Sampling strategies, equilibrium phase

STRATEGY First, locate the initial and final conditions on the phase diagram. The region in which each of these points lie shows the stable phase of the sample under those conditions. If a point lies on one of the curves, then both phases are present in mutual equilibrium. 

Compounds with similar clogZ) values can be plated on the same plate for the convenience of adding similar amounts of liquid (octanol or buffer) simultaneously in 96-wells. After equilibrium, compounds in the octanol and the aqueous buffer can be transferred to new 96-well injection plates, which are placed in the autosampler for direct injection into the LC/MS. The results are analyzed, and the experimentally determined logZ) values (elogD) are stored in the database. If necessary, samples taken from one or two phases may be diluted before being placed inside the autosampler vials. The specific dilution factors can be selected following strategies described previously and the 96-needle Apricot pipettor can be used to add octanol and buffer instead of a manual pipette. 

The strategy for the selection of the CSP—eluent combination for enantioseparations using SMB technology has been described in numerous publications [11,47—50]. The basic requirements for the mobile phase include selectivity, safety, recycling ability, and cost. One property, which is less important in analytical-scale operations but of crucial importance in SMB, is the solubility of the sample in the mobile phase. The last requirement means that it is desirable for the CSP to be stable and exhibit sufficient (enantio)selectivity in the solvent, where the maximum amount of a sample can be dissolved. Together with the productivity of the process, this is required for running the system under equilibrium conditions. It is almost impossible to automate an SMB separation before it reaches steady-state conditions. 

Also in solid-phase microextraction (SPME) analytes are typically not extfacted quantitatively from the matrix. However, when partition equilibrium is reached, the extracted amount of an analyte is proportional to its initial concentration in the sample matrix phase. As indicated by Ai [33], application of SPME for quantitative analysis is feasible also when the partition equilibrium is not attained. Pawliszyn [34] has reviewed the quantitative aspects of SPME. Provided proper calibration strategies are followed, SPME can yield quantitative data and excellent precision, reproducibility and linearity (detection limits of 15 ng/L). In terms of precision, linearity and sensitivity SPME equals HS techniques. 

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