Factors affecting the selectivity and efficiency of solid-phase extraction

Abstract

The modified surface of solid phase extraction sorbents is studied with respect to the impact on the isolation and purification of analytes. Interactions at the interface are characterized by quantifying recoveries of a broad range of analytes, on a variety of surfaces, and under various extraction conditions. Bonded phases of varying hydrocarbon chain length are studied. A hydrophobic surface (e.g., C18) favors the retention of small polar compounds, while a more polar surface (C2) favors the elution of larger hydrophobic compounds. A compromise phase (C8) improves overall recoveries, while analyte recoveries were optimized by extraction onto stacked and layered phases. Analytes are retained by different mechanisms and under different solvent conditions. Selective elution of analytes is achieved by judiciously choosing the elution solvent. Data obtained from comparing the time requirements for drying various phases are consistent with previously developed models of the bonded silica surface. The impact of the presence of water on the elution of analytes is also studied. Experiments are presented where increasing concentrations of organic solvent are added to the sample matrix. Recoveries for polar compounds dropped as the matrix became more energetically favorable. Recoveries improved for hydrophobic species as the formation of agglomerations was disrupted. The impact of sample loading rates on analyte recoveries is studied. No significant differences in recoveries of a broad range of non-ionizable analytes are observed for loading rates ranging from 8 to 30 mL per minute on a 13 mm diameter x 15 mm height sorbent bed. The impact of the porous nature of the extraction sorbent on analyte recoveries, under different conditions of temperature and solvent contact time, is studied. A dependence on the diffusion of analytes into and out of the pores is observed. Experiments are devised to characterize the role of particulates in the sample matrix during solid phase extraction. Parameters studied include size of particles in the matrix, in the sorbent bed, porosity of the frit retaining the sorbent, and utility of a depth filter. Samples laden with particulates are spiked with trace analytes and show no reduction in recoveries resulting from the presence of particulate matter.