Liquid – liquid extraction is primarily applied where direct separation methods such as distillation and crystallization cannot be used or are too costly. Liquid – liquid extraction is also employed when the components to be separated are heat-sensitive such as antibiotics or relatively non-volatile, for example, mineral salts.9 Liquid – liquid extraction is used in industry for the many purposes. First, separation of systems with similar boiling points such as separation of aromatics from aliphatic hydrocarbons. Second, separation of high boilers and low-concentration solutes from aqueous solutions such as phenol. Third, separation of mixtures with high boiling points such as vitamins.
Fourth, separation of temperature-sensitive compounds such as acrylates and biotechnology. Fifth, separation of azeotropic mixtures, for example, extraction of acetic or formic acid from aqueous media using MTBE as solvent. Sixth, extraction of organic compounds from salt solutions such as caprolactam. Seventh, extraction of salts from polymer solutions such as ketone resins and polyols. Eighth, extraction of metal salts from low-grade ores such as copper. Ninth, extraction of metal salts from wastewater such as copper and lastly, recovery of nuclear fuels such as Purex process.9Solid Phase Microextraction (SPME) Solid Phase Microextraction (SPME) is a solvent-free adsorption or desorption technology. It is rapid, economical, and versatile.
A fibre coated with a liquid which is polymer or a solid which is sorbent, or a combination of both is used by SPME. The fibre coating extracts the compound of interest from the sample by absorption in the case of liquid coatings or solid coatings. Then, insert the fibre directly into the chromatograph for desorption and analysis is done.
10 In addition, the concept of SPME may have been derived from the idea of an immersed GC capillary column. The SPME apparatus is a very simple device. It looks like modified syringe consisting of a fibre holder and a fibre assembly, the latter containing a 1–2 cm long retractable SPME fibre. The SPME fibre itself is a thin fused-silica optical fibre, coated with a thin polymer film (such as polydimethylsiloxane (PDMS)), conventionally used as a coating material in chromatography.11There are three basic modes which can be performed by Fibre SPME which are direct extraction, in a headspace configuration, and in a membrane-protected approach. For direct extraction, dip the fibre coating into the aqueous sample and the analytes can partition between the coating and the matrix. For headspace extraction, place the fibre in the headspace above the aqueous matrix during extraction. Extracting the volatile analytes only, but this method is advantageous for high molecular weight interfering samples.
When a sample contains both high molecular weight interfering and non-volatile compounds, such as proteins, it is quite difficult to applied direct or headspace SPME. In such cases, restricted-access materials or membrane-protected SPME is used and a better reproducibility and accuracy result will be produced.12First, the fibre is drawn into the needle. Then, the needle is passed through the septum which seals the vial. Next, the plunger is depressed to expose the fibre to the sample or headspace that above the sample.
Then, the organic analytes are adsorbed to the coating on the fibre. After adsorption equilibrium is attained, which can be anywhere from 2 minutes to 1.5 hours, the fibre is then drawn back into needle and is withdrawn from the sample vial.
Lastly, the needle is introduced into the GC injector or SPME/HPLC interface, where adsorbed analytes are thermally desorbed and delivered to the instruments column. Besides, there are two type of extraction which are fibre extraction and in-tube extraction. In environmental applications, HS-SPME is used for analysing volatile and semi volatile compounds in solid samples such as soils sediments and sludges. HS-SPME has been used to determine aromatics and PAHs in spiked sand and clay matrices, volatile organic compounds in landfill soils, organometallic compounds in sediments in soil, and in plasma samples and inorganic mercury samples in soil. It has also been used for the determination of odorants, chloro- and nitrobenzene and chloro- and nitroanilines in a broad variety of soils. SPME extraction is also can be applied for the direct determination of different components of air samples, which is analogous to the conventional HS extraction.For application in food chemistry, SPME extraction technique became commercially available it has been used for the analysis of different foods and food materials. Various SPME methods have been applied to the analysis of various components and contaminants in a range of different food samples.
Besides, HS-SPME is used popularly for the characterization of different alcoholic drinks based on their volatile composition or to extract specific trace components from the HS. In addition, SPME is also used for hair analysis. 11Micro Extraction by Packed Sorbent (MEPS) is a new development in the fields of sample preparation and sample handling. MEPS is the miniaturization of conventional SPE packed bed devices from millilitre bed volumes to microliter volumes. The MEPS approach to sample preparation is suitable a few phases such as reversed phases, normal phases, mixed mode or ion exchange chemistries.
13 MEPS is worked with much smaller samples which as small as 10 ?L than full-scale SPE. It can be fully automated where the sample processing, extraction and injection steps are performed on-line using the same syringe. MEPS reduces the volume of sample and also solvent needed.
In MEPS the sorbent, 1–2 mg, is either inserted into the syringe of 100–250 ?L barrel as a plug or between the needle and the barrel as a cartridge. The cartridge bed can be packed or coated to provide selective and suitable sampling conditions. Any sorbent material such as silica-based material, for example, C2, C8, C18, SCX, restricted access material (RAM), carbon, polystyrene-divinylbenzene copolymer (PS-DVB) or molecular imprinted polymers (MIPs) can be used. Besides, a liquid polymer coating can be provided. 14First, the sample is pumped through the MEPSBIN which some volumes may be taken. Next, the MEPS BIN is washed once bypumping 20 µL to 50 µL of wash solution through the BIN in order to removeinterferences.
Then, the analyte is eluted by drawing solvent through the BINinto the syringe barrel. Finally, the analyte is injected directly into theinjector. 50 µL solvent is pumped and followed by 50 µL wash solution toprepare BIN for the next sample.13