across the gap and on into the mass spectrometer whereas the lighter helium atoms are deflected
outwards by the vacuum and are pumped away. Increased enrichment of the sample vapour can be
achieved by incorporating a second pair of jets into the device. The diameter and hence the volumetric
gas flow through a capillary column is much lower than that through a packed column which obviates
the need for a separator as the carrier gas can be readily pumped away by the vacuum system of the
spectrometer. Furthermore, the column can be inserted directly into the spectrometer reducing dead
volume (Figure 4.29(c)).
In many GC separations, a large solvent peak may precede those of sample components, and minor
components sometimes elute on the tails of major component peaks. It is advantageous to be able to
divert solvent peaks and sometimes other large peaks from the mass spectrometer to avoid swamping it.
This is easily accomplished by means of a solvent dumping valve positioned between the end of the
column and the separator; carefully timed operation of this valve enables selected peaks to be vented to
the atmosphere. As in other instances where GC is interfaced with another technique (e.g. GC-IR) any
transfer tubing used between the end of the column and the spectrometer must be heated to prevent
condensation of samples during the transfer stage. In the case of GC-MS, the dumping valve and
separator must be heated also, either with a separate heater or by enclosing them within the GC oven.
The total volume of the separator, dumping valve and transfer line (dead volume) must be kept to a
minimum so as not to degrade chromatographic resolution, and glass or glass-lined metal components
are preferable as hot metal surfaces can catalyse the decomposition of thermally sensitive compounds.
A stream-splitter may be used at the end of the column to allow the simultaneous detection of eluted
components by destructive GC detectors such as an FID. An alternative approach is to monitor the total
ion current (TIC) in the mass spectrometer which will vary in the same manner as the response of an
FID. The total ion current is the sum of the currents generated by all the fragment ions of a particular
compound and is proportional to the instantaneous concentration of that compound in the ionizing
chamber of the mass spectrometer. By monitoring the ion current for a selected mass fragment (m/z)
value characteristic of a particular compound or group of compounds, detection can be made very
selective and often specific. Selected ion monitoring (SIM) is more sensitive than TIC and is therefore
particularly useful in trace analysis.
As early GC peaks elute in a few seconds or less, rapid scanning of the mass range of interest is
necessary. Fast scanning also allows partially resolved GC peaks to be sampled several times, peak
slicing, to facilitate identification of the individual components (Figure 12.5) provided that the dead
volume of the interface is small compared to peak volumes. For the speedy interpretation of spectral
data from complex chromatograms a