Soxhlet extraction is probably the most common technique used for the extraction of materials from solid samples, and has been used for more than a century. It was originally used for the determination of fat in milk . As such, all other extraction techniques are generally compared with Soxhlet extraction.
The sample is typically placed in a thimble (usually made from cellulose) but doesn’t need to be and the solvent is placed in a flask below the thimble. The solvent is heated and recondenses into the thimble. Once the liquid reaches a defined level, it is siphoned back to the flask, transporting the extracted species back into the bulk liquid. The solvent is then distilled again, thereby delivering pure solvent to the material being extracted. The number of cycles (‘turnovers’) can affect the extraction. Hence, a minimum number is normally expected and this varies from material to material and species to species being extracted.
The advantage of Soxhlet is the repeated delivery of fresh pure distilled solvent to the sample, allowing continual shifting of the transfer equilibrium. It is also a very simple system requiring a heater, condenser and inexpensive glassware. Soxhlet has one advantage over reflux in that fine-particulate matrices can be used because these are contained in the extraction thimble, resulting in only extracted analytes in solution and is more amenable to potentially thermally challenging samples.
A standard Soxhlet apparatus can use large solvent volumes, a potential disadvantage to the technique. However, automated Soxhlet systems are available that offer additional functionality, such as warm Soxhlet, hot extraction and continuous extraction. Several solvent-extraction systems based on the Soxhlet device allow fast and safe extraction from materials. Some of this include the ‘Soxtec Systems’ from FOSS. These are automated or semi-automated analysers which extract lipids rapidly and accurately. These instruments carry out boiling, rinsing and solvent recovery. Similarly, Soxtherm extractors from Gerhardt GmbH have been developed to reduce extraction times. The sample to be analysed is weighed into cellulose thimbles and inserted in the extraction device. The Büchi Extraction System UniversalExtractor E-800 is an automated system which can be used to carry out an extraction according to the original Soxhlet principle. Four different extraction methods are possible without making changes to the unit: Soxhlet standard, Soxhlet warm, hot extraction, and continuous extraction Basically, the system has an inert-gas supply to avoid oxidation during extraction and to accelerate the evaporation and drying process even with high-boiling-point solvents (up to 150 °C).
Solvent selection is important with Soxhlet extraction. Take for example an acidified or basified solvent. Only the pure solvent would be distilled to be in contact with the sample. However, the species extracted would be in prolonged contact with the adjusted solvent (probably under exaggerated conditions). These exaggerated conditions would be produced by the distillation of water leaving an increased concentration of the modify agent (acid or base). So a more acidic or alkali solution would be present which could increase the chance of the extracted analytes reacting at this higher of lower pH
If possible the extracting solvent used in an extraction study should be the solvent used in the product. An extreme example of this would be trying to use the solvent in a pressurised metered-dose inhaler that uses a hydrofluoroalkane (HFA) propellant. The solvent (typically HFA 227 or HFA 134a, but could move to HFa 152) is a hydrofluorocarbon with a boiling point of less than –15 °C for HFA 227 or ‑26 °C for HFA 134a.
The HFA can be added to a Soxhlet apparatus to run a sub-ambient Soxhlet in which the cooling of the solvent needs to be quite extreme because the solvent needs to be cooled to at least –30 °C (hence the term ‘sub-ambient’). The cold temperature is required to condense the propellant to allow continued extraction. This can be achieved several approaches, the simplest being the use of a dry-ice and acetone condenser.
Several challenges affect the use of sub-ambient Soxhlet apparatus. The dominant factor is the kinetics because the extraction is being carried out at –20 °C. A common rule of thumb is that for every 10 °C change in temperature the rate of reaction doubles. Hence, sub-ambient Soxhlet under the same conditions will be (very approximately) 20-times slower than Soxhlet with DCM as the solvent simply due to the kinetics of the reaction. This would require a 20-day extraction compared with a typical 24-h extraction, and for this duration it is becoming closer to a leachable study. To combat this, the surface area of the material can be increased, typically requiring cryomilling. Secondly, by maximising the sample-to-solvent loading, the amount extracted is maximised. In addition, running the extraction for much longer than one working day is extremely challenging. A dry-ice/acetone condenser needs to be periodically recharged with additional dry ice and acetone. A re-circulating condenser operating at –30 °C requires a specialist solvent to work (such as ethylene glycol or a fluorocarbon).
- Soxhlet, Die Gewichtsanalytische Bestimmung des Milchfettes, Polytechnisches J. (Dingler’s), 1879, 232, 461.