Proton Transfer - HP 5973 MSD Hardware Manual

 Chemical Ionization Theory
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Proton transfer

Proton transfer can be expressed as
+
+ M → MH
BH
where the reagent gas B has undergone ionization resulting in protonation. If the
proton affinity of the analyte (sample) M is greater than that of the reagent gas, then
the protonated reagent gas will transfer its proton to the analyte, forming a
positively charged analyte ion.
The most frequently used example is the proton transfer from CH
molecular analyte, which results in the protonated molecular ion MH
The relative proton affinities of the reagent gas and the analyte govern the proton
transfer reaction. If the analyte has a greater proton affinity than the reagent gas,
then proton transfer can take place. Methane (CH
gas because its proton affinity is very low.
Proton affinities can be defined according to the reaction:
+ +
→ BH
B + H
where the proton affinities are expressed in kcal/mole. Methane's proton affinity is
127 kcal/mole. The following tables list the proton affinities of several possible
reagent gases and of several small organic compounds with various functional
groups.
The mass spectrum generated by a proton-transfer reaction depends on several
criteria. If the difference in proton affinities is large (as with methane), substantial
excess energy may be present in the protonated molecular ion. This can result in
subsequent fragmentation. For this reason, isobutane, with a proton affinity of
195 kcal/mole, may be preferred to methane for some analyses. Ammonia has a
proton affinity of 207 kcal/mole, making it less likely to protonate most analytes.
Proton-transfer chemical ionization is usually considered to be "soft" ionization, but
the extent of the softness is dependent on the proton affinities of both the analyte
and the reagent gas, as well as on other factors, including ion source temperature.
26
+
+ B
+
to the
+
5
.
) is the most common reagent
4