HP 5973 MSD, Руководство по оборудованию

Страница: 28 / 122

28
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
–
, then the
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
+
and C
2
H
5
+
are capable of hydride abstraction. These
species have large hydride-ion affinities, which results in the loss of H
–
for long-
chain alkanes, according to the general reaction
R
+
+ M
→
[M-H]
+
+ RH
For methane, R
+
is CH
5
+
and C
2
H
5
+
, and M is a long-chain alkane. In the case of
CH
5
+
, the reaction proceeds to form [M-H]
+
+ CH
4
+ H
2
. The spectra resulting from
hydride abstraction will show an M-1 amu peak resulting from the loss of H
–
. This
reaction is exothermic so fragmentation of the [M-H]
+
ion is often observed.
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
+
peak at m/z = 299 is created by proton transfer, and
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
2
H
5
]
+
and
[M+C
3
H
5
]
+
ions, which result in M+29 and M+41 amu mass peaks.
Addition reactions are particularly important in ammonia CI. Because the NH
3
has
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
4
+
, [NH
4
NH
3
]
+
, and [NH
4
(NH
3
)
2
]
+
. In particular,
the ammonium ion, NH
4
+
, will give rise to an intense [M+NH
4
]
+
ion observed at
M+18 amu, either through condensation or association. If this resulting ion is
unstable, subsequent fragmentation may be observed. The neutral loss of H
2
O or
NH
3
, observed as a subsequent loss of 18 or 17 amu, respectively, is also common.