High solid-phase extractors

Accelab offers a complete fully automated high-throughput synthesis laboratory (Fig. 7), which includes synthesizer, solid-phase extraction module, balance, vor-texer, liquid-liquid extractor, and a vacuum centrifuge. A robot arm is in charge of all vessel transfers. 

Dynamic PFE usually requires implementing a concentration step prior to the determinative step, and because the extracted analytes are dissolved in a liquid (usually aqueous) phase, SPE is a highly useful tool for avoiding the dilution effect. For this purpose, SPE cartridges and columns packed with appropriate sorbents and coupled online to the extractor outlet can be employed in the same way as commented on for static PEE. Miniaturized retention has also been developed by using solid-phase microextraction (SPME). 

Figure 4.23 Illustrates another alternative for the separation of both phases once the batch liquid-liquid extraction has been finished. Formerly conceived for solid-liquid extractions, this complex mechanical assembly [20] consists of two automatic burettes for addition of the two phases, the extractor —moveable in various fashions— and a vertically moving paddie stirrer. The extraction vessel rotates at a high speed, which promotes phase separation, as shown in the figure. The lighter phase creeps up the walls and passes to an upper receptacle —the separation is facilitated by adding more aqueous phase. Once separation Is complete, an aspiration probe withdraws the organic phase. Finally, a mechanical system turns the vessel over for cleaning.

The two-phase systems of interest here are SCF-liquid and SCF-solid. Lahiere et al. (1987) have studied the transfer of ethanol or propanol from an aqueous solution into supercritical CO2. The experimental extraction behavior in a sieve-tray extractor was compared with that predicted from a model used for common liquids. This model for subcrit-ical liquids uses overall mass-transfer coefficients obtained from the individual film coefficients by the conventional sum of the two resistances approach. Lahiere et al. (1987) observed reasonable correspondence between experimentally observed values and the performance predicted from conventional models. For SCF-solid systems with pure solids, Debenedetti and Reid (1986) have observed that the mass-transfer coefficient for the SCF phctse is very strongly influenced by natural convection. This happens because the SCF has very low viscosity and yet has a high density, leading to a much more important role of natural convection than in normal liquids. 

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