Metabolites_**2014**, \(4\), 71-97; doi:10.3390/metabo4010071  
_Metabolites_  
**ISSN 2218-1989**  
www.mdpi.com/journal/metabolites/  
_Review_  
**Microextraction by Packed Sorbent (MEPS) and Solid-Phase Microextraction (SPME) as Sample Preparation Procedures**  
**for the Metabolomic Profiling of Urine**  
**[PERSON] 1, [PERSON] 1, [PERSON] 1, [PERSON] 1 and [PERSON] 1,2,**  
Footnote 1: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 2: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 3: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 4: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 5: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 6: [[http://www.cds.org/](http://www.cds.org/)]([http://www.cds.org/](http://www.cds.org/))  
Footnote 7: http:  
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//www.cds.org/  
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that can be achieved in terms of reliability, accuracy, precision and sensitivity, as well as the time and cost of analysis. In several cases, over 80% of analysis time is spent on sampling and sample preparation steps, including homogenization, extraction, concentration and clean-up. This is necessary for several matrices, such as the biological ones, once the analytical instruments cannot handle the sample complexity directly. Therefore, sample preparation has been recognized as the main bottleneck of the analytical process, particularly for the analysis of trace components [1,2].  
In this sense, an ideal sample preparation should present the following features: (i) minimal sample loss and a maximum recovery of the target analyte; (ii) elimination of coexisting components with a high yield; (iii) a simple, fast and inexpensive method; (iv) compatibility with the following analytical instruments; and (v) in conformity with green chemistry demands [3, 4, 5, 6, 7]. Microextraction techniques (METs), which use a minimal extractant amount (sorbent or liquid phase) offer these benefits and are becoming widely used in different fields, such as the biomedical, food, forensic and environmental ones, just to name the most frequently described applications (reviewed in [8, 9, 10, 11, 12, 13]).  
The recent advances in this field have converged on the miniaturization and integration of sample preparation online with analytical instrumentation, in order to reduce laboratory workload and to increase analytical performance [14]. From this perspective, METs have emerged in the last few years as powerful sample preparation approaches suitable for easily automating with liquid and gas chromatographic systems applied in a diversity of bioanalytical areas. Nowadays, there are several MET formats available that can be grouped in liquid-phase and solid-phase METs (Figure 1). In the  
Figure 1: Classification of microextraction techniques (METs).  
first group, we have the single-drop and the membrane-assisted microextractions variants. The second group, the solid-phase METs, can be organized according to the diffusion process in stirring and flow through versions. Nevertheless, solid-phase microextraction (SPME) in its different formats and microextraction by packed sorbent (MEPS) are certainly two of the most successful METs currently used with an increasing range of applications. Moreover, the aim of this review is to discuss the main advantages of using urine as a biological matrix suitable for diagnostic purposes combined with high throughput analytical techniques.