Recent Advances in Pressurized-Fluid Extraction

May 02, 2014

Pressurized-fluid extraction (PFE) is an automated extraction technique that uses elevated temperature and pressure to increase the rate and efficiency of the extraction process. PFE reduces the amount of time and solvent required, while significantly improving laboratory throughput relative to traditional extraction techniques such as Soxhlet. This article discusses current advances in commercial PFE instrumentation and shows how these systems can benefit scientists in analytical laboratories who wish to perform automated extraction.

Table 1: Physiochemical processes influenced by pressurized-fluid extraction (PFE).
Pressurized-fluid extraction (PFE) is a technique performed to extract solid or semi-solid samples using organic solvents. Elevated temperatures (up to 200 °C) are used to increase the kinetics of the extraction process while applying high pressures (for example, 1500 psi) to maintain the organic solvents in the liquid state. PFE is unique in that extractions are performed rapidly with reduced solvent use, compared with traditional extraction techniques. For example, PFE can reduce the extraction time down to 20 min per sample versus hours using Soxhlet and reduce solvent consumption to 30 mL per sample. The physiochemical processes influenced by PFE are described in Table 1.

PFE instrumentation follows a common pathway to produce extracts. An extraction cell containing the sample is loaded into an oven and a pump transfers extracting solvent into the cell from a reservoir. The cell is then pressurized and heated to a preset temperature. The temperature and pressure in the extraction cells rises above ambient levels and the hot solvent enhances the extraction rate of the analytes from the matrix. PFE systems are designed so that solvent will flow through the extraction cell and be collected into a bottle or tube at the end of the flow path. Once the extraction is complete, the extraction cell is purged with nitrogen gas to remove residual solvent and the collected extract is ready for concentration and analysis. PFE systems are currently manufactured by three vendors: Thermo Scientific (the accelerated solvent extraction [ASE] system); Fluid Management Systems, Inc. (FMS) (the pressurized liquid extraction [PLE] system); and Büchi (the SpeedExtractor system). Each system offers automation capabilities for the analytical laboratory to reduce the amount of time spent on sample preparation. These systems use elevated temperature and pressure to improve extraction efficiency and productivity compared with traditional extraction techniques such as Soxhlet. A summary of each system along with significant features is provided in this article.

Accelerated Solvent Extraction (ASE)

Figure 1: Accelerated solvent extraction (ASE) system schematic.
Accelerated solvent extraction (ASE) was first introduced at Pittcon in 1995 by Dionex (now part of Thermo Fisher Scientific). ASE increases extraction efficiency by using elevated temperatures of up to 200 °C with a fixed pressure of 1500 psi using both stainless steel and zirconium extraction cells (1–100 mL). These latter cells are especially resistant to low concentrations of mineral acids and strong bases. Currently, two ASE instruments are available from Thermo Fisher: The ASE 150 Accelerated Solvent Extraction instrument, a single-cell system for the extraction of solid and semisolid samples; and the multiple cell ASE 350 instrument. The ASE 350 instrument can process up to 24 samples in a batch and store up to 24 extraction methods. Extraction methods can be preprogrammed and multiple extraction methods run in a single batch, providing sequence control. Each sample is processed sequentially under the same programmed method conditions. The total extraction time is usually less than 20 min and the amount of solvent used is approximately 1.5 times the volume of the sample cell (for example, 15 mL for a 10-mL cell). Extracts are delivered to the collection vessels through a filter inserted at the bottom of the cell, and in many cases do not need any additional preparation prior to analysis. The schematic shown in Figure 1 demonstrates the operating principles of this system.

Table 2: Summary of Thermo Scientific accelerated solvent extraction (ASE) features.
The features of the ASE method are summarized in Table 2. The ASE system is capable of performing in-line clean up through use of adsorbents to the extraction cell. These absorbents are layered at the bottom of the cell prior to sample introduction and selectively remove interfering compounds during the extraction. For example, adding activated alumina oxide eliminates the co-extraction of lipids when extracting organic compounds. Several different absorbents can be used and the correct selection is documented in reference 1. Once the extraction is complete, the collection vial can be interfaced with a Rocket Evaporator (Genevac Ltd.) for automated sample concentration and evaporation. This method significantly improves the sample preparation process by eliminating the need for off-line clean-up steps and by enabling the direct transfer of samples from the system to an automated evaporator. With the use of in-cell adsorbents and the Rocket Evaporator, ASE is able to combine sample filtration, cleanup, and evaporation into one workflow.

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