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

A maioria dos átomos nas moléculas pode ocorrer como isótopos diferentes, resultando em diferentes massas atômicas. Portanto, o espectro de massa contém sinais de massa correspondentes a essas diferenças.
As posições dos sinais de massa de vários isótopos são determinadas por suas diferenças de massa. As intensidades relativas dos sinais de massa geralmente dependem da abundância relativa dos isótopos.
Por exemplo, moléculas contendo carbono mostram um pico adicional de 13C, uma unidade acima do pico do íon molecular. Este pico adicional é conhecido como pico M+1.
Um aumento no número de átomos de carbono dá mais chances de incluir 13C e, portanto, aumenta a intensidade relativa do pico M + 1.
Os espectros de massa de cloretos e brometos exibem um sinal M + 2, pois os isótopos primários de cloro e bromo diferem em duas unidades de massa.
As intensidades relativas dos picos de M+2 ajudam a identificar se é cloro ou bromo na molécula do analito.

espectrometria de massa: efeito isotópico

Mass Spectrometry: Isotope Effect

A maioria dos elementos existe na natureza como uma mistura de isótopos. Os isótopos diferem em peso devido ao respectivo número de nêutrons. O peso molecular de uma molécula é diferente dependendo do isótopo específico de seus elementos envolvidos. Como resultado, o espectro de massa da molécula exibe picos do mesmo fragmento em várias posições. As posições desses sinais de massa dependem da diferença entre a massa molecular. Além disso, a intensidade desses sinais depende da proporção relativa de abundância isotópica na natureza.
Por exemplo, o carbono tem dois isótopos estáveis de ocorrência natural de massa molecular 12 e 13 u que coexistem em uma composição relativa de 1:0,011. Os sinais de massa para um íon molecular contendo carbono ou um fragmento carregado são posicionados em sua massa molecular (M) e uma unidade acima de sua massa molecular (M + 1). A proporção relativa do pico M + 1 para o pico M depende do número de átomos de carbono na molécula específica. Os picos M e M+1 de metano, decano e icosana são comparados. À medida que o número de átomos de carbono aumenta, a intensidade relativa do pico M + 1 também aumenta devido à crescente probabilidade de a molécula conter um átomo de 13C.
Os picos de isótopos no espectro de massa e sua intensidade relativa são usados para identificar a presença de elementos específicos no analito desconhecido. Por exemplo, um pico M+2 na molécula indica a presença de cloro ou bromo na molécula, uma vez que os isótopos de haletos diferem em dois u. Além disso, a partir da intensidade relativa do pico M+2, é possível diferenciar entre o cloro e o bromo no analito. A abundância de isótopos de cloro é de 75% de 35Cl e 25% de 37Cl, enquanto que, para isótopos de bromo, é de aproximadamente 50% de 79Br e 81Br. A diferença na razão relativa entre os picos M e M + 2 nos espectros de massa de clorobenzeno e bromobenzeno é notável.

Espectrometria de massa: efeito isotópico

A maioria dos elementos existe na natureza como uma mistura de isótopos. Os isótopos diferem em peso devido ao respectivo número de nêutrons. O peso molecular de ...

Education

JoVE Core

Analytical Chemistry

Química Analítica

Unit 1

Unidade 1

Principles of Mass Spectrometry

Princípios da Espectrometria de Massas

KEY TERMS AND CONCEPTS

TERMOS E CONCEITOS-CHAVE

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

analytical chemistry

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Isotope Peaks in Mass Spectra

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