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In GC, which is short for Gas Chromatography, the sample (1-5 µl) is vaporized in an injector and carried by a gas stream through a column where the compounds are separated. They are then registered as separate peaks by means of a detector. Two types of columns are used: packed columns and capillar columns. For environmental analyses, mainly capillar columns are used today. The inner diameter is in most cases ca 0,1 -0,3 mm and the length is between 25 and 60 m.
FID (Flame Ionization Detector) is the most general detector for GC. Substances that are eluted from the column are burned in a hydrogen/air flame so that ions are formed, which results in a measurable electric current in the detector. C-H-bonds are ionized in the first place, which makes FID sensitive to organic compounds. The detector response is proportional to the number of carbon atoms. FID is sometimes called "carbon counter". The advantages of this detector are high sensitivity and a wide linear range. It is non-specific, which is a disadvantage if one wants to determine a few compounds in low concentrations in a complex sample matrix.
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An ECD (Electron Capture Detector) has high sensitivity and selectivity for halogenated compounds, e g chlorinated pesticides and PCB. Substances eluted from the column pass a radioactive foil, often Ni-63, which emits beta radiation. The carrier gas (e g nitrogen) is ionized and produces electrons which generate a background current. Electronegative compounds, such as halogens, capture the electrons which results in a measurable decrease in the current. ECD is also sensitive to organic peroxides and nitro compounds. A disadvantage is that the linear range is small.
In GC-MS, MS is short for mass spectrometry, denoting the detector used. Mass spectrometry here not only provides detection of the substance eluted from the column, but also gives much information on its molecular structure by means of the fragmentation pattern in the mass spectrum. Substances eluted from the column are ionized in an ion source. The two most common ionization techniques used in analyses for persistent organic pollutants are EI (electron impact ionization) och ECNI (electron capture negative ionization, generating negative ions). Ionized molecules and/or fragments are separated according to their m/z (mass/charge) ratio. In order to increase sensitivity and selectivity, a few specific high-intensity ions can be selected for measurement (GC-MS-SIM).
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