Name: mass spectrometry fragmentation patterns
Uploaded: 2024-04-04T10:51:33.000Z
Duration: 1 min 20 s
Channel: JoVE

В масс-спектрометрии молекула аналита ионизируется в молекулярный ион на один электрон меньше, чем молекула аналита.
Потеря электрона ослабляет некоторые связи в молекулярном ионе, который впоследствии фрагментируется по путям, ведущим к относительно стабильным, более мелким формам.
Например, рассмотрим фрагментацию молекулярного иона пентана. Расщепление происходит преимущественно в связи между вторым и третьим атомом углерода, а не между концевым и прилегающим к нему атомом углерода. Первый дает относительно стабильный карбокатион.
Фрагментация порождает катион и радикал, где расположение неспаренного электрона приводит к двум модам расщепления. Расщепление, которое приводит к стабильному карбокатиону, предпочтительнее, чем расщепление, образующее стабильный радикал.
Фрагментация также происходит за счет расщепления двух связей. Например, из молекулярных ионов спиртов получается радикальный катион и нейтральная молекула.
Другой паттерн фрагментации — это образование резонансно-стабилизированных карбокатионов, обусловленных присутствием пи-связей или несвязывающих электронов в молекуле анализируемого вещества.

Масс-спектрометрия: обзор молекулярной фрагментации

Mass Spectrometry: Molecular Fragmentation Overview

Ионизация молекулы в молекулярный ион внутри масс-спектрометра вызывает нестабильность структуры молекулы из-за потери электрона. В конечном итоге это приводит к фрагментации или разрыву некоторых связей в молекуле. Фрагментация происходит преимущественно по определенным связям с образованием относительно стабильных фрагментов.

Одним из типов фрагментации является разрыв одинарной связи в молекулярном ионе. Расщепление приводит к радикалу и катиону. Расщепление может происходить по различным одинарным связям в молекуле, но преимущественно происходит по положениям, которые генерируют стабильный карбокатион и радикал. По определенной связи неспаренный электрон может быть передан любому фрагменту, образуя два набора продуктов. Предпочтительная ориентация переноса электрона происходит таким образом, чтобы генерировать более стабильный карбокатион, а не стабильный радикал. Рассмотрим картину фрагментации пентана, показанную на рисунке 1.

<img alt="Figure1" src="/files/ftp_upload/12298/12298-PT_Figure_1.svg" style="text-align: center; height: 375px;" />

Figure 1. Фрагментация молекулярного иона пентана.

Из содержания карбокатионов в масс-спектрах, как показано на рисунке 2, ясно, что расщепление молекулярных ионов может происходить между любыми двумя соседними атомами углерода. Однако расщепление между двумя внутренними атомами углерода более предпочтительно, чем расщепление между концевым и соседним с ним углеродом. Расщепление внутренних атомов углерода дает относительно стабильный первичный карбокатион и первичный радикал. Расщепление связи между концевым атомом углерода и соседним с ним углеродом приводит к образованию первичного карбокатиона и метильного радикала или первичного радикала и метилкарбокатиона. Если такое расщепление и происходит, то именно через способ расщепления, который приводит к бутильному карбокатиону и метиловому радикалу.

<img alt="Figure2" src="/files/ftp_upload/12298/12298-PT_Figure_2.svg" style="text-align: center; height: 275px;" />

Figure 2. Масс-спектр пентана.

Другой тип расщепления — это диссоциация множественных связей с образованием фрагментов. Этот тип фрагментации приводит к образованию катион-радикала меньшего размера и нейтральной молекулы. Например, спирты могут фрагментироваться на более мелкие катион-радикалы, вытесняя молекулу воды. Следовательно, в масс-спектрах наблюдается пик М-18. На рис. 3 представлена ​​схема фрагментации 1-гептанола.

<img alt="Figure3" src="/files/ftp_upload/12298/12298-PT_Figure_3.svg" style="text-align: center; height: 100px;" />

Figure 3. Фрагментация 1-гептанола.

Молекулярная масса гептанола составляет 116 ед., а в его масс-спектре виден пик при m/z = 98, показанный на рисунке 4. Аналогичным образом, циклоалкены подвергаются ретро-реакции Дильса-Альдера с вытеснением алкена и катион-радикала меньшего размера.

<img alt="Figure4" src="/files/ftp_upload/12298/12298-PT_Figure_4.svg" style="text-align: center; height: 275px;" />

Figure 4. Масс-спектр гептанола.

Третий тип фрагментации наблюдается в молекулах с неподеленными электронами или π-связями. В молекулах с неподеленной парой электронов связь углерод-углерод рядом с атомом, несущим неподеленную пару, разрывается с образованием резонансно стабилизированного карбокатиона. На рисунке 5 в качестве примера изображена фрагментация эфира.

<img alt="Figure5" src="/files/ftp_upload/12298/12298-PT_Figure_5v2.svg" style="text-align: center; height: 140px;" />

Figure 5. Фрагментация эфиров.

π-связи в молекулярном ионе способствуют фрагментации молекулы с образованием аллильного карбокатиона, стабилизированного резонансом. На рис. 6 показана картина фрагментации концевого алкена.

<img alt="Figure6" src="/files/ftp_upload/12298/12298-PT_Figure_6v2.svg" style="text-align: center; height: 120px;" />

Figure 6. Фрагментация алкенов.

Ионизация молекулы в молекулярный ион внутри масс-спектрометра вызывает нестабильность структуры молекулы из-за потери ...

mass-spectrometry-molecular-fragmentation-overview

Education

JoVE Core

Analytical Chemistry

Аналитическая химия

Unit 1

Раздел 1

Principles of Mass Spectrometry

Принципы масс-спектрометрии

KEY TERMS AND CONCEPTS

ОСНОВНЫЕ ТЕРМИНЫ И ПОНЯТИЯ

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 than the more stable radical.

Pentane shows this idea clearly. Its molecular ion can break between any two adjacent carbon atoms, but cleavage between two inner carbon atoms is preferred over cleavage near the end of the chain. That inner break gives a relatively stable primary carbocation and primary radical. When the terminal bond breaks, the products can include a primary carbocation and a methyl radical, or a primary radical and a methyl carbocation. In the preferred case, the cleavage leads to a butyl carbocation and a methyl radical.

A second pattern is loss of a neutral fragment from the ion. This type of fragmentation gives a smaller radical cation plus a neutral molecule. Alcohols can lose water in this way, so their mass spectra often show an M−18 peak. For example, 1-heptanol has a molecular mass of 116 u, and its spectrum shows a peak at m/z = 98. Cycloalkenes can also follow a retro-Diels–Alder reaction to expel an alkene and leave a smaller radical cation.

A third pattern appears in molecules with lone pairs or π bonds. In compounds with lone pairs, the carbon-carbon bond next to the atom bearing the lone pair can break to form a resonance-stabilized carbocation, as seen with ethers. In molecules with π bonds, fragmentation often creates an allylic carbocation that is stabilized by resonance. A terminal alkene is a common example of this behavior.

Mass Spectrometry Fragmentation Patterns

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

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 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...

mass spectrometers

How Mass Spectrometers Sort Ions

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...

mass spectrum

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...

tandem mass spectrometry

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)

How GC-MS Separates and Identifies Compounds

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 molecular ions fragment in mass spectrometry, including single-bond cleavage, water loss from alcohols, and resonance-stabilized products.

Mass Spectrometry Fragmentation Patterns | JoVE K12

analytical chemistry

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Mass Spectrometry Fragmentation Patterns

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