Deuterium Exchange
5
Note on Molecular Weight Determination
Compounds often don’t give a large parent ion, M
+
, peak. Alcohols, for example readily loose
H
2
O and give no parent ion. Auto-ionization also can give a parent peak for the protonated
molecular ion, MH
+
, which occurs at 1 m/z higher than the parent mass, M
+
. (See the note on
Auto-Ionization, below.) Chemical ionization, CI, is a very useful ionization method to
circumvent these problems. In CI a low pressure of a reagent gas is admitted to the source of the
mass spectrometer. The electron beam of the source ionizes this reagent gas. Ions from the
reagent gas then react chemically in the gas phase with your sample to generate MH
+
ions.
Acetonitrile, C
2
H
3
N, is a common reagent gas for CI. The electron impact produces a number of
reagent ions including the protonated acetonitrile ion,
[
C
2
H
4
N
]
+
. This reagent ion then reacts
with your compound:
M +
[
C
2
H
4
N
]
+
→
MH
+
+ C
2
H
3
N
Therefore, the peak has a mass of one greater than your molecular weight. CI is a “soft
ionization” technique. That is, the ionization process is very gentle and therefore produces little
or no fragmentation. A CI spectrum is then very simple. There will usually be one prominent
peak, the MH
+
peak, and maybe a few much lower intensity fragment ions. The
“m50t200m12CI” method is set up do easily do CI for your sample.
For some types of compounds CI also produces adduct ions. These ions are at M+13 or M+40.
The M+40 peak is produced by reactions with the
[
C
2
H
2
N
]
+
cation:
M +
[
C
2
H
2
N
]
+
→
[M+C
2
H
2
N
]
+
The adduct peaks are usually much lower in intensity than your parent MH
+
peak. For some types
of samples the normal M
+
ion is also produced by a charge transfer process:
M +
[
C
2
H
3
N
]
•
+
→
M
•
+
+ C
2
H
3
N
For example, acetonitrile CI works well with compounds containing nitrogen atoms, producing
a characteristic MH
+
ion and a confirmatory M+13 adduct ion (probably from the reaction with
[CH3CNCH2]
+
). Saturated hydrocarbons also produce abundant
[
M+40
]
+
adducts, and alkenes
give both
[
M+40
]
+
and
[
M+54
]
+
ions.
Note on Auto Ionization
The M+1/M ratios for various peaks are very helpful for predicting the formula for the ion.
However, the ionization process in some mass spectrometers can produce inaccurate ratios for
the parent ion. This problem is caused by auto ionization, which is particularly prevalent in
internal ionization ion trap mass spectrometers. The ions in an ion trap are held for relatively
long periods (10 msec) in the trap before being scanned out to determine the mass spectrum.
During this time ion-molecule reactions can occur. An ion in the trap can transfer a proton to an
unionized molecule, M,
NH
+
+ M -> N + MH
+
Unfortunately, this alternative ionization mechanism produces an M+1 peak that can be much
larger than the M+1 isotope peak from the M
+
• ion. Therefore, you should always be careful to
consider auto-ionization when using an internal ionization ion trap mass spectrometer. Auto-
ionization is expected for basic compounds, like amines, and to a lesser extent esters and
ketones.
The prevalence of auto-ionization depends on several factors. Auto-ionization increases as the
number of ions in the trap increases and as the time in the trap increases. Both of these
