Discussion
Which of the following is a gas-phase ionization technique ?
Answer: A
The gas-phase ionization technique is electron impact.
In electron impact (EI) ionization, the sample is first vaporised, and the gas-phase molecules are then struck by a beam of electrons (usually 70 eV), which knock out an electron and produce molecular ions and fragments. It is the standard source in GC-MS.
The other three are desorption techniques, in which ions are formed directly from a condensed sample: fast atom bombardment ionises a sample dissolved in a liquid matrix, field desorption ionises a sample coated on an emitter in a strong electric field, and laser desorption uses laser pulses on a solid. They suit non-volatile and thermally fragile compounds that cannot be vaporised.
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