Chapter 20
Molecular Mass Spectrometry
Mass spectrometry
information about
is
capable
of
providing
(1) the elemental composition of samples of matter.
(2) the structures of inorganic, organic, and
biological molecules.
(3) the qualitative and quantitative composition of
complex mixtures.
(4) the structure and composition of solid surfaces.
(5) isotopic ratios of atoms in samples.
MOLECULAR MASS SPECTRA
Ethyl benzene, molecular mass of 106 dalton. The
analyte vapor was bombarded with a stream of electrons
that led to the loss of an electron by the analyte and
formation of the molecular ion M+
C6H5CH2CH3 + e-  C6H5CH2CH.+3 + 2eThe charged species C6H5CH2CH.+3 is the molecular ion.
Relaxation occurs by fragmentation of part of the
molecular ions to produce ions of lower masses. The
positive ions produced on electron impact are attracted
through the slit of a mass spectrometer where they are
sorted according to their mass-to-charge ratios and
displayed in the form of a mass spectrum. The largest
peak, termed the base peak.
ION SOURCES
The starting point for a mass spectrometric
analysis is the formation of gaseous analyte
ions. Ion sources fall into two major categories:
gas-phase sources and desorption sources. In
the former, the sample is first vaporized and
then ionized. In the latter, the sample in a solid
or liquid state is converted directly into gaseous
ions.
ION SOURCES
Ion sources are also classified as being hard
sources and soft sources. Hard sources impart
sufficient energy to analyte molecules,
relaxation involves rupture of bonds, producing
fragment ions that have mass-to-charge ratios
less than that of the molecular ion. Soft sources
cause little fragmentation. Consequently, the
resulting mass spectrum often consists of the
molecular ion peak and only a few, if any other,
peaks.
The Electron-Impact Sources
The sample is brought to a temperature high
enough to produce a molecular vapor, which is
then ionized by bombarding the resulting
molecules with a beam of energetic electrons.
M + e-  M.+ + 2ewhere, M = analyte molecule,
M.+ = molecular ion.
Relaxation then usually takes place by
extensive fragmentation, giving a large number
of positive ions of various masses that are less
than that of the molecular ion. These lower
mass ions are called daughter ions.
Isotope Peaks
Sometimes peaks occur at masses that are greater
than that of the molecular ion. These peaks are
attributable to ions having the same chemical
formula but different isotopic compositions. e.g.,
for methylene chloride, the more important
isotopic species are 12C1H235Cl2 (m = 84),
13C1H 35Cl (m = 85), 12C1H 35Cl37Cl (m = 86),
2
2
2
13C1H 35Cl37Cl (m = 87), 12C1H 37Cl (m = 88).
2
2
2
The size of the various peaks depends upon the
relative natural abundance of the isotopes.
Advantages and Disadvantages of El Sources
Electron–impact sources are convenient to use because
of good sensitivities. The extensive fragmentation and
consequent large number of peaks is also an advantage
because it often makes unambiguous identification of
analytes possible. This fragmentation can also be a
disadvantage, however, when it results in the
disappearance of the molecular ion peak so that the
molecular weight of analytes cannot be established.
Another limitation of the electron-impact source is the
need to volatilize the sample, which may result in
thermal degradation of some analytes before ionization
can occur. Electron-impact sources are only applicable
to analytes having molecular weights smaller than about
103 daltons.
Chemical Ionization Sources
In chemical ionization, gaseous atoms of the
sample are ionized by collision with ions
produced by electron bombardment of an excess
of a reagent gas. A gaseous reagent is introduced
into the ionization region in an amount such that
the concentration ratio of reagent to sample is 103
to 104. Because of this large concentration
difference the electron beam reacts nearly
exclusively with reagent molecules.
Chemical Ionization Sources
One of the most common reagents is methane
which reacts with high-energy electrons to give
several ions such as CH+4, CH+3, and CH+2.
These ions react rapidly with additional methane
molecules as follows:
CH4+ + CH4  CH5+ + CH3
CH3+ + CH4  C2H5+ + H2
Chemical Ionization Sources
Collisions between the sample molecule MH and CH5+
or C2H5+ are highly reactive and involve proton or
hydride transfer. e.g.,
CH5+ + MH  MH2+ + CH4
Proton transfer
C2H5+ + MH  MH2+ + C2H4
Proton transfer
C2H5+ + MH  M+ + C2H6
Hydride transfer
Proton transfer reaction give the (M + 1)+ ion whereas
the hydride transfer produces an ion with a mass one
less than the analyte, or the (M - 1)+ ion. With some
compounds, an (M + 29)+ peak is also produced from
transfer of a C2H5+ ion to the analyte.
Instrument Components
The block diagram in Fig. 20-11 shows the
major components of mass spectrometers. The
purpose of the inlet system is to introduce a very
small amount of sample into the mass
spectrometer, where its components are
converted to gaseous ions.
Ion sources of mass spectrometers convert the
components of a sample into ions. The function
of the mass analyzer is analogous to that of the
grating in an optical spectrometer.
Instrument Components
In the former, however, dispersion is based upon
the mass-to-charge ratios of the analyte ions
rather than upon the wavelength of photons.
A mass spectrometer contains a transducer (for
ions) that converts the beam of ions into an
electrical signal that can then be processed,
stored in the memory of a computer, and
displayed or recorded in a variety of ways.