Large, non-polar analytes such as some chlorinated pesticides, PAHs and PCBs can often be retained from aqueous samples using ISOLUTE® C2 or C8 (‘weakly retentive’) sorbents. Smaller, more polar analytes such as acid herbicides often require much more retentive sorbents such as ISOLUTE C18 or ENV+ for efficient extraction from aqueous samples. But, if the more retentive sorbent is used to extract the large, non-polar analytes, they will be retained very strongly, and may be difficult to elute (requiring large elution volumes).
Layers of sorbent with different retention and elution characteristics can be combined in a single SPE column to ensure efficient retention of analytes with a range of polarity/solubility. When a less retentive sorbent is layered above a more retentive sorbent, the large, non-polar analytes will be retained on the top layer, while analytes requiring a more retentive phase will be retained on the bottom layer.
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