📚 Mass Spectrometry in IB CCEA Chemistry | IB CCEA 化学：质谱考点精讲
Mass spectrometry is one of the most powerful analytical techniques for determining the relative atomic mass of an element, the relative molecular mass of a compound, and detailed structural information through fragmentation patterns. For IB CCEA Chemistry, you need to understand the principles of operation, the interpretation of mass spectra, and how to apply isotope abundance data to calculate relative atomic mass. This article covers all the essential examination topics in a clear, bilingual format to help you master mass spectrometry.
质谱法是测定元素相对原子质量、化合物相对分子质量以及通过碎片模式获取详细结构信息的最强大的分析技术之一。对于 IB CCEA 化学，你需要理解质谱仪的工作原理、质谱图的解读方法，以及如何应用同位素丰度数据计算相对原子质量。本文以清晰的双语格式覆盖所有核心考点，帮助你彻底掌握质谱法。
1. Introduction to Mass Spectrometry | 质谱技术简介
Mass spectrometry is an instrumental method that separates gaseous ions according to their mass-to-charge ratio (m/z). It can analyse elements and compounds with high precision and sensitivity. Unlike many other spectroscopic techniques, mass spectrometry does not involve the absorption of electromagnetic radiation; instead, it measures the relative abundance of ions produced from a sample. A mass spectrum is a plot of relative intensity (or relative abundance) against m/z ratio.
质谱法是一种根据离子质荷比（m/z）对气态离子进行分离的仪器分析方法。它能以高精度和高灵敏度分析元素和化合物。与许多其他光谱技术不同，质谱法不涉及电磁辐射的吸收，而是测量样品产生的离子的相对丰度。质谱图是相对强度（或相对丰度）对质荷比（m/z）的标绘图。
2. Basic Components of a Mass Spectrometer | 质谱仪的基本组成
All mass spectrometers share the same fundamental processes: ionisation, acceleration, deflection, and detection. The key components include the sample inlet, the ion source, the mass analyser, and the detector. The entire system is maintained under a high vacuum to prevent collisions that would interfere with the flight of the ions. The mass analyser uses either a magnetic or electric field to separate ions based on their m/z values.
所有质谱仪都遵循相同的基本过程：电离、加速、偏转和检测。关键部件包括进样口、离子源、质量分析器和检测器。整个系统保持高真空状态，以防止碰撞干扰离子的飞行。质量分析器利用磁场或电场根据离子的 m/z 值将其分离。
3. Ionisation: Electron Impact (EI) | 电离方法：电子轰击（EI）
In electron impact ionisation, the gaseous sample is bombarded with high-energy electrons (typically 70 eV). This knocks out an electron from the molecule, forming a radical cation known as the molecular ion. The equation is:
M + e⁻ → M⁺· + 2e⁻
The molecular ion M⁺· has the same mass as the original molecule but carries a positive charge and an unpaired electron. EI often causes extensive fragmentation because the energy imparted is much greater than the bond dissociation energies, providing rich structural information from the fragment peaks.
在电子轰击电离中，气态样品受到高能电子（通常为 70 eV）的轰击。这会从分子中打出一个电子，形成一个称为分子离子的自由基阳离子。方程式为：
M + e⁻ → M⁺· + 2e⁻
分子离子 M⁺· 的质量与原分子相同，但带有一个正电荷和一个未成对电子。由于所传递的能量远大于键解离能，EI 通常会引起大量碎片化，碎片峰提供了丰富的结构信息。
4. Ionisation: Electrospray Ionisation (ESI) | 电离方法：电喷雾电离（ESI）
Electrospray ionisation is a softer technique used primarily for large, polar, and non‑volatile molecules such as proteins. The sample is dissolved in a volatile solvent and sprayed through a fine needle at high voltage, producing charged droplets. As the solvent evaporates, the analyte gains a proton to form [M+H]⁺ ions. Fragmentation is minimal, so ESI mass spectra typically show the intact molecular ion cluster. This process can be summarised as:
M + H⁺ → [M+H]⁺
电喷雾电离是一种较温和的技术，主要用于像蛋白质这样的大分子、极性分子和非挥发性分子。样品溶解在挥发性溶剂中，通过加有高电压的细针头喷雾，产生带电液滴。随着溶剂蒸发，分析物获得一个质子，形成 [M+H]⁺ 离子。碎片化极少，因此 ESI 质谱图通常显示完整的分子离子簇。该过程可概括为：
M + H⁺ → [M+H]⁺
5. Acceleration, Deflection, and Detection | 加速、偏转与检测
Positively charged ions are accelerated through a series of negatively charged plates with a known potential difference, giving all ions with the same charge the same initial kinetic energy. They then enter a magnetic field that applies a force perpendicular to their motion, causing them to travel in a curved path. The radius of curvature depends on the m/z ratio: lighter ions and those with higher charges are deflected more. By varying the magnetic field strength, ions of different m/z values are brought successively to the detector, which records the ion current and generates the mass spectrum.
带正电的离子通过一系列具有已知电势差的带负电的极板加速，使所有带相同电荷的离子具有相同的初始动能。随后，它们进入一个磁场，该磁场施加一个垂直于其运动方向的力，使其沿曲线路径飞行。曲率半径取决于 m/z 比值：较轻的离子和带电量较高的离子偏转更大。通过改变磁场强度，不同 m/z 值的离子可被依次引导至检测器，检测器记录离子电流并生成质谱图。
6. Understanding the Mass Spectrum | 解读质谱图
A mass spectrum displays m/z values on the horizontal axis (often in atomic mass units, u, for singly charged ions) and relative abundance on the vertical axis. The most abundant peak is assigned an arbitrary height of 100 and is called the base peak. All other peaks are measured relative to this. The spectrum not only reveals the molecular mass of the compound but also provides characteristic fragmentation patterns useful for structure elucidation.
质谱图的横轴是 m/z 值（对于单电荷离子，通常以原子质量单位 u 表示），纵轴是相对丰度。最高丰度的峰被任意指定为高度 100，称为基峰。所有其他峰都以此为基准进行测量。质谱图不仅揭示了化合物的分子质量，还提供了用于结构解析的特征性碎片模式。
7. Molecular Ion Peak and Base Peak | 分子离子峰与基峰
The molecular ion peak (M⁺) is the peak that appears at the highest m/z value, corresponding to the unfragmented radical cation. It tells you the relative molecular mass of the sample compound. In some cases, the molecular ion peak may be very weak or even absent due to complete fragmentation. The base peak is the tallest peak in the spectrum, representing the most stable cation formed during ionisation and fragmentation. Notably, the base peak is not always the molecular ion peak.
分子离子峰（M⁺）是在最高 m/z 值处出现的峰，对应于未碎裂的自由基阳离子。它告诉你样品的相对分子质量。在某些情况下，由于完全碎片化，分子离子峰可能很弱甚至不出现。基峰是谱图中最高的峰，代表电离和碎裂过程中形成的最稳定的阳离子。请注意，基峰并不总是分子离子峰。
8. Fragmentation Patterns | 碎片模式
When a molecular ion possesses excess internal energy, it can break apart into smaller fragments. Some fragments are charged and detected in the mass spectrum, while neutral fragments are lost undetected. The difference between the m/z values of successive peaks indicates the mass of the neutral fragment lost. Common neutral losses include CH₃ (15 u), OH (17 u), H₂O (18 u), and CO (28 u). Recognising these losses helps identify functional groups. For example, a peak at M−15 often points to the loss of a methyl group.
当分子离子具有过多的内能时，它会碎裂成较小的碎片。一些碎片带有电荷，并在质谱图中被检测到，而中性碎片的丢失则无法检测。相邻峰 m/z 值的差值即表示丢失的中性碎片的质量。常见的中性丢失包括 CH₃（15 u）、OH（17 u）、H₂O（18 u）和 CO（28 u）。识别这些丢失有助于确定官能团。例如，M−15 处的峰通常表明丢失了一个甲基。
9. Isotopic Patterns: Chlorine and Bromine | 同位素模式：氯与溴
Elements with significant natural isotopes produce characteristic patterns in mass spectra. Chlorine consists of ³⁵Cl (75%) and ³⁷Cl (25%). A compound containing one chlorine atom shows two molecular ion peaks at M and M+2 in a 3:1 ratio. Bromine has two isotopes ⁷⁹Br (51%) and ⁸¹Br (49%), giving an almost 1:1 M to M+2 pattern. For two chlorine or two bromine atoms, the intensity ratios follow the binomial expansion. These patterns are vital for identifying halogen‑containing compounds.
具有显著天然同位素的元素会在质谱图中产生特征模式。氯由 ³⁵Cl（75%）和 ³⁷Cl（25%）组成。含有 1 个氯原子的化合物会在 M 和 M+2 处显示出 3:1 比率的两个分子离子峰。溴有 ⁷⁹Br（51%）和 ⁸¹Br（49%）两个同位素，产生近乎 1:1 的 M 与 M+2 模式。对于 2 个氯原子或 2 个溴原子，其强度比遵循二项式展开。这些模式对于鉴定含卤素化合物至关重要。
| Number of Cl or Br atoms | Ratio M : M+2 : M+4 |
|---|---|
| 1 × Cl | 3 : 1 |
| 2 × Cl | 9 : 6 : 1 |
| 1 × Br | 1 : 1 |
| 2 × Br | 1 : 2 : 1 |
10. Calculating Relative Atomic Mass from Mass Spectra | 由质谱计算相对原子质量
For an atomic sample, the mass spectrum shows peaks corresponding to each isotope. The relative atomic mass, Aᵣ, is calculated by taking the weighted average of the isotopic masses using their percent abundances. The formula is:
Aᵣ = Σ (isotopic mass × percent abundance) / 100
For example, naturally occurring copper contains ⁶³Cu (69.17%, mass = 62.93 u) and ⁶⁵Cu (30.83%, mass = 64.93 u), giving Aᵣ ≈ (62.93×69.17 + 64.93×30.83) / 100 ≈ 63.55. You must show the correct method in exams.
对于原子样品，质谱图会显示对应每种同位素的峰。相对原子质量 Aᵣ 是通过使用其百分丰度计算同位素质量的加权平均值而得到的。公式为：
Aᵣ = Σ（同位素质量 × 百分丰度）/ 100
例如，天然存在的铜包含 ⁶³Cu（69.17%，质量 = 62.93 u）和 ⁶⁵Cu（30.83%，质量 = 64.93 u），计算得 Aᵣ ≈ (62.93×69.17 + 64.93×30.83)/100 ≈ 63.55。考试中你必须展示正确的计算方法。
11. The M+1 and M+2 Peaks | M+1峰与M+2峰
In addition to halogen isotopic patterns, organic compounds show small peaks at M+1 due to the presence of ¹³C (1.1% natural abundance). The intensity of the M+1 peak relative to the M peak can be used to estimate the number of carbon atoms. For a compound with n carbon atoms, the M+1 peak intensity is roughly n × 1.1% of the M peak. The M+2 peak can arise from ³⁴S (4.3%), ³⁷Cl, or ⁸¹Br, and its abundance helps distinguish which heteroatom is present.
除了卤素的同位素模式外，有机化合物还会因存在 ¹³C（天然丰度 1.1%）而在 M+1 处出现小峰。M+1 峰相对于 M 峰的强度可用于估算碳原子数目。对于一个含有 n 个碳原子的化合物，M+1 峰的强度大约是 M 峰的 n × 1.1%。M+2 峰可能源自 ³⁴S（4.3%）、³⁷Cl 或 ⁸¹Br，其丰度有助于区分存在哪种杂原子。
12. The Nitrogen Rule | 氮规则
The nitrogen rule is a simple but powerful tool for identifying whether a molecule contains an odd number of nitrogen atoms. It states that an organic compound containing an even number of nitrogen atoms (including zero) will have an even‑numbered molecular ion mass. If the number of nitrogen atoms is odd, the molecular ion mass will be an odd number. This rule helps narrow down possible molecular formulas when interpreting mass spectra.
氮规则是一个简单却有力的工具，用于判断分子是否含有奇数个氮原子。该规则指出，含有偶数个氮原子（包括零）的有机合物，其分子离子质量为偶数。若氮原子数为奇数，则分子离子质量为奇数。这个规则有助于在解读质谱图时缩小可能的分子式范围。
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