PREDICTION FROM LIQUID-CHROMATOGRAPHIC DATA OF OBLIGATORY BACKFLUSH DESORPTION FROM SOLID-PHASE EXTRACTION CARTRIDGES PACKED WITH POROUS GRAPHITIC CARBON

The potential of porous graphitic carbon sorbents (PGC) for extracting polar and water-soluble analytes from aqueous samples was demonstrate d. However, even though breakthrough of analytes does not occur during the loading of samples, the recoveries are sometimes much lower than 100%, owing to the difficult desorption of some analytes from the soli d-phase extraction cartridge. Extensive work was carried out to measur e the retention factors of some polar and apolar analytes with analyti cal columns packed with the same PGC sorbent. Some analytes can be str ongly retained in methanol, acetonitrile, tetrahydrofuran or dichlorom ethane, explaining why desorption from a cartridge used for solid-phas e extraction with several millilitres of organic solvent is not as eff ective as from a cartridge packed with octadecylsilica (C-18). It is s hown that the desorption volume required can be predicted from the dat a generated by liquid chromatographic measurements of retention factor s in solvents. When compounds with a wide range of polarity are to be extracted, the desorption volume can be large, and it is obligatory to desorb the analytes from the cartridge by backflushing in order to ha ve a 3-5-ml desorption volume. Then methanol can be used. Applications to the trace level analysis of polar pesticides in environmental wate rs are presented.