Name: using reagent gas in ci mass spectrometry
Uploaded: 2024-12-05T18:32:21.000Z
Duration: 1 min 21 s
Channel: JoVE

電子衝突は、質量分析でサンプル分子をイオン化するための従来の方法です。しかし、分子によっては、高エネルギーの電子ビームが一次分子イオンの過剰な分裂を引き起こします。
このような質量スペクトルには分子イオンピークがないため、サンプルの同定が困難になります。
別の化学イオン化法は気相プロトン化反応であり、最初に、サンプル分子の比較的安定した共役酸が生成されます。その後、この種は低分子量成分に断片化されます。 
例えば、ジ-sec-ブチルエーテルの化学イオン化を考えてみましょう。エーテルはメタンガスと混合されます。次に、高エネルギーの電子ビームがメタンを優先的にイオン化します。
形成されたメタンラジカルカチオンは、別のメタン分子と衝突し、メタニウムイオンとメタンラジカルを生成します。
メタニウムイオンは超酸であり、エーテルをプロトン化する気相プロトンの供給源です。電子衝突イオン化によって形成される分子イオンと比較して、プロトン化エーテルは安定しており、質量スペクトルでM+1のピークが得られます。

化学イオン化 (ci) 質量分析法

Chemical Ionization (CI) Mass Spectrometry

質量スペクトルにおける分子の分子イオンピークは、分子の識別に不可欠な情報を提供します。ただし、従来の電子衝撃イオン化では、分子イオンが検出器に到達する前に急速に解離する可能性があります。このようなイオン化された分析物分子の寿命を延ばすには、より穏やかなイオン化方法が必要です。化学イオン化 (CI) は、容易にプロトン化されて対応する共役酸を生成する分析物分子の質量分析に役立つ気相プロトン化反応です。このプロセスでは、サンプルを過剰な試薬ガスと混合し、電子衝撃が主に試薬ガス上で発生するようにします。試薬から形成された荷電種が分析物分子をプロトン化し、分子イオンと比較して比較的安定したプロトン化分析物 (共役酸) を生成します。これにより、質量スペクトルに M+1 ピークが生じます。その後、共役酸がフラグメンテーションを起こし、追加の信号が生成されます。

たとえば、メタンガス中のジ-sec-ブチルエーテルの CI はこのプロセスを示しています。エーテルを試薬として過剰のメタンガスと混合すると、電子衝撃はエーテルではなくメタンに発生します。結果として生じるメタンラジカルカチオンは、別のメタン分子と反応してメタンラジカルとメタニウムイオンを生成します。メタニウムイオンは気相プロトンの供給源であり、エーテルをプロトン化して共役酸を形成できます。ジ-sec-ブチルエーテルの化学イオン化中の連続反応は、図 1 に示されています。

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="height: 175px;">

図 1: ジ-sec-ブチルエーテルメタン混合物の化学イオン化。

このエーテルの共役酸 (m/z = 131) は、従来の電子衝撃イオン化によって形成されたエーテルの分子イオン (m/z = 130) よりも比較的安定しています。従来の方法では、分子イオンはα開裂によってフラグメンテーションを受け、m/z = 101 で信号を生成します。図 2 は、ジ-sec-ブチルエーテルで直接電子衝撃イオン化中に発生する反応を示しています。

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="height: 175px;">

図 2: 電子衝撃ジ-sec-ブチルエーテルのイオン化と分子イオンのフラグメンテーション。

したがって、CI によってイオン化されたジ-sec-ブチルエーテルの質量スペクトルは、m/z = 131 で M+1 ピークを特徴としています。一方、電子衝撃イオン化によってイオン化されたジ-sec-ブチルエーテルの質量スペクトルは、その分子量の m/z 値でピークを示しません。図 3a と 3b は、それぞれ電子衝撃法と化学イオン化法によってイオン化されたジ-sec-ブチルエーテルの質量スペクトルを示しています。

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="height: 225px;">

図 3: a) ジ-sec-ブチルエーテルの電子衝撃イオン化によって得られた質量スペクトルには、m/z = 130 のピークは見られません。 b) ジ-sec-ブチルエーテルの化学イオン化によって得られた質量スペクトルには、m/z = 131 のピークがはっきりと見られます。

化学イオン化 (CI) 質量分析法

マススペクトル内の分子の分子イオンピークは、分子の同定に重要な情報を提供します。しかし、従来の電子衝突は...

Education

JoVE Core

Analytical Chemistry

分析化学

Unit 1

ユニット1

Mass Spectrometry Fragmentation Methods

質量分析におけるフラグメンテーション法

KEY TERMS AND CONCEPTS

主要用語と概念

using reagent gas in ci mass spectrometry

Chemical ionization mass spectrometry uses a reagent gas to help detect molecules that break apart too quickly in electron impact ionization. It is a milder ionization method that can extend the life of the ionized analyte, which makes molecular identification easier.

In chemical ionization, the sample is mixed with an excess reagent gas. This setup makes the electron impact occur mainly on the reagent gas instead of on the analyte. The charged species formed from the reagent then protonates the analyte molecule and forms a stable conjugate acid.

The protonated analyte is usually more stable than the original molecular ion. As a result, the mass spectrum often shows an M+1 peak. The conjugate acid can still fragment and produce extra signals.

The chemical ionization of di-sec-butyl ether in methane shows this process clearly. Methane acts as the reagent gas, and electron impact first creates a methane radical cation. That species reacts further to form a methanium ion, which donates a gas-phase proton to the ether and forms its conjugate acid.

The protonated di-sec-butyl ether appears at m/z = 131 and is more stable than the molecular ion at m/z = 130. With conventional electron impact ionization, the molecular ion fragments by alpha cleavage and gives a signal at m/z = 101. In the mass spectrum, CI shows the M+1 peak at m/z = 131, while electron impact ionization does not show a peak at the molecular weight of the ether.

Using Reagent Gas in CI Mass Spectrometry

using-reagent-gas-in-ci-mass-spectrometry

mass spectrometry: long-chain alkane fragmentation

mass-spectrometry-long-chain-alkane-fragmentation

Mass Spectra of Linear Alkane Chains

Mass spectra of linear alkanes show a clear fragmentation pattern. The molecular ion often breaks at a carbon-carbon bond away from the end of the chain. This cleavage forms a stable carbocation and a stable radical, so the middle of the mass-to-charge plot usually has the strongest signals while the ends are weaker. As each bond breaks, a methyl group is lost and the main peaks in the spectrum appear 14 u apart. Smaller peaks near each main signal can also appear. These come from secondary...

mass spectrometry: branched alkane fragmentation

mass-spectrometry-branched-alkane-fragmentation

Branched Alkanes in Mass Spectra

Branched alkanes in mass spectrometry fragment quickly because branching can create stable carbocations. When cleavage happens at a branching point, a secondary or tertiary carbocation may form. That stability helps explain why the molecular ion peak is often very weak or missing compared with a linear alkane. The lesson uses 2-methylbutane, neopentane, and n-pentane to show how this pattern appears in mass spectra. 2-methylbutane and neopentane break apart to form secondary and tertiary...

mass spectrometry: cycloalkane fragmentation

mass-spectrometry-cycloalkane-fragmentation

Cycloalkane Ions and Mass Peaks

Cycloalkane ions in mass spectrometry often give strong peaks because the ring structure helps stabilize the molecular ion. Compared with linear or branched alkanes, this added stability changes how the molecule breaks apart. As a result, cycloalkanes can produce prominent signals in the mass spectrum. Cyclohexane is a clear example. Its molecular ion has an m/z of 84, and this peak is often stronger than the signal from hexane, a linear alkane. The closed ring helps hold the ion together,...

mass spectrometry: alkene fragmentation

Alkene Ion Fragmentation Patterns

Alkene ion fragmentation patterns in mass spectrometry begin when the molecule loses one electron from the unsaturated π bond. This ionization forms a stable molecular ion. The ion can then break apart by three main reaction pathways. The most common pathway is cleavage at the allylic position, which is the carbon next to the double bond. This produces an allylic carbocation that is stabilized by resonance. In terminal alkenes, this fragment appears at a mass-to-charge ratio of 41. In internal...

mass spectrometry: cycloalkene fragmentation

mass-spectrometry-cycloalkene-fragmentation

Cycloalkene Side-Chain Cleavage in Mass Spectra

Cycloalkenes can break apart in a mass spectrometer through retro-Diels–Alder fragmentation. In this process, two carbon-carbon bonds in the ring split, and ethene is released. The remaining fragment is a dienyl radical cation that is 28 u less than the molecular ion. This pathway is similar to the fragmentation of cycloalkanes that also releases ethene. The difference is the type of charged fragment that remains. Cycloalkanes form an alkyl species, while cycloalkenes form a dienyl species.

mass spectrometry: alkyne fragmentation

Alkyne Mass Spectra and Key Peaks

Alkyne mass spectra often show strong peaks from two main fragmentation paths. One path breaks the carbon-carbon bond between the alpha and beta carbons of the alkyne bond. This produces a 3-propynyl cation, also called a propargyl cation. In terminal alkynes, this is the only fragmentation path that gives the 3-propynyl cation. The unsubstituted 3-propynyl cation appears at a mass-to-charge ratio of 39. In internal alkynes, the 3-propynyl cation is substituted. For example, 2-pentyne forms a...

mass spectrometry: alcohol fragmentation

Alcohol Fragmentation Patterns in Mass Spectra

Alcohols fragment in mass spectrometry in predictable ways. When an alcohol, written as R-OH, ionizes, it can lose one non-bonded electron from the oxygen atom. That forms a molecular ion, but the molecular ion peak is often weak or may not appear because alcohols break apart quickly. Two main fragmentation routes are important for alcohols: β-cleavage and dehydration. In β-cleavage, the bond at the β-position next to the hydroxyl group breaks. This gives a resonance-stabilized cation and a...

mass spectrometry: aromatic compound fragmentation

mass-spectrometry-aromatic-compound-fragmentation

Mass Spectra of Benzene and Toluene

Mass spectra of aromatic compounds show a strong molecular ion peak. In ionization, the molecule becomes a molecular ion, which appears as a prominent peak in the spectrum. Benzene gives a molecular ion at m/z 78, and toluene gives one at m/z 92. Benzene’s molecular ion is especially stable. It does not fragment readily because breaking the aromatic benzene ring requires a large amount of energy. Alkyl-substituted benzenes behave differently in mass spectrometry. Toluene can lose a hydrogen...

mass spectrometry: amine fragmentation

Amines in Mass Spectra: Nitrogen Rule

Amines can be identified in mass spectrometry by their fragmentation patterns and by the nitrogen rule. The molecular ions of amines often break apart through β-cleavage, which cuts a carbon-carbon bond next to the nitrogen. This process forms an alkyl radical and a resonance-stabilized nitrogen-containing cation. The nitrogen rule helps predict the mass of the molecular ion. A compound with one nitrogen atom, or any odd number of nitrogen atoms, gives a molecular ion with an odd molecular...

mass spectrometry: alkyl halide fragmentation

mass-spectrometry-alkyl-halide-fragmentation

Mass Spectra of Alkyl Halides

Mass spectra of alkyl halides show clear isotope patterns from the halogen atom. Chlorine has two common isotopes, 35Cl and 37Cl, in a 3:1 ratio. Bromine has two common isotopes, 79Br and 81Br, in a 1:1 ratio. These isotope ratios create two molecular ion peaks, [M] and [M + 2], with relative heights that match the halide abundance. This pattern is easy to see in examples such as 2-chloropropane and 1-bromopropane. 2-Chloropropane gives peaks in a 3:1 ratio. 1-Bromopropane gives peaks in a 1:1...

mass spectrometry: aldehyde and ketone fragmentation

mass-spectrometry-aldehyde-and-ketone-fragmentation

Carbonyl Fragment Patterns in Mass Spectra

Aliphatic aldehydes and ketones break apart in mass spectrometry through a few key carbonyl fragment patterns. The main pathways are alpha-cleavage, inductive cleavage, and the McLafferty rearrangement. These patterns help chemists read the mass spectrum and spot a carbonyl group. Alpha-cleavage is a common break next to the carbonyl group. The bond beside the carbonyl splits, and this forms an alkyl radical and an acylium cation. The acylium cation is often seen in the spectrum because it is...

mass spectrometry: carboxylic acid, ester, and amide fragmentation

mass-spectrometry-carboxylic-acid-ester-and-amide-fragmentation

Mass Spectra of Carboxylic Compounds

Mass spectra of carboxylic acids, esters, and amides often show two key fragmentation routes: β-cleavage and McLafferty rearrangement. In β-cleavage, the carbon-carbon bond at the β-position next to the carboxylic group breaks. This produces a neutral radical and a cation. Long-chain compounds can also fragment by McLafferty rearrangement if they have a hydrogen on the γ-carbon. The γ-carbon is the third carbon away from the carboxylic group. This rearrangement gives a radical cation and a...

chemical ionization (ci) mass spectrometry

Chemical ionization mass spectrometry uses a reagent gas to help detect molecules that break apart too quickly in electron impact ionization. It is a milder ionization method that can extend the life of the ionized analyte, which makes molecular identification easier. In chemical ionization, the sample is mixed with an excess reagent gas. This setup makes the electron impact occur mainly on the reagent gas instead of on the analyte. The charged species formed from the reagent then protonates...

electrospray ionization (esi) mass spectrometry

electrospray-ionization-esi-mass-spectrometry

How ESI Turns Biomolecules Into Ions

Electrospray ionization, or ESI, is a mass spectrometry method used to detect biomolecules that are too large or nonvolatile for conventional electron impact ionization. These molecules may break down before they can ionize or evaporate. ESI helps prevent rapid fragmentation and lets scientists record a mass signal for the whole biomolecule. ESI uses electrical energy to move ions from the liquid phase into the gas phase. The analyte biomolecule is mixed with an ionic liquid, which is a...

matrix-assisted laser desorption ionization (maldi)

matrix-assisted-laser-desorption-ionization-maldi

How MALDI Turns Samples Into Ions

Matrix-assisted laser desorption ionization (MALDI) is a mass spectrometry technique that turns biomolecules into ions for analysis. It is used to identify and characterize proteins, peptides, nucleic acids, and carbohydrates. Because of this, it is common in biological and medical research, as well as pharmacology and biochemistry. In MALDI, the analyte of interest is mixed with a matrix material. The matrix is usually an organic compound, such as a small organic acid or a crystalline...

key terms and concepts

See how chemical ionization uses a reagent gas to protonate analytes, form a stable conjugate acid, and produce an M+1 peak in mass spectra.

Using Reagent Gas in CI Mass Spectrometry | 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。

Using Reagent Gas in CI Mass Spectrometry

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