The Detector
Once a component of the analyte makes it all the way through the stationary phase it has to be detected. Remember that each component is a vapor mixed with the inert mobile phase. Thus, detectors in GC have to be able to detect changes in the composition of a vapor solution. These detectors are designed so that they do not respond to pure mobile phase and so they do not show any activity until the mobile phase contains one of the analyte components. GC detectors work by converting the signal they receive when analyte reaches them into a small electric current. This is amplified and then computerized so that it can be seen on a monitor and printed. Each component is displayed as a triangular peak. The retention time of each component is the time it takes for that component to traverse the injector and column, and is measured to the top of the peak. The area under the peak is proportional to the amount of that compo nent in the mixture so that GC can be used for quantitative analysis. Ideally, each substance in the mixture would take a different amount of time to traverse the column and reach the detector. A chromatogram is a plot of the response of the detector versus the retention time. Every time GC analyzes the same substance under the same conditions, its retention time should be about the same, as long as the conditions of the experiment are not changed. Thus, the retention time gives a tentative identification of the substance. It is important to understand that the retention time is not a characteristic that definitely identifies a substance. For example, an exhibit may show a peak at 2.4 min in a gas chromatogram. A sample of known heroin also has a retention time of 2.4 min using the same instrument under the same conditions. The fact that both the unknown and the known have the same retention time is indicative of their being the same substance, but it does not prove it. There are millions of substances in the world and several might have the same retention time. This is why chromatography is a separation technique and not an identification technique. A few of the more common types of GC detectors are described below. Flame Ionization Detector This detector produces a small flame from the reaction of hydrogen taken from a tank and oxygen from the air. This flame does not affect the mobile phase carrier gas; however, when a component of the analyte reaches the flame it loses an electron and becomes ionized. These ions create an electric current that is amplified and sent to a computer for display. The magnitude of the current is proportional to the amount of substance present. This is a nonselective detector. It responds pretty much to all organic compounds. The gas chromatogram in Figure 6.8 above was generated on an instrument using a flame ionization detector. Mass Spectrometer Detector In Chapter 5, MS was discussed as a stand-alone instrument for the identification of pure substances. One of the concepts mentioned there was combining a chromatograph such as a gas chromatograph with a mass spectrometer. As each substance elutes from the stationary phase of the GC, it is sent immediately to the ionization chamber of mass spectrometer. A mass spectrometer can detect and identify each analyte component as it elutes from the column. A mass spectrum of each substance can be generated very quickly.
The identification process can be very efficient if the gas chromatography/mass spectrometry (GC/MS) system contains a spectral library. This is a collection of up to thousands of mass spectra of known compounds. A reasonably powerful personal computer can take the mass spectrum of an unknown substance and use it to search the spectral library. This process may take less than 1 min for 50,000 compounds. The result of the search will usually be a list of about 10 compounds whose mass spectra most resemble the unknown and a number that indicates how closely each one matches. A very high match number indicates that there is a high likelihood that the known and unknown are the same substance. The MS detector is also a quantitative tool. A chromatogram can be produced that is developed by plotting the total ions produced in the mass spectrometer versus retention time. The area under the peaks in this total ion chromatogram are proportional to the amount of material in each component of the analyte. Other GC Detectors There are several other types of GC detectors that are not as widely used in forensic applications. They are listed here for completeness. The responses of all of these detectors, like those discussed above are displayed as peaks that can be used for quantitative analysis.
Nitrogen–phosphorous – This type of detector is similar to the flame ionization detector except that it can detect only substances that contain nitrogen or phosphorous. This type is widely used in biological, toxicological, and environmental applications and is very sensitive.
Thermal conductivity – This type of detector relies on the change in the ability of the mobile phase gas to conduct heat as it is mixed with an analyte. It is simple to engineer and use and is very versatile.
Electron capture – This is an extremely sensitive detector that is used on substances that have a halide such as chloride or bromide, or oxygen in the molecule.
