Extraction insoluble residue, either it is solid or

Extraction is a
process which used to remove a desired compound from a solid or a liquid
mixture using a suitable solvent, or it can also be defined as removal of the
soluble material from an insoluble residue, either it is solid or liquid by
treatment with suitable solvent. There will be left behind a little amount of
impurities after extraction because it is never possible to completely extract
or remove a compound from a given solvent ideally. 1

                For the analytical
process, 60% of the time taken is needed for the sample preparation. Therefore,
it is important to choose the correct sample preparation technique and
understand the underlying separation mechanisms to fully maximize the process
to give optimum results.2

            There
are many types of extraction methods including Solid phase extraction (SPE),
Liquid-liquid extraction (LLE), Solid phase micro extraction (SPME), Micro
extraction by packed sorbent (MEPS) and Stir bar sorptive extraction (SBSE).
However, in the processing of medicinal plants, significant advances have been
made such as microwave-assisted (MAE), ultrasound-assisted extraction (UAE) and
supercritical fluid extraction (SFE). Increasing yield at lower cost is the aim of these advances. 3

Solid Phase Extraction

            Solid-phase
extraction (SPE) is one of the sample preparation technique which is sensitive.
SPE has same principles of column chromatography and it is also generally used
in analytical laboratories for the extraction experiment such as extraction of
analytes from a complex mixture. Besides, prior to the quantification, SPE enables the clean-up, extraction and concentration of analytes. Most of
the problems that have encountered with liquid-liquid extraction has been
prevented by solid phase extraction. In addition, quantitative recovery yields
can be improved by SPE. BY using SPE technique, the most extraction is in less
than 30 minutes, so it is a rapid and easy to perform. In addition, automation
can be used in this technique. Moreover, SPE is only needed to handle low
amount of solvents. However, when
solid phase extraction is compared to other sample preparation techniques, for
example, protein precipitation, QuEChERS (quick, easy, cheap, effective,
rugged, and safe) or liquid–liquid
extraction (LLE) , SPE is often requires more effort toward method development
and is considerably more time consuming. However, the rewards are certainly
worth it, with highly selective extractions typically producing very pure
samples.2,4

            Besides,
SPE sorbent phases fall into four primary categories which are nonpolar, polar,
ion exchange (cation and anion), and mixed mode.2

Procedure of Solid phase extraction

            Solid phase extraction depends on the difference of
affinity between an analyte and interferents, which is present in a liquid
matrix, for a solid phase (sorbent). The separation of the target analyte from
the interferents depends on the affinity.4 Besides, retained or
unretained analytes are found. For retained analyte, it is while matrix
interference is washed through. For unretained, it is while the matrix
interference is adsorbed.
Solid phase extraction involves four steps:

Conditioning: It must be exposed to a common
solvent and solution for the material in the cartridge to be solvated and
effective.
Loading: The loading solution containing the
analyte is percolated through the solid phase. Ideally, the analyte and
some impurities are retained on the sorbent.
Washing: A solvent or solution is used to wash
the interferents from SPE material without removing the analytes of
interest.
Elution: During this elution step, the compound
of interest is collected.

Applications

           Solid-Phase Extraction (SPE) is a
sample preparation step which is important in determination of specific
compounds in foods. SPE offers many opportunities not only for analysis of a
large diversity of food samples but also for optimization and advances. 5

           Besides, in recent years, SPE
technique enables a high-throughput DNA extraction. Moreover, in food analysis, SPE technique is widely
used for clean-up. Classical gravity column chromatography has been largely
replaced by SPE. 6

           Furthermore, the applications of SPE
are determination of sympathomimetic amines in blood, urine and tissues,
Benzodiazepines screening in urine, determination of cocaine and cocaine
metabolites and anabolic steroids in urine and Lysergic acid diethylamide (LSD)
in blood, serum or plasma.

Liquid-liquid Extraction

          Liquid – liquid extraction is also known as solvent extraction. It is a
separation process which is based on the different distribution of the
components to be separated between two liquid phases. It depends on the mass
transfer of the component to be extracted from a first liquid phase to a second
one. For instance, the isolation of trimyristin form nutmeg is an example of
solid-liquid phase extraction. It is also possible to partition the components
of a mixture between two immiscible liquids such as liquids that will not
dissolve in each other and form two distinct phases when combined.7

            There are two general types of
liquid-liquid extractions which are organic solvent extraction and acid-base
extraction. For an organic solvent extraction, an organic solvent with high
affinity for the desired compound is used to extract the compound from another
solution. For an acid-base extraction, in which an organic acid or base is
extracted from an organic solvent by using an aqueous solution of an inorganic
base or acid, respectively. A neutralization occurs which converts the compound
into an ionic, water-soluble salt, causing it to transfer from the organic
phase to the aqueous phase.7

Extraction with organic solvents

            Liquid-liquid
extractions normally involve an organic solvent and water. Organic solvent is
immiscible in water and the most common organic solvents are diethyl ether,
ethyl acetate, toluene and methylene chloride. If 50 mL of ethyl acetate and 50
mL of water are placed in a flask and the solution is stirred to mix it, a
homogeneous solution will not be obtained. After stirring, if the solution can
stand, two distinct liquid phases will form in the flask: the lower layer is
the denser solvent and the upper layer is less dense solvent.7

            Most
organic solvents are much less polar than water. Polar compounds are more
soluble in polar solvents than in nonpolar solvents, and non-polar is more
soluble in non-polar solvent.

Depending on the selective solubility of
different compounds in polar versus nonpolar solvents, separation by LLE is
carried out.7

            The
main principle of liquid-liquid extraction is the distribution
coefficient or the partition coefficient, Kd,
where7

K is the ratio of solubility of solute
dissolved in the organic layer to the solubility of material dissolved in the
aqueous layer. 

 

Applications

            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.9

Solid 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.12

First, 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.

Applications

            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. 11

            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. 11

Microextraction by Packed Sorbent (MEPS)

            Micro
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. 14

Procedure of MEPS

First, the sample is pumped through the MEPS
BIN which some volumes may be taken. Next, the MEPS BIN is washed once by
pumping 20 µL to 50 µL of wash solution through the BIN in order to remove
interferences. Then, the analyte is eluted by drawing solvent through the BIN
into the syringe barrel. Finally, the analyte is injected directly into the
injector. 50 µL solvent is pumped and followed by 50 µL wash solution to
prepare BIN for the next sample.13

Applications

            MEPS has been successfully applied
in many fields. First, in environmental application, it is used to determine
organic priority pollutants. In addition, MEPS is also used in determining emerging
compounds in snow samples and wastewater too. Besides, MEPS is also used in
forensics industry. For example, contribution of miroextraction in packed
sorbent for analysis of cotinine in human urine by GC-MS. Moreover, MEPS is
widely used in pharmaceutical industry. For instance, liquid chromatographic
analysis of oxcarbazepine and its metabolites in plasma and saliva. MEPS is
used in determination of 2,4,6-trichloroanisole and 2,4,6-tribromoanisole in
wine. Lastly, MEPS is useful in life science industry. For example, rapid and
sensitive method for determination of cyclophosphamide in patients’ plasma
samples. 13

Stir Bar Sorptive Extraction (SBSE)

            Stir
bar sorptive extraction (SBSE) is a solvent-free extraction technique. Most
conventional extraction techniques such as solid phase extraction, liquid-liquid
extraction and so forth are slower than SBSE. There is more sorbent volume can
be taken by the stir bar. Besides, due to the efficient stirring, SBSE can also
extract and concentrate analytes from a sample volume which is large. Hence, SBSE
is more sensitive than SPME which is up to 1000x. The Twister might be look
alike a conventional magnetic stirring rod, but while stirring the samples,
such as water, or other beverages, it will absorb and concentrate organic
compounds into its sorbent coating. By using multi-position stir plates, extraction
of many samples can be carried out simultaneously. The result of this is high
productivity. By using SBSE, some preparation steps are eliminated such as labour-
and resource-intensive sample preparation steps. By using thermal desorption,
desorption of analytes from the Twister is taken place. If the analytes are
very high-boiling, thermally labile, or when they must be determined by HPLC,
liquid desorption using a solvent is performed.15

            In
addition, the basic concept of SBSE is based upon sorption, which is a form of
partition based upon the analyte’s dissolution in a liquid-retaining polymer
from a liquid or vapor sample, thus, originating a bulk retention. The main
advantages of sorption are related to high inertness of PDMS, which gives
better performance for labile, polar, or reactive compounds; absence of
catalytic degradation reactions; analyte recovery mechanism based upon a
well-known chromatographic process; and linearity of sorption isotherms, which is
fundamental for quantitative analysis.16

Applications

            By
using PDMS Twister, the application are pesticides in water, PAH in marine
tissue, determination of 2,4,6-TCA in wine and flavor compounds in food.
Furthermore, by using Silicone Twister, the application is analysis the flavor
compounds in beverages. 15

            Besides,
SBSE is also in production of ultra-pure water, heavy metal recovery, recovery
of organic acids from salts and in oil emulsion and pulp mill waste treatment.
In addition, it also used in the enzyme and pharmaceutical preparation.

Recent Advances/ Development

            A
recent advance in the application of HS-SPME is an internally cooled coated
fibre device (CCF). It is also called as cold fibre HS-SPME device. The main purpose
for developing CCF is there is lots of improved release of analytes from the
interfering phases in complex matrices.17

            It
is used to increase the temperature of extraction for desorption of the analytes
from solid particles. However, exothermic process is carried out when the
absorption of analytes by the fibre coating. Thus, a decrease of the partition
coefficients will be caused by the process. The objective is to increase the
mass transfer process, by this, it increases the distribution constants of
analytes too. For that purpose, an internally cooled coated fibre device which
is known as CCF was developed which the sample matrix is heated at the same
time while the fibre coating is cooled down. CCF is used for the matrices especially
it is high viscosity or for volatiles with low partition coefficients. The
extraction of analytes from various environmental matrices was successfully
done by this method. In addition, the application of CCF is food analysis too. Besides,
a cold-fibre HS-SPME is more sensitive and having higher sample throughput than
conventional HS-SPME. But, there is a drawback which is increased fibre
capacity through this cooled coated fibre leads to the loss of selectivity. It
is not only the analytes, the interferences are also affected which are
extracted exhaustively onto the coating.17