Liquid Solid Extraction Techniques

Soxhlet techniques The Soxhlet extractor (Figure 8)

is one of the oldest extraction systems available but is still very common. A solid sample is placed in an extraction thimble inside the middle chamber. Upon boiling, the solvent vapours from the bottom flask travel up to the condenser and then drip through the sample. The sample is soaked in solvent (a two-phase distribution equilibrium), which then returns to the flask when the liquid reaches the top of the siphon. The sample is exposed to fresh solvent after every siphon cycle, usually at a rate of about six cycles per hour. Typical extraction times are 6-24 h. Once assembled and operating, there is little that can go wrong with this system. However, operators must be aware of the following general hints:

• Proper cycling is required: the rate (cycles per hour) is usually specified in the method, and the operator must ensure that the unit siphons in distinct events rather than continuously draining.

• Solvent level in the thimble: if too high, sample may be lost from the thimble, contaminating the extract.

• Moisture content: dry samples work best; add a drying agent to remove free moisture.

The system requires a large volume of organic solvent, and extraction time is long. Despite these limitations, the Soxhlet extractor is still in widespread the evaporation and collection of solvent, further improving efficiency. These alternatives offer considerable advantages in terms of time and solvent use, and results are generally comparable to the traditional method.

Solid-phase extraction Solid-phase extraction (SPE) is an alternative to liquid-liquid extraction where the extraction solvent is replaced with a solid sorbent. The sorbent is usually packed into a cartridge (Figure 9) that can vary in size from about 1 mL to more than 50 mL. The quantity of sorbent can range from about 500 mg to 10 g. Extraction is accomplished by forcing the aqueous sample past the sorbent (via vacuum or pressure), causing analytes in the sample to be sorbed. This two-phase distribution is similar to the partitioning that occurs in chromatog-raphy. After the sample has passed through the sorbent bed, the sorbed analytes are eluted with a strong solvent, such as methanol, acetonitrile or carbon disulfide.

The SPE process involves the following sequential steps:

• Conditioning/cleaning of the sorbent with an organic solvent such as methanol

• Extraction of the sample.

• Air drying or rinsing to remove any remaining sample.

• Elution of analytes using a strong organic solvent.

Figure 8 Soxhlet extractor. (Reproduced with permission of ACCTA, Inc.)

use, primarily because of its excellent reputation for providing complete extraction. Indeed, Soxhlet values are often used as the standard against which other extraction methodologies are compared.

Modified Soxhlet extractors The lengthy Soxhlet extraction times have prompted the development of modified extractors, such as the Soxtec® system (Foss Tecator AB, Hoganas, Sweden). The sample is placed in an extraction thimble, but the thimble is then directly immersed in boiling solvent, rather than bathed in cooler condensed solvent. The increased temperature means faster extraction kinetics. After about 1 h equilibration, the sample is removed from the solvent and flushed with fresh condensed solvent for an additional hour. The apparatus even allows

SPE offers three primary advantages over conventional liquid-liquid extraction: reduced solvent usage, extraction speed and selective chemistry. In an ideal method, only a few millilitres of organic solvent may be necessary for an extraction and it is possible to extract and elute 10 100-mL samples or more in as little as 15-20 min. Finally, by varying the nature of the sorbent, it is possible to achieve selective extraction and/or selective elution. For example, a minor change in bonded phase from a C18 phase to a C8 phase can actually result in a significant change in selectivity. The shorter chain C8 phase is less retentive towards more hydrophobic molecules and exposes somewhat more of the polar character from the underlying silica. This trend can be extended using even shorter aliphatic bonded phases or by adding a polar functional group to the chain (e.g. cyano- or phenyl-). There are no analogous series in liquidliquid systems.

There are a host of sorbents available, including more polar functional groups, polymer-based, ion exchange, affinity and chelating materials. Nearly every liquid-liquid extraction method has an SPE counterpart, and almost all provide equal if not better

Figure 9 Solid-phase extraction (SPE) cartridge design. (Reproduced with permission of ACCTA, Inc.)

results, with considerably less effort. It should also be noted that SPE can be performed on nonaqueous samples using a polar sorbent, but this application is usually used for sample clean-up rather than extraction.

Finally, it is important to note SPE's limitations:

• High particulate samples will often plug the frits.

• Extracting capacity (total extractable mass) is more limited than with conventional solvent extraction.

• Reproducibility (batch-to-batch) can be a problem, although this is less of a concern now than during early development of the technique.

Membrane disc extraction Membrane extraction discs, first sold under the brand name Empore® (3M, St Paul, MN, USA), are an alternative SPE system. In the membrane discs the sorbent is enclosed in a support network rather than simply being packed into a cartridge. The unique Empore® design consists of 90% (w/w) sorbent particles (8-10 |im diameter), in a network of polytetrafiuoroethylene fibrils, in a disc format that resembles a thicker version of conventional synthetic membrane filters. A typical disc is about 0.5 mm thick with diameters ranging from 1 to 90 cm.

A membrane disc extraction method would typically consist of the following steps:

• Pre-washing the disc with the final eluting solvent.

• Pre-wetting the disc with methanol or some other solvent that is miscible with the sample (which is usually aqueous).

• Extraction, i.e. drawing the sample through the disc.

• Elution of analytes, which involves a soak with the elution solvent for a period of time, followed by elution with the aid of a vacuum. This elution step may be repeated with different solvents if necessary.

Membrane discs have the same advantages over liquid-liquid extraction as SPE but are superior to conventional SPE because the extraction rate is faster; flow rates of 100-200 mL min"1 are typical. The small particles also provide greater capacity and uniformity of packing. Unfortunately, the discs are more sensitive to the presence of particulates, so a pre-filter is often necessary.

Early applications of membrane discs focused on environmental analysis, where large sample sizes made the fast extraction rates attractive. Membrane discs can also be formulated into SPE-like cartridges, allowing the efficient processing of small clinical samples (e.g. serum, urine, etc.). Use of membrane disc applications continues to grow, although the number of reported applications is not as high as SPE, due to the relative age of the two techniques.

Solid-phase microextraction Solid-phase microextraction (SPME) is another version of liquid-solid extraction techniques. In this system, the extraction

Figure 10 Solid-phase microextraction system. (Reproduced with permission of ACCTA, Inc.)

phase consists of a fused silica fibre coated with a sorbent (e.g. dimethylsilicone or other immobilized polymer) with thicknesses ranging from about 10 to 100 |im (Figure 10). The fibre is placed in contact with the liquid or gas sample and analytes are sorbed on to the phase, from which they are directly desor-bed into a chromatograph.

Unlike the other techniques discussed here, SPME is a completely solvent-free extraction method. The extraction step tends to be rapid, usually requiring 10-20 min, and desorption can take only a few seconds. Thus, a fast analysis with a good lower limit of detection is possible, since the entire extract is analysed. Furthermore, the selectivity and extractability can be affected by changes in fibre chemistry as well as solution pH, ionic strength, etc. Sampling by immersion in the sample or extraction from the head-space above the liquid (often a faster extraction) provides additional flexibility. However, SPME, by its nature, is not applicable to as wide a range of samples as other techniques, and there are currently a more limited number of sorbents available compared to SPE. However, SPME offers some unique advantages that make it an attractive alternative for many applications.

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