# Recent Advances in Extraction and Stirring Integrated Techniques
## 3 Stir Membrane Extraction
### Stir Membrane Extraction in the Liquid Phase Microextraction Context  
The original SME device can be adapted to the liquid phase microextraction (LPME) context with slight modifications. The simple introduction of a cap in the lower part of the unit creates a small chamber where an extractant phase can be located, as can be observed in Figure 5. The membrane is used for achieving the confinement of the liquid extractant phase in the chamber while the unit is stirred in the sample. The stir membrane unit in LPME can operate under the two [33] and three [34; 35] phase mode. In the two phase mode, an organic solvent is located in the chamber, wetting the membrane, and the analytes are extracted by the different solubilities they present in the aqueous (sample) and organic (extractant) phases. This mode is specially indicated for the extraction of non-polar compounds and it is fully compatible with gas chromatography. In the three phase mode, the organic solvent only wets the membrane (forming the so-called supported liquid membrane, SLM) while an aqueous phase is located in the internal chamber. This mode, which is based on the transference of the analytes through the SLM thanks to an existing pH gradient between the sample and the extractant (both of aqueous nature), is useful for the extraction of ionisable non-polar compounds.  
The conventional stir membrane unit, both in the solid and liquid phase microextraction context, is stirred in the sample. Sample volumes larger than 20 mL (the exact volume depends on the extraction vessel) are required which restricts the application to some biological samples with limited volume. For this reason, the stir membrane extraction has been also adapted to this scenario. This was accomplished by slightly modifying the device, as can be observed in Figure 6, by just increasing the height of the plastic adapter. This modification allows the creation of an upper chamber with a small volume where the sample is located [36]. Although this approach, which was evaluated for the extraction of paracetamol from saliva samples, cannot be strictly considered as a stirring device (the unit is agitated in a vortex), it clearly shows the versatility of the membrane-based units.  
In the same way, the original device has been adapted for solid-liquid-liquid extraction [37]. Although the main idea remains the same, the unit was built using a different construction block.  
Figure 5: Stir membrane extraction device working in the liquid phase microextraction context.  
Figure 6: Modification of the stir membrane liquid-liquid-liquid extraction device to process a low volume of sample. SLM, supported liquid membrane.  
In this case, a conventional plastic eppendorf tube is used as extraction device (Figure 7) where the solid sample is dispersed in an organic solvents mixture. An aqueous extractant phase is situated in the eppendorf cap and protected by a polymeric membrane. Once deployed, the unit is rotated and agitated. During the extraction, the analytes are transferred from the solid sample to the organic solvent and then from the organic solvent to the aqueous phase, passing through the membrane. This approach was initially used for the determination of parabens in lyophilized human breast milk with very good sensitivity.