Towards forensic application of solid-phase microextraction: Development and validation of an SPME approach to study pharmaceuticals and illicit drugs

Forensic toxicology is a discipline in toxicology that aids in the legal investigation of death, poisoning or drug use. A small, yet complex part of forensic toxicology is postmortem toxicology, which aims to interpret toxicological findings after death. An important factor complicating this interpretation is postmortem drug redistribution. As active processes that drive drug concentration gradients stop postmortem, passive diffusion will cause artefactual changes in blood concentrations. Many compounds show some level of postmortem redistribution, but especially basic drugs show high levels of postmortem redistribution, due to a postmortem decrease in body pH.
To be able to study kinetic processes, such as postmortem drug redistribution, it is essential to sample drug concentrations at several time points. Limitations in forensic studies include the amount of available sample and sampling can be constrained from an ethical or legal point of view. The most important limitation, however, is that by removing blood or tissue, the existing equilibrium is disturbed, thereby changing diffusion processes and making further sampling not representative of the true process of postmortem redistribution. To overcome this problem, specific sampling techniques can be applied that leave the sample undisturbed. Solid-phase microextraction (SPME) represents such a sampling technique. With SPME, an equilibrium extraction is obtained, making repeated sampling possible in certain conditions such as negligible depletion. Another important feature of SPME is that only the free concentration partitions towards the fiber.
In this thesis, a prototype SPME fiber is described, which utilizes two mechanisms of extraction. The fiber is coated with C18 chains and propylsulfonic acid, which act as strong cation exchange groups. This dual extraction mechanism gives the fiber its name: mixed-mode fiber or C18/SCX fiber. The C18/SCX fiber is especially efficient at sorbing relatively hydrophobic, cationic compounds (such as basic drugs).
The experimental work in this thesis shows the benefit of the C18/SCX fiber for the sorption of cationic compounds. This fiber can be used to measure freely dissolved concentration and with that understand distribution processes for cationic compounds. Unlike the partitioning of neutral compounds, the distribution of cationic compounds is still a relatively little studied topic. Their distribution in vivo seems to be governed by different proteins and lipids compared to neutral compounds. Using partitioning data on different binding matrices, a simple model can be constructed to predict distribution in vivo. Improved knowledge on this distribution will aid in assessing the bioaccumulation potential of cationic compounds. SPME can be useful to measure freely dissolved concentrations, and with that calculate sorption and assess distribution processes. Furthermore, the C18/SCX fiber is used to study protein binding and uptake from tissue. The C18/SCX fiber was shown to perform well in extracting basic drugs from semi-solid tissues, which is an essential step towards the application of SPME-based methods in forensic toxicology. This proves the benefit of SPME in aqueous and semi-solid complex samples, as the method is simple and it provided good quantitative results.