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

L'impatto degli elettroni è il metodo convenzionale per ionizzare le molecole del campione nella spettrometria di massa. Tuttavia, per alcune molecole, il fascio di elettroni ad alta energia provoca un'eccessiva frammentazione dello ione molecolare primario.
L'assenza di un picco di ioni molecolari in tali spettri di massa rende difficile l'identificazione del campione.
Un metodo alternativo di ionizzazione chimica è la reazione di protonazione in fase gassosa, in cui viene creato prima l'acido coniugato relativamente stabile della molecola del campione. Questa specie si frammenta poi in componenti a basso peso molecolare. 
Ad esempio, si consideri la ionizzazione chimica dell'etere di-sec-butilico. L'etere è mescolato con gas metano. Un fascio di elettroni ad alta energia ionizza quindi il metano in modo preferenziale.
Il catione radicale metano formatosi si scontra con un'altra molecola di metano, generando uno ione metanio e un radicale metano.
Lo ione metanio è un superacido e fonte di un protone in fase gassosa, che protona l'etere. Rispetto allo ione molecolare formato dalla ionizzazione a impatto elettronico, l'etere protonato è stabile, dando il picco M+1 nello spettro di massa.

spettrometria di massa a ionizzazione chimica (ci)

Chemical Ionization (CI) Mass Spectrometry

Il picco di ioni molecolari di una molecola nello spettro di massa fornisce informazioni fondamentali per l'identificazione molecolare. Tuttavia, la ionizzazione convenzionale per impatto elettronico può portare alla rapida dissociazione di alcuni ioni molecolari prima che raggiungano il rilevatore. È necessario un metodo di ionizzazione più blando per aumentare la vita di tali molecole di analita ionizzate. La ionizzazione chimica (CI) è una reazione di protonazione in fase gassosa utile per l'analisi di massa di molecole di analita che sono facilmente protonate per produrre l'acido coniugato corrispondente. In questo processo, il campione viene miscelato con un eccesso di gas reagente, che assicura che l'impatto elettronico si verifichi principalmente sul gas reagente. La specie carica formata dal reagente protona la molecola di analita, producendo un analita protonato relativamente stabile (acido coniugato) rispetto allo ione molecolare. Ciò si traduce in un picco M+1 nello spettro di massa. L'acido coniugato può quindi subire una frammentazione che genera segnali aggiuntivi.

Ad esempio, la CI del di-sec-butil etere nel gas metano illustra questo processo. Quando l'etere viene miscelato con un eccesso di gas metano come reagente, l'impatto elettronico si verifica sul metano anziché sull'etere. Il catione radicale del metano risultante può reagire con un'altra molecola di metano per generare un radicale metano e uno ione metanio. Lo ione metanio è una fonte di protoni in fase gassosa, che possono protonare l'etere per formare il suo acido coniugato. Le reazioni sequenziali durante la ionizzazione chimica del di-sec-butil etere sono illustrate in Figura 1.

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="height: 175px;">

Figura 1: Ionizzazione chimica della miscela di metano di-sec-butil etere.

Questo acido coniugato dell'etere (m/z = 131) è relativamente più stabile dello ione molecolare (m/z = 130) dell'etere formato tramite la ionizzazione convenzionale per impatto elettronico. Nel percorso tradizionale, lo ione molecolare subisce una frammentazione tramite scissione α, dando un segnale a m/z = 101. La Figura 2 illustra le reazioni che si verificano durante la ionizzazione per impatto elettronico direttamente sul di-sec-butil etere.

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="height: 175px;">

Figura 2: Ionizzazione per impatto elettronico del di-sec-butil etere e frammentazione dello ione molecolare.

Quindi, gli spettri di massa del di-sec-butil etere ionizzato tramite CI, presentano un picco M+1 a m/z = 131. D'altro canto, gli spettri di massa del di-sec-butil etere ionizzato tramite ionizzazione per impatto elettronico non mostrano alcun picco al valore m/z relativo al suo peso molecolare. Le Figure 3a e 3b mostrano gli spettri di massa del di-sec-butil etere ionizzato rispettivamente tramite i metodi di impatto elettronico e ionizzazione chimica.

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="height: 225px;">

Figura 3: a) Nessun picco a m/z = 130 è visibile nello spettro di massa ottenuto tramite ionizzazione per impatto elettronico del di-sec-butil etere. b) Il picco a m/z = 131 è chiaramente visibile nello spettro di massa ottenuto tramite ionizzazione chimica del di-sec-butil etere.

Spettrometria di Massa a Ionizzazione Chimica (CI)

Il picco ionico molecolare di una molecola nello spettro di massa fornisce informazioni vitali per l'identificazione molecolare. Tuttavia, l'impatto elettronico convenzionale ...

Education

Didattica

JoVE Core

Analytical Chemistry

Chimica Analitica

Unit 1

Unità 1

Mass Spectrometry Fragmentation Methods

Metodi di frammentazione della spettrometria di massa

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

analytical chemistry

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Using Reagent Gas in CI Mass Spectrometry

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