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

El impacto de electrones es el método convencional para ionizar moléculas de muestra en espectrometría de masas. Sin embargo, para algunas moléculas, el haz de electrones de alta energía causa una fragmentación excesiva del ion molecular primario.
La ausencia de un pico de iones moleculares en tales espectros de masas dificulta la identificación de la muestra.
Un método alternativo de ionización química es la reacción de protonación en fase gaseosa, en la que primero se crea el ácido conjugado relativamente estable de la molécula de muestra. A continuación, esta especie se fragmenta en componentes de bajo peso molecular. 
Por ejemplo, considere la ionización química del éter di-sec-butílico. El éter se mezcla con gas metano. A continuación, un haz de electrones de alta energía ioniza preferentemente el metano.
El catión radical de metano formado colisiona con otra molécula de metano, generando un ion de metano y un radical de metano.
El ion metanio es un superácido y fuente de un protón en fase gaseosa, que protona el éter. En comparación con el ion molecular formado por ionización por impacto electrónico, el éter protonada es estable, lo que da el pico M+1 en el espectro de masas.

espectrometría de masas por ionización química (ci).

Chemical Ionization (CI) Mass Spectrometry

El pico de ion molecular de una molécula en el espectro de masas proporciona información vital para la identificación molecular. Sin embargo, la ionización por impacto de electrones convencional puede provocar la disociación rápida de algunos iones moleculares antes de que lleguen al detector. Se requiere un método de ionización más suave para aumentar la vida útil de dichas moléculas de analito ionizadas. La ionización química (CI) es una reacción de protonación en fase gaseosa útil para el análisis de masas de moléculas de analito que se protonan fácilmente para producir el ácido conjugado correspondiente. En este proceso, la muestra se mezcla con un exceso de gas reactivo, lo que garantiza que el impacto de electrones se produzca principalmente en el gas reactivo. La especie cargada formada a partir del reactivo protona la molécula de analito, lo que produce un analito protonado relativamente estable (ácido conjugado) en comparación con el ion molecular. Esto da como resultado un pico M+1 en el espectro de masas. El ácido conjugado puede sufrir entonces una fragmentación que genera señales adicionales.

Por ejemplo, la CI del di-sec-butil éter en gas metano ilustra este proceso. Cuando el éter se mezcla con un exceso de gas metano como reactivo, el impacto de electrones ocurre en el metano en lugar de en el éter. El catión radical metano resultante puede reaccionar con otra molécula de metano para generar un radical metano y un ion metanio. El ion metanio es una fuente de protones en fase gaseosa, que pueden protonar el éter para formar su ácido conjugado. Las reacciones secuenciales durante la ionización química del di-sec-butil éter se representan en la Figura 1.

<img alt="Figure1" src="/files/ftp_upload/12300/12300_PT_Figure_1.svg" style="height: 175px;">

Figura 1: Ionización química de la mezcla de di-sec-butil éter metano.

Este ácido conjugado del éter (m/z = 131) es relativamente más estable que el ion molecular (m/z = 130) del éter formado mediante la ionización por impacto de electrones convencional. En la ruta tradicional, el ion molecular sufre fragmentación a través de una escisión α, lo que da una señal en m/z = 101. La Figura 2 ilustra las reacciones que ocurren durante la ionización por impacto electrónico directamente sobre el éter di-sec-butílico.

<img alt="Figure2" src="/files/ftp_upload/12300/12300_PT_Figure_2.svg" style="height: 175px;">

Figura 2: Ionización por impacto electrónico del éter di-sec-butílico y fragmentación de iones moleculares.

Por lo tanto, los espectros de masas del éter di-sec-butílico ionizado mediante CI presentan un pico M+1 en m/z = 131. Por otro lado, los espectros de masas del éter di-sec-butílico ionizado mediante ionización por impacto electrónico no muestran ningún pico en el valor m/z de su peso molecular. Las Figuras 3a y 3b muestran los espectros de masas del éter di-sec-butílico ionizado mediante los métodos de impacto electrónico e ionización química, respectivamente.

<img alt="Figure3" src="/files/ftp_upload/12300/12300_PT_Figure_3.svg" style="height: 225px;">

Figura 3: a) No se observa ningún pico m/z = 130 en el espectro de masas obtenido mediante ionización por impacto electrónico del di-sec-butil éter. b) El pico m/z = 131 es claramente visible en el espectro de masas obtenido mediante ionización química del di-sec-butil éter.

Espectrometría de Masas por Ionización Química (CI).

El pico de iones moleculares de una molécula en el espectro de masas proporciona información vital para la identificación molecular. Sin embargo, el impacto convencional de electrones ...

Education

Educación

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidad 1

Mass Spectrometry Fragmentation Methods

Métodos de fragmentación en espectrometría de masas

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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

analytical chemistry

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Using Reagent Gas in CI Mass Spectrometry

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