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Einzel lens and steering plates, through an aperture, onto the belt forming a spot covering an area of
about 5 mm . The beam strikes the belt with an energy of about 3 keV. The laser light can be focused
2
on the same spot by a suitable set of mirrors and lenses. The transport medium, ultimately employed by
the authors, is a carbon steel belt that has a blackened surface, and consequently absorbed more energy
from the laser beam. Providing laser energies are kept reasonably low, there is no degradation of the
surface. Comparing the results obtained from the two methods of ionization, laser desorption appears to
have the advantage, in that it provides more reliable molecular weight information. Furthermore, by
varying the power of the laser, different degrees of fragmentation can be achieved and consequently
more structural information can be obtained. However, as it is difficult to control the laser energy
absorbed, the mass spectra are less reproducible. In contrast, continuous ionization by primary ion
bombardment provides more characteristic fragment peaks, without losing molecular weight
information and both the mass spectra and the chromatograms are less noisy and more reproducible.

Direct Inlet LC/MS Interfaces

Direct inlet interfaces allow a portion of the column eluent to be injected straight into ion source via a
suitable restriction without any prior concentration procedure. A diagram of a simple direct inlet
interface is shown in Figure 9.16.

Figure 9.16
The Direct Inlet LC/MS Interface
The restriction is usually composed of an orifice 2-5 µm in diameter through which the column element
is forced. A liquid jet is first formed