Flame ionization detector mechanism

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35], At room temperature, only olefins are detected at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with 03 and O atoms. 

Detailed kinetic models almost never include all species that are known to be present in the reactor. As an example, it is well known to everyone who has used a gas chromatograph with a flame-ionization detector, that ions are present in hydrocarbon flames. However, mechanisms for methane flames do not, in general, include the reactions of ions. The fact is that implicitly reduced mechanisms are used more often than not in modelling work understanding how objectively reduced mechanisms can be generated is, therefore, of primary importance. 

Spikes can arise due to physical or chemical contamination of the column or detector. For flame ionization detectors, these contaminants can come from the air. However, the simplest possible reason for the presence of spikes is the mechanical vibration of the instrument during analysis. Typical appearance of spikes is shown in Figure 2.14. 

Later, it was possible to improve substantially the separation of deuterated olefins. Atkinson et al. [67], for example, separated all ethylene isomers differing by one unit of mass. It was shown that the accuracy of the gas chromatographic analysis of deuterium-labelled ethylene isotopes is the same as that in the mass spectral method. Silver nitrate solution in ethylene glycol (5M), saturated at room temperature, was used as the stationary phase [67]. This solution was added to Chromosorb P (45—60 mesh) in the ratio of 1 4 and this mixture was mechanically stirred for 4 h. The sorbent obtained was packed into nylon tube sections of length 15 m and diameter 3 mm. A flame-ionization detector was used. 

Mass detector. The liquid chromatographer s demand for a universal detector which overcomes some of the problems encountered with the RI detector, (such as poor sensitivity and temperature instability) led to the development about ten years ago of the mass detector described here. The transport detectors of the 1960s detected the solute by means of a flame ionization detector after removal of the solvent from the eluent stream. They were abandoned, owing to lack of sensitivity and mechanical problems associated with the moving belt or wire. The new mass detector is similar in principle, but here the eluent leaves the column and is pumped into a nebulizer, assisted by an air supply. The atomized liquid is passed into a heated evaporation column where all the solutes less volatile than the solvent are carried down the column as a cloud of fine particles. A light source and photomultiplier arranged at the bottom of the column, perpendicular to the flow, detect the cloud of particles. The output from the photomultiplier, which is proportional to the concentration, can be amplified and directed to a recorder or data system. 

Holm T (1999) Aspects of the mechanism of the flame ionization detector Journal of Chromatography A 842 221-228. 

The nitrogen-phosphorus detector (NPD) is also called the alkali flame ionization detector (AFID), or thermionic NPD if no flame is used. The flame NPD is similar to the FID, but with an additional unit, usually a rubidium silicate bead, which is heated by an electrical current (Figure 2.9). When a compound enters the detection compartment, the ion current for compounds containing N or P increases. The mechanism for N detection may briefly described by the following ... 

Workstations and robotic systems are very expensive, so inexpensive alternatives such as flow configurations have been developed for automated sample preparation. The earliest flow systems for sample preparation were used for GC determination (with flame ionization detector [FID] or electron capture detector [EGD] detection) of organic compounds, which requires no special extraction or derivatization, in environmental matrices [30-34]. Automated GC-MS systems for the determination of volatiles in water or air [35-38] are the most commonly reported. Detailed descriptions of these systems can be found elsewhere in this book. Few continuous flow systems (CFSs) for the automated pretreatment of biological fluids in combination with GC-MS have been developed to date. The intrinsically discrete nature of the GC-MS sample introduction mechanism makes online coupling to continuous flow systems theoretically incompatible for reasons such as the different types of fluids used (liquid and gas) and the fact that the chromatographic column affords volumes of only 1 to 2 j,l of cleaned-up extract. Therefore, the organic extracts from CFSs have traditionally been collected in glass vials and aliquots for manual transfer to the GC-MS instrument (off-line approach) only in a few cases is an appropriate interface used to link the CFS to the GC-MS instrument (on-line approach). These are the topics dealt with below. 

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