Hydride Abstraction; Addition - HP 5973 MSD Hardware Manual

 Chemical Ionization Theory
3RVLWLYH &, WKHRU\

Hydride abstraction

In the formation of reagent ions, various reactant ions can be formed that have high
–
hydride-ion (H ) affinities. If the hydride-ion affinity of a reactant ion is higher than
the hydride-ion affinity of the ion formed by the analyte's loss of H
thermodynamics are favorable for this chemical ionization process. Examples
include the hydride abstraction of alkanes in methane chemical ionization. In
+
methane CI, both CH
5
species have large hydride-ion affinities, which results in the loss of H
chain alkanes, according to the general reaction
+
+ M → [M-H]
R
+
For methane, R is CH
+
CH , the reaction proceeds to form [M-H]
5
hydride abstraction will show an M-1 amu peak resulting from the loss of H
reaction is exothermic so fragmentation of the [M-H]
Often, both hydride-abstraction and proton-transfer ionization can be evident in the
sample spectrum. One example is the methane CI spectrum of long-chain methyl
esters, where both hydride abstraction from the hydrocarbon chain and proton
transfer to the ester function occur. In the methane PCI spectrum of methyl
stearate, for example, the MH
+
the [M-1] peak at m/z 297 is created by hydride abstraction.

Addition

For many analytes, proton-transfer and hydride-abstraction chemical ionization
reactions are not thermodynamically favorable. In these cases, reagent gas ions are
often reactive enough to combine with the analyte molecules by condensation or
association (addition reactions). The resulting ions are called adduct ions. Adduct
ions are observed in methane chemical ionization by the presence of [M+C
+
[M+C H ] ions, which result in M+29 and M+41 amu mass peaks.
3 5
Addition reactions are particularly important in ammonia CI. Because the NH
a high proton affinity, few organic compounds will undergo proton transfer with
ammonia reagent gas. In ammonia CI, a series of ion-molecule reactions takes place,
resulting in the formation of NH
the ammonium ion, NH
M+18 amu, either through condensation or association. If this resulting ion is
unstable, subsequent fragmentation may be observed. The neutral loss of H
NH , observed as a subsequent loss of 18 or 17 amu, respectively, is also common.
3
28
+
and C H are capable of hydride abstraction. These
2 5
+
+ RH
+ +
and C H , and M is a long-chain alkane. In the case of
5 2 5
+
+ CH
+
peak at m/z = 299 is created by proton transfer, and
+ +
, [NH NH ]
4 4 3
+
, will give rise to an intense [M+NH
4
–
, then the
+ H . The spectra resulting from
4 2
+
ion is often observed.
+
, and [NH (NH ) ] . In particular,
4 3 2
+
] ion observed at
4
–
for long-
–
. This
+
H ] and
2 5
has
3
O or
2