Page 29 - Tandem Techniques

Page 11

There are a number of different types of injection devices available and these will be discussed in some
detail later, in conjunction with the type of column with which they are used. The injector is connected
directly to the column which is situated in an oven, and its temperature can be either controlled
isothermally or programmed between chosen limits at selected rates. The carrier gas leaves the column
and passes directly to the detector which is situated in its own thermostatically controlled oven. This is
normally controlled isothermally at a temperature at least 20°C higher than the maximum temperature
the column oven will reach during the development of the separation. This prevents any solute
condensing in the detector or detector connecting tubes. If the chromatograph is connected to some
other instrument, and the detector is destructive (e.g. the FID), the carrier gas is split into two streams
between the column and the detector; one stream passes to the detector and the other to the associated
instrument. If the detector is non-destructive, (i.e. the katherometer), then all the carrier gas can pass
through the detector and part, or all, the exit gas fed to the second instrument. In some instances, the
sensor of the associated equipment can be used as the chromatographic detector, in which case the
whole column eluent can pass directly to the tandem instrument bypassing the GC detector. This
procedure assumes that the response of the sensor in the associated instrument is linear and that the
quantitative accuracy is not impaired.

It is not germane to the subject of this book to discuss detectors in detail, but some points will be made
that are important with respect to tandem systems. The most practical and useful detectors for combined
techniques employing gas chromatography as the separation process are the two previously mentioned,
the katherometer and the FID. The katherometer has the advantage of being non-destructive and thus,
all the eluent can pass to the second instrument, conserving the sample, and providing the maximum
material for measurement. On the other hand, the katherometer has limited sensitivity relative to the
FID, and thus requires more sample, which precludes its use with certain types of column. The
katherometer has also a limited linear dynamic range but, within that range, can be used for accurate
quantitative work. The destructive nature of the FID demands the use of a split system and thus the
second