Name: mass analyzers and ion separation
Uploaded: 2024-04-04T10:51:34.000Z
Duration: 1 min 13 s
Channel: JoVE

Les analyseurs de masse courants comprennent les analyseurs à secteur magnétique, à double focale, à quadripôle, à temps de vol et à piège ionique.
La capacité de tout analyseur de masse peut être évaluée par sa résolution, qui décrit la capacité à différencier des ions ayant des rapports masse/charge similaires.
La résolution est exprimée comme le rapport entre la valeur nominale m/z d'un pic et la plus petite différence résoluble entre celui-ci et un pic adjacent. Les facteurs limitatifs de résolution de l’analyseur dépendent du type d’analyseur.
Par exemple, un simple analyseur de secteur magnétique a besoin que les ions aient à peu près la même énergie cinétique lorsqu’ils pénètrent dans le champ magnétique afin que les ions ayant des valeurs m/z différentes soient déviés de manière fiable vers différentes trajectoires.
Ainsi, même des variations mineures d’énergie cinétique causées lors de la fragmentation et de l’accélération rendent cette séparation moins efficace.
Cependant, un analyseur à double focale utilise un secteur de champ électrique appliqué supplémentaire pour corriger les différences d’énergie cinétique, ce qui peut donner jusqu’à dix fois plus de résolution.

analyseurs de masse : présentation

Mass Analyzers: Overview

L'analyseur de masse est un élément crucial du spectromètre de masse. Dans la chambre d'ionisation, l'échantillon vaporisé est bombardé avec un faisceau d'électrons à haute énergie pour générer un cation radical et se fragmenter davantage en molécules neutres, radicaux et cations. Une série de plaques accélératrices chargées négativement accélèrent les cations dans l’analyseur de masse. L'analyseur de masse sépare les ions en fonction de leurs rapports masse/charge (m/z), puis les dirige vers le détecteur. Les types courants d'analyseurs de masse comprennent le secteur magnétique, la double focalisation, le quadripôle, le temps de vol et le piège à ions.

Un champ magnétique est appliqué dans l'analyseur de secteur magnétique et les charges en mouvement sont déviées vers une trajectoire courbe. Pour un champ magnétique donné, le rayon de courbure dépend de la valeur m/z, et les ions d'une valeur m/z particulière passent au détecteur. En conséquence, le champ magnétique de l'instrument varie en continu pour transmettre séquentiellement des ions dans une plage m/z plus large vers le détecteur.

Un analyseur de masse à double focalisation utilise une combinaison du secteur électrostatique et du secteur magnétique. Lorsque les ions sortent de la source, le champ électrostatique focalise le faisceau ionique pour minimiser la propagation de l'énergie cinétique, puis conduit les ions vers le secteur magnétique. Le secteur magnétique trie les ions en fonction de la déviation. Cette double focalisation du faisceau permet d'obtenir une haute résolution.

La résolution du spectromètre de masse est définie comme sa capacité à différencier deux spectres de masse rapprochés. Une résolution unitaire indique la capacité de l'instrument à différencier les masses avec une différence d'une unité entière. Par exemple, un instrument avec une résolution unitaire peut distinguer m/z 50 de m/z 51, ou m/z 100 de m/z 101, ou m/z 500 de m/z 501. Généralement, la résolution des spectromètres commerciaux est entre 500 et 500 000.

Analyseurs de masse : présentation

Les analyseurs de masse courants comprennent les analyseurs à secteur magnétique, à double focale, à quadripôle, à temps de vol et à piège ionique.
La capacité de n’importe quel analyseur de masse ...

Education

Enseignement

JoVE Core

Analytical Chemistry

Chimie analytique

Unit 1

Unité 1

Principles of Mass Spectrometry

Principes de la spectrométrie de masse

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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, double-focusing, quadrupole, time-of-flight, and ion trap analyzers. In a magnetic sector analyzer, a magnetic field bends the moving ions into a curved path. The radius of that curve depends on the ion's m/z value. By changing the magnetic field, the instrument can pass ions with different m/z values to the detector one after another.

A double-focusing mass analyzer combines an electrostatic sector and a magnetic sector. As ions leave the source, the electrostatic field focuses the beam and reduces the spread in kinetic energy. The beam then enters the magnetic sector, which separates the ions by deflection. This double-focusing design helps the instrument achieve high resolution.

Resolution describes how well a mass spectrometer can tell apart two closely spaced mass spectra. Unit resolution means the instrument can distinguish masses that differ by one integer unit, such as m/z 50 and 51, or 100 and 101, or 500 and 501. In commercial spectrometers, resolution is generally in the range of 500 to 500,000.

Mass Analyzers and Ion Separation

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-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...

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 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

See how mass analyzers separate ions by m/z, how magnetic sector and double-focusing designs work, and what resolution means in mass spectrometry.

Mass Analyzers and Ion Separation | JoVE K12

analytical chemistry

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Mass Analyzers and Ion Separation

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