Mass spectrometry is the analytical
technique that provides the most structural information for the least amount of
analyte material. It provides qualitative and quantitative information about
the atomic and molecular composition of inorganic and organic materials and
their chemical structures. As an analytical technique it possesses distinct
advantages such as:
1. Increased sensitivity over most
other analytical techniques because the analyzer, as a mass-charge filter,
reduces background interference
2. Excellent specificity from
characteristic fragmentation patterns to identify unknowns or confirm the
presence of suspected compounds.
3. Information about molecular
weight.
4. Information about the isotopic
abundance of elements.
A few of the disadvantages of the
method is that often fails to distinguish between optical and geometrical
isomers and the positions of substituent in o-, m- and p- positions in an
aromatic ring. Also, its scope is limited in identifying hydrocarbons that
produce similar fragmented ions.
The mass spectrum obtained is a
fingerprint for each compound because no two molecules are fragmented and
ionized in exactly the same manner.
History
of Mass Spectrometry
Mass spectrometry (MS) was developed
at the beginning of the 20th century by J.J. Thomson (1910) Dempster
(1918) and F.W. Aston (1920) to separate and identify isotopes. In the
thirties, Smyth (1931) and Tate (1935) recognized that ionization of organic
molecules by electron impact leads to characteristic fragment ions which can be
measured by mass spectrometry and which allow conclusions on the structure of
the chemical compound.
In the forties, mass spectrometry
was first commercially used in the petroleum and rubber industry of the U.S.A.
In the sixties mass spectrometry was introduced to organic chemistry as a
powerful spectroscopic method, especially for the determination of accurate
molecular masses and for the calculation of elemental compositions. It was also
used for the identification of unknown compounds, since the mass spectrometer
reproducible forms fragment ions which are typical for an organic molecule.
In the seventies, mass spectrometry
was directly coupled with chromatographic methods such as gas chromatography.
This coupling significantly improved the analysis of complex mixtures of
organic compounds because of its high specificity and sensitivity.
In the eighties GC-MS became the
most powerful method for the analysis of organic compounds such as drugs,
pesticides, pollutants and their metabolites in clinical, forensic and food
chemical determinations. It is also used in doping control as well as in
environmental and occupational toxicology.
The general operation of a mass spectrometer is:
The
Basics of Mass Spectrometer / Mass
Spectrometry
Mass spectrometers use the difference in mass-to-charge ratio
(m/z) of ionized atoms or molecules to separate them. Therefore, mass spectrometry allows
quantitation of atoms or molecules and provides structural information by the
identification of distinctive fragmentation patterns.
The general operation of a mass spectrometer is:
- Create gas-phase ions
- Separate the ions in space or time based on their mass- to-charge ratio
- Measure the quantity of ions of each mass-to-charge ratio
The sample is introduced through the
inlet into the ion source which is under vacuum. The
sample components are ionized in the ion source and the ions formed are accelerated
in the direction of the mass analyzer. Without
the vacuum, the molecular mean free path would be very short and ions would
collide with air molecules before they could reach the mass analyzer. The
resulting distribution of molecular fragments is characteristic of the molecule
and it is called a mass spectrum.
The mass spectrum
obtained consists of a unique bar graph in which the height of the bars
represents the relative abundance of the most abundant ions of each individual
compound as a function of the mass (Fig. 2). These ions give information
concerning the molecular weight and the most electronically stable ion fragments from the original
molecule. These unique fragments may be matched or interpreted to characterize
a molecule based on its atomic structure. A typical electron impact (EI) mass
spectrum of acetone is shown in Fig. 2. Ion fragments appear at the
mass-to-charge ratios (m/z) 43 and 15 (the m/z = 15 is not shown). These ion
fragments represent the parts of the molecule that broke off from the parent
molecule. Also the parent molecule of acetone appears at m/z = 58, a valuable
information about the molecular weight of the analyzed compound.
Fig. 2: Electron-impact
mass spectrum of acetone showing the molecular ion at m/z 58 and the base peak
at m/z 43.
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The ion sources
used for the ionization of samples are: chemical ionization (CI),
atmospheric pressure chemical ionization (APCI), electron impact
(EI), electrospray ionization (ESI), thermospray ionization (TI),
fast atom bombardment (FAB), matrix assisted laser desorption (MALDI).
The two most accepted ways to create
ions in mass spectrometers are by electron impact (EI) and chemical ionization
(CI). The largest amount of mass spectra have been obtained by electron impact.
Electron impact (EI) is the oldest
and most simple technique. The ionization source is heated and is under vacuum
so most samples will vaporize and then ionize. Ionization is usually
accomplished by impact of a high energetic (70 eV) electron beam. The electrons
are drawn out from a tungsten filament when a voltage of 70 eV is applied. The
voltage applied to the filament defines the energy of the electrons.
These high energy electrons strike
the neutral analyte molecule causing ionization – loss of an electron – and
fragmentation. This ionization technique produces almost exclusively positive
ions:
M +
e- →
M+ + 2e-
The EI technique is applicable to
all volatile compounds ( >103 Da) and gives reproducible mass spectra with fragmentation
to provide structural information.
The Chemical Ionization (CI) is the
other major technique for ionizing samples. In CI, a reagent gas like methane
is admitted to the ionization chamber where it is ionized, producing a cation
that undergoes further reactions to produce secondary ions. For example:
CH4 + e- → CH4+ +
2e-
CH4 + CH4+ → CH5+ + CH3
The secondary ion produced CH5+
in this case serves as a reagent to ionize the sample gently. Usually this
process results in less fragmentation and more simple spectra. The major peaks
that normally result are M+1, M, M-1, M+29 where M is the mass of the analyte
under examination.
The CI technique gives mainly
molecular weight information.
The mass analyzers used:
quadrupoles, magnetic sectors, time-of –flight (TOF), Fourier transform and
quadrupole ion traps.
The detectors used are the electron
multiplier or Faraday cup. These devices have high gain amplification
characteristics in the range of 107. The high gain capabilities of
the electron multiplier are what allow the detection of very low femtoampere
ion currents.
REFERENCES
- D. Harvey, “Modern Analytical Chemistry”, McGraw-Hill Companies Inc., 2000
- “Gas Chromatography”, J. Willett, John Wiley &Sons, 1987
- K. Pfleger et al. “Mass Spectral and GC Data of Drugs, Poisons, Pesticides, Pollutants and their Metabolites”, 2nd Edition, VCH, 1992
-
H.M. McNair, J.M. Miller, “Basic Gas Chromatography”, John Wiley &Sons, 1997