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

Die meisten Atome in Molekülen könnten als unterschiedliche Isotope auftreten, was zu unterschiedlichen Atommassen führt. Das Massenspektrum enthält also Massensignale, die diesen Unterschieden entsprechen.
Die Positionen der Massensignale verschiedener Isotope werden durch ihre Massenunterschiede bestimmt. Die relativen Intensitäten der Massensignale hängen im Allgemeinen von der relativen Häufigkeit der Isotope ab.
Zum Beispiel weisen kohlenstoffhaltige Moleküle einen zusätzlichen Peak ab 13 °C auf, eine Einheit höher als der molekulare Ionenpeak. Dieser zusätzliche Peak wird als M+1-Peak bezeichnet.
Eine Erhöhung der Anzahl der Kohlenstoffatome erhöht die Chancen, 13C einzuschließen, und erhöht somit die relative Intensität des M+1-Peaks.
Die Massenspektren von Chloriden und Bromiden weisen ein M+2-Signal auf, da sich die primären Chlor- und Bromisotope um zwei Masseneinheiten unterscheiden.
Die relativen Intensitäten der M+2-Peaks helfen zu identifizieren, ob es sich im Analytmolekül um Chlor oder Brom handelt.

massenspektrometrie: isotopeneffekt

Mass Spectrometry: Isotope Effect

Die meisten Elemente kommen in der Natur als Gemisch von Isotopen vor. Die Isotope unterscheiden sich im Gewicht aufgrund ihrer jeweiligen Anzahl an Neutronen. Das Molekulargewicht eines Moleküls ist unterschiedlich, abhängig vom spezifischen Isotop seiner beteiligten Elemente. Infolgedessen weist das Massenspektrum des Moleküls an mehreren Positionen Peaks aus demselben Fragment auf. Die Positionen dieser Massensignale hängen von der Differenz zwischen der Molekülmasse ab. Darüber hinaus hängt die Intensität dieser Signale vom relativen Verhältnis der Isotopenhäufigkeit in der Natur ab.
Zum Beispiel hat Kohlenstoff zwei stabile, natürlich vorkommende Isotope der Molekülmasse 12 und 13 u, die in einer relativen Zusammensetzung von 1:0,011 koexistieren. Die Massensignale für ein kohlenstoffhaltiges Molekülion oder ein geladenes Fragment werden bei seiner Molekülmasse (M) und eine Einheit höher als seine Molekülmasse (M+1) positioniert. Das relative Verhältnis des M+1-Peaks zum M-Peak hängt von der Anzahl der Kohlenstoffatome in dem jeweiligen Molekül ab. Die M- und M+1-Peaks von Methan, Decan und Icosan werden verglichen. Mit zunehmender Anzahl der Kohlenstoffatome nimmt auch die relative Intensität des M+1-Peaks zu, da die Wahrscheinlichkeit steigt, dass das Molekül ein 13-C-Atomenthält.
Die Isotopenpeaks im Massenspektrum und ihre relative Intensität werden verwendet, um das Vorhandensein bestimmter Elemente in dem unbekannten Analyten zu identifizieren. Zum Beispiel zeigt ein M+2-Peak im Molekül das Vorhandensein von Chlor oder Brom im Molekül an, da sich die Isotope der Halogenide um zwei u unterscheiden. Zusätzlich ist es möglich, aus der relativen Intensität des M+2-Peaks zwischen dem Chlor und dem Brom im Analyten zu unterscheiden. Die Häufigkeit von Chlorisotopen beträgt 75 % von 35Cl und 25 % von 37Cl, während sie bei Brom-Isotopen etwa 50 % von 79Br und 81Br beträgt. Bemerkenswert ist der Unterschied im relativen Verhältnis zwischen M- und M+2-Peaks in den Massenspektren von Chlorbenzol und Brombenzol.

Massenspektrometrie: Isotopeneffekt

Die meisten Elemente kommen in der Natur als Gemisch von Isotopen vor. Die Isotope unterscheiden sich im Gewicht aufgrund ihrer jeweiligen Anzahl an Neutronen. Das Molekulargewicht von ...

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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

mass spectrometry: overview

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...

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...

mass spectrometry: molecular fragmentation overview

mass-spectrometry-molecular-fragmentation-overview

Mass Spectrometry Fragmentation Patterns

Mass spectrometry fragmentation patterns show how a molecular ion breaks apart after ionization. When a molecule loses an electron inside the mass spectrometer, its structure becomes unstable. The ion then fragments at bonds that often produce the most stable pieces. One common pattern is cleavage of a single bond in the molecular ion. This break forms a radical and a cation. The bond can split in more than one way, but the favored path is the one that gives the more stable carbocation rather...

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A mass spectrometer sorts charged ions by their mass-to-charge ratio and records their relative abundance. It turns vaporized analyte molecules into ions, separates them, and then measures the signal from each type of ion. The process begins in the ionization chamber. The analyte is in the gas phase inside a vacuum, and it is struck by high-energy electrons. The electrons have about 70 eV of energy, which is enough to remove an electron from the analyte and form a molecular ion. After...

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Reading Peaks in a Mass Spectrum

A mass spectrum shows the charged fragments in an analyte as peaks plotted by mass-to-charge ratio, or m/z. The x-axis gives the mass of each charged fragment divided by the number of charges it carries. The y-axis shows the relative abundance of each charged species based on the signal intensity recorded at the detector. The tallest peak is called the base peak. It is assigned an abundance of 100%, and the other peaks are scaled against it. The peak from the molecular ion is called the parent...

mass spectrum: interpretation

Decoding Molecular Ions in Mass Spectra

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...

mass analyzers: overview

Mass Analyzers and Ion Separation

Mass analyzers separate ions by their mass-to-charge ratio, or m/z, in a mass spectrometer. After a sample is vaporized in the ionization chamber, it is hit with a high-energy electron beam. This creates a radical cation and also produces neutral molecules, radicals, and cations. Negatively charged accelerator plates then push the cations into the mass analyzer, where they are sent on to the detector. Several mass analyzer designs are commonly used. These include magnetic sector,...

mass analyzers: common types

Quadrupole, TOF, and Ion Trap Analyzers

Quadrupole, time-of-flight, and ion trap mass analyzers each separate ions in a different way. They are common tools in mass spectrometry and help sort ions by their mass-to-charge ratio, or m/z. A quadrupole mass analyzer uses four cylindrical metal rods arranged in a diamond shape. The rods carry a direct current, or DC, voltage and a radio-frequency AC voltage. Ion motion depends on the field strength, so only ions with a certain m/z resonate and reach the detector at that setting. This...

high-resolution mass spectrometry (hrms)

Mass Accuracy in HRMS Analysis

High-resolution mass spectrometry separates ions by very small differences in mass. This matters because atomic masses are not always exact whole numbers, even though they are often rounded for simple calculations. Atomic mass is usually estimated by adding the masses of protons and neutrons. In reality, the proton and neutron do not have exactly the same mass. That creates small differences when atomic masses are compared to the simplest hydrogen atom. Helium, for example, is made of 2...

mass spectrometry: complex analysis

Hyphenated Mass Spectrometry Methods

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

Tandem mass spectrometry uses two or more mass analyzers in series to improve selectivity and lower chemical noise during analyte detection. A set of analyzers separated by an interaction cell lets the instrument break ions apart and study the fragment ions that form. Fragmentation happens in the interaction cell and can be caused in several ways. Collision-induced dissociation, or CID, uses inert gases such as nitrogen, argon, or helium. Other methods include surface-induced dissociation...

gas chromatography&ndash;mass spectrometry (gc&ndash;ms)

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Gas chromatography-mass spectrometry (GC-MS) separates compounds in a mixture and then identifies them. A gas chromatograph first splits the mixture into its parts. A mass spectrometer then analyzes each part on its own to determine molecular masses and molecular structures. The gas chromatograph uses a long, narrow capillary column. The inner wall of the column has a polysiloxane coating. This coating serves as the stationary phase. It has a high boiling point, low viscosity, and low polarity.

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

inductively-coupled-plasmamass-spectrometry-icpms-overview

ICP-MS Sample Ionization and Detection

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...

inductively coupled plasma-mass spectrometry (icp-ms): interferences

inductively-coupled-plasma-mass-spectrometry-icp-ms-interferences

ICP-MS Signal Interference Sources and Fixes

Inductively coupled plasma-mass spectrometry (ICP-MS) is a sensitive method for elemental analysis, but its spectra can be affected by interferences. These interferences can make the mass signal less accurate, even when the instrument is working well. Spectroscopic interferences happen when ions in the plasma have the same mass-to-charge ratio, or m/z, as the analyte ion. This group includes isobaric interferences, polyatomic ion interferences, and refractory oxide ion interferences. Isobaric...

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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