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

O impacto de elétrons é o método convencional para ionizar moléculas de amostra em espectrometria de massa. No entanto, para algumas moléculas, o feixe de elétrons de alta energia causa fragmentação excessiva do íon molecular primário.
A ausência de um pico de íons moleculares em tais espectros de massa dificulta a identificação da amostra.
Um método alternativo de ionização química é a reação de protonação em fase gasosa, onde primeiro, o ácido conjugado relativamente estável da molécula da amostra é criado. Esta espécie então se fragmenta em componentes de baixo peso molecular. 
Por exemplo, considere a ionização química do éter di-sec-butílico. O éter é misturado com gás metano. Um feixe de elétrons de alta energia ioniza o metano preferencialmente.
O cátion radical metano formado colide com outra molécula de metano, gerando um íon metanio e um radical metano.
O íon metânio é um superácido e fonte de um próton em fase gasosa, que protona o éter. Comparado ao íon molecular formado pela ionização por impacto de elétrons, o éter protonado é estável, dando o pico M + 1 no espectro de massa.

espectrometria de massas por ionização química (ci)

Chemical Ionization (CI) Mass Spectrometry

O pico do íon molecular de uma molécula no espectro de massas fornece informações vitais para a identificação molecular. No entanto, a ionização convencional por impacto de elétrons pode levar à rápida dissociação de alguns íons moleculares antes que eles atinjam o detector. É necessário um método de ionização mais suave para aumentar a vida útil dessas moléculas de analito ionizadas. A ionização química (CI) é uma reação de protonação na fase gasosa útil para a análise em massa de moléculas de analito que são facilmente protonadas para produzir o ácido conjugado correspondente. Nesse processo, a amostra é misturada com um gás reagente em excesso, o que garante que o impacto do elétron ocorra principalmente sobre o gás reagente. As espécies carregadas formadas pelo reagente protonam a molécula do analito, produzindo um analito protonado relativamente estável (ácido conjugado) em comparação com o íon molecular. Isso resulta em um pico M+1 no espectro de massas. O ácido conjugado pode então sofrer fragmentação, gerando sinais adicionais.

Por exemplo, a ionização química (CI) do éter di-sec-butílico em gás metano ilustra esse processo. Quando o éter é misturado com excesso de gás metano como reagente, o impacto de elétrons ocorre no metano e não no éter. O cátion radical metano resultante pode reagir com outra molécula de metano para gerar um radical metano e um íon metânio. O íon metânio é uma fonte de prótons na fase gasosa, que pode protonar o éter para formar seu ácido conjugado. As reações sequenciais durante a ionização química do éter di-sec-butílico estão representadas na Figura 1.

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="text-align: center; height: 175px;">

Figura 1: Ionização química da mistura de éter di-sec-butílico e metano.

Este ácido conjugado do éter (m/z = 131) é relativamente mais estável do que o íon molecular (m/z = 130) do éter formado por meio da ionização convencional por impacto de elétrons. Na rota tradicional, o íon molecular sofre fragmentação por meio de clivagem α, gerando um sinal em m/z = 101. A Figura 2 ilustra as reações que ocorrem durante a ionização por impacto de elétrons diretamente no éter di-sec-butílico.

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="text-align: center; height: 175px;">

Figura 2: Ionização do éter di-sec-butílico por impacto de elétrons e fragmentação do íon molecular.

Assim, os espectros de massa do éter di-sec-butílico ionizado via CI apresentam um pico M+1 em m/z = 131. Por outro lado, os espectros de massa do éter di-sec-butílico ionizado via ionização por impacto de elétrons não mostram nenhum pico no valor m/z de seu peso molecular. As Figuras 3a e 3b mostram os espectros de massas do éter di-sec-butílico ionizado por meio dos métodos de impacto de elétrons e de ionização química, respectivamente.

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="text-align: center; height: 225px;">

Figura 3: a) Nenhum pico m/z = 130 é visível no espectro de massa obtido via ionização por impacto de elétrons do éter di-sec-butílico. b) O pico m/z = 131 é claramente visível no espectro de massa obtido por ionização química do éter di-sec-butílico.

Espectrometria de Massas por Ionização Química (CI)

O pico de íons moleculares de uma molécula no espectro de massa fornece informações vitais para a identificação molecular. No entanto, o impacto de elétrons convencional ...

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Mass Spectrometry Fragmentation Methods

Métodos de Fragmentação em Espectrometria de Massas

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

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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

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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

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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

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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

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mass spectrometry: aromatic compound fragmentation

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Mass Spectra of Benzene and Toluene

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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...

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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...

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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

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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

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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...

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How MALDI Turns Samples Into Ions

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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

analytical chemistry

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Using Reagent Gas in CI Mass Spectrometry

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