Solid-phase extraction for HPLC analysis of dietary folates

Phenyl and cyclohexyl bonded silicas used for the solid-phase extraction (SPE) of dietary folates were studied and compared in terms of their retention capacities and recoveries. The SPE procedure was optimised to achieve the maximum recovery. The stabilities of folates in elution buffers and during the SPE procedure were also studied. The use of ascorbic acid throughout the SPE procedure was found to be important for the stabilisation of tetrahydrofolate - the most labile folate compound, whereas the use of both ascorbic acid and 2-mercaptoethanol was necessary to stabilise tetrahydrofolate during the storage of the eluates in the autosampler until analysis. Two purification methods, using phenyl endcapped (PH EC) cartridges or the combination of strong anion-exchange (SAX) and PH EC cartridges, were applied to the determination of folates in different food extracts, and verified by correlation with the usual purification method that uses SAX cartridges. The combination of SAX and PH EC cartridges was found to give the cleanest extracts.