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

La maggior parte degli atomi nelle molecole potrebbe presentarsi come isotopi diversi, con conseguente masse atomiche diverse. Quindi, lo spettro di massa contiene segnali di massa corrispondenti a queste differenze.
Le posizioni dei segnali di massa provenienti da vari isotopi sono determinate dalle loro differenze di massa. Le intensità relative dei segnali di massa dipendono generalmente dall'abbondanza relativa degli isotopi.
Ad esempio, le molecole contenenti carbonio mostrano un picco aggiuntivo a partire da 13C, un'unità superiore al picco di ioni molecolari. Questo picco aggiuntivo è noto come picco M+1.
Un aumento del numero di atomi di carbonio dà più possibilità di includere 13C, e quindi aumenta l'intensità relativa del picco M+1.
Gli spettri di massa dei cloruri e dei bromuri mostrano un segnale M+2, poiché gli isotopi primari del cloro e del bromo differiscono di due unità di massa.
L'intensità relativa dei picchi M+2 aiuta a identificare se nella molecola dell'analita è cloro o bromo.

spettrometria di massa: effetto isotopico

Mass Spectrometry: Isotope Effect

La maggior parte degli elementi esiste in natura come una miscela di isotopi. Gli isotopi differiscono in peso a causa del rispettivo numero di neutroni. Il peso molecolare di una molecola è diverso a seconda dell'isotopo specifico dei suoi elementi coinvolti. Di conseguenza, lo spettro di massa della molecola mostra picchi dallo stesso frammento in più posizioni. Le posizioni di questi segnali di massa dipendono dalla differenza tra la massa molecolare. Inoltre, l'intensità di questi segnali dipende dal rapporto relativo di abbondanza isotopica in natura.
Ad esempio, il carbonio ha due isotopi stabili presenti in natura di massa molecolare 12 e 13 u che coesistono in una composizione relativa di 1:0,011. I segnali di massa per uno ione molecolare contenente carbonio o un frammento carico sono posizionati alla sua massa molecolare (M) e un'unità superiore alla sua massa molecolare (M+1). Il rapporto relativo tra il picco M+1 e il picco M dipende dal numero di atomi di carbonio in una particolare molecola. Vengono confrontati i picchi M e M+1 di metano, decano e icosane. All'aumentare del numero di atomi di carbonio, aumenta anche l'intensità relativa del picco M+1 a causa della crescente probabilità che la molecola contenga un atomo di 13C.
I picchi isotopici nello spettro di massa e la loro intensità relativa vengono utilizzati per identificare la presenza di elementi specifici nell'analita sconosciuto. Ad esempio, un picco M+2 nella molecola indica la presenza di cloro o bromo nella molecola, poiché gli isotopi degli alogenuri differiscono di due u. Inoltre, dall'intensità relativa del picco M+2, è possibile distinguere tra cloro e bromo nell'analita. L'abbondanza degli isotopi del cloro è del 75% del 35Cl e del 25% del 37Cl, mentre, per gli isotopi del bromo, è circa il 50% del 79Br e dell'81Br. La differenza nel rapporto relativo tra i picchi M e M+2 negli spettri di massa del clorobenzene e del bromobenzene è notevole.

Spettrometria di massa: effetto isotopico

La maggior parte degli elementi esiste in natura come una miscela di isotopi. Gli isotopi differiscono in peso a causa del rispettivo numero di neutroni. Il peso molecolare di ...

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Analytical Chemistry

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Principles of Mass Spectrometry

Principi della spettrometria di massa

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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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mass spectrometry: isotope effect

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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