## 2 Extraction Techniques
### Microextraction by Packed Sorbent (MEPS)  
MEPS was introduced by [PERSON] in 2004 [11]. It is a miniaturization of the conventional solid-phase extraction (SPE)-packed bed devices from milliliter bed volumes to microliter volumes, which can be connected online to gas chromatography (GC) and/or liquid chromatography (LC) without any further modifications [33]. This technique has been successfully used to extract a wide range of analytes in different biological matrices, such as urine, plasma, saliva and blood [11,34,35].  
In MEPS, approximately 1-2 mg of the sorbent is packed inside a syringe (100-250 \(\mu\)L) as a plug or between the barrel and the needle as a cartridge. Sample extraction takes place in this packed bed, which can be coated to provide selective and suitable sampling conditions. The MEPS approach to sample preparation is suitable for reversed phases (extraction of hydrophobic analytes or polar organic analytes from aqueous matrices), normal phases (extraction of polar analytes from non-polar organic solvents) and mixed mode and ion exchange chemistries (extraction of charged analytes from aqueous or non-polar organic samples) [9,34,35]. There are several available MEPS sorbent materials (Figure 3), including reversed phase (C18, C8 and C2), normal phase (silica), restricted access material (RAM), HILIC (hydrophilic interaction liquid chromatography), carbon, polystyrene-divinylbenzene copolymer (PS-DVB), molecular imprinted polymers (MIPs), strong cation exchange (SCX) and mixed mode (C8/SCX) chemistries (reviewed in [9,35]).  
Figure 3: Simplified flowchart for MEPS sorbents selection. The parameters used were the matrix properties (aqueous or organic), the polarity and solubility of the target analytes (soluble in water or organic solvents) and the extraction mode (reverse-phase (RP), ion-exchange (IE) or normal-phase (NP)). Adapted from [9]. APS, AminoPropyl Siloxane; AX, Anion eXchange; CX, Cation eXchange; HDVB, Highly cross-linked polystyrene DiVinylBenzene; PEP, Polar Enhanced Polymer; SAX, Strong Anion eXchange; SCX, Strong Cation eXchange; SDVB, polyStyrene DiVinylBenzene.  
This extraction method differs from commercial solid-phase extraction (SPE) in that the packing is integrated directly into the syringe and not into a separate column. Furthermore, the packed sorbent can be used more than 100 times, even when using plasma or urine samples, but a conventional SPE column is used only once. Moreover, MEPS can handle low sample volumes (10 \(\upmu\)L) to large volumes (1,000 \(\upmu\)L). The analytes are then eluted with small volumes of an organic solvent, such as methanol or other mobile phases, allowing for a very significant concentration of the target when large sample volumes are used. The combination of MEPS and chromatographic techniques, such as GC-MS, HPLC and LC-MS/MS, is an excellent tool for the screening and determination of biomarkers in biological samples. This approach for sample preparation is therefore very promising for many reasons, namely that: (1) it is fast and easy to use; (2) it can be fully automated for online procedure; (3) it reduces the solvent and sample volume, as well as the waste produces; and (4) the cost of analysis is minimal when compared to conventional SPE. Overall, it is one of the most user- and environmental-friendly METs available for sample extraction.  
MEPS is a very simple and straightforward MET, but it nevertheless involves a wide range of optimization steps that allow for a fine tuning of the extraction efficiency.