# Recent Advances in Extraction and Stirring Integrated Techniques
## 5 Stir Disk Extractions  
Disks offer a higher superficial area than bars and, in some cases, they present a higher porosity that enhances the diffusion of the analytes through the sorptive phase. Rotating disk sorptive extraction (RDSE) was the first approach in this context [56]. The RDSE unit (Figure 10A) consists of a thin film of PDMS deposited over a PTFE disk containing an integrated magnetic bar. Similarly to SCSE, this configuration protects the sorptive phase from direct contact with the vial, allowing for the application of faster stirring rates than conventional SBSE. RDSE operates in a similar fashion than SBSE since the unit is stirred into the sample for a defined period of time for the isolation of the target compounds. After the extraction the unit is recovered for the final instrumental analysis. When chromatographic techniques are employed, the analytes must be eluted with a proper solvent [57; 58]. However, the configuration of the unit allows for the detachment of the film after the extraction for its direct spectroscopic analysis, avoiding the dilution inherent to the elution step [59; 60]. RDSE has evolved in the last few years following two different trends, namely: the development of new extraction phases and the potential automation of the technique [61]. PDMS has a clear potential as a sorptive phase but presents some disadvantages that have been previously described. The development of new phases in RSDE will increase the versatility of the technique, opening the door to the extraction of analytes of intermediate or high polarity. The first approach is this context involved the use of conventional C18 solid phase extraction (SPE) disks for the extraction of hexachlorobenzene from water [62]. These SPE disks, which are membranes with embedded particles, are linked to the device using silicone as a binder. The results were promising but these disks have non-polar compounds as targets. Some polymeric SPE sorbents have been specially designed for the extraction of polar compounds. [PERSON] et al. proposed an evolution of the classic RDSE in order to exploit this characteristic. In this case, a cavity is dug into the unit and finally loaded with polymeric sorbent (HLB) particles (Figure 10B) [63]. The cavity is covered with a filter to confine the particles, avoiding their losses during the extraction. The great variety of commercially available SPE sorbents and their easy loading on the unit makes this approach highly versatile. In addition, lab-synthesized sorbents can be also applied. In this sense, researchers have proposed the use of MIPs [64] and ionic liquids intercalated in montmorillonite [65] as a sorptive phase in this device. Polyethylene disks, conventionally used as frits in solid phase extraction cartridges, can be also used as stirred units by simply piercing them with a metallic wire. This material is hydrophobic and it is indicated for the extraction of hydrophobic compounds [66]. The material exhibits sufficient thermal stability to allow for its mild thermal desorption in a conventional headspace vial. Borosilicate disks can also be excellent supports for RDSE due to their mechanical strength. However, as polar materials, they are not able to extract organic compounds from aqueous samples. Borosilicate disks have been modified with carbon nanohorns on the surface for the extraction of benzophenone-3 from swimming pool water [67]. These modified disks can be easily adapted to a portable drill (Figure 11) that stirs them into the sample for the on-site extraction of the target compound. The enrichment factor and extraction recovery, which are 1379 and 68.9%, respectively, reveal the great potential of the technique. Figure 11: Borosilicate disk modified with carbon nanohorns extraction device. Reproduced with permission of from [67], copyright Elsevier, 2014.