On-column preconcentration of phenolic compounds by sequential injection chromatography

Phenolic compounds are secondary metabolites of plants involved in defense against chemical and biological attacks. They are also highly important constituents of food, mainly fruits, vegetables, and beverages, such as tea and wine, because of the benefits to human health (e.g. antioxidant, antimicrobial, and anticancer properties). As phenolic compounds are generally found at low concentrations in food, analyte preconcentration is often needed for analytical determination. This work proposes the on-column preconcentration of phenolic compounds in SIC by exploiting analyte retention at the head of a C18 fused core packed column (Supelco Ascentis Express, 2.7 µm, 30 x 4.6 mm i.d.). The on-column preconcentrantion involved aspiration of water as carrier, followed by the sample aliquot to a holding coil. The sample zone was then directed to the chromatographic column and, because of the low eluent strength, the analytes were effectively accumulated at the head of the column. Then, the mobile phase (acetonitrile and water, 35:65 v/v, with pH adjusted to 2.5) eluted the analytes for spectrophotometric detection at 254 nm. The approach was successful, without peak broadening or significant changes in retention times with the increase of the sample volume (Figure 1). Figure 2 shows the linear relation between the peak height and the sample volume, thus increasing the detectability of the analytes. For example, a preconcentration factor of 13 was estimated for kaempferol with 750 µL of sample. The proposed procedure can be extended for preconcentration of different analytes, after optimization of both stationary and mobile phases.