2.3.4 Detectors
While the focus of this book is utilizing mass spectrometry (Chapter 4) as the detector, it is informative to note that a variety of detection systems are available for GC. The most common and commercially available ones are listed in the table below with information on their detection limits and analytes of interest.
Table 2.2 Commercially Available GC Detectors.
| Detector | General Type |
Analytes it is used to measure | Typical Detection Limits |
|---|---|---|---|
| Flame Ionization Detector (View the FID Animation below) | Selective |
Any chemical that will burn in a H2/O2 flame | parts per million |
| Thermoconductivity Detector (View the Thermocon-ductivity Animation below) | Universal |
Any chemical with a thermal conductivity (~specific heat) different from He | parts per thousand or hundred |
| Electron Capture (View the ECD Animation below) | Selective |
Electrophores such as halogenated hydrocarbons | parts per billion or less |
| Flame Photometric | Specific |
P and S containing compounds | parts per million or less |
| Fourier Transform Infra-Red | Specific |
Chemicals with specific molecular vibrations | parts per thousand or hundred |
| Mass Spectrometry | Universal |
Any chemical species | parts per million or less |
Three of the most common GC detectors will be discussed in detail here, while types of mass spectrometers will be presented in Chapter 4. One of the earliest GC detectors was the thermal conductivity detector (TCD). The basis for this detector is that most analytes have a thermal conductivity lower then that of helium. Helium is used as the carrier gas and as it passes a wire with a current applied to it the wire heats up via electrical resistance. Helium molecules remove the maximum amount of heat and the wire reaches thermal equilibrium and a constant current reading. As an analyte with a different thermal conductivity enters the detector, the wire heats up with increasing electrical resistance and the measured current decreases. Since the analytes pass through the detector as a “chromatographic plug” a bell-shaped current reading results known as a chromatographic peak. After the analyte has passed through the detector, the current returns to the original baseline reading as the helium re-cools the wire. Usually two matched columns and detectors are used, where only He is passed through one setup and samples are injected into the other. While this detector responds to any chemical with a thermal conductivity different than helium (which includes almost every other compound), these detectors suffer from relatively poor detection limits (parts per thousand to parts per hundred). Animation 2.2 illustrates the operation of a TCD.
Animation 2.2 Illustration of a Thermal Conductivity Detector.
The most common detector in gas chromatography is the Flame Ionization Detector (FID). This detector is based on the fact that most chemicals will burn in an H2-air flame and current can be passed through the path of ions produced in the flame. Helium is again used as the carrier gas and analytes are injected in the standard split-splitless injector. As individual packets of analytes are separated in the column and enter the detector they burn in the flame. As illustrated below in Animation 2.3, a potential is placed across the flame jet and an electron collector plate is placed above the flame. As ions are produced, electrons are passed through the ion cloud, and a current is measured that is proportional to the mass of analytes produced in the flame. The FID is also considered a universal detector, although not every chemical will burn in an H2-air flame. FID are relatively sensitive with detection limits of 1 ppm for most chemicals.
Animation 2.3 Illustration of a Flame Ionization Detector.
The electron capture detector (ECD) is perhaps the most sensitive detector for a GC and was developed primarily to detect chlorinated hydrocarbons in the environment. It relies on the electrophilic nature of halogens contained in an organic chemical, but can be used to detect other electrophilic-contained elements such as oxygen. The detector is a sealed unit and contains a radioactive isotope of nickel, 63Ni. This isotope gives off a steady supply of beta particles that are essentially high speed electrons. These high-speed electrons collide with trace amounts of methane carrier gas that enters the column after the column effluent and produces slower speed electrons (thermal electrons). These thermal electrons are captured by the anode in the middle of the detector and provide a constant current in the absence of any electrophilic analytes. As electrophilic analytes enter the detector they attract the thermal electrons and carry them out of the detector. This removal process results in a lowering of the current measured by the detector and the change in current is measured as an inverse chromatographic peak. ECDs are extremely sensitive and yield detection limits of pg or sub-parts per billion concentrations in the injection solvent. The operation of an ECD is illustrated in Animation 2.4.
Animation 2.4 Illustration of an Electron Capture Detector.
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