Flame ionization detection oxidized compounds

Flame ionization detectors are capable of detecting virtually all organic compounds and show a lower limit of detection of approximately 1 X 10-9 mol. They also show good linearity of response and the fact that they do not respond to oxides of carbon or nitrogen or to water makes them particularly convenient for aqueous samples. They have the disadvantage, however, that samples are destroyed unless a stream-splitting device is incorporated. 

Detection. Nearly all of the vapor-phase organic compounds will respond when added to a flame ionization detector, Consequently, this detector is most commonly used. Other special-purpose detectors include photoionization, mass spectrometry, atomic emission, ion mobility, mercury oxide reduction, and chemiluminescence detectors. 

Reversed-phase HPLC has been used to analyze the oxidation products of triacylglycerols in edible oils. The detection is often based on monitoring the conjugated dienes with an ultraviolet detector (234-235 nm). However, the UV detector provides no information about oxidation products without a conjugated diene structure, e.g., products of oleic acid. Information about these compounds is important when oils with a high oleic acid content are studied. The most common universal detector types—refractive index and flame ionization detectors—are not sensitive enough to detect small amounts of oxidation products. 

More recently, chemiluminescence detectors based on redox reactions have made possible the detection of many classes of compounds not detected by flame ionization. In the redox chemiluminescence detector (RCD), the effluent from the column is mixed with nitrogen dioxide and passed across a catalyst containing elemental gold at 200-400°C. Responsive compounds reduce the nitrogen dioxide to nitric oxide. The nitric oxide is reacted with ozone to give the chemiluminescent emission. The RCD yields a response from compounds capable of undergoing dehydrogenation or oxidation and produces sensitive emissions from alcohols, aldehydes, ketones, acids, amines, olifins, aromatic compounds, sulfides, and thiols. 

Karlsson et al. reported the supercritical fluid chromatography of methaqualone, cotinine, and reclopride, among other compounds, using capillary columns of different polarities [19]. Detection was either thermionic nitrogen-phosphorus or flame ionization. Supercritical nitrous oxide was used as the mobile phase. The detection limits obtained were in the range of 2-4 ppm and the precision was in the range of 3-12%. 

For FID detection, hydrogen is mixed with the carrier gas, and the column effluent stream is burned in air. The FID measures the ionization of the sample stream, which is proportional to the mass of the sample in the gas. Flame ionization detector has high sensitivity (MDL is lO g/sec), but it has two main disadvantages FID cannot detect compounds that do not produce ions in a hydrogen-oxygen flame (e.g., H2O, CO2, CO, CS2, N2, H2S, formic acid, and nitrogen oxides). Further, FID destroys the sample material in the detection process. Therefore, if the gas effluent is to be examined further, an effluent splitter is required prior to the FID detector. 

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