Name: mass spectrometry fragmentation patterns
Uploaded: 2024-04-04T10:51:33.000Z
Duration: 1 min 20 s
Channel: JoVE

在质谱分析中，分析物分子被电离，形成比分析物分子少一个电子的分子离子。
失去一个电子会削弱分子离子中的某些化学键，导致其通过特定途径断裂，生成相对稳定的较小物种。
例如，考虑戊烷分子离子的裂解。裂解优先发生在第二个与第三个碳原子之间的化学键，而非末端碳与其相邻碳原子之间的键。前者可生成相对稳定的碳正离子。
裂解产生一个阳离子和一个自由基，其中未成对电子的排列方式导致两种裂解模式。相较于生成稳定自由基的裂解方式，生成稳定碳正离子的裂解方式更占优势。
裂解同样通过两个化学键的断裂而发生。例如，醇类分子离子会生成一个自由基阳离子和一个中性分子。
另一种裂解模式是形成由共振稳定的碳正离子，这种裂解由分析物分子中的π键或非键合电子所驱动。

质谱分析法：分子断裂概述

Mass Spectrometry: Molecular Fragmentation Overview

分子在质谱仪内会被电离成分子离子，由于失去的电子会导致分子的结构变得不稳定。这最终会导致分子中某些键发生断裂。断裂主要发生在特定的键上，使其以产生相对稳定的片段。

一种类型的碎裂模式是分子离子中的单键断裂。裂解会产生自由基和阳离子。裂解可以发生在分子中的各种单键处，但裂解会优先发生在产生稳定的阳离子碳和自由基的位置。在特定的键上，不成对的电子可以转移到任一片段，从而产生两组产物。电子转移以最优取向的方式发生便能够产生更稳定的碳正离子而不是稳定的自由基。思考如图 1 中所示的戊烷裂解模式。

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

图 1. 戊烷分子离子的裂解。

如图 2 所示，从质谱中碳阳离子的丰度来看，可以很明显的看出分子离子的裂解可以发生在任何两个相邻的碳原子之间。然而，两个内部碳原子之间的裂解比末端与其相邻碳原子之间的裂解更为优先。内部碳的裂解能够产生相对稳定的伯碳正离子和伯自由基。末端与其相邻碳原子之间的键发生断裂所产生伯碳阳离子和甲基基团或伯基团和甲基碳阳离子。如果发生这种裂解，则会通过这种裂解模式来产生丁基碳阳离子和甲基自由基。

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

图 2. 戊烷的质谱。

另一种类型的裂解是通过多重键的解离所产生的片段。这种类型的裂解能够产生较小的自由基阳离子和中性分子。例如，醇可以通过排出水分子而分裂成更小的自由基阳离子。因此，质谱会显示为 M−18 峰。从图 3 中可以看到 1-庚醇的裂解示意图。

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

图 3. 1-庚醇的裂解。

庚醇的分子量为 116 u，在如图 4 所示捕获的质谱中可以看到 m/z = 98 处的峰。类似地，环烯烃在参与逆狄尔斯-阿尔德反应后能够释放出烯烃和较小的自由基阳离子。

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

图 4. 庚醇的质谱。

第三种类型的碎裂是在其具有未共享电子或 π 键的分子中所观察到的。在具有孤对电子的分子中，带有孤对电子的原子旁边的碳-碳键则会发生断裂，从而产生共振稳定的碳正离子。以图 5 中醚的裂解为例。

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

图 5. 醚的裂解。

分子离子中的 π 键能够促进分子发生裂解，从而能够产生通过共振稳定的烯丙基碳正离子。图 6 中则显示了末端烯烃的裂解模式。

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

图 6. 烯烃的裂解。

分子在质谱仪内电离形成分子离子时，由于失去一个电子，导致分子结构不稳定……

mass-spectrometry-molecular-fragmentation-overview

Education

教育

JoVE Core

Analytical Chemistry

分析化学

Unit 1

第一单元

Principles of Mass Spectrometry

质谱分析原理

KEY TERMS AND CONCEPTS

关键术语与概念

mass spectrometry fragmentation patterns

Mass spectrometry fragmentation patterns show how a molecular ion breaks apart after ionization. When a molecule loses an electron inside the mass spectrometer, its structure becomes unstable. The ion then fragments at bonds that often produce the most stable pieces.

One common pattern is cleavage of a single bond in the molecular ion. This break forms a radical and a cation. The bond can split in more than one way, but the favored path is the one that gives the more stable carbocation rather than the more stable radical.

Pentane shows this idea clearly. Its molecular ion can break between any two adjacent carbon atoms, but cleavage between two inner carbon atoms is preferred over cleavage near the end of the chain. That inner break gives a relatively stable primary carbocation and primary radical. When the terminal bond breaks, the products can include a primary carbocation and a methyl radical, or a primary radical and a methyl carbocation. In the preferred case, the cleavage leads to a butyl carbocation and a methyl radical.

A second pattern is loss of a neutral fragment from the ion. This type of fragmentation gives a smaller radical cation plus a neutral molecule. Alcohols can lose water in this way, so their mass spectra often show an M−18 peak. For example, 1-heptanol has a molecular mass of 116 u, and its spectrum shows a peak at m/z = 98. Cycloalkenes can also follow a retro-Diels–Alder reaction to expel an alkene and leave a smaller radical cation.

A third pattern appears in molecules with lone pairs or π bonds. In compounds with lone pairs, the carbon-carbon bond next to the atom bearing the lone pair can break to form a resonance-stabilized carbocation, as seen with ethers. In molecules with π bonds, fragmentation often creates an allylic carbocation that is stabilized by resonance. A terminal alkene is a common example of this behavior.

Mass Spectrometry Fragmentation Patterns

mass spectrometry: overview

Mass Spectrometry and Molecular Identity

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mass spectrometry: molecular fragmentation overview

Mass spectrometry fragmentation patterns show how a molecular ion breaks apart after ionization. When a molecule loses an electron inside the mass spectrometer, its structure becomes unstable. The ion then fragments at bonds that often produce the most stable pieces. One common pattern is cleavage of a single bond in the molecular ion. This break forms a radical and a cation. The bond can split in more than one way, but the favored path is the one that gives the more stable carbocation rather...

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mass spectrum: interpretation

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mass analyzers: overview

Mass Analyzers and Ion Separation

Mass analyzers separate ions by their mass-to-charge ratio, or m/z, in a mass spectrometer. After a sample is vaporized in the ionization chamber, it is hit with a high-energy electron beam. This creates a radical cation and also produces neutral molecules, radicals, and cations. Negatively charged accelerator plates then push the cations into the mass analyzer, where they are sent on to the detector. Several mass analyzer designs are commonly used. These include magnetic sector,...

mass analyzers: common types

Quadrupole, TOF, and Ion Trap Analyzers

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high-resolution mass spectrometry (hrms)

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mass spectrometry: complex analysis

Hyphenated Mass Spectrometry Methods

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gas chromatography&ndash;mass spectrometry (gc&ndash;ms)

How GC-MS Separates and Identifies Compounds

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inductively coupled plasma&#8211;mass spectrometry (icp&#8211;ms): overview

inductively-coupled-plasmamass-spectrometry-icpms-overview

ICP-MS Sample Ionization and Detection

ICP-MS uses an inductively coupled plasma torch to turn samples into ions for mass analysis. Solid samples are first dissolved and volatilized before they enter the hot argon plasma. Solution samples are nebulized, which means they are converted into a fine spray, and then passed through the same plasma. Inside the plasma, the analytes break apart. Their atoms become positively charged ions, along with some molecular species. These positive ions are sent to the mass spectrometer, where they...

inductively coupled plasma-mass spectrometry (icp-ms): interferences

inductively-coupled-plasma-mass-spectrometry-icp-ms-interferences

ICP-MS Signal Interference Sources and Fixes

Inductively coupled plasma-mass spectrometry (ICP-MS) is a sensitive method for elemental analysis, but its spectra can be affected by interferences. These interferences can make the mass signal less accurate, even when the instrument is working well. Spectroscopic interferences happen when ions in the plasma have the same mass-to-charge ratio, or m/z, as the analyte ion. This group includes isobaric interferences, polyatomic ion interferences, and refractory oxide ion interferences. Isobaric...

key terms and concepts

Learn how molecular ions fragment in mass spectrometry, including single-bond cleavage, water loss from alcohols, and resonance-stabilized products.

Mass Spectrometry Fragmentation Patterns | JoVE K12

analytical chemistry

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Mass Spectrometry Fragmentation Patterns

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