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

Most atoms in molecules could occur as different isotopes, resulting in different atomic masses. So, the mass spectrum contains mass signals corresponding to these differences.
The positions of mass signals from various isotopes are determined by their mass differences. The relative intensities of the mass signals generally depend on the relative abundance of the isotopes.
For example, carbon-containing molecules show an additional peak from 13C, one unit higher than the molecular ion peak. This additional peak is known as the M+1 peak.
An increase in the number of carbon atoms gives more chances to include 13C, and so increases the relative intensity of the M+1 peak.
The mass spectra of chlorides and bromides exhibit an M+2 signal, as the primary chlorine and bromine isotopes differ by two mass units.
The relative intensities of the M+2 peaks helps to identify whether it is chlorine or bromine in the analyte molecule.

mass spectrometry: isotope effect

Mass Spectrometry: Isotope Effect

Most elements exist in nature as a mixture of isotopes. The isotopes differ in weight due to their respective number of neutrons. The molecular weight of a molecule is different depending on the specific isotope of its elements involved. As a result, the mass spectrum of the molecule exhibits peaks from the same fragment at multiple positions. The positions of these mass signals depend on the mass differences between isotopes. Furthermore, the intensity of these signals is dependent on the relative ratio of isotopic abundance in nature.For example, carbon has two stable naturally occurring isotopes of molecular mass 12 and 13 u that coexist in a relative composition of 1:0.011. The mass signals for a carbon-containing molecular ion or a charged fragment are positioned at its molecular mass (M) and one unit higher than its molecular mass (M+1). The relative ratio of the M+1 peak to M peak depends on the number of carbon atoms in the particular molecule. The M and M+1 peaks of methane, decane, and icosane are compared. As the number of carbon atoms increases, the relative intensity of the M+1 peak also increases because of the increasing likelihood of the molecule containing a 13C atom.The isotope peaks in the mass spectrum and their relative intensity are used to identify the presence of specific elements in the unknown analyte. For example, an M+2 peak in the molecule indicates the presence of chlorine or bromine in the molecule, since the isotopes of halides differ by two u. Additionally, from the relative intensity of the M+2 peak, it is possible to differentiate between the chlorine and bromine in the analyte. The abundance of chlorine isotopes is 75% of 35Cl and 25% of 37Cl, whereas, for bromine isotopes, it is approximately 50% of 79Br and 81Br. The difference in the relative ratio between M and M+2 peaks in the mass spectra of chlorobenzene and bromobenzene is notable.

Most elements exist in nature as a mixture of isotopes. The isotopes differ in weight due to their respective number of neutrons. The molecular weight of ...

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Analytical Chemistry

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Principles of Mass Spectrometry

KEY TERMS AND CONCEPTS

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 and Molecular Identity

Mass spectrometry helps identify a compound by measuring its molecular mass and molecular formula. It does this by turning the analyte molecule into ions and then comparing ion abundance with mass-to-charge ratio. One common ionization method is electron ionization, or EI. In EI, high-energy electron beams strike analyte molecules in the gas phase. The beam removes an electron and leaves a molecular ion behind. A molecular ion is a radical cation, so it has a positive charge and an unpaired...

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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Mass spectra help scientists identify an unknown compound by finding the molecular ion peak. The molecular ion peak shows the mass of the intact molecule, but it is often weak or missing because high-energy electron beams cause strong fragmentation. When that happens, a soft-energy electron beam can be used to scan the spectrum and improve the molecular ion signal. Other ionization methods can also give a stronger molecular ion peak. Chemical ionization, field ionization, and desorption...

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Mass Accuracy in HRMS Analysis

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Mass spectrometry can be paired with separation methods to study complex mixtures. This combination is called a hyphenated method. It helps when a pure mass spectrum is too hard to read because a mixture causes too much fragmentation and too many overlapping signals. GC-MS is one common hyphenated method. GC stands for gas chromatography, which separates volatile, thermally stable compounds in narrow capillary columns before they enter the mass spectrometer. This approach is widely used in...

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MS/MS Scan Modes and Fragment Ions

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inductively coupled plasma&#8211;mass spectrometry (icp&#8211;ms): overview

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ICP-MS uses an inductively coupled plasma torch to turn samples into ions for mass analysis. Solid samples are first dissolved and volatilized before they enter the hot argon plasma. Solution samples are nebulized, which means they are converted into a fine spray, and then passed through the same plasma. Inside the plasma, the analytes break apart. Their atoms become positively charged ions, along with some molecular species. These positive ions are sent to the mass spectrometer, where they...

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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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