Alkali metal salt flame ionization detector

Alkali Metal Salt Flame Ionization Detector... 

A modiHcation of the alkali metal salt flame ionization detector is the thermionic detector. Instead of a flame, a low-temperature H2 plasma is used. The alkali metal salt is embedded in a ceramic matrix which is heated to about 600-800 °C. P or N containing radicals and ions... 

Modem thermionic ionization detectors evolved out of earlier studies of alkali-metal-doped flame ionization detectors [253]. Adding an alkali metal salt to a flame enhanced the response of the detector to compounds containing certain elements, such as N, P, S, B, as well as some metals (e.g. Sb, As, Sn, Pb). In its early versions, however, the detector response was unreliable and critically dependent on experimental parameters. Recent studies involving the continuous introduction of alkali metal salt solutions or aerosols into the flame demonstrated more reliable performance but have not been taken up [268,269]. 

The thermionic or alkali flame ionization detector (FID) is a variant of the FID that has enhanced sensitivity to nitrogen (N) and phosphorous (P) containing compounds. It is essentially a hydrogen flame around a pellet of an alkali metal salt with an electric potential across the flame. Organic compounds in the air are drawn into the flame and ionized. The alkali metal enhances the response to N and P-containing compounds and, since many chemical agents contain N and P, it is selective for their detection. This detector is rarely used without prior separation by GC. 

The alkali flame-ionization detector, sometimes called an NP or nitrogen-phosphorus detector, contains a thermionic source, such as an alkali-metal salt or a glass element containing rubidium or other metal, that results in the efficient ionization of organic nitrogen and phosphorus compounds. It is a selective detector that shows little response to hydrocarbons. 

The thermionic specific detector (TSD), also called the alkali flame ionization detector, is really a FID with a bead of an alkali metal salt such as Rb or Cs. A schematic is shown in Fig. 4.28. The TSD shows... 

The popularity of the BCD can be attributed to the high sensitivity to organohalogen compounds, which include many compounds of environmental interest, including polychlorinated biphenyls and pesticides. It is the least selective of the so-called selective detectors but has the highest sensitivity of any contemporary detector. The NPD or thermionic ionization or emission detector is a modified FID in which a constant supply of an alkali metal salt, such as rubidium chloride, is introduced into the flame. It is a detector of choice for analysis of organophosphorus pesticides and pharmaceuticals. The FPD detects specific luminescent emission originating from various excited state species produced in a flame by sulfur- and phosphorus-containing compounds. 

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