，基峰）和 29（C₂H₅⁺）。识别这些典型碎裂是常见考点。
10. Using Mass Spectrometry to Identify Compounds | 利用质谱鉴定化合物
The molecular ion peak gives Mᵣ. Combine this with empirical or molecular formula information. Use the nitrogen rule: an odd Mᵣ suggests an odd number of nitrogen atoms. Even Mᵣ suggests zero or even number of N atoms.
分子离子峰给出 Mᵣ，结合经验式或分子式信息。利用氮规则：奇数 Mᵣ 表明含奇数个氮原子，偶数 Mᵣ 表明不含或含偶数个 N 原子。
Isotope patterns help deduce the presence of Cl, Br, S, Si, etc. The base peak often indicates a particularly stable fragment. By piecing together fragment ions, you can reconstruct the molecule’s structure.
同位素峰形可推断 Cl、Br、S、Si 等元素的存在。基峰常指示特别稳定的碎片。把碎片离子拼凑起来，就能重建分子结构。
11. Common Exam Pitfalls and Tips | 常见考试陷阱与提示
Don’t confuse the base peak with the molecular ion peak. The molecular ion may be absent for tertiary alcohols and branched alkanes. Always check for M+2 and M+1 peaks before assigning Mᵣ.
不要混淆基峰与分子离子峰。叔醇和支链烷烃可能没有分子离子峰。在确定 Mᵣ 之前，务必检查 M+2 和 M+1 峰。
When calculating Aᵣ, ensure the m/z values correspond to singly charged monatomic ions. If the spectrum includes doubly charged ions (z=2), the m/z would be half the mass. IB and CIE usually avoid this complexity.
计算 Aᵣ 时，确保 m/z 对应的是单电荷单原子离子。若谱图含双电荷离子 (z=2)，m/z 会是质量的一半。IB 和 CIE 通常避开这一复杂情况。
In fragmentation, remember that the charge tends to stay on the more stable carbocation. Write plausible fragmentation equations using single-barbed and double-barbed arrows where required.
在碎裂中，要记住正电荷往往留在更稳定的碳正离子上。按需要用单钩和双钩箭头写出合理的碎裂方程式。
12. Summary of Key Points | 要点总结
Mass spectrometry provides m/z and abundance data. The molecular ion peak reveals Mᵣ, isotopic peaks indicate certain elements, and fragment peaks allow structural elucidation. Calculations of relative atomic mass and identification of functional groups are regularly examined.
质谱提供 m/z 和丰度数据。分子离子峰揭示 Mᵣ，同位素峰指示特定元素，碎片峰可解析结构。相对原子质量的计算和官能团的鉴定是常考内容。
Review typical fragmentation patterns of alkanes, haloalkanes, alcohols, aldehydes, ketones, and carboxylic acids. Practise predicting mass spectra of given molecules and explaining the origin of major peaks.
复习烷烃、卤代烷、醇、醛、酮、羧酸的典型碎裂模式。练习预测给定分子的质谱，并解释主要峰的来源。
With a clear understanding of these principles, mass spectrometry questions become systematic and rewarding. Always annotate your spectrum, show your working, and link fragments to molecular structure.
清晰掌握这些原理后，质谱题就能变得系统且易于得分。务必在谱图上标注、展示计算过程，并将碎片与分子结构关联起来。
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