As the analyte leaves the GC column, it is passed over this emitter, which typically consists of nickle or tritium. The electrons from the emitter ionize the nitrogen carrier gas and cause it to release a burst of electrons. In the absence of organic compounds, a constant standing current is maintained between two electrodes. With the addition of organic compounds with electronegative functional groups, the current decreases significantly as the functional groups capture the electrons.
The advantages of ECDs are the high selectivity and sensitivity towards certain organic species with electronegative functional groups. However, the detector has a limited signal range and is potentially dangerous owing to its radioactivity. In addition, the signal-to-noise ratio is limited by radioactive decay and the presence of O2 within the detector.
Atomic Emission Detectors. Atomic emission detectors AED are element-selective detectors that utilize plasma, which is a partially ionized gas, to atomize all of the elements of a sample and excite their characteristic atomic emission spectra.
AED is an extremely powerful alternative that has a wider applicability due to its based on the detection of atomic emissions. MIP is the most commonly employed form and is used with a position-able diode array to simultaneously monitor the atomic emission spectra of several elements.
The components of the Atomic emission detectors include 1 an interface for the incoming capillary GC column to induce plasma chamber,2 a microwave chamber, 3 a cooling system, 4 a diffraction grating that associated optics, and 5 a position adjustable photodiode array interfaced to a computer. Atomic emission detector. GC Chemiluminescence Detectors. Chemiluminescence spectroscopy CS is a process in which both qualitative and quantitative properties can be be determined using the optical emission from excited chemical species.
It is very similar to AES, but the difference is that it utilizes the light emitted from the energized molecules rather than just excited molecules. Moreover, chemiluminescence can occur in either the solution or gas phase whereas AES is designed for gaseous phases. The light source for chemiluminescence comes from the reactions of the chemicals such that it produces light energy as a product. This light band is used instead of a separate source of light such as a light beam.
CS also has its limitations and the major limitation to the detection limits of CS concerns with the use of a photomultiplier tube PMT. A PMT requires a dark current in it to detect the light emitted from the analyte. Chromatography detectors biochemistry chemistry GasChromatography ClassificationofDetectors MassSpectrometry electronmultiplier Flameionizationdetectors Thermalconductivitydetectors Electroncapturedetectors Atomicemissiondetectors Chemiluminescencespectroscopy UZ UniversityofZimbabwe.
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The He plasma energy emitted is extremely high and capable of ionizing all compounds other than He, which is used to create the plasma, and Ne, which has extremely high ionization energy.
Principle of Ionization. Compounds eluted from the column are ionized by light energy from the plasma. The light energy from the He plasma is The ECD is a selective, high-sensitivity detector for electrophilic compounds. The ECD is capable of detecting organic halogen compounds, organic metal compounds, diketone compounds, etc. Schematic Diagram of the ECD.
The principle of detection used by the ECD is as follows. The ECD detects ions by reading the change in voltage value that maintains a constant ion current gathered at the collector.
PCB- is much larger and heavier than e- and so takes more time to reach the collector. The FTD is a selective, high-sensitivity detector for organic nitrogen compounds and inorganic and organic phosphorus compounds. The FTD does not react to inorganic nitrogen compounds. Schematic Diagram of the FTD. The principle of detection used by the FTD is as follows. The FTD detects ions by reading the change in ion current gathered at the collector. When a current is passed through the platinum coil with an alkali source attached to the coil rubidium salt , the coil increases in temperature, which creates plasma around the alkali source.
The FPD is a selective, high-sensitivity detector for phosphorus P compounds, sulfur S compounds, and organic tin Sn compounds. The FPD is highly selective as it detects element-specific light emitted within a hydrogen flame. Schematic Diagram of the FPD. The principle of detection used by the FPD is as follows. Sulfur compounds, phosphorus compounds, and organic tin compounds each emit light at unique wavelengths when burned.
By passing light through a filter, only light of these unique wavelengths reaches a photomultiplier tube. The photomultiplier tube then converts the detected light intensity into an electrical signal. The SCD is a selective, high-sensitivity detector for sulfur S compounds. The SCD is highly sensitive and capable of detecting infinitesimal amounts of sulfur compounds.
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Typical uses of GC include testing the purity of a particular substance, or separating the different components of a mixture.
In some situations, GC may help in identifying a compound. In preparative chromatography, GC can be used to prepare pure compounds from a mixture.
Instrumentation of GC 3 4. Detectors Devices used to or designed to respond to a very small quantity of sample present in the column gas 4 5. Minute amount of solute can be detected Gives linear response As it responds to the number of C- atoms entering the detector per unit time, it is mass sensitive rather than concentration sensitive Insensitive towards water, CO2, SO2, NO2 More complicated and more expensive Destructive of the sample Functionals groups like carbonyl, alcohol, halogen, amine are not detected 10 Advantages Disadvantages
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