Name: isotope peaks in mass spectra
Uploaded: 2025-12-12T12:30:35.000Z
Duration: 1 min 13 s
Channel: JoVE

La plupart des atomes des molécules peuvent se présenter sous la forme d’isotopes différents, ce qui entraîne des masses atomiques différentes. Ainsi, le spectre de masse contient des signaux de masse correspondant à ces différences.
Les positions des signaux de masse de divers isotopes sont déterminées par leurs différences de masse. Les intensités relatives des signaux de masse dépendent généralement de l’abondance relative des isotopes.
Par exemple, les molécules contenant du carbone présentent un pic supplémentaire à partir de 13°C, une unité plus élevé que le pic d’ions moléculaires. Ce pic supplémentaire est connu sous le nom de pic M+1.
Une augmentation du nombre d’atomes de carbone donne plus de chances d’inclure 13C, et augmente ainsi l’intensité relative du pic M+1.
Les spectres de masse des chlorures et des bromures présentent un signal M+2, car les isotopes primaires du chlore et du brome diffèrent de deux unités de masse.
L’intensité relative des pics M+2 permet d’identifier s’il s’agit de chlore ou de brome dans la molécule d’analyte.

spectrométrie de masse : effet isotopique

Mass Spectrometry: Isotope Effect

La plupart des éléments existent dans la nature sous la forme d’un mélange d’isotopes. Les isotopes diffèrent en poids en raison de leur nombre respectif de neutrons. Le poids moléculaire d’une molécule est différent en fonction de l’isotope spécifique de ses éléments impliqués. En conséquence, le spectre de masse de la molécule présente des pics à partir du même fragment à plusieurs positions. Les positions de ces signaux de masse dépendent de la différence entre la masse moléculaire. De plus, l’intensité de ces signaux dépend du rapport relatif de l’abondance isotopique dans la nature.
Par exemple, le carbone a deux isotopes stables naturels de masse moléculaire 12 et 13 u qui coexistent dans une composition relative de 1:0,011. Les signaux de masse d’un ion moléculaire contenant du carbone ou d’un fragment chargé sont positionnés à sa masse moléculaire (M) et une unité au-dessus de sa masse moléculaire (M+1). Le rapport relatif entre le pic M+1 et le pic M dépend du nombre d’atomes de carbone dans la molécule particulière. Les pics M et M+1 du méthane, du décane et de l’icosane sont comparés. À mesure que le nombre d’atomes de carbone augmente, l’intensité relative du pic M+1 augmente également en raison de la probabilité croissante que la molécule contienne un atome de carbone 13.
Les pics isotopiques dans le spectre de masse et leur intensité relative sont utilisés pour identifier la présence d’éléments spécifiques dans l’analyte inconnu. Par exemple, un pic M+2 dans la molécule indique la présence de chlore ou de brome dans la molécule, puisque les isotopes des halogénures diffèrent de deux u. De plus, à partir de l’intensité relative du pic M+2, il est possible de différencier le chlore et le brome dans l’analyte. L’abondance des isotopes du chlore est de 75 % de 35Cl et de 25 % de 37Cl, alors que pour les isotopes du brome, elle est d’environ 50 % de 79Br et 81Br. La différence dans le rapport relatif entre les pics M et M+2 dans les spectres de masse du chlorobenzène et du bromobenzène est notable.

Spectrométrie de masse : effet isotopique

La plupart des éléments existent dans la nature sous la forme d’un mélange d’isotopes. Les isotopes diffèrent en poids en raison de leur nombre respectif de neutrons. Le poids moléculaire de ...

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isotope peaks in mass spectra

Isotope peaks in mass spectra show how natural isotope mixtures change the signal of a molecule. Most elements occur as a mix of isotopes, which are atoms of the same element with different numbers of neutrons. Because isotopes have different masses, the same fragment can appear at more than one position in a spectrum.

The mass of a molecule depends on which isotopes are present in its atoms. The size of each isotope peak also depends on how common those isotopes are in nature. Carbon is a useful example. It has two stable isotopes, 12 and 13 u, in a relative composition of 1:0.011.

For carbon-containing ions, the main signal appears at M, which is the molecular mass. A second signal appears at M+1. The size of the M+1 peak compared with the M peak depends on how many carbon atoms the molecule has. Methane, decane, and icosane show this trend clearly. As the number of carbon atoms increases, the M+1 peak becomes more intense because there is a greater chance of including a 13C atom.

Isotope peaks can also help identify specific elements in an unknown analyte. An M+2 peak often points to chlorine or bromine, because their isotopes differ by two u. The relative intensity of that M+2 peak can help distinguish between them. Chlorine is about 75% 35Cl and 25% 37Cl, while bromine is about 50% 79Br and 81Br. This difference creates distinct M to M+2 patterns, such as those seen in chlorobenzene and bromobenzene.

Isotope Peaks in Mass Spectra

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Isotope peaks in mass spectra show how natural isotope mixtures change the signal of a molecule. Most elements occur as a mix of isotopes, which are atoms of the same element with different numbers of neutrons. Because isotopes have different masses, the same fragment can appear at more than one position in a spectrum. The mass of a molecule depends on which isotopes are present in its atoms. The size of each isotope peak also depends on how common those isotopes are in nature. Carbon is a...

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key terms and concepts

Learn how isotope peaks appear in mass spectra, why carbon gives M and M+1 signals, and how M+2 peaks can reveal chlorine or bromine.

Isotope Peaks in Mass Spectra | JoVE K12

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Isotope Peaks in Mass Spectra

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