Page 356 - Tandem Techniques

Page 340

emitter surface would make it rather impractical. However, the introduction of a different emitter
surface that could be part of a transport interface might render it more feasible.

Liquid Chromatography/Mass Spectrometry (LC/MS) Techniques

The liquid chromatograph/mass spectrometer combination can only be successful if the mobile phase is
eliminated, either before entering or from inside the mass spectrometer. This is necessary to ensure that
the production of ions is not adversely affected or that the spectrum is not swamped with fragment ions
from the solvent. The evaporation of the mobile phase generates a considerable volume of solvent vapor
and if allowed to enter the ion source must be rapidly pumped away. The problem can be reduced by
using a splitting system, in much the same way as the early GC/MS tandem systems operated, or by
employing the solvent vapor as a chemical ionization agent. In fact the first LC/MS system developed
was that by McLafferty [3], who used the direct sampling technique passing the eluent directly into the
mass spectrometer. A small portion of the column eluent was split from the main stream, and fed by
means of capillary tube, directly into the source of a high-resolution mass spectrometer. The solvent
was used as a chemical ionization agent and thus produced chemical ionization spectra of the eluted
solutes.

There are two solutions to the solvent problem in LC/MS tandem instruments. The first is to remove the
solvent exterior to the mass spectrometer while contained on a suitable transport medium. The dry
solutes on the surface of the transport medium are then moved into the ion source for ionization. The
transport concept is not used extensively as an interface for LC/MS in contemporary tandem
instruments, but has been found to be extremely useful as an interface for LC/FTIR, as already
discussed. The second solution to the elimination of the solvent is to utilize the direct inlet approach of
McLafferty, but with suitable interfaces that will cope with the solvent and provide a variety of
ionization techniques. The transport interfaces will first be discussed, largely because historically these
were the first to become commercially available. In addition, as alternative transport media were
identified, and evoked the development