When selecting an appropriate SPE sorbent to perform an extraction, consideration is given first to the sample matrix, and then the analyte to be extracted. Reversed phase extractions require the sample matrix to be predominantly aqueous in nature (reversed phase extractions are commonly applied to urine, serum/plasma, whole blood, drinking water, waste water etc), normal phase extractions are generally used when a non-polar solvent has been used to extract non-polar analytes from a solid matrix. Ion exchange will typically be applied when analytes have a positive or negative charge.
Mixed mode sorbents incorporate both reversed phase, and ion exchange characteristics. They therefore allow for the simultaneous extraction of both neutral species (through reversed phase extraction) and either cationic (basic) or anionic (acidic) analytes. This coextraction provides the user with the possibility to extract a wider range of analytes using one protocol.
The most common use of such cartridges is the analysis of drugs of abuse from urine. Utilising a cartridge containing both reversed phase and cation exchange retention characteristics allows for THC metabolites to be extracted at the same time as basic drugs such as opiates and amphetamines.
It is important to note that, when selecting washing conditions they are selected based on the most polar analyte which is being retained via reversed phase. The pH should be maintained during washing such that the analytes retained via ion exchange continue to be charged.
When it comes to elution, you can selectively elute the neutral analytes first by eluting with 100% organic solvent, methanol or acetonitrile would be typically be used for this purpose. Ionic analytes can then be eluted in a second step using an appropriate pH modifier with organic solvent. It is also possible to elute in one step by going directly to organic solvent with pH modifier if so desired.
A good example of this approach can be seen by viewing this technical note on Strata-X Drug B.
