## 6 Other Microextraction Techniques  
An interesting stir-bar-supported \(\upmu\)SPE was proposed for the quantification of polychlorinated biphenyl congeners in serum samples followed by GC-MS analysis [83]. The proposed configuration involved the placement of a small stir bar inside the porous polypropylene membrane along with the sorbent material. The edges of the membrane were heat-sealed to secure the contents. The authors concluded that the best extraction results were obtained with C\({}_{18}\) material. Similarly to [81], sonication greatly affects the desorption process, and therefore a time span of 15 min was adequate for the complete desorption of the analytes. Various solvents with varying polarity indexes (toluene, n-hexane, methanol, and acetonitrile) were tested, and it was found that toluene (at a volume of 250 \(\upmu\)L) provided the best extraction recoveries for all compounds. Enrichment factors in the range of 11.7-18.4 were obtained, but the absolute recoveries varied between 59.0 and 86.7% as \(\upmu\)SPE is not an exhaustive technique like SPE. The inter-day (3.2-9.1%) and intra-day (3.1-7.2%) RSDs were within acceptable ranges. An analogous ultrasound-assisted \(\upmu\)SPE was proposed by the research group of [PERSON] for the simultaneous determination of methadone and tramadol in serum samples [84]. Various modern sorbents were tested, including carbon nanotubes, oxidized carbon nanotubes, and TiO\({}_{2}\) nanoparticles. Among these materials, oxidized carbon nanotubes provided the best performance due to the strong affinity between the drugs and certain adsorbents. Using a non-polar HP-5 MS capillary column, the analytes were chromatographed in less than 10 min and quantified under SIM mode. One milligram of sorbent was sufficient for the extraction of the drugs at a sample volume of 1200 \(\upmu\)L and a pH value of 9.0. As expected, the volume of the desorption solvent (\(N\),\(N\)-dimethylformamide) decreased, and the preconcentration factor increased; thus, the authors selected the value of 50 \(\upmu\)L for this purpose. The method also indicated good repeatability and linearity over a wide concentration range. Low LODs were achieved (0.5 and 0.8 ng/mL), while the relative recoveries were within the range of 96.4-105.0%. Two sorptive-based extraction protocols were proposed for the determination of synthetic opioids in oral fluid samples [85] and basic drugs in blood [86]. The first approach was based on the utilization of a fabric-phase sorptive extraction (FPSE) for the isolation of fentanyl analogs from saliva prior to GC-MS analysis. The combination of London dispersion forces, hydrogen bonding capability, and dipole-dipole interactions within the sol-gel CW 20M sorbent enabled strong intermolecular interactions between the FPSE membrane and the analytes. More information about the FPSE can be found elsewhere [87]. Cotton swabs were used for sample collection, followed by their extraction using a mixed mode CW 20 M/sol-gel PTHF/sol-gel PDMDPS fabric membrane with dimensions of 1 \(\times\) 1 cm. Each material was characterized by FT-IR. The target compounds exhibited intermediate-to-low polarity, and ethyl acetate was better suited for releasing the compounds retained in the polar CW20M coating into a less polar solvent. NemrodW\({}^{\copyright\otimes}\) software was utilized to create an asymmetric screening design for the examination of the most influential factors of the FPSE procedure. The method was validated according to FDA guidelines. A representative total ion chromatogram is illustrated in Figure 7.  
The LODs ranged from 1 to 15 ng/mL, except for furanyl fentanyl (25 ng/mL), and the intra-day and inter-day precision did not exceed 8.2% and 8.6% RSD, respectively. A total of 15 samples from volunteers and 8 samples from patients with drug addiction undergoing detoxification treatment were analyzed with the proposed method. In the latter method, the authors took advantage of using cellulose paper material for the extraction of basic compounds from blood samples [86].