Detector coupling alternating current

Unlike pre-column derivatlzation, the post-column mode is currently implemented with a series of configurations coupled on-line to the chromatographic column and connected directly to the continuous detector. The off-line alternative is of little practical use today. Hence, there is a significant reduction of human intervention in this context. 

If one alternating is followed by three static dipoles the detector signal is an alternating ion current of 25MHz. A Faraday-cup coupled to a high sensitive, high frequency receiver, e. g. a short wave receiver, will be sufficient to detect the signal. 

There are several types of detectors that can be coupled to a gas chromatograph. One example is the flame ionization detector (FID), where, as the name implies, the effluent flows through a flame as it leaves the column, which generates ions as one of the products. These ions are then detected via an electrical current monitor. When coupled with suitable pre-concentration of the analyte, GC-FID can approach compound detection sensitivities as low as a few pptv. However, the FID works best for compounds such as hydrocarbons and is therefore not a universal detector. Other types of well-known GC detectors include the electron capture detector (BCD) and the thermal conductivity detector (TCD), and as with FID these alternatives also have their strengths and weaknesses but we will not discuss these in this book. 

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