INTRODUCTION:
Solvent extraction
(SFE) is one of the old methods of separation known AND certainly dates back to
Paleolithic age. The science of solvent extraction has evolved over a long
period of time and much progress has been made in the understanding of solvation and the properties of liquid mixtures used in
extraction processes. Hannay and Hogarth’s (1879) early observations of the dissolution of medium. However, it
is only quite recently (around1960) that commercial process applications of
supercritical fluid extraction have been
extensively examined.
Since the end of the 1970s, supercritical fluids have been used to isolate
natural products; industrial applications of SFE have experienced a strong
development since the early 1990s in terms of patents As
will be seen throughout this paper, the main supercritical solvent used is
carbon dioxide. Carbon dioxide (critical conditions tc=31.3◦C
and pc= 72.8 bar,dc=0.467gm/ml)
is cheap, environmentally friendly and generally recognized as safe by FDA and EFSA.[1,2]
Definition:
Supercritical fluid
extraction:
Supercritical fluid extraction
(SFE) may be defined as separation of chemicals, flavors from the products such
as coffee, tea, hops, herbs, and spices which are mixed with supercritical
fluid to form a mobile phase. In this process, the mobile phase is subjected to
pressures and temperatures near or above the critical point for the purpose of
enhancing the mobile phase solvating power. The process begins with CO2 in
vapor form. It is then compressed into a liquid before becoming supercritical.
While supercritical, the extraction takes place. [3]
Critical conditions:
Temperature
(tc)= 30.9
◦C
Pressure
(pc)=73.8 bar
Density
(dc)=0.467gm/ml [4,5]
Principle:
The first guiding
principle is the optimization of the solubility of materials to be extracted
(lipids, heavy metals, natural products) in supercritical CO2 and
the improvement of the fractionation with respect to a particular lipid species,
natural products. [2]
History:
The first reported observation of the occurrence of a
supercritical phase was made by Baron Cagniard de la
Tour in 1822.
He
noted visually that the gas-liquid boundary disappeared when the temperature of
certain materials was increased by heating each of them in a closed glass
container.
From these early experiments, the critical point of a
substance was first discovered.
The first workers have been done to demonstrate the
solvating power of supercritical fluids for solids in 1879.
In 1970 a significant development in supercritical fluid
extraction (SFE), provided incentive for extensive future work, which involved
decaffeination of green coffee with CO2.
Supercritical fluid:
The supercritical fluid
extraction (SFE) has been applied only recently to sample preparation on
an analytical scale This technique resembles” Soxhlet extraction” except that the solvent used is a supercritical fluid,
substance above its critical temperature and pressure. This fluid provides a
broad range of useful properties. One main “advantage” of using
SFE is the elimination of organic solvents, thus reducing the problems of their
storage and disposal in the lipidologist laboratory.
Furthermore, several legislative protocols (such as the EPA Pollution
Prevention Act in the USA) have focused on advocating a reduction in the use of
organic solvents which could be harmful to the environment.
Besides ecological
benefits, one of the most interesting properties of SFE is the high diffusion
coefficients of lipids in supercritical fluids, far greater than in
conventional liquid solvents. Thus, the extraction rates are enhanced and less
degradation of solutes occurs. Several studies have shown that SFE is a
replacement method for traditional gravimetric techniques. In addition, carbon
dioxide, which is the most adopted supercritical fluid has low cost, is a
nonflammable compound and devoid of oxygen, thus protecting lipid samples
against any oxidative degradation. [6]
The definition of a
supercritical fluid is best described by using a typical pressure-temperature
phase diagram as shown in Figure No: 1.
The range of solvating
power of practical supercritical fluids for SFC is of primary importance,
and ultimately defines the limits of application. The solubility of analytes typically increases with density and a maximum
rate of increase in solubility with pressures generally observed near the
critical pressure, where the rate of increase of density with pressure is
greatest. There is often a linear relationship at constant temperature between
log [solubility and fluid density for dilute solutions of nonvolatile compounds
(up to concentrations where solute-solute interactions become important). At
constant pressure, when solute volatility is extremely low, and at densities
less than or near the critical density, increasing temperature will typically
decrease solubility. However, solute entrainment in the fluid may increase at
sufficiently high temperatures, where solute vapor pressure also becomes
significant .Under conditions of constant density, solubility generally
increases with temperature. Thus, while the highest supercritical fluid
densities (at constant temperature) are obtained near the critical temperature,
the greatest solubilities and lowest chromatographic
retention will often be obtained at somewhat lower densities, but at higher
temperatures. As with liquids, polar solutes are most soluble
in polar supercritical fluids, although nominally non polar fluids can be
remarkably good solvents for many moderately polar compounds. Carbon
dioxide, for example, can exhibit solvating properties at higher pressures,
intermediate between liquid n-pentane and dichloromethane. A comparison of the effective solvent
polarity of seven fluids as a function of reduced density is shown in Figure
No: 2 Solvent polarity is defined in terms of solvent polarizability
(x*) which was developed by Kamlet et aL75 to
correlate different solventsolute interactions based
on the solvatochromic effect of the solvent on the
x-x* electronic transition of probe solutes. In this plot, x* contains terms to
account for solvent polarity (i.e., dipolarity) and polarizability, but does not include effects from potential
hydrogen bonding interactions. At equal reduced densities, the various fluids
have quite different x* values, indicating that there are large differences in
their effective polarities/polarizabilities. Ammonia
has the largest x* value, which supports the fact that it is the most polar
solvent.
Figure 1: Shows Pressure-temperature
phase diagram demonstrating the supercritical fluid region and its relation to
liquid- and gas-phase regions. [6]
Figure 2: Shows Solvent
polarizability/polarity parameter (x*) for various supercritical
fluids as a function of reduced density at a reduced temperature of 1.03.
Supercritical fluids: (a)NH,, (b) CO,,
(c) N,O, (d) Xe, (e) CCl,(F)
C,H, (g) SF.[7]
The solvatochromic
method also demonstrates the variable solvent properties of a supercritical
fluid as a function of density. Many polar solvents would offer highly specific
solvating power but have excessively high critical temperatures, precluding
practical operation with current stationary phases. The thermostability
limits of the analytes themselves can also be
exceeded. This has generated interest in mixed or binary fluid mobile phases
that can have enhanced solvating power at lower critical temperatures. Solvatochromic studies suggest that such fluid mixtures
have a net enrichment of polar modifiers in the cybotactic
region (neares neighbor solvation
sphere) of the analyte [8]
Characteristics of
SF:
It is both the
liquid-like and gas-like characteristics of supercritical fluids that make them
unique for chemical separation. In particular, supercritical fluid densities,
diffusivities, and viscosities fall into ranges between those of liquids and
gases. Under practical analytical operating conditions, pressures from
50-5OOatm and temperatures from ambient to 3OO0C, densities of supercriticalfluids range from one to eight-tenths of their
liquid densities. Diffusivities of analytes in
supercritical fluids throughout this operating range vary between10-3
and 10-4cm’/s compared to values of less than 10-5cm2/s
for liquids. Viscosities of supercritical fluids are typically 10-100 times
less than those of liquids. On the other hand, viscosities of supercritical
fluids are considerably higher and diffusivities considerably lower than in
gases. Moreover, densities of supercritical fluids can be 100-1000 times
greater than those of gases. Advantages of supercritical fluids over liquid
phases rest with improved mass transfer processes due to lower fluid
viscosities and higher analyte diffusivities, while
advantages over gas phases rest with increased molecular interactions due to
higher densities.
Other
characteristics of supercritical fluids
That is important to
consider include the operational temperature and pressure range. Table.1
provides a list of nine of the most common supercritical fluids used in
extraction and chromatography along with temperature, pressure, density, and
dipole moment information. These nine are chosen primarily because of the
convenience of their critical temperatures and critical pressures. These
temperatures and pressures are low enough for use with commercial
instrumentation. The polarity of the supercritical fluid, as
reflected in its dipole moment and polarizability.
The density at 400 atm (p and I; = 1.03 was
calculated from compressibility data. ‘Measurements were made under saturated
conditions if no pressure is specified or were
performed at 25°C if no temperature is specified. [8]
Properties of
supercritical fluids:-
A supercritical fluid is any substance above its critical
temperature and critical pressure. In the supercritical area there is only one
state-of-the-fluid and it possesses both gas- and liquid-like properties.
A supercritical fluid
exhibits physicochemical properties intermediate between those of liquids and
gases.
Characteristics of a
supercritical fluid:
Dense gas
Solubility’s
approaching liquid phase
Diffusivities
approaching gas phase. [9,10]
Figure No: 3 Shows Phase diagram (P---T):
Critical Temperature
(Tc):
The
highest temperature at which a gas can be converted to a liquid by an increase
in pressure.
Critical pressure (Pc):
The
highest pressure at which a liquid can be converted to a traditional gas by an
increase in temperature.
Triple point (Tp):
A point at which the
gas, liquidAND solide
phases all exist in equilibrium. Therefore, the properties of gas-like
diffusivity, gas-like viscosity, and liquid-like density combined with
pressure-dependent solvating power have provided the impetus for applying
supercritical fluid technology to various problems. [11] All
the above terms are mentioned in Figure No: 3.
Density
considerations:
For a material at
temperatures just above the critical temperature of the substance, liquid-like
densities are rapidly approached with modest increases in pressure. Higher
pressures are required to attain liquid-like densities for temperatures further
above the critical temperature Lists the densities at the critical point and at
400 atm and Tc for various
fluids employed for SFE.
Characteristics of
Super-critical Fluids Relevant to Separation Science:
In the absence of actual phase equilibria
data, simple mole fraction additivity methods used to
obtain mixture critical parameters can result in considerable error and lead to
inadvertent operation in the vapor-liquid region. More complex predictive
methods utilizing equations of state[12],[13]
or surface fraction functions (Chueh and Prausnitz method)[14] generally provide
more accurate estimates of the true critical parameters. These considerations
are important when pressure programming methods are used, but are of lesser
importance when relatively high isobaric pressures are used.
Extraction method:
Often the analysis of
complex materials requires as a preliminary step separation of the analyte or analytes form a sample
matrix. Ideally, an analytical separation method should be rapid, simple and
inexpensive; should give quantitative recovery of analytes
without loss or degradation; should yield a solution of the analyte
this is sufficiently concentrated to permit the final measurement to be made
without the need for concentration; and should generate little or no laboratory
wastes that have to be disposed of.[16]
Figure 4: Shows Relation between the Extraction time(min.) and Extracted
amount(%)
It must be noticed that
the fast back-diffusion of analytes in the
supercritical fluid reduces the extraction time since the complete extraction
step is performed in about 20 min instead of several hours, shown in Figure
No: 4. A common practice in SFE, which must be mentioned in connection
-with the physicochemical properties of supercritical fluids, is the use of
modifiers (co-solvents). [17]
Modifiers
(co-solvents):
These are compounds
that are added to the primary fluid to enhance extraction efficiency. Thus,
addition of 1 to 10% of methanol or ethanol to CO2 expands its
extraction range to include more polar lipids. When the extraction was
performed with supercritical carbon dioxide and 20% of ethanol, more than 80%
of the phospholipids were recovered from salmon roe.[18]
Instrumentation:
Instrument components
include a fluid source, commonly a tank of carbon dioxide followed by a syringe
pump having a pressure rating of at least 400 atm a
valve to control the flow of the critical fluid into a heated extraction cell
having a capacity of a few ml, and lastly an exit valve leading to a flow
restrictor that depressurizes the fluid and transfers it into a collection
device. Figure No: 5 shows the flow diagram of
SFE aparatus.
Figure No: 5 Shows SFE Flow diagram.
1.
Mobile phase:
Mobile phase: The most widely used mobile phase for SFE is carbon
dioxide. It is an excellent solvent for a variety of organic molecules. In
addition, it transmits in the ultraviolet and is odorless, nontoxic, readily
available, and remarkably inexpensive when compared with other chromatographic
mobile phases which has been shown in Table No: 1.
Table No: 1 Shows Comparison of the physical properties of supercritical CO2 and those
of ordinary gases and liquids [19]
|
Phases |
Density (g/cm2) |
Viscosity (g/cm.s) |
Diffusion coefficient
(cm2/s) |
|
Gasses |
0.0001-0.002 |
0.0001-0.0003 |
0.1-0.4 |
|
Supercritical CO2 |
0.47 |
0.0003 |
0.0007 |
|
Liquids |
0.6-1.6 |
0.002-0.03 |
0.000002-0.00002 |
Sample
Matrix Parameters that influence Supercritical Fluid Extraction:
Particle
size and shape
Surface
area and porocity
Moisture
content
Changes
in morphology
Sample
size
Extractables level
The
parameters effect on solubility:
The vapor pressure of the component
Interaction
with the supercritical fluid
Temperature, pressure, density and additives.
A
generalized solubility isotherm for a solute-supercritical fluid system as a
function of pressure and at two different temperatures, r, and T2, is shown in Figure
No: 6. Upon initial pressurization of the system, there is a decrease in
solute solubility in going from the respective pressures designated by points A
and A’ to B and B’. At a certain pressure beyond B and B’, the solute’s
solubility begins to increase with pressure. Frequently, this pressure regime
is called the “threshold pressure” [20], since there
is a large measurable solubility -enhancement of the solute in the dense fluid
solvent. However, it has been noted [21] that the above-reported
solubility trends and threshold pressures are very dependent on the technique
that is utilized to measure the solute’s solubility in the supercritical fluid
media. However, the differential extraction behavior-
Figure 6: Shows Generalized solubility isotherms as a function of pressure.
-exhibited
between points A and A’ or B and B’ can obviously be
used as a basis for the selective extraction of target analytes.
Similarly, fractionation of solute mixtures can be performed in the pressure
interval between B or B’ and C and C’, although the relative separation factor
between individual solutes is not always large. Note that the solubility
isotherms may cross at a particular pressure called the “cross-over pressure” [22],
at which the solubility of one solute can- be enhanced in the fluid phase
relative to the other. Solute fractionation at the solubility
maxima, C and c’. As shown in Figure-14, is also possible, but the
resultant a values may be low, since many solutes will extract into the
supercritical fluid at these high pressures. For this reason, some analysts
avoid conducting extractions in the solubility maxima region. However, as shown
by King and co-workers [23], this pressure region is to be
preferred for exhaustively extracting bulk phases, such as lipid materials from
insoluble sample matrix components. Also, extractions conducted in this region
generally can be completed much more rapidly, since the solutes have
considerably higher solubility in the supercritical fluid under these
conditions of equal importance in the above solubility criteria are the mass
transfer properties of the extracted solutes in the supercritical fluids.
Solute extraction fluxes from a sample matrix are directly proportional to the
product of the solute’s solubility in the supercritical fluid times its
diffusivity in the fluid. Therefore, as a solute’s solubility increases with
pressure, its corresponding diffusivity in the super-critical fluid can
decrease over two orders of magnitude. The net effect of the above two trends
can best be measured in terms of mass transfer coefficients or dimensionless
transport numbers. For example, the ratio of the Reynolds number (Re) for CO,
at 200 atm and 55°C to those for the liquid solvents
cited in Table No: 2, at an equivalent fluid velocity, is 6.5, 5.0, and
1.74 for methanol, n-hexane and methylene chloride,
respectively.
Table No: 2 Shows Comparison of physical properties of supercritical co2 with
liquid solvents at 250c (T 1.9)
|
Parameter |
CO2 |
n-Hexane |
Methylene Chloride |
Methanol |
|
Density
(g/mL) |
0.746 |
0.660 |
1.326 |
0.791 |
|
Kinematic
Viscosity (m2/s x 107) |
1.00 |
4.45 |
3.09 |
6.91 |
|
Diffusivity
of Benzoic acid (m2/s x 109) |
6.0 |
4.0 |
2.9 |
1.8 |
In
this case, the larger fluid turbulence that occurs in the CO2 should greatly
enhance the rate of solute extraction. The kinetics for solute extraction into
a supercritical fluid follow a similar pattern to that observed for liquid
extraction. As we know initial stage of the extraction is governed by the
distribution coefficient of the solute between the dense fluid -phase and the
sample matrix, giving way to a. diffusion-controlled process in the latter
stages of the extraction. The implications of the curve shown in Figure No:
7 on the extent and time of SFE has been treated theoretically by Bartle
and co-workers [24] in terms of the “hot ball” model, where
the mass of extractable material remaining in the sample matrix m to the mass
of original extractable material mo is given by
(1)
Where,
n is an integer; D is the diffusion coefficient of the Solute in the
hypothetical spherical matrix of radius r; and t is the extraction time. This
Figure 7: Shows Generalized extraction curve of percent solute extracted as a function
of volume of extraction fluid or time of extraction.
Expression
can be rewritten in terms of reduced time tr
= π2Dt/r2 , to
yield an expression for m/mo in terms of an exponential decay series
expansion. The final expression, given in (2) is
(2)
The
latter equation has been found to describe analytical SFE kinetics from such
diverse sample matrices as railroad bed soil, crushed rosemary, and comminuted
polypropylene pellets.
In
many cases, slow solute extraction kinetics or limiting analyte
solubility in the fluid phase, can be overcome by the addition of modifiers or cosolvents to the supercritical fluid phase. Examples of
solubility enhancements for selected solutes that have been realized by adding modifiers
into supercritical CO2 are shown in Table No: 3.
Table.3 Shows Solubility enhancement with supercritical co2 with various
modifiers
|
Solute |
Modifier |
Enhancement
Factor |
|
Acridine |
3.5%
MeOH |
2.3 |
|
2-Amino
benzoic acid |
3.5%
MeOH |
7.2 |
|
Cholesterol |
9%
MeOH |
100 |
|
Hydroquinone |
2%Tributyl
phosphate |
>300 |
|
Tryptophan |
0.53%
AOT,w0=10 |
>>100 |
|
|
5%Octanol |
|
The
addition of methanol to CO2 not only enhances the solubilization of polar solutes, such as acridine and 2-amino benzoic acid, but increases the
solubility of highly soluble lipophilic solutes, like
cholesterol, over lOO-fold. Certain specific
modifiers, such as tributyl phosphate, act as complexing agents [25], thereby enhancing
the extraction of a donor molecule, hydroquinone, over 300-fold.
Reduced
solubility parameter:
δ1 : solubility parameter of the fluid
Pc : the critical pressure
ρ : the density of the supercritical fluid
ρliq:
the density of the liquid gas
δ2 : solubility parameter of solute
Δε: the
energy of vaporization at a given temperature
Δν:
the corresponding molar volume
Fluid
reservoir:
A gas cylinder provides a
source of SF (e.g., CO2).Both syringe and reciprocating pumps can be used as solvent delivery
systems
Pumps:
a)
Reciprocating pump, [26] b) Syringe pump, [27] c)
Other pump modules (like supplementary modifier pump)
For the instrumentation used in
some analysis, a syringe pump was employed. Although syringe pumps are
relatively expensive, they deliver pulse-free flow over a large range of flow
rates.
Example:
Quantitative Analysis of Additives in Low Density
Polyethylene Using On-line Supercritical Fluid Extraction. A supplementary
modifier pump is used if the analyte/ matrix to be
extracted requires a polar modifier. Stainless steel or fused silica tubing is
used to connect the various parts of the extraction apparatus.
Extraction
cell (or) Columns (stationary phase): The extraction chamber or vessel is the
compartment where the sample is placed for subjection to the action of the SF.
It must be capable of withstanding high pressure (300-600 atm).
The extraction vessel is usually a stainless steel cylinder of varying length
and inner diameter shown in Figure No: 8. The
high pressure rating and the absence of leaks are characteristic of SFE vessels
1. The vessel is in turn placed in a temperature-controlled zone, which is
required, since the critical temperature of most SFs is above room temperature.
Figure 8: Shows Types of extraction cells
a)
Open tubular capillary columns:-
Open tubular columns for SFE
must possess the usual qualities of high efficiency, inertness, and lasting
stability, which .are characteristic of open tubular columns for GC. The main
differences in the preparation of the columns are related to the smaller
internal diameters characteristic of SFE columns. Immobilization (generally
cross-linking of the polymeric phase) is an essential ingredient in the
preparation of open tubular columns. It must be performed to resist
dissolution, but without lowering solute diffusion within the phase. [9]
b)
Packed columns:-
In the packed column, the
stationary phase is normally near monomolecular thickness and is polymerized
and chemically bonded to the support. Both
open-tubular and packed columns are used for SFC although currently the former
are favored. Open-tubular columns are similar to the fused-silica columns with
internal coatings of bonded and crossed-linked siloxanes
of various types. For example In the on-line SFE-SFC
system used in the additive analtsis, a linear fused silicacapillary was employed as a vessel outlet restrictor. [28],[29]
Restrictors:
The pressure change from
supercritical conditions in the extraction vessel to the prevailing atmospheric
conditions is effected via an interface known as restrictor. Commercially
available restrictors are of two types: fixed restrictors, shown in Figure
No: 9 which are manufactured in various designs (e.g., linear, tapered, integral,
pinhole, and frit), and variable restrictors 1. Heating of the restrictor is
usually required to avoid plugging through freezing. [30]
a)
Fixed restrictors:
i) Linear restrictor (fused-silica)
ii) Tapered desire
iii) Integral restrictor
iv)
Ceramic frit restrictor
v) Metal restrictor
(platinum, platinum-iridium or steel)
b)
Variable restrictors:
i) Variable nozzle (HP)
ii) Backpressure regulator (BPR) (Jasco)
Figure No: 9 Shows Fixed restrictors
Collector
(trapping system):
Following the restrictor is a
trapping device. There are three
basic types of SFE systems characterized by the way in which the solutes are isolated
from the SFE media used
In the first type,
solutes are separated from the extraction media based on pressure reduction,
which causes a solubility decrease.
In the second
type, a temperature change isused to bring about a
decrease in solubility from the extraction media,
And in the third typesolutes are absorbed onto an appropriate absorbate.
Often a combination of the
first and second types is used, where after extraction the SF is simply
evaporated to leave the solutes of interest. The simplest way of collection is
when the restrictor outlet is inserted through the septum of a collection vial
containing a few milliliters of solvent. The most
common way of collection is solid phase trapping. The
materials used for this purpose are column packings
or inert surfaces. The solid phase trapping system is often heated or cooled
depending on the volatility of the target analytes.
In any case, this collection mode involves an additional step which is
desorption of the analytes from the adsorbent by
elution with a small amount of solvent for subsequent analysis or,
alternatively, thermal desorption and sweeping of the trap by the eluent if an on-line coupled system is used. The trapping
temperature depends on whether the analytes are to be
isolated from the fluid. The collection chamber should be sealed in order to
avoid losses of the analytes. In this research, a
cryogenic trap served as the interface between SFE and SFC. Thermal desorption
and sweeping the trap with SF CO2 was employed to flush analytes
onto the SFC column
Detectors:
A
major advantage of SFC /SFE over HPLC is that the flame ionization detector of
gas chromatography can be employed. Mass
spectrometers are also more easily adapted as detectors for SFE than HPLC. [31]
i) UV detector
ii)
Fluorescence detector
iii)
Flame ionization detector
iv) Electron capture detector
v)
Mass spectrometric detector
Different
modes of Supercritical fluid extraction:
(i) Static
extraction mode (steady state extraction),
(ii)
Dynamic extraction mode (non-steady
state extraction),
(iii)
Recirculating
mode
Contact between the SF and
sample from which extraction takes place can be established in a static or
dynamic mode 1. In a static extraction, the sample matrix is soaked in a fixed
amount of SF. This type of extraction is often compared to a teabag in a cup of
water. In a dynamic extraction, SF continuously passes through the sample
matrix. This is analogous to a coffee maker 1. Typically a dynamic extraction
can be more exhaustive than a static extraction. SFE can be performed in the
dynamic mode, static mode or a combination of the two. In order to develop an
efficient and quantitative extraction method, many experimental parameters must
be optimized. The extraction pressure is an important variable because the
density, and hence the solvating power of SF is directly related to the
pressure. The effect of temperature is more complicated than that of pressure.
Increasing the temperature increases the diffusion coefficients of the solutes,
whereas at the same time it also decreases the density. In addition, the
considerations of fluid flow rate, addition of a modifier, and extraction time
should be explored to achieve highest recoveries.
Types
of SFEs:
SFE is generally not selective
enough to isolate specific solutes from the matrix without further clean-up or
resolution from co-extracted species prior to qualitative and quantitative
analysis. Consequently, for analytical applications, SFE is usually used in
conjunction with chromatographic techniques, to improve the overall selectivity
of the process in isolating specific solutes. SFE combined with chromatography
can be either
“off-line”
or “on-line”.[32]
i)
Off-line:- In the off-line process, SFE takes place as a separate and isolated
process to the chromatography. A block diagram is shown in Figure No: 10.
ii)
On-line:- In the on-line process, SFE and chromatography are coupled to form an
integrated process. In other words, the extracted species are passed directly
to the chromatograph, usually via a trap or sample loop and a valves witching
device shown in Figure No: 11.
Among all these coupling
techniques, on-line SFE/SFC is the most feasible combination.
a)
SFE-GC
b)
SFE-MS
c)
SFE-LC
d) SFE-SFC
Figure 10: Shows Off-line SFE:
Figure 11: Shows On-line SFE
Advantages
of on-line SFE:
1.
Direct coupling of the analyte-containing
supercritical fluid to a chromatographic separation system with appropriate
detection.
2.
Eliminating sample handling after loading in the extraction
Disadvantages
of on-line SFE:
1.
Long periods of time
2.
Understand the nature of analytes
Scaling-up SFE and preparation of the crude extract: After the SFE conditions were optimized,
an ideal optimization conditions are shown in Table No: 6 the extraction
was scaled up by 100 times using a preparative system. 5 kilograms amount of
sample (40–60 mesh) was placed into an extraction vessel with a 1.0×104 ml
capacity, and extracted statically for 1 h followed by another 5 h dynamically
under the optimized conditions at 45 ◦C, 25MPa. The flow-rate of carbon
dioxide supercritical fluid was set at 40 kg/h, and the extract in
supercritical fluid was depressed directly into a separate vessel. The SFE
extract before methanol washing (crude extract I) was light yellow semi-solid
and then re-dissolved in methanol, and the methanol soluble fraction
(crude extract II)was obtained and evaporated to dryness under reduced pressure
at 60 ◦C, which was subjected to subsequent HSCCC isolation and
separation.[33]
Applications
SFE:
1. Application of SFE to enantiomeric
separations: Chiral
separation is a very important issue for the pharmaceutical industry. The
applicability of SFE as an effective and green technique for enantioseparations is known since the late 1990s. In these
processes, diastereomeric salts or complexes of the racemic compounds and resolving agents are formed before
the extraction step. The selected resolving agent is added in less than stoichiometric ratio to the racemic
compound. The unreacted enantiomers
are extracted with the supercritical solvent, and are collected as a powder
after depressurization of the solution[34]
2. Metals recovery using
supercritical fluids: Removal
of heavy metals from solid matrices and liquids remain a big challenge and,
although various methods have been described for this purpose, SFE seems to be
one of the most promising. Complexing agents used in
conventional solvent extraction can also be used in SFE complexation
of metal ions
. [