GC Instrument Component Design Detectors

There are around a dozen GC detectors in common use. Spectroscopic instruments can be interfaced to the effluent of a GC and act as a form of detector that has the compound identification power of a spectroscopic measurement. This mating of a separation instrument to a spectroscopic instrument is called a hyphenated technique. The acronyms for the two classes of instruments are separated by a hyphen (or sometimes a slash [/]), as in GC-MS. These will be discussed later in Section 12.8. Some of the general characteristics of GC detectors that need to be considered are the following  

Destructive versus nondestructive Some detectors destroy the analyte as part of the process of their operation (e.g., by burning it in a flame, fragmenting it in the vacuum of a mass spectrometer, or by reacting it with a reagent). Others leave it intact and in a state in which it may be passed on to another type of detector for additional characterization. 

Mass flow versus concentration response These two modes of detector response are described in Section 11.4. In general, destructive detectors are mass-flow detectors. If the flow of analyte in effluent gas stops, the detector quickly destroys whatever is in its cell, and the signal drops to zero. A nondestructive detector does not affect the analyte, and the concentration measurement can continue for as long as the analyte continues to reside in the detector cell, without decline in the signal. Some types of nondestructive detectors (e.g., the BCD) measure the capture of an added substance (e.g., electrons). The saturation of this process results in a loss of signal to analyte proportionality, and they are mass-flow detectors. 

Requirement for auxiliary gases Some detectors do not function well with the carrier gas composition or flow rates from a capillary column effluent. Makeup gas, sometimes the same as the carrier gas, may be required to increase flow rates through the detector to levels at which it responds better and/or to suppress detector dead volume degradation of resolution achieved on the column. Some detectors require a gas composition different from that used for the GC separation. Some detectors require both air and hydrogen supplied at different flow rates than the carrier to support an optimized flame for their operation. Makeup flow dilutes the effluent but does not change the detection mechanism from concentration to mass-flow detection. 

Sensitivity and linear dynamic range Detectors (both universal and of course selective ones) vary in their sensitivity to analytes. Sensitivity refers to the lowest concentration of a particular analyte that can be measured with a specified signal-to-noise ratio. The more sensitive the detector. 

Universality vi. selectivity If a detector responds with similar sensitivity to a very wide variety of analytes in the effluent it is said to be universal (or at least almost so—no GC detector is absolutely universal). Such detectors are valuable when one needs to be sure that no components in the separated 

---