Page 262 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GTM/GRL P2: GNH Final Pages
Encyclopedia of Physical Science and Technology EN009N-406 July 18, 2001 23:32

150 Mass Spectrometry

processes provides clues to the original structure of the it encounters will most likely be a methane molecule. The
molecule. A balance must be attained between fragmenta- ion/molecule reaction between the two involves a pro-
+
tion extensive enough to provide reasonable clues to struc- ton transfer to create CH and CH . Several other sub-

5 3
ture of the neutral molecule, and a relative intensity of the sequent reactions occur, and the final distribution of ions
molecular ion large enough so that the molecular ion can depends explicitly on the source temperature and pres-
be identified, and the mass of the sample molecule de- sure, but the primary reactant ion is usually CH . This
+
5
termined. Determination of molecular mass is a physical ion acts as a strong gas-phase acid that protonates any-
measurement, in contrast to the insight and intuition of thing more basic than methane; neutral sample molecules
interpretation. The appearance of both M + and fragment are sufficiently basic to accept a proton to form the proto-
+
ions derived from the molecular ion—the deduction of nated molecule. The mass of the (M + H) ion reveals the
both molecular mass and molecular structure—is the core molecular mass, and the protonated molecule then frag-
use that has supported the growth of electron ionization ments in accordance with the amount of internal energy
mass spectrometry. contained within it. Methane is not the only reagent gas
that forms a reagent ion that transfers a proton to the gas-
phase sample molecule. Isobutane introduced into the CI
2. Chemical Ionization +
source at a pressure of about 1 torr generates C 4 H as an
9
In electron ionization, an encounter with an energetic elec- acidic reagent ion, which also transfers a proton to the neu-
tron causes the ejection of a single electron from a gas- tral sample molecule, but gives it less energy so that the
phase sample molecule M to form the odd-electron molec- protonated molecule fragments less. Ammonia reacts un-
+
ular ion M . If too much energy is deposited into M + der CI conditions to create NH . The ammonium ion can
+
4
during the ionization process, or if the molecule is prone transfer a proton to the sample molecule, or it can trans-
to dissociate, fragment ions may be seen in the mass spec- fer the intact ammonium group to create (M + NH 4 ) .A
+
trum, but the M + may be reduced to such a low inten- simple rule in CI mass spectrometry is use methane to ob-
sity that it is indistinguishable above the background sig- tain the protonated molecule and some fragmentation, and
nal level. Without the molecular ion, the determination of then use isobutane when you want to minimize the frag-
molecular mass is difficult. Chemical ionization (CI) was mentation and concentrate ion current in the protonated
developed to overcome this difficulty and provide molec- molecule (as might be required when low limits of detec-
ular ions for compounds used for determination of the tion are sought). Since the ammonium ion will transfer
molecular mass. its proton only to compounds more basic than ammonia
Chemical ionization involves a collision and reaction itself, selectivity in ionization can be achieved.
between an ion and a molecule, both in the gas phase. The
ion is called the reagent ion and the molecule is the neutral
3. Electrospray Ionization
sample molecule. Since the process is an ion/molecule re-
action, parameters such as source temperature and source Among the ionization methods discussed here, ESI is
pressure, sample pressure and reagent gas pressure, and unique in that it generates ions directly from within a so-
the presence of impurities in the gas stream that enters lution that is sprayed from a fine needle at atmospheric
the source must be carefully controlled. The CI source pressure. At the core of the ionization source is a stain-
is a variation of the standard EI source, with modifica- less steel capillary tube that carries solvent (effluent from
tions required to achieve a higher source pressure (about a LC column, typically) at a flow rate of 2–5 µl/min. A
1 torr) while keeping the mass analyzer pressure within potential difference of 3000–4000 V is maintained be-
acceptable limits. The source filament emits electrons, but tween the needle and a counter electrode, which can be
these are now accelerated to several hundred volts (250– a wall of the source, or a skimmer cone with an aperture
500 eV is typical) compared to the standard 70 eV of that passes the ions into the mass spectrometer. A spray is
an EI source. This higher energy allows the electrons to generated at the tip by the solvent flow emerging at atmo-
penetrate through the higher gas pressure in the source. spheric pressure, and the potential difference ensures that
Since the pressure of 1 torr is mostly the reagent gas (of- the droplets emerging from the needle are charged, aiding
ten methane), the electron emitted from the filament is in their dispersal. A supplemental flow of gas is also some-
likely to encounter a methane molecule. When it does, an times used to aid in desolvation of the droplets. The ions
electron ionization process occurs, viz. that are within the bulk solvent, or more accurately, those
ions that are formed in the droplet as the last evaporation
CH 4 + e − −→ CH + + e − + e − .
filament 4 filament methane of neutral solvent molecules occurs, are focused by ion
TheCH + iondoesnottravelfarbeforeitencountersaneu- lenses, and sampled through apertures, to be passed into
4
tral gas molecule, and at 1 torr of methane, the molecule the mass spectrometer.