Page 417 - Tandem Techniques

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volatility is enhanced by raising the temperature, the solutes start to decompose, and the spectra
obtained are mixed and generally meaningless. Surface ionization is one solution to the problem, where
the material, dispersed on a metal surface, is bombarded with high-energy particles. In secondary ion
mass spectrometry (SIMS), Ar , 02 and Cs ions, produced by a special ion-gun, can be used to
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bombard the sample providing the sample is dispersed on a conducting sheet, so that the charge leaks
away, and does not affect the subsequent ion focusing. Fast atom bombardment (FAB) is a similar
process, in which the colliding atoms are uncharged, but their kinetic energy is sufficient to produce
sample desorption. Plasma desorption ionization, promoted by either a radioactive substance such as
252 Cf or an inductively coupled plasma can also be effective for ionizing substances on a surface. Laser
desorption ionization (LDI) is another method of producing ions of substances of high molecular weight
and this can be assisted (Matrix Assisted Laser Desorption Ionization, MALDI) by dispersing the
material in substances such as glycerol which help the desorption process. Field desorption ionization
involves the ejection of ions by a very strong electric field from a sample deposited on a surface
containing points having a very small radius of curvature. The first LC/MS interfaces involved either a
wire or belt transport system, that were cumbersome and difficult to operate, but could provide both
electron impact spectra and chemical ionization spectra of substances having m/z values of several
hundred. Other transport interfaces were developed with special nebulizing jets, and one device had the
propensity of providing both laser desorption and secondary ion mass spectra from LC column eluents.
In fact, the most important types of contemporary LC/MS interfaces are the direct inlet systems. There
are a number of different types of direct inlet system. There is the thermal interface where the column
eluent passes through a heated jet into the ion source. Another direct inlet system, probably the most
popular today, is the electrospray where a strong electric field acts on the surface of a sample solution
as it is sprayed into a dry gas such as nitrogen. This process produces a cloud of charged droplets that
rapidly evaporate and, as a consequence, shrink and become smaller in diameter. The accompanying
increase in charge density that results from the decrease in volume, surface area, and radius of curvature
of the droplets causes very strong