Accelerated Solvent Extraction With Acid Pretreatment for Improved Laboratory Productivity

Sample preparation, specifically solvent extraction, is an important beginning step in the analytical process. If the extraction procedure is incomplete, the results are inaccurate analyses or poor recoveries. Analysts have used an array of solvent extraction techniques ranging from Soxhlet, shaking, sonication, and blending. Accelerated solvent extraction (ASE®) technology is a flow-through solvent extraction system that helps increase productivity and sample throughput while reducing preparation cost and providing a platform for automation.

Complex matrices such as food samples or polar compounds such as substituted phenols often require acid hydrolysis or pretreatment prior to solvent extraction. For food matrices, acid hydrolysis is often necessary to facilitate the complete extraction of lipids by organic solvents. Acidic pretreatment of soils or tissues will protonate the phenolic groups and increases their solubility in organic solvent.1

Figure 1 - ASE 350 (left) and 150 (right) systems.

A new standard for flow-through solvent extraction, the ASE 150 and ASE 350 systems (Dionex Corp., Sunnyvale, CA) perform extraction of matrices that have undergone acid or alkaline pretreatment or digestion (see Figure 1). Samples pretreated in this way can corrode stainless steel cells and pathways such as those used in previous ASE systems. The new ASE technology uses Dionium™ (patent applied for) to prevent corrosion under acidic or alkaline conditions used in standard pretreatments. The ability to extract these pretreated matrices expands the capabilities of ASE technology and widens the scope of ASE applications.

The ASE 150 is a single-cell system that provides fast extraction and significantly reduced solvent use. The ASE 350 combines the flexibility of two systems, offering cell and collection vessels that accommodate sample sizes ranging from 500 mg to 100 g. The ASE 350 can extract up to 24 samples sequentially while offering fast, automated extractions and reduced consumption of solvent (see Figure 2 for a flow schematic).

Figure 2 - Schematic of extraction using ASE: 1) load cell into ASE, 2) fill cell with solvent (0.5–1.0 min), 3) heat and pressurize cell (5 min), 4) static extraction (5 min) (repeat if necessary), 5) flush cell with fresh solvent (0.5 min), 6) purge cell with nitrogen (1–2 min), and 7) extract ready. Total extraction time: 12–18 min depending on number of static cycles.

The new pH-hardened Dionium components enable ASE to support a wider variety of applications. Such applications include solvent extraction of hydrolysates after acid hydrolysis, pretreatment of waste vegetation using sulfuric acid, mild digestion coupled with solvent extraction of encapsulated pollutants, and in-cell cleanup of solvent extract using acidic or alkaline sorbents.

Review of new applications and procedures

Hydrolysis

Food samples often require acid hydrolysis prior to solvent extraction for gravimetric or GC-MS determination of lipids. ASE can now accommodate the classic Mojonnier method, since the Dionium components provide resistance to the hydrochloric acid found in the hydrolysate after the hydrolysis step. By replacing classic liquid–liquid extraction with ASE, chemists can improve precision and productivity while reducing sample handling.2

Encapsulated pollutants

Incineration of commercial and residential waste may generate dioxins and furans. During incineration, these pollutants may become coated or encapsulated with carbonates. The encapsulation will prevent complete extraction of these analytes, since organic solvents are unable to penetrate or solubilize the carbonate. Many procedures require a pretreatment of the matrix with hydrochloric acid (HCl) to dissolve the carbonate layer. In addition, some matrices such as fly ash require mild digestion prior to solvent extraction. However, this kind of in-cell pretreatment causes corrosion of stainless steel cell walls and frits by the HCl.

The HCl pretreatment step can now be performed together with extraction, using ASE with acid-resistant Dionium cells and components. The sample is mixed with ASE Prep DE (diatomaceous earth) then HCl is added, and the mixture is transferred to a Dionium cell. This pretreatment of the encapsulated matrices facilitates full recovery of pollutants for analysis. Combining pretreatment and solvent extraction streamlines the sample preparation process.

Pretreatment of waste vegetation for ethanol production

The analytical determination of sugars for ethanol production from waste vegetation requires a challenging extraction. Matrices such as corn stover must be ground, pretreated with sulfuric acid, washed, then extracted with solvent in preparation for HPLC analysis. Current extraction techniques employ mixing, stirring, or shaking with sulfuric acid. This step is followed by washing with water, and finally extraction with an organic solvent. This pretreatment can now be performed in one step with extraction, using ASE. The ground vegetation is added to a Dionium cell with glass fiber filters, then extracted at elevated temperature with 0.1 M sulfuric acid. The resulting extract is analyzed by HPLC to determine the presence and amount of specific sugars available for ethanol production.

Conclusion

The ASE 150 and 350 systems with pH-hardened pathways provide an alternative to extraction steps of the traditional Mojonnier method for determining lipids in foods, and support in-cell pretreatment with strong mineral acid in low concentrations. The ASE 350 system also provides a platform for automated pretreatment and extraction of waste vegetation. ASE technology allows automation of sample pretreatment and solvent extraction for streamlined implementation compared to traditional techniques, which require labor-intensive sample handling. Moreover, the ASE technique reduces solvent usage by as much as 90% compared to traditional techniques, and total extraction time by up to 50%. The ASE 350 system expands the capabilities of ASE technology and provides greater flexibility in operation.

References

  1. Determination of unbound fat in various food matrices using accelerated solvent extraction (ASE). Application note 321, Dionex Corp., Sunnyvale, CA.
  2. Extraction of total fat from food samples after acid hydrolysis using accelerated solvent extraction (ASE) with GC-MS Analysis. Application note 361, Dionex Corp., Sunnyvale, CA.

The authors are with Dionex Corp., Salt Lake City Technical Center, 1182 W. 2400 S., Ste. A, Salt Lake City, UT 84119, U.S.A.; tel.: 801-972-9292; fax: 801-972-9291; e-mail: brian.dorich@dionex.com.

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