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

L’impact électronique est la méthode conventionnelle d’ionisation des molécules d’échantillon en spectrométrie de masse. Cependant, pour certaines molécules, le faisceau d’électrons de haute énergie provoque une fragmentation excessive de l’ion moléculaire primaire.
L’absence d’un pic d’ions moléculaires dans de tels spectres de masse rend difficile l’identification de l’échantillon.
Une autre méthode d’ionisation chimique est la réaction de protonation en phase gazeuse, où l’on crée d’abord l’acide conjugué relativement stable de la molécule de l’échantillon. Cette espèce se fragmente ensuite en composants de faible poids moléculaire. 
Prenons l’exemple de l’ionisation chimique de l’éther di-sec-butylique. L’éther est mélangé à du méthane. Un faisceau d’électrons de haute énergie ionise ensuite préférentiellement le méthane.
Le cation radical méthane formé entre en collision avec une autre molécule de méthane, générant un ion méthanium et un radical méthane.
L’ion méthanium est un superacide et une source d’un proton en phase gazeuse, qui protone l’éther. Par rapport à l’ion moléculaire formé par ionisation par impact d’électrons, l’éther protoné est stable, ce qui donne le pic M+1 dans le spectre de masse.

spectrométrie de masse à ionisation chimique (ci)

Chemical Ionization (CI) Mass Spectrometry

Le pic d'ion moléculaire d'une molécule dans le spectre de masse fournit des informations vitales pour l'identification moléculaire. Cependant, l'ionisation par impact électronique classique peut conduire à la dissociation rapide de certains ions moléculaires avant qu'ils n'atteignent le détecteur. Une méthode d'ionisation plus douce est nécessaire pour augmenter la durée de vie de ces molécules d'analyte ionisées. L'ionisation chimique (CI) est une réaction de protonation en phase gazeuse utile pour l'analyse de masse des molécules d'analyte qui sont facilement protonées pour produire l'acide conjugué correspondant. Dans ce processus, l'échantillon est mélangé à un excès de gaz réactif, ce qui garantit que l'impact électronique se produit principalement sur le gaz réactif. L'espèce chargée formée à partir du réactif protone la molécule d'analyte, produisant un analyte protoné relativement stable (acide conjugué) par rapport à l'ion moléculaire. Cela se traduit par un pic M+1 dans le spectre de masse. L'acide conjugué peut ensuite subir une fragmentation qui génère des signaux supplémentaires.

Par exemple, la CI de l'éther di-sec-butylique dans le méthane gazeux illustre ce processus. Lorsque l'éther est mélangé à un excès de méthane gazeux comme réactif, l'impact électronique se produit sur le méthane plutôt que sur l'éther. Le cation radical méthane résultant peut réagir avec une autre molécule de méthane pour générer un radical méthane et un ion méthanium. L'ion méthanium est une source de protons en phase gazeuse, qui peuvent protoner l'éther pour former son acide conjugué. Les réactions séquentielles au cours de l'ionisation chimique de l'éther di-sec-butylique sont décrites dans la Figure 1.

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="height: 175px;">

Figure 1: Ionisation chimique du mélange d'éther di-sec-butylique et de méthane.

Cet acide conjugué de l'éther (m/z = 131) est relativement plus stable que l'ion moléculaire (m/z = 130) de l'éther formé via l'ionisation par impact électronique classique. Dans la voie traditionnelle, l'ion moléculaire subit une fragmentation par clivage α, donnant un signal à m/z = 101. La figure 2 illustre les réactions qui se produisent lors de l'ionisation par impact électronique directement sur l'éther di-sec-butylique.

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="height: 175px;">

Figure 2: Ionisation par impact électronique de l'éther di-sec-butylique et fragmentation des ions moléculaires.

Ainsi, les spectres de masse de l'éther di-sec-butylique ionisé par CI présentent un pic M+1 à m/z = 131. En revanche, les spectres de masse de l'éther di-sec-butylique ionisé par impact électronique ne présentent aucun pic à la valeur m/z de son poids moléculaire. Les figures 3a et 3b montrent les spectres de masse de l'éther di-sec-butylique ionisé par les méthodes d'impact électronique et d'ionisation chimique, respectivement.

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="height: 225px;">

Figure 3: a) Aucun pic m/z = 130 n'est visible dans le spectre de masse obtenu par ionisation par impact électronique de l'éther di-sec-butylique. b) Le pic m/z = 131 est clairement visible dans le spectre de masse obtenu par ionisation chimique de l'éther di-sec-butylique.

Spectrométrie de masse à ionisation chimique (CI)

Le pic d’ions moléculaires d’une molécule dans le spectre de masse fournit des informations essentielles pour l’identification moléculaire. Cependant, l’impact d’électrons conventionnel ...

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Analytical Chemistry

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Mass Spectrometry Fragmentation Methods

Méthodes de fragmentation en spectrométrie de masse

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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

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Cycloalkene Side-Chain Cleavage in Mass Spectra

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Alkyne Mass Spectra and Key Peaks

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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...

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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

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Using Reagent Gas in CI Mass Spectrometry

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