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

Der Elektronenaufprall ist die herkömmliche Methode zur Ionisierung von Probenmolekülen in der Massenspektrometrie. Bei einigen Molekülen führt der hochenergetische Elektronenstrahl jedoch zu einer übermäßigen Fragmentierung des primären Molekülions.
Das Fehlen eines molekularen Ionenpeaks in solchen Massenspektren erschwert die Identifizierung der Probe.
Eine alternative chemische Ionisationsmethode ist die Gasphasenprotonierungsreaktion, bei der zunächst die relativ stabile konjugierte Säure des Probenmoleküls erzeugt wird. Diese Spezies zerfällt dann in niedermolekulare Bestandteile. 
Betrachten wir zum Beispiel die chemische Ionisation von Di-sec-Butylether. Der Äther wird mit Methangas vermischt. Ein hochenergetischer Elektronenstrahl ionisiert dann bevorzugt Methan.
Das gebildete Methanradikalkation kollidiert mit einem anderen Methanmolekül, wodurch ein Methanion und ein Methanradikal entstehen.
Das Methanionat ist eine Supersäure und Quelle eines Gasphasenprotons, das den Äther protoniert. Im Vergleich zu dem molekularen Ionion, das durch Elektronenstoßionisation gebildet wird, ist der protonierte Äther stabil und ergibt den M+1-Peak im Massenspektrum.

chemische ionisation (ci) massenspektrometrie

Chemical Ionization (CI) Mass Spectrometry

Der Molekülionen-Peak eines Moleküls im Massenspektrum liefert wichtige Informationen für die Identifizierung des Molekül. Die herkömmliche Elektronenstoßionisation kann jedoch zur schnellen Dissoziation einiger Molekülionen führen, bevor sie den Detektor erreichen. Um die Lebensdauer solcher ionisierten Molekülproben zu erhöhen, ist eine mildere Ionisierungsmethode erforderlich. Die chemische Ionisation (CI) ist eine Protonierung in der Gasphase, die für die Massenanalyse von Molekülproben nützlich ist, die leicht protoniert werden können, um die entsprechende konjugierte Säure zu ergeben. Bei diesem Verfahren wird die Probe mit einem überschüssigen Reagenzgas gemischt, wodurch sichergestellt wird, dass der Elektronenstoß hauptsächlich auf das Reagenzgas erfolgt. Die aus dem Reagenz gebildeten geladenen Spezies protonieren die Molekülproben und erzeugen einen relativ stabilen protonierten Analyten (konjugierte Säure) im Vergleich zum Molekülion. Dies führt zu einem M+1-Peak im Massenspektrum. Die konjugierte Säure kann dann eine Fragmentierung erfahren, die zusätzliche Signale erzeugt.

Beispielsweise veranschaulicht die CI von Di-sec-butylether in Methangas diesen Prozess. Wenn der Ether mit überschüssigem Methangas als Reagenz gemischt wird, tritt ein Elektronenstoß auf Methan statt auf den Ether auf. Das resultierende Methanradikalkation kann mit einem anderen Methanmolekül reagieren und ein Methanradikal und ein Methanion erzeugen. Das Methanion ist eine Quelle von Protonen in der Gasphase, die den Ether protonieren können, um seine konjugierte Säure zu bilden. Die aufeinanderfolgenden Reaktionen während der chemischen Ionisierung von Di-sec-butylether sind in Abbildung 1 dargestellt.

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="height: 175px;">

Abbildung 1: Chemische Ionisierung der Di-sec-butylether-Methanmischung.

Diese konjugierte Säure des Ethers (m/z = 131) ist relativ stabiler als das Molekülion (m/z = 130) des Ethers, das durch die herkömmliche Elektronenstoßionisierung gebildet wird. Beim herkömmlichen Verfahren wird das Molekülion durch α-Spaltung fragmentiert, was ein Signal bei m/z = 101 ergibt. Abbildung 2 veranschaulicht die Reaktionen, die während der Elektronenstoßionisierung direkt an Di-sec-butylether auftreten.

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="height: 175px;">

Abbildung 2: Elektronenstoßionisierung von Di-sec-butylether und Fragmentierung des Molekülions.

Die Massenspektren von Di-sec-butylether, der über CI ionisiert wurde, weisen also einen M+1-Peak bei m/z = 131 auf. Andererseits zeigen Massenspektren von Di-sec-butylether, der über Elektronenstoßionisierung ionisiert wurde, keinen Peak beim m/z-Wert seines Molekulargewichts. Abbildung 3a und 3b zeigen die Massenspektren von Di-sec-butylether, der über die Elektronenstoß- bzw. chemische Ionisierungsmethoden ionisiert wurde.

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="height: 225px;">

Abbildung 3: a) Im Massenspektrum, das durch Elektronenstoßionisation von Di-sec-butylether erhalten wurde, ist kein Peak bei m/z = 130 sichtbar. b) Im Massenspektrum, das durch chemische Ionisation von Di-sec-butylether erhalten wurde, ist der Peak bei m/z = 131 deutlich sichtbar.

Chemische Ionisation (CI) Massenspektrometrie

Der molekulare Ionenpeak eines Moleküls im Massenspektrum liefert wichtige Informationen für die molekulare Identifizierung. Herkömmliche Elektronenpralle ...

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Lehre

JoVE Core

Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Mass Spectrometry Fragmentation Methods

Fragmentierungsmethoden der Massenspektrometrie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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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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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