Name: mass accuracy in hrms analysis
Uploaded: 2024-04-04T10:51:34.000Z
Duration: 1 min 15 s
Channel: JoVE

Mit Ausnahme des Isotops 12C liegen die Massen der Elementkerne nahe an ganzen Zahlen – aber nicht genau.
Massenspektren zeigen jedoch in der Regel die Teilchenmasse auf die nächste ganze Zahl gerundet, die als nominale Atom- oder Molekülmasse bezeichnet wird.
Diese Näherung macht es schwierig, Moleküle mit ähnlichen Molekülmassen zu unterscheiden, die sich nur durch Dezimalstellen unterscheiden.
Zum Beispiel haben Cyclopentanon und Cyclohexan die gleiche nominale Molekülmasse, wenn sie auf die nächste ganze Zahl gerundet werden. Solche Moleküle können mit Hilfe der hochauflösenden Massenspektrometrie unterschieden werden.
HRMS verwendet schmale Magnetfeldtische, um Teilchenmassen mit einer Genauigkeit von 0,0001 einheitlichen atomaren Masseneinheiten zu detektieren. Die durch hochauflösende Massenspektrometrie ermittelte Masse wird als exakte Molekülmasse bezeichnet.
Die Details der genauen Molekülmasse können verwendet werden, um Moleküle mit der gleichen nominalen Molekülmasse zu unterscheiden. Dies wiederum hilft bei der Identifizierung der Summenformel der Probe.

hochauflösende massenspektrometrie (hrms)

High-Resolution Mass Spectrometry (HRMS)

Die Auflösung eines Massenspektrometers hängt von der Effizienz der Trennung von Ionen mit unterschiedlichen Ionenmassen ab. Die Masse eines Atoms ergibt sich näherungsweise aus der Summe der Massen der Protonen und Neutronen im Inneren, wobei man davon ausgeht, dass die Massen von Protonen und Neutronen gleich sind. Allerdings sind die Massen des Protons (1.6726 × 10−24 g) und des Neutrons (1.6749 × 10−24 g) nicht wirklich gleich. Es gibt einen kleinen Fehler bei der Angabe der Atommassen im Verhältnis zum einfachsten WasserSubstanzatom. Beispielsweise ist die Masse von Helium (2 Protonen + 2 Neutronen) nicht genau viermal so groß wie die Masse von WasserSubstanz (1 Proton). Wenn mehrere Nukleonen im Kern eingeschlossen sind, führt dies außerdem zu einem Anstieg der potentiellen Energie und einer Abnahme der Masse (durch die gegenseitige Umwandlung von Masse und Energie). Dies ist der Grund für die geringere Masse von 12C im Vergleich zur Masse von sechs Deuteriumatomen (2H), obwohl die Gesamtzahl der Protonen und Neutronen in beiden Fällen gleich ist. Atommassen werden oft in einheitlichen Atommasseneinheiten (u) ausgedrückt, definiert als die Ruhemasse eines freien 12C-Atoms im Grundzustand dividiert durch 12. (Die einheitliche Atommasseneinheit wird auch als Dalton mit dem Symbol Da bezeichnet. Sie wird oft als „atomare Masseneinheit“ mit dem Symbol „amu“ bezeichnet, was der Name und das Symbol einer ähnlichen auf SauerSubstanz basierenden Einheit war, die durch die einheitliche atomare Masseneinheit ersetzt wurde.)

Für alle Atomkerne mit Ausnahme des freien, ruhenden 12C-Grundzustands ist die genaue Atommasse keine ganze Zahl. Daher ist die Molekülmasse nicht unbedingt eine ganze Zahl. Normalerweise werden die Atom- und Molekülmassen auf die nächste ganze Zahl (sogenannte Nominalmasse) angenähert. Die Masse eines bestimmten Isotops kann auch als relative Masse ausgedrückt werden, d. h. die Masse des Isotops dividiert durch die Atommassenkonstante, die 1 u beträgt. Table 1 listet die relative Masse, die genaue Masse und die Nennmasse für verschiedene Isotope auf.

<table style="margin-left: auto; margin-right: auto;">
	<tbody>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Isotop</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Anzahl der Protonen</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Anzahl der Neutronen</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Relative Masse</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Genaue Masse</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Nennmasse</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">1H</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">0</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1.0078 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">4He</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4.0026 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">4 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">12C</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12.0000 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">12 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">14N</td>
			<td style="color: black; border: 1px solid black; text-align: center;">7</td>
			<td style="color: black; border: 1px solid black; text-align: center;">7</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14.0032 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">14 u</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">16O</td>
			<td style="color: black; border: 1px solid black; text-align: center;">8</td>
			<td style="color: black; border: 1px solid black; text-align: center;">8</td>
			<td style="color: black; border: 1px solid black; text-align: center;">16</td>
			<td style="color: black; border: 1px solid black; text-align: center;">15.9949 u</td>
			<td style="color: black; border: 1px solid black; text-align: center;">16 u</td>
		</tr>
	</tbody>
</table>

Table 1: Die relative Masse, die genaue Masse und die Nennmasse der am häufigsten vorkommenden Isotope von WasserSubstanz, Helium, KohlenSubstanz, StickSubstanz und SauerSubstanz.

Normale Massenspektrometer können Ionen unterscheiden, die sich um mindestens 1 u unterscheiden. Dies erschwert die Unterscheidung verschiedener Moleküle mit denselben nominellen Massenwerten. Betrachten Sie das Beispiel von Cyclopentanon und Cyclohexan: Beide haben die gleiche nominelle Molekülmasse von 84 u; Sie haben jedoch unterschiedliche exakte Molekülmassen, chemische Strukturen und Atomzahlen (Cyclopentanon: (CH2)4CO, 84,0573 u; Cyclohexane: C6H11, 84,0936 u). Allerdings kann ein hochauflösendes Massenspektrometer sogar Ionen unterscheiden, die sich nur um 0,0001 u unterscheiden. Ein hochauflösendes Massenspektrometer nutzt winzige Magnetfeldschritte, um die geladenen Spezies in der Analysekammer zu scannen. Die winzigen Schritte sind effizient bei der Trennung von Ionen mit ähnlichen Molekulargewichten, indem sie die Pfade der Ionen entsprechend ihrem genauen Molekulargewicht ablenken.

Hochauflösende Massenspektrometrie (HRMS)

Die Auflösung eines Massenspektrometers hängt von der Effizienz der Trennung von Ionen mit unterschiedlichen Ionenmassen ab. Die Masse eines Atoms wird angenähert an die ...

Education

Lehre

JoVE Core

Analytical Chemistry

Analytische Chemie

Unit 1

Einheit 1

Principles of Mass Spectrometry

Prinzipien der Massenspektrometrie

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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 protons and 2 neutrons, but its mass is not exactly four times the mass of hydrogen.

Mass can also change because of energy inside the nucleus. When many nucleons are packed together, potential energy increases and mass decreases through mass-energy interconversion. This is why 12C has a lower mass than six deuterium atoms, even though both contain the same total number of protons and neutrons.

Atomic masses are commonly given in unified atomic mass units, or u. This unit is defined as one-twelfth of the rest mass of a free, ground-state 12C atom. It is also called the dalton, or Da. The older term amu was used for a similar oxygen-based unit that has been replaced.

For most nuclei other than free, ground-state, at-rest 12C, the exact atomic mass is not a whole number. That means molecular mass is not always a whole number either. Scientists often round to the nearest whole number and call that the nominal mass. The mass of an isotope can also be written as a relative mass, which is the isotope mass divided by the atomic mass constant, equal to 1 u. Table 1 shows relative mass, exact mass, and nominal mass for common isotopes such as 1H, 4He, 12C, 14N, and 16O.

Normal mass spectrometers usually separate ions that differ by at least 1 u. That can make it hard to tell apart molecules with the same nominal mass. Cyclopentanone and cyclohexane are one example. Both have a nominal molecular mass of 84 u, but their exact masses, structures, and atom counts are different. Cyclopentanone is (CH2)4CO at 84.0573 u, while cyclohexane is C6H11 at 84.0936 u.

A high-resolution mass spectrometer can separate ions that differ by only 0.0001 u. It uses very small magnetic field steps in the analyzing chamber to scan charged species. These fine steps deflect ions according to exact molecular weight and help distinguish compounds with very similar masses.

Mass Accuracy in HRMS Analysis

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 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)

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

Explore how high-resolution mass spectrometry separates ions by exact mass, distinguishes isotopes, and tells apart molecules with the same nominal mass.

Mass Accuracy in HRMS Analysis | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

Mass Accuracy in HRMS Analysis

Description

Transcript

Tags