Average Kinetic Energy of Ions after EI in MS?

[BlakeLeonard]

Hello,

From what I've been reading, 70eV electrons are the standard for electron impact ionization in mass spectrometry. I'm trying to get a feel for how much kinetic energy on the average is imparted to an ion. I know this gets complex, when you start talking about multiple fragments, but in just the simplest case, a Hydrogen atom, how much of the remaining 55eV is converted to kinetic energy of the ion? Thanks.

Blake

 

[turin]

Well, I don't know what fraction of the KE of the electron is imparted to the H-ion (proton), but I can tell you that, at only 56 eV available for KE, the proton will not even "feel" it. One complication is that there are two electrons and one proton in the final state (which you alluded to). One consideration is that the smaller particle tends to take the larger portion of the KE. For instance, when M>>m, then the lighter projectile tends to take almost all of the available KE, and the heavier target tends to obtain only a fraction on the order of m/M. In the case of a hydron atom, that is a fraction of only 1/2000. So, I would expect that the proton doesn't even obtain a single eV. For the case of a heavier ion, this is even much smaller.

 

[astrokreng]

,

Thank you for your inquiry. The average kinetic energy of ions after electron impact ionization in mass spectrometry can vary depending on the specific conditions and molecules involved. However, as you mentioned, 70eV electrons are commonly used in this process. In the simplest case of a hydrogen atom, the remaining 55eV of energy can be converted to kinetic energy of the ion. This conversion occurs through the ionization process, where the electron is removed from the atom, leaving behind a positively charged ion with the remaining kinetic energy.

It is important to note that in more complex molecules, the energy may be distributed among multiple fragments, making it difficult to determine a specific value for the average kinetic energy of ions. Additionally, the exact amount of energy transferred to the ion may also depend on the specific ionization mechanism used. In any case, the kinetic energy of ions can be further manipulated and controlled through the use of electric and magnetic fields in the mass spectrometer.

I hope this helps answer your question. If you have any further inquiries, please don't hesitate to ask. Thank you for your interest in mass spectrometry and its applications in scientific research.