📚 Mass Spectrometry: Principles and Analysis | 质谱法：原理与分析
Mass spectrometry (MS) is an analytical technique used to measure the mass-to-charge ratio (m/z) of ions. It is widely applied to determine relative isotopic masses, relative atomic masses, and to identify unknown compounds by their fragmentation patterns. In A-level chemistry, you need to understand how a mass spectrometer works, interpret simple mass spectra, and calculate relative atomic masses from isotopic abundance data.
质谱法是一种分析技术，用于测量离子的质荷比（m/z）。它被广泛应用于测定相对同位素质量、相对原子质量，以及通过碎裂模式鉴定未知化合物。在A-Level化学中，你需要了解质谱仪的工作原理，解析简单的质谱图，并利用同位素丰度数据计算相对原子质量。
1. Introduction to Mass Spectrometry | 质谱法简介
Mass spectrometry has become an indispensable tool in modern chemistry. It can detect trace amounts of substances, provide accurate molecular masses, and offer structural information through fragmentation. The basic principle involves converting sample molecules into ions, separating them according to their mass-to-charge ratio, and recording the intensity of each ion beam. The output is a mass spectrum, a plot of relative abundance against m/z.
质谱法已成为现代化学不可或缺的工具。它可以检测微量物质，提供精确的分子质量，并通过碎裂提供结构信息。基本原理是将样品分子转化为离子，根据质荷比将其分离，并记录每种离子束的强度。输出结果是质谱图，即相对丰度相对于质荷比的图。
2. The Five Key Stages | 五个关键阶段
A simple mass spectrometer operates through five main stages: vaporisation, ionisation, acceleration, deflection, and detection. Some modern instruments may combine or modify these steps, but the core concepts remain essential.
简单质谱仪通过五个主要阶段运行：气化、离子化、加速、偏转和检测。一些现代仪器可能合并或修改这些步骤，但核心概念依然至关重要。
- Vaporisation – the sample is heated and turned into a gas.
气化 – 样品被加热并转变为气体。 - Ionisation – gaseous atoms or molecules are bombarded with high-energy electrons to form positive ions.
离子化 – 气态原子或分子被高能电子轰击形成正离子。 - Acceleration – positive ions are accelerated by an electric field to a constant kinetic energy.
加速 – 正离子被电场加速至恒定的动能。 - Deflection – the ion beam passes through a magnetic field, causing ions to follow curved paths according to their m/z.
偏转 – 离子束通过磁场，使离子根据其质荷比沿弯曲路径运动。 - Detection – ions strike a detector, generating an electric current proportional to their abundance.
检测 – 离子撞击检测器，产生与其丰度成正比的电流。
3. Ionisation: Electron Impact | 离子化：电子轰击
In electron impact ionisation, the sample is first vaporised. A beam of high-energy electrons (typically 70 eV) is fired at the gaseous sample. When an electron collides with a sample molecule, it can knock out an outer electron, producing a radical cation:
在电子轰击离子化中，样品首先被气化。一束高能电子（通常为70 eV）射向气态样品。当一个电子与样品分子碰撞时，可以击出一个外层电子，产生自由基阳离子：
M(g) + e⁻ → M⁺• + 2e⁻
The molecular ion M⁺• often has enough excess energy to break bonds, leading to fragmentation.
分子离子M⁺•通常具有足够过剩的能量来断裂化学键，导致碎裂。
This method works well for small, volatile organic molecules. However, extensive fragmentation may make it difficult to observe the molecular ion peak. The spectrum shows many fragment peaks, which provide valuable structural clues.
该方法适用于小分子、挥发性有机化合物。然而，广泛的碎裂可能使得分子离子峰难以观测。谱图显示许多碎片峰，它们提供了有价值的结构线索。
4. Ionisation: Electrospray | 离子化：电喷雾
Electrospray ionisation (ESI) is a softer technique often used for large biomolecules. The sample is dissolved in a volatile solvent and passed through a fine needle at high voltage. This creates a fine aerosol of charged droplets. As the solvent evaporates, the droplets shrink until the repulsion between charges ejects ions from the surface. Often, the species detected is [M+H]⁺ (protonated molecule) or [M–H]⁻ (deprotonated molecule) in negative mode.
电喷雾离子化（ESI）是一种较温和的技术，常用于大分子生物分子。样品溶解在挥发性溶剂中，通过高压细针头喷出。这会产生带电液滴的细雾。随着溶剂蒸发，液滴收缩，直到电荷间的排斥力使离子从表面喷射出来。通常检测到的物种是[M+H]⁺（质子化分子）或在负离子模式下的[M–H]⁻（去质子化分子）。
Because ESI produces little fragmentation, it gives a clear molecular ion peak, making it ideal for determining relative molecular mass. A-level specifications often contrast EI and ESI.
由于ESI产生的碎裂很少，它能给出清晰的分子离子峰，因此是测定相对分子质量的理想选择。A-Level课程大纲通常会将EI和ESI进行对比。
5. Acceleration | 加速
After ionisation, the positive ions are attracted towards a series of negatively charged plates. The electric field accelerates the ions to a high velocity, giving them the same kinetic energy:
离子化后，正离子被引向一系列带负电的板。电场将离子加速至高速，赋予它们相同的动能：
KE = ½mv² = zV
where z is the charge and V is the accelerating voltage. Thus, ions with smaller mass will reach a higher speed for a given charge. This ensures that when they enter the magnetic field, their separation depends purely on m/z.
其中z是电荷数，V是加速电压。因此，对于给定电荷，质量较小的离子将达到更高的速度。这确保了当它们进入磁场时，分离仅仅取决于质荷比。
6. Deflection (Mass Analysis) | 偏转（质量分析）
The accelerated ions travel into a magnetic field applied perpendicular to their direction. A charged particle moving through a magnetic field experiences a force that causes it to move in a curved path. The radius of curvature r is proportional to m/z for a fixed magnetic field and accelerating voltage:
加速后的离子进入与其运动方向垂直的磁场。带电粒子在磁场中运动时会受到一个力，使其沿曲线路径运动。对于固定的磁场和加速电压，曲率半径r与m/z成正比的平方根关系：
r ∝ √m/z
This means lighter ions with smaller m/z are deflected more and follow a tighter curve. By varying the magnetic field strength, ions of different m/z can be focused onto the detector one at a time, thus scanning the mass range.
这意味着较轻的离子（m/z较小）偏转更大，路径更弯曲。通过改变磁场强度，不同m/z的离子
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