Novel materials as capillary coatings for in‐tube solid‐phase microextraction for bioanalysis

In‐tube solid‐phase microextraction with a capillary column as extraction device can be directly coupled with high‐performance liquid chromatography systems (HPLC). The in‐tube solid‐phase microextraction technique has been continuously developed since it was introduced in 1997. New couplings have also been evaluated on the basis of state‐of‐the‐art HPLC instruments. Different types of capillaries (wall‐coated open tubular, porous layer open tubular, sorbent‐packed, porous monolithic rods, or fiber‐packed) with selective stationary phases (monoliths, magnetic nanoparticles, conducting polymers, restricted access materials, ionic liquids, carbon, deep eutectic solvents, and hybrid materials) have been developed to boost in‐tube solid‐phase microextraction performance (sorption capacity and selectivity). This technique has been successfully applied to analyze biological samples (serum, plasma, whole blood, hair, urine, milk, skin, and saliva) for therapeutic drug monitoring, to study biomarkers, to detect illicit drugs, to conduct metabolomics studies, and to assess exposure to drugs. This review describes current advances in in‐tube solid‐phase microextraction extraction devices and their application in bioanalysis.