Classical Detectors for GC-MS Systems

Classical detectors are important for the consideration of GC-MS coupling if an additional specific means of detection is to be introduced parallel to MS. The classical detectors can be grouped by the response characteristics into the concentration dependant detectors (analyte concentration in the carrier gas) such as the thermal conductivity detector (TCD), and the mass-flow-dependent detectors such as FID, NPD as is the MS (Hill, 1992). A change of make-up gases do not affect the signal in mass flow dependant detectors. 

A parallel detection to MS with a thermal conductivity detector is not found often in practice as the mass spectrometric analysis of gases is mostly carried out with specially configured MS systems (RGA, residual gas analyser, mass range 100 Da). Also, the parallel coupling of an FID does not lead to results which are complementary to those of MS, as both detection processes give practically identical total ion chromatograms. The response factors for most of the organic substances are comparable. A parallel MS detection to the FID can be used for a routine FID method development (see applications section). 

Applications can cover the rapid screening, for example, on the intensity of halo-genated compounds using an ECD with an intelligent decision for a subsequent MS analysis of the same positively screened sample for a mass selective quantitation. 

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