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

質量分析では、分析種分子は、分析種分子よりも電子が1つ少ない分子イオンにイオン化されます。
電子が失われると、分子イオンの一部の結合が弱まり、その後、比較的安定した小さな種につながる経路を通じて断片化されます。
たとえば、ペンタン分子イオンの断片化を考えてみましょう。切断は、末端とその隣接する炭素との間ではなく、第2炭素と第3炭素の間の結合で優先的に発生します。前者は比較的安定したカルボカチオンをもたらします。
フラグメンテーションはカチオンとラジカルを生成し、不対電子の配置により2つの開裂モードが生じます。安定なカルボカチオンを生成する切断は、安定したラジカルを生成する切断よりも好ましいです。
断片化は、2つの結合の切断によっても発生します。例えば、アルコールの分子イオンは、ラジカルカチオンと中性分子を生成します。
別のフラグメンテーションパターンは、分析種分子内のπ結合または非結合性電子の存在によって駆動される共鳴安定化カルボカチオンの形成です。

質量分析: 分子断片化の概要

Mass Spectrometry: Molecular Fragmentation Overview

質量分析計内で分子が分子イオンにイオン化すると、電子が失われるため、分子の構造が不安定になります。これにより、最終的には分子内の一部の結合が断片化または切断されます。断片化は主に特定の結合で発生し、比較的安定した断片が取得します。

断片化パターンの 1 つのタイプは、分子イオン内の単結合の切断です。切断によりラジカルとカチオンが生成されます。切断は分子内のさまざまな単結合で発生しますが、切断は安定なカルボカチオンとラジカルを生成する位置で優先的に発生します。特定の結合では、不対電子がいずれかのフラグメントに移動し、2 セットの生成物が生成されます。電子移動の優先配向は、安定したラジカルではなく、より安定したカルボカチオンを生成するような方法で起こります。Figure 1 に示すペンタンの断片化パターンを考えてみましょう。

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. ペンタン分子イオンの断片化。

Figure 2 に示すように、質量スペクトルはにおけるカルボカチオンの豊富さから、分子イオンの切断は隣接する 2 つの炭素原子間で発生する可能性があることが明らかです。しかしながら、内部の２つの炭素原子間の開裂は、末端とその隣接する炭素間の開裂よりも好ましい。内部炭素の開裂により、比較的安定した一次カルボカチオンと一次ラジカルが生成されます。末端とその隣接する炭素の間の結合が開裂すると、一級カルボカチオンとメチルラジカル、または一級ラジカルとメチルカルボカチオンが生成されます。この切断が起こる場合、切断モードによってブチルカルボカチオンとメチルラジカルが生成されます。

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. ペンタンの質量スペクトルは。

切断のもう 1 つのタイプは、複数の結合が解離してフラグメントが生成されることです。このタイプの断片化により、より小さなラジカルカチオンと中性分子が生成されます。たとえば、アルコールは水分子を追い出すことによって、より小さなラジカルカチオンに断片化することがあります。その結果、質量スペクトルはは M-18 ピークを示す。Figure 3 は、1-ヘプタノールの断片化の概略を示しています。

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. 1-ヘプタノールの断片化。

ヘプタノールの分子量は 116 u で、Figure 4 の質量スペクトルはでは m/z = 98 のピークが見られます。同様に、シクロアルケンは逆ディールス アルダー反応を受けて、アルケンとより小さなラジカル カチオンを放出します。

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. ヘプタノールの質量スペクトルは。

3 番目のタイプの断片化は、非共有電子または π 結合を持つ分子で観察されます。非共有電子対を持つ分子では、非共有電子対を持つ原子の隣の炭素-炭素結合が切れて、共鳴安定化されたカルボカチオンが生成されます。Figure 5 は、例としてエーテルのフラグメンテーションを示しています。

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. エーテルの断片化。

分子イオンのπ結合は分子の断片化を促進し、共鳴によって安定化されたアリルカルボカチオンを示す。Figure 6 は、末端アルケンの断片化パターンを示しています。

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. アルケンの断片化。

質量分析計内で分子が分子イオンにイオン化されると、分子の構造が不安定になります。

mass-spectrometry-molecular-fragmentation-overview

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Principles of Mass Spectrometry

質量分析法の原理

KEY TERMS AND CONCEPTS

主要用語と概念

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

Mass spectrometry helps identify a compound by measuring its molecular mass and molecular formula. It does this by turning the analyte molecule into ions and then comparing ion abundance with mass-to-charge ratio. One common ionization method is electron ionization, or EI. In EI, high-energy electron beams strike analyte molecules in the gas phase. The beam removes an electron and leaves a molecular ion behind. A molecular ion is a radical cation, so it has a positive charge and an unpaired...

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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...

mass spectrometers

How Mass Spectrometers Sort Ions

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Reading Peaks in a Mass Spectrum

A mass spectrum shows the charged fragments in an analyte as peaks plotted by mass-to-charge ratio, or m/z. The x-axis gives the mass of each charged fragment divided by the number of charges it carries. The y-axis shows the relative abundance of each charged species based on the signal intensity recorded at the detector. The tallest peak is called the base peak. It is assigned an abundance of 100%, and the other peaks are scaled against it. The peak from the molecular ion is called the parent...

mass spectrum: interpretation

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mass analyzers: overview

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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,...

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high-resolution mass spectrometry (hrms)

Mass Accuracy in HRMS Analysis

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Hyphenated Mass Spectrometry Methods

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

inductively-coupled-plasmamass-spectrometry-icpms-overview

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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

JoVE Core 分析化学 は、複雑な化学原理をアニメーションで視覚化することで、SI 単位から NMR 分光法を含むトピックをよりわかりやすくします。実験を撮影した実写動画では、これらの概念が世界中の研究所でどのように応用されているかを取り上げています。

analytical chemistry

Mass Spectrometry Fragmentation Patterns

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