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structure of a completely unknown substance, although this might be possible in certain cases, if a high
resolution mass spectrometer is employed. An IR spectrum simultaneously produced by the GC/MS/IR
triplet system would help significantly in the structure elucidation. The GC/MS system, however, can
confirm the identity of a given solute with a high degree of confidence and for this purpose the GC/MS
tandem system is most commonly used. Some supplier of mass spectrometers that have been employed
with tandem instruments are given in Table 5.1.

Synopsis

The GC/MS combined instrument was introduced soon after the invention of GC itself, and initially, the
mass spectroscopists looked upon GC as a novel sampling system for MS. In the early days, packed
columns were used which required high flow rates, and thus sample concentrators were developed. The
two most popular were the Ryharge concentrator, that operated on the principle of differential gas
diffusion between concentric jets, and the Bieman concentrator, that operated on the principle of
differential gas diffusion through a porous membrane. There are two popular types of ionizing
processes used in GC/MS systems, electron impact ionization, and chemical ionization. Electrons,
having energies between 5 and 100 V, are usually employed in electron impact ionization, and the ions
formed are magnetically and/or electrostatically impelled into the analyzer region of the mass
spectrometer. The accelerating voltage is adjusted to provide the optimum energy that would give the
desired ion fragment pattern. Chemical ionization is achieved by producing ions from a reagent gas,
such as methane by electron impact, and allowing the reagent ions to collide with sample atoms,
producing sample ions. The ionization is gentle, (ca 5 eV), so very few fragment ions are produced, and
those that are produced have m/z values close to that of the parent ion. Different reagent gases give
different ions, and can provide different structural information. Most chemical ionization sources have a
number of reagent gases available, and many ion sources can perform the dual role of electron impact
and chemical ionization. The inductively coupled plasma ionization source, produces ions in an argon
plasma maintained by inductively coupled rf energy, and about 50% all the elements present in the
sample produce ions in the plasma region. The sample atoms account