Dionex ASE 150, Руководство оператора

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ASE 150 Operator’s Manual
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Doc. 065207-02 9/08
the temperature also leads to faster diffusion rates. This means that analytes move
faster from the boundary layer near the surface of the matrix from which they are
extracted to the bulk solvent at higher temperatures. Higher temperatures also
mean lower solvent viscosities, meaning that the solvent can penetrate the pores
of the matrix more easily. Finally, increased temperature makes it easier to disrupt
solute-matrix interactions (dipole attractions, van der Waals forces, hydrogen
bonding, etc.) and remove analytes from the matrix. Taken all together, the net
effect is that doing extractions at elevated temperatures means that the extractions
happen much faster and use less solvent. However, temperature alone is not
enough because many of the organic solvents used in extractions boil at relatively
low temperatures. This is one limitation of techniques such as Soxhlet or
automated Soxhlet. The highest temperature at which extractions take place in
these techniques is the boiling point of the solvent.
If sufficient pressure is exerted on the solvent during the extractions, temperatures
above the boiling point can be used. This means that all of the advantages of
working at elevated temperature can be realized even with solvents with relatively
low boiling points. Operating at elevated pressures also helps the overall
extraction process to happen more quickly. Pumping solvent through a packed bed
is easier at elevated pressures. Pressurized solvent will be forced into the pores of
the sample matrix, coming in more intimate contact with analytes in those areas.
Hence, the combination of elevated temperatures and pressures allows extractions
to occur rapidly and completely. For a more detailed discussion of the theoretical
consideration of ASE, consult Richter et al.,
Anal. Chem. 1996 , 68 , 1033-1039.