Classification of FI Gas-liquid Separation Systems

FI gas-liquid separation systems may be classified according to the chemical processes involved. In most cases the analyte is transformed into a volatile species by means of a suitable acid-base chemical reaction before the separation such as for the release of carbon dioxide, sulphur dioxide, hydrogen cyanide, hydrogen sulfide. Sometimes the analyte is sufficiently volatile to be separated from the liquid phase under elevated temperatures without further reaction. Among these are ethanol, ozone, and chlorine dioxide. 

The separated gas phase containing the analyte may be transported, usually with the help of a suitable carrier gas. directl to the detector, such as in hydride generation AAS, or it may be absorbed by a liquid acceptor stream and then transported, with or without a derivatization reaction, to the detector, as usually performed in spectrophotometric systems. 

Introduction of a reagent into the acceptor stream not only enhances the selectivity but also the sensitivity by creating more favourable kinetic conditions for mass transfer. When the separated gaseous analyte species reacts with a reagent to form a different species on the acceptor side, the concentration of the gaseous analyte is maintained at a very low level at the separation interface, which favours the further release of the analyte. Thus, the transfer of carbon dioxide is enhanced by using a basic acceptor stream, and the transfer of ammonia is enhanced by an acidic acceptor. 

FI gas-liquid separation systems may also be classified according to the physical means of separation. Two main approaches are available for performing on-line gas-liquid separations in FIA systems, i.e.  

Separation by gas diffusion through a liquid impermeable micro-porous membrane. 

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