There are many detectors available for GC (see Table 1). These detectors can be divided into two categories: general detectors and selective detectors. The thermal conductivity detector (TCD) is a general detector that can be used for both organic and inorganic compounds. It produces a signal by measuring the thermal conductivity of the carrier gas/analyte mixture leaving the GC column. Hydrogen and helium are the two carrier gases that work best with a TCD, because they have the greatest difference in thermal conductivities from most analytes. The main advantage of a TCD is its ability to respond to any compound, provided that this substance is different from the carrier gas and is present in a sufficient quantity for monitoring. A disadvantage of a TCD is it will respond to impurities in the carrier gas, to stationary phase bleeding from the column, or to air leaking into the GC system. The TCD is also highly sensitive to changes in flow-rate, such as might occur during temperature programming or flow programming. Another disadvantage of a TCD is it has a relatively poor lower limit of detection compared with other common GC detectors.
Figure 2: Gas Chromatography-Mass Spectrometer GC/MS System from Agilent.
A big advantage of GC/MS is it can be used to either universally or selectively detect compounds as they leave the column. When used as a universal detector, the total number of measured ions is used to plot the mass chromatogram. This method involves using the mass spectrometer to quickly scan through a wide range of mass-to-charge ratios while collecting information on each ion that occurs within this range. This detection format is also known as the “full-scan mode” of GC/MS. This mode is useful when the goal is to look for a broad range of compounds in a single analysis (e.g., during drug screening) or when a mass spectrum is desired to determine the identity of an unknown compound (e.g., determining the drug taken by a patient suffering from an overdose). A more selective mode for performing GC/MS is selected ion monitoring (SIM). In this approach, only a few ions characteristic of the compounds of interest are examined. This mode is employed when low detection limits are desired and when it is known in advance what compounds are to be analyzed.