Sequential Solid-Phase Microextraction with Biocompatible Coating Materials to Address Displacement Effects in the Quantitative Analysis

Solid-phase microextraction (SPME) is a simple and effective sample-preparation technique for the analysis of complex samples. However, sample matrices containing high concentrations of nonpolar substances or spiked analytes in free form can cause swelling, saturation, and/or competition phenomena in the coating material. This results in a displacement effect wherein polar analytes with low affinities for the solid coating material are displaced by nonpolar substances in the matrix or spiked analytes with a high affinity. Therefore, the quantitative analysis of polar analytes can be challenging, as the displacement effect causes non-linearity in the calibration curves. This paper presents a comprehensive investigation of the conditions under which the displacement effect occurs and how it influences the quantitative analysis of polar analytes. To remedy this issue, a sequential SPME strategy using two SPME blades with different selectivities is applied. SPME blades offer a large surface area and coating volume─and thus, greater extraction capacity─which may mitigate the displacement effect. In addition, the biocompatible coatings on the SPME blades are comprised of small amounts of sorbent particles embedded by a polyacrylonitrile (PAN) binder, which allows them to be directly immersed into complex matrixes such as biological and food samples, as the PAN acts as a barrier that prevents the adsorption of large macromolecules (e.g., cells and proteins). As such, a C18/PAN-coated blade was applied for the first extraction step, which significantly decreased the concentrations of nonpolar compounds in the sample. In the second step, a hydrophilic-lipophilic balanced (HLB)/PAN-coated blade was employed to extract the polar analytes and any remaining nonpolar analytes. The proposed sequential SPME strategy successfully enabled the quantitative determination of polar and nonpolar drugs of abuse with log P values ranging from 0.16 to 4.98 in biological matrices while also providing good linearities.