a) The meachnisms of analyte ion formation in API and APCI are very similar. First of all, what is the major instrumental difference between these methods?
In API, the elder technique, a 63Ni (beta emitter) foil is employed to provide electrons to initiate the ionization process. Just one year later, the same team of researchers developed APCI where the source of primary ionization is delivered by a DC corona discharge at the tip of a sharp needle set to high voltage (4-5 kV).
b) Write down the sequence from primary ionization to [M+H]+ analyte ion formation in APCI.
1) N2 + corona discharge → N2+• + e–
2) N2+• + N2 + N2(3rdbody) → N4+• + N2(3rdbody)
3) N4+• + H2O → H2O+• + 2 N2
4) H2O+• + H2O → [H3O]+ + OH•
5) [H3O]+ + n H2O → [nH2O+H]+
6) [nH2O+H]+ + M → [M+H]+ + n H2O
c) Positive ions other than [M+H]+ can also occur in APCI.
Which other types of analyte ions are common? Also name the correponding processes leading to their formation.
1) Molecular ions, M+•, by charge transfer.
2) Adduct ions, [M+NH4]+, by electrophilic addition.