Basic Gas-diffusion Separation Systems

The flow-rates of the donor and acceptor streams typically vary in the range 0.5-1.5 ml min and are usually identical for the two streams to achieve optimum transfer efficiencies (cf. Sec. 5.3.4). 

When the acceptor stream is stopped for a predetermined period while the sample in the donor stream is pumped continuously through the donor channel of the gas dif sion separator, a preconcentration of the gaseous analyte may be effected in the acceptor 

6 FI manifold with gas-diffusion separator nested in sample loop of the injection valve used for preconcentration of volatile species by time-bas sampling (sample loading sequence). AS, autosampler, T, heating thermostat (optional) CDS, gas-diffusion separator, V, injection valve Ri, reagent for generation of volatile species R2. acceptor reagent stream R3. derivatization reagent (optional) D, detector W, waste a, valve position in sample injection sequence. Crossed circles in valve represent blocked channels [20]. 

5 rather inconvenient. A time-based sampling gas-diffusion preconcentration FI system with a sandwich type separator nested in the sample loop is shown schematically in Fig 

A disadvantage of the above system is in the mode of sample changeover, which requires synchronization of the action of an autosampier with that of the injection valve. This not only complicates the instrumentation but also introduces air segments into the donor flow, bringing deleterious effects on the membrane due to pressure fluctuations in the separator channels. This last defect may be avoided by stopping the pump when the sample probe of the autosampler is lifted out of the liquid surface, or by including an additional valve in the system, which further complicates the system. 

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