Page 30 - Tandem Techniques

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instrument is partially deprived of sample. However, the FID is far more sensitive than the katherometer
and consequently, only needs a very small proportion of the eluent for monitoring. Furthermore, it has a
very wide linear dynamic range and thus, in general, the quantitative accuracy is not impaired.

The only two other detectors that are used to any significant extent with GC tandem systems are the
Nitrogen-Phosphorous Detector (NPD) a modified form of the FID and the electron capture detector.
Both these detectors are extremely sensitive and both are normally used with splitters, the former
because it is destructive, the latter because its volume is relatively large and the resulting dispersion
would impair the performance of the associated instrument. This is an important aspect of interfaces
between the two instruments and will be discussed in considerable detail later. The NPD has a
reasonable linear dynamic range and provides accurate quantitative analyses.

The electron capture detector, on the other hand, has a limited linear range and thus, the sample size
must be carefully selected to satisfy both the quantitative accuracy required and the needs of the
associated instrument. In some instances the associated instrument, e.g. a mass spectrometer, can be
used as the detector. In such cases the total column eluent can be made to bypass the GC detector and
enter the mass spectrometer directly. If this procedure is adopted, again care must be taken to ensure
that the quantitative accuracy of the analysis is not impaired due to an inadequate linear response of the
associated instrument.

The signal from the detector sensor is processed in a suitable manner by the detector amplifier and the
output is usually passed to the chromatograph computer. The computer acquires the data and processes
the results, and also controls the operation of the chromatograph and automatic sampler (if part of the
equipment). The chromatograph can also communicate with the computer of the associated instrument
to help synchronize measurements as the peaks are eluted. Interface design and construction will be
discussed in detail in those chapters dealing with the