Mass spectrometry - Sources ion generation in the gas phase
9 important questions on Mass spectrometry - Sources ion generation in the gas phase
Whit what techniques can you analyse samples in the gas, liquid and solid phase?
- EI Electron Ionisation
- CI Chemical Ionisation
Samples in liquid phase, atmospheric pressure ionization
- ESI ElectroSpray Ionisation
- APCI Atmospheric Pressure Chemical Ionization
- APPI Atmospheric Pressure
Photochemical Ionization Samples in solid phase
- MALDI Matrix Assisted Laser Desorption Ionisation
- DESI Desorption Electrospray Ionization
How does electron ionization (EI) work?
Electrons are ejected from the molecules.
Radical cations are formed (M+.).
Intense fragmentation
Only singly charged ions are usually formed
Hard ionization, most of the molecule is usually fragmented
Only possible for small compounds (< 1000 Da), volatiles, thermally stable.
It is the most common MS interfaced with GC
How does chemical ionization (CI) work?
A gas is ionized by the electron beam and reacts with the analytes, usually transferring protons.
Cations (M+) are formed.
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Are there multicharged ions in ESI?
Yes they can be pressent
Exaple with an hypotherical molecule having MW of 1000
- MH+ is 1001Da with 1 charge: m/z=1001
- M(2H+ ) is 1002Da with 2 charges: m/z=501
- M(3H+ ) is 1003Da with 3 charges: m/z = 334.3
- M(4H+ ) is 1004Da with 4 charges: m/z = 251
How can you intepret a multi chared spectrum in ESI?
- m1 , m2 are two consecutive m/z MS peaks observed for the same molecule
- We assume the molecule ionizes through protonation (as it is most often the case) m1 , m2 are due to the molecule (having a mass M) + n protons (in one case) and the same molecule plus n-1 protons in the other case
- If we assume that a proton is taken up, the mass adding to M is X=1
- The last two equations have two unknown terms (n and M) so they can be solved
What can be ionization issues in ESI?
- Two molecules having the same response will not necessarily be present in the same amount
- The same molecule is not giving the same response in different conditions (different instrument, days, interference level, solvents) -
- Matrix can vary the efficiency of ionization a lot
- Ionization efficiency is mostly NOT predictable
How does atmospheric pressure chemical ionization (APCI) work?
It is suited to apolar molecules having an average MW (500-2000 Da) which usually do not ionize easily in ESI.
How does matrix assisted laser desorption ionisation (MALDI) work?
- A photon beam of a laser ionizes the analyte dispersed in a solid matrix
- Product ions very often singly charged
- Soft ionization technique
- Obviously not directly linkable to chromatography
- Very sensitive More tolerant than ESI to contaminant of the samples
How does desorption electrospray ionization (DESI) work?
- The charged spray hits a surface, ioizing the molecules on the surface.
- MS spectrum directly on solid samples without any pretreatment
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