MARKET 1 Lecture Notes - Lecture 17: Mass Spectrometry, Electronvolt, Reagent
Document Summary
Hard ionisation techniques (e. g. ei: more fragmentation occurs (due to more energy being involved, sometimes difficult to see m+ Soft ionisation techniques (e. g. ci, esi: less fragmentation, good for determining the molecular mass. Molecules are ionised during collision with a beam of high energetic electrons (typical 70 ev energy) Electrons are boiled off in a filament (f) Due to high internal energy high degree of fragmentation (which occurs in ion source) Sometimes difficult to see m+ (due to lack of stability) Most useful in identification of structures compare with library. Electron bombardment of reagent gas produces reagent ions. Methane (ch4), iso-butane or ammonia (nh3) as reagent gas. Each gas has a different proton-affinity (pa) with the molecule (higher pa usually gives less fragmentation) Useful in determination of the molecular ion. + + 2e: ch4 + e- ch4, ch4, m + ch5. + [m+h]+ + ch4: ammonia as reagent gas.
