# New Challenges in (Bio)Analytical Sample Treatment Procedures for Clinical Applications
## 3 Sample Preparation Techniques for Diagnostic Purposes
### Microextraction by Packed Sorbent (MEPS)  
Microextraction by packed sorbent (MEPS) is a miniaturized SPE technique with very interesting potential benefits, such as low solvent volume for analytes elution, small sample volume (10-250 uL), direct injection without further treatments, simple operation, and cost-effectiveness. In this procedure, about 1-2 mg of sorbent (such as C18, C8) is packed in the barrel of a gas-tight syringe positioned between the needle and the barrel as a cartridge. When the sample passed through the solid bed, the analytes were absorbed into the sorbent [1,38].  
[PERSON] et al. validated an innovative method that combined MEPS with ultra-high-performance liquid chromatography coupled to electrospray ionization-triple quadrupole mass spectrometry (UHPLC-ESI-MS/MS) for the simultaneous determination of 8-isoprostaglandin F2\(\alpha\) (8-isoPGF2\(\alpha\)) and cortisol in saliva samples collected from patients with heart failure. During the optimization of this method, they maximized the extraction efficiency of 8-isoPGF2\(\alpha\) and cortisol with the dilution of saliva samples with water (ratio 1:5, \(v/v\)) and filtration by syringe filter prior to MEPS extraction to avoid proteins, mucins, and other interferences that may cause deterioration of the sorbent and cartridge clogging, without loss of analytes. A LOD of 10 pg/mL was obtained for each target analyte; the coefficient of determination (R\({}^{2}\)) was 0.7, and the \(p\)-value was \(<\) 0.001.  
Compared to other techniques, such as enzyme-linked immunosorbent assays (ELISA), which may lead to partially inaccurate results or solid-phase extraction (SPE) procedures, which required requires a larger volume of samples, a substantial problem for saliva analysis, the obtained results with MEPS showed that this is a good alternative with different advantages. MEPS procedure required a low volume of organic solvents (\(<\)100 uL/measurement) and a reusable sorbent (over 100 extractions with the same cartridge), which permits automating the extraction procedure with a reduction of the cost of analysis. In addition, this procedure showed a satisfactory recovery (95-110%) and an adequate limit of detection, which guarantees a reliable determination of low concentration levels of 8-isoPGF2\(\alpha\) and cortisol in real saliva samples. The obtained results confirmed that the MEPS method allows reliable quantification of 8-isoPGF2\(\alpha\) and cortisol as new potential non-invasive biomarkers for monitoring patients with heart failure and is useful for clinical purposes [45].  
[PERSON] et al. developed a highly sensitive MEPS/UHPLC method to detect and quantify lipid peroxidation biomarkers, such as leukotrienes E4 (LTE4), B4 (LTB4), and 11\(\beta\)-prostaglandin F2\(\alpha\) (11\(\beta\)PGF2\(\alpha\)) related to asthma, in the urine of child patients. The targeted analytes were isolated from urine using a semiautomatic miniaturization of solid-phase extraction, the MEPS; the LOD for LTB4 and 11\(\beta\)PGF2\(\alpha\) was, respectively, 0.04 ug L\({}^{-1}\) and 1.12 ug L\({}^{-1}\) while LOQ values ranged between 0.10 ug L\({}^{-1}\) for the LTB4 and 2.11 ug L\({}^{-1}\) for 11\(\beta\)PGF2\(\alpha\). Compared to other methodologies, this technique showed more advantages, such as lower elution solvent volume consumption, high precision due to the semi-automatic procedure, excellent recoveries, and extraction efficiency. Moreover, the extraction procedure is fast, with a total analysis time of 31 min. The obtained valuesfor the 118 PGF2\(\alpha\) and LTB4 were about 1.8 times higher, and for the LTE4 were about 1.4 times higher in asthmatic patients than in healthy individuals suggesting the potential of these eicosanoids on asthma diagnosis. Consequently, MEPS/UHPLC-PDA represents a promising method for the simultaneous detection of these and other eicosanoids present in other biological matrices as plasma or saliva of patients with different inflammatory diseases [46].