What is electron ionization process?
Electron ionization (EI), formerly known as electron impact, is an ionization method in which energetic electrons interact with gas-phase atoms or molecules to produce ions. This technique was early introduced by Dempster in 1918 , and is widely used for gases and volatile molecules.
Why does electron impact cause fragmentation?
Although the mass spectra are very reproducible and are widely used for spectral libraries, EI causes extensive fragmentation so that the molecular ion is not observed for many compounds. Fragmentation is useful because it provides structural information for interpreting unknown spectra.
How does ESI MS work?
ESI uses electrical energy to assist the transfer of ions from solution into the gaseous phase before they are subjected to mass spectrometric analysis. Ionic species in solution can thus be analysed by ESI-MS with increased sensitivity.
What happens in electron impact ionization?
Electron impact (also known as electron ionisation) This usually knocks off one electron from each particle forming a 1+ ion. The 1+ ions are then attracted towards a negative electric plate where they are accelerated.
Why is 70 eV electron ionization?
At around 70 eV, the de Broglie wavelength of the electrons matches the length of typical bonds in organic molecules (about 0.14 nm) and energy transfer to organic analyte molecules is maximized, leading to the strongest possible ionization and fragmentation.
What are the advantages of electron ionization techniques?
Advantages and Disadvantages of EI The classic EI method has many advantages. EI is non-selective ionization and can be ionized as long as the sample can be vaporized. EI has high ionization efficiency and sensitivity. EI spectrum provides a wealth of structural information and is the “fingerprint” of the compound.
What is LC-MS analysis?
Liquid Chromatography with tandem mass spectrometry (LC-MS-MS) is a powerful analytical technique that combines the separating power of liquid chromatography with the highly sensitive and selective mass analysis capability of triple quadrupole mass spectrometry.
What is electron impact used for?
Electron impact is used for elements and substances with low formula masses as it would otherwise cause larger organic molecules to fragment.
What is the standard electron energy used in electron ionization?
Since the ionization cross section depends on the chemical nature of the sample and the energy of ionizing electrons a standard value of 70 eV is used. At low energies (around 20 eV), the interactions between the electrons and the analyte molecules do not transfer enough energy to cause ionization.
What is the effect of chemical ionization on fragmentation?
Chemical ionization yields even electron ions that are less prone to fragmentation. The fragmentation that does occur is often predicted through an understanding of cation chemistry.
What is the difference between electron impact ionization and ionization?
Electron impact ionization yields molecular radical ions, also called odd electron ions. Their fragmentation is very common and is often predicted through an understanding of free radical chemistry. Chemical ionization yields even electron ions that are less prone to fragmentation.
What is hard ionization method?
It is an ionization method in which high energy electrons interact with atoms or molecules in solid or gas phases to produce ions. This technique is considered to be a hard ionization method (high fragmentation) because it uses high-energy electrons to generate ions.
Which ionization technique is used in mass spectrometry?
EI was one of the first ionization techniques developed for mass spectrometry. However, this method is still a popular ionization technique. This technique is considered a hard (high fragmentation) ionization method, since it uses highly energetic electrons to produce ions.