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 demonstrated. 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 solid-phase extraction cartridge. Extensive work was carried out to measure the retention factors of some polar and apolar analytes with analytical columns packed with the same PGC sorbent. Some analytes can be strongly retained in methanol, acetonitrile, tetrahydrofuran or dichloromethane, explaining why desorption from a cartridge used for solid-phase extraction with several millilitres of organic solvent is not as effective as from a cartridge packed with octadecylsilica (C-18). It is shown that the desorption volume required can be predicted from the data generated by liquid chromatographic measurements of retention factors 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 have a 3-5-ml desorption volume. Then methanol can be used. Applications to the trace level analysis of polar pesticides in environmental waters are presented.