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Related Concept Videos

Electrospray Ionization (ESI) Mass Spectrometry01:12

Electrospray Ionization (ESI) Mass Spectrometry

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Higher molecular weight biomolecules are nonvolatile compounds that may decompose before ionizing or vaporizing during mass analysis with conventional electron impact ionization methods. Accordingly, electrospray ionization (ESI) is the favored method for vaporizing and ionizing biomolecules as it circumvents rapid fragmentation and enables the recording of mass signals for the entire biomolecule.
ESI utilizes electrical energy to transfer ions from the liquid phase of the sample into the...
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Chemical Ionization (CI) Mass Spectrometry01:21

Chemical Ionization (CI) Mass Spectrometry

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The molecular ion peak of a molecule in the mass spectrum provides vital information for molecular identification. However, conventional electron impact ionization can lead to the rapid dissociation of some molecular ions before they reach the detector. A milder ionization method is required to increase the lifetime of such ionized analyte molecules. Chemical ionization (CI) is a gas-phase protonation reaction useful for mass-analyzing analyte molecules that are easily protonated to yield the...
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Mass Spectrum: Interpretation01:24

Mass Spectrum: Interpretation

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An unknown compound can be established by identifying the molecular ion peak in the mass spectrum. The molecular ion peak is often weak or absent due to the predominance of fragmentation in high-energy electron beams. In such cases, a soft-energy electron beam can be used to scan the spectrum to enhance the intensity of the molecular ion peak. Additionally, chemical ionization, field ionization, and desorption ionization spectra are used to obtain a relatively intense molecular ion peak.To...
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Mass Spectrometry: Overview01:19

Mass Spectrometry: Overview

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Mass spectrometry is an analytical technique used to determine the molecular mass and molecular formula of a compound. The basic principle of mass spectrometry is to generate ions from the analyte molecule and measure these ion abundances against their molecular mass. One common type of ionization, known as electron ionization or EI, bombards the analyte molecules in the gas phase with high-energy electron beams. The electron beams displace an electron from the molecule and leave behind a...
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Mass Spectrometry: Molecular Fragmentation Overview01:20

Mass Spectrometry: Molecular Fragmentation Overview

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The ionization of a molecule into a molecular ion inside the mass spectrometer causes instability in the molecule's structure due to the loss of an electron. This eventually leads to the fragmentation or breaking of some bonds in the molecule. The fragmentation occurs predominantly at specific bonds to yield relatively stable fragments.
One type of fragmentation pattern is the cleavage of a single bond in the molecular ion. The cleavage leads to a radical and a cation. The cleavage can occur at...
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Ionization Energy03:12

Ionization Energy

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The amount of energy required to remove the most loosely bound electron from a gaseous atom in its ground state is called its first ionization energy (IE1). The first ionization energy for an element, X, is the energy required to form a cation with 1+ charge:
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Related Experiment Video

Updated: Apr 1, 2026

Photoelectron Imaging of Anions Illustrated by 310 Nm Detachment of F−
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Soft Cold EI - approaching molecular ion only with electron ionization.

Aviv Amirav1, Uri Keshet1, Albert Danon2

  • 1School of Chemistry, Tel Aviv University, Tel Aviv, 69978, Israel.

Rapid Communications in Mass Spectrometry : RCM
|October 8, 2015
PubMed
Summary
This summary is machine-generated.

Soft Cold EI using gas chromatography/mass spectrometry (GC/MS) enhances molecular ion detection. Increasing the nozzle-skimmer distance in Cold EI minimizes undesirable fragmentation, improving identification probabilities for complex samples.

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Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation
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Analysis of Volatile and Oxidation Sensitive Compounds Using a Cold Inlet System and Electron Impact Mass Spectrometry
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Molecular Beam Mass Spectrometry With Tunable Vacuum Ultraviolet VUV Synchrotron Radiation
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Area of Science:

  • Analytical Chemistry
  • Mass Spectrometry
  • Chemical Physics

Background:

  • Cold Electron Ionization (EI) in supersonic molecular beams (SMB) offers enhanced molecular ions alongside fragment ions for GC/MS analysis.
  • While beneficial for identification, conventional Cold EI may not be ideal for matrices requiring soft ionization yielding only molecular ions.

Purpose of the Study:

  • To investigate low electron energy Cold EI for achieving molecular ion-only detection.
  • To mitigate undesirable collision-induced dissociation (CID) of labile molecular ions.

Main Methods:

  • Utilized GC/MS with a fly-through ion source employing Cold EI at selected low electron energies.
  • Adjusted helium pressure by increasing nozzle-skimmer distance to minimize ion source reheating and CID.
  • Analyzed hydrocarbon standards, including a squalane isomer (C30H62) and n-C24H50.

Main Results:

  • Observed that supersonic expansion cooling can be counteracted by reheating from scattered helium atoms.
  • Identified significant CID of labile molecular ions, particularly in hydrocarbons, due to collisions.
  • Demonstrated that increasing nozzle-skimmer distance effectively reduced helium pressure, suppressing CID.

Conclusions:

  • An increased nozzle-skimmer distance significantly boosted the ratio of molecular ions to fragment ions.
  • Low electron energy Cold EI combined with a large nozzle-skimmer distance transforms EI into Soft Cold EI.
  • This method advances towards the goal of a molecular ion-only ionization technique for GC/MS.