*8.2.3 Pressurized liquid extraction (PLE) or accelerated solvent extraction (ASE)*

Pressurized liquid extraction (PLE) also known as pressurized fluid extraction (PFE), accelerated solvent extraction (ASE), and pressurized solvent extraction (PSE), or as enhanced solvent extraction system (ESE) [67].

Dionex Corporation introduced PLE in 1995 as an alternative to maceration, percolation, sonication, Soxhlet extraction, etc. It is an automated technique for extracting solid samples with liquid solvents (either aqueous or organic, single or mixtures) above their boiling point, combine high pressures (4–12 MPa) and moderate to high temperatures (50–300°C) [68]. When water is the extraction solvent,

**Figure 4.** *Schematic representation of an ultrasound-assisted extraction equipment.*

*Extraction of Bioactive Compounds from Medicinal Plants and Herbs DOI: http://dx.doi.org/10.5772/intechopen.98602*

#### **Figure 5.**

*Scheme of pressurized liquid extraction equipment [68].*

different terms are used to define the method, that includes hot water extraction (HWE), subcritical water extraction (SWE), high-temperature water extraction (HTWE), hot water extract pressurized (PHWE), liquid water extraction or superheated water extraction [67]. Sample size, solvent, pressure, temperature, pH, flow rate, extraction time are the standard parameters influencing the PLE process, with temperature and solvent type being the most significant ones [69–71].

In this process, for a short period of time (5–10 min), a cartridge in which the ample has been placed is filled with an extracting solvent and used to statically extract the sample under elevated temperature and pressure. To purge the sample extract from the extraction cell into a collector flask pressurized gas is used (**Figure 5**) [68].

To increase the efficiency of this extraction process, environmentally friendly liquid solvents are used at moderate to elevated temperature and pressure [72]. The increased temperature causes dramatic changes in the physical–chemical properties of water, enhances the analytes' solubility, breaks matrix-analyte interactions achieving a higher diffusion rate, and accelerates the extraction process by increasing the diffusivity of the solvent. The increased pressure in contrast, keeps the solvent in a liquid state without boiling and forces the solvent to penetrate the matrix pores [55, 73–75].

The main advantages of this technique are: (i) faster extraction from 15 to 50 min, (ii) low quantity of solvents (15–40 mL), and no filtration is required. However, costly equipment and the need for a throughout optimization of variables to avoid a matrix-dependent efficiency are the main demerits [72–74].

#### *8.2.4 Supercritical fluid extraction (SFE)*

SFE is used for separating components from the matrix with the application of supercritical fluids as the extracting solvent (**Figure 6**) [30].

Using CO2 as the extracting fluid has many advantages. Besides, its lower boiling point (31°C) and its critical pressure (74 bar). Moreover, carbon dioxide is abundant in nature, safe and inexpensive. But while carbon dioxide is the preferred fluid for SFE, it possesses several polarity limitations. When extracting polar solutes and when strong analyte-matrix interactions are present solvent polarity is crucial. Carbon dioxide fluid is usually mixed with organic solvents to alleviate the polarity limitations (**Figure 7**) [2].

## **Figure 6.**

*Schematic diagram of supercritical fluid extraction (SFE) set-up [76].*

#### **Figure 7.** *Schematic representation of a supercritical fluid extraction (SFE) system [62].*


Some specific disadvantages of this method are:

i.high investment cost;


SFE finds extensive application in extracting pesticides, environmental samples, foods and fragrances, essential oils, polymers, and natural products [50, 77]. Conde-Hernández and collaborators extracted the essential oil of rosemary (*Rosmarinus officinalis*) by S-CO2 extraction, hydro distillation and steam distillation. They found that both yields of essential oil and antioxidant activity of SFC extract were higher than those from the other two methods [78, 79].
