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SUPERCRITICAL FLUID

SUPERCRITICAL FLUID EXTRACTION (SFE)

SUPERCRITICAL FLUID CHROMATOGRAPHY (SFC)

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What is a supercritical fluid?

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A supercritical fluid is any substance at a temperature and pressure above its critical point.

It can diffuse through solids like a gas, and dissolve materials like a liquid.

Additionally, close to the critical point, small changes in pressure or temperature result in large changes in density, allowing many properties to be "tuned".

Supercritical fluids are suitable as a substitute for organic solvents in a range of industrial and laboratory processes.

Supercritical Fluid

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In the Supercritical region the substance is neither a gas nor a liquid – it is a fluid that has properties of both.

There are no sharp boundaries between gas and liquid.

Properties of SCFs can be very different from the normal liquid phase.

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Critical properties of various solvents

Solvent Molecular weight

Critical temperature

Critical pressure

Critical density

g/mol K MPa (atm) g/cm3

Carbon dioxide (CO2) 44.01 304.1 7.38 (72.8) 0.469

Water (H2O) 18.015 647.096 22.064 (217.755)

0.322

Methane (CH4) 16.04 190.4 4.60 (45.4) 0.162

Ethane (C2H6) 30.07 305.3 4.87 (48.1) 0.203

Propane (C3H8) 44.09 369.8 4.25 (41.9) 0.217

Ethylene (C2H4) 28.05 282.4 5.04 (49.7) 0.215

Propylene (C3H6) 42.08 364.9 4.60 (45.4) 0.232

Methanol (CH3OH) 32.04 512.6 8.09 (79.8) 0.272

Ethanol (C2H5OH) 46.07 513.9 6.14 (60.6) 0.276

Acetone (C3H6O) 58.08 508.1 4.70 (46.4) 0.278

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Some Characteristics of Supercritical fluid

• Above the critical temperature no phase transition regardless of the applied

pressure

• supercritical fluid has physical and thermal properties that are between those of the pure liquid and gas fluid density is a strong function of the temperature

and pressure diffusivity much higher a liquid

readily penetrates porous and fibrous solids Low viscosity Recovery of analytes

Return T and P

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Carbon dioxide is known to be the most stable and an excellent solvent compound and is normally used in mobile phases for supercritical fluid chromatography (SFC) as well as in supercritical fluid extraction (SFE)

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Carbon dioxide pressure-temperature phase diagram

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This is not …

Above the critical point, the phase boundary (meniscus)between liquid and vapor phases disappears, and thesubstance is a single homogeneous fluid.

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One homogenous phase called the "supercritical fluid”

Carbon Dioxide

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Comparison of Gases, Supercritical Fluids and Liquids

Density (kg/m3)

Viscosity (µPa∙s)

Diffusivity (mm²/s)

Gases 1 10 1-10

Supercritical Fluids

100-1000 50-100 0.01-0.1

Liquids 1000 500-1000 0.001

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SUPERCRITICAL FLUID EXTRACTION (SFE)

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Extraction in analysisOften the analysis of complex materials requires as a preliminary step, separation of the analyte or analytes from 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 that 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.

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Supercritical Fluid Extraction

• SF combines desirable properties of gases and liquids Solubility of liquids Penetration power of gases

• Process flexibility: Density of SF and solubility of a solute in it can be changed in a continuous manner by change of pressure

• Environmental perspective: Innocuous substances such as water and carbon dioxide can be used as extracting solvents instead of organics

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A practical approach of SFE

• Sample is placed in thimble or extraction cell• Supercritical fluid is pumped through the

thimble extraction of the soluble compounds is

allowed to take place as the supercritical fluid passes into a collection trap through a restricting nozzle

fluid is vented in the collection trap solvent to escapes or is recompressed

• Material left behind in the collection trap is the product of the extraction batch process

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SFE principle: (1)Solute solubility

Solubilityof solute in

SF(m/v)

Pressure

• Density of a SF increases with pressure i.e. more solvent molecules per unit volume

• Pressure packs the solvent molecules closer and facilitates the entrapment of more solute molecules

• Dependence of solubility on pressure can be utilized to fine-tune the SFE process

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(2) Enhanced penetration

• Diffusivity of solvent molecules in a SF approach gaseous state diffusivity

• Solute diffusivity within a SF approaches that shown in gaseous phase

Solvent (SF)

Solute

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Supercritical Fluid Extraction

Advantages of supercritical fluid extraction (SFE):

a. SFE is generally fast. The rate of mass transfer between a sample matrix and an extraction fluid is determined by the rate of diffusion of a species in the fluid and the viscosity of the fluid—the greater the diffusion rate and the lower the viscosity, the greater will be the rate of mass transfer.

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b. The solvent strength of a supercritical fluid can be varied by changes in the pressure and to a less extent in the temperature.

c. Many supercritical fluids are gases at ambient condition.

d. Some supercritical fluid are cheap, inert, and nontoxic.

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SFE 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.

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Instrumentation – important components

1. a tank of the mobile phase, usually CO2, 2. a pump to pressurize the gas, 3. an oven containing the extraction vessel, 4. a restrictor to maintain a high pressure in the extraction line, 5. a trapping vessel.

Analytes are trapped by letting the solute-containing supercritical fluid decompress into an empty vial, through a solvent, or onto a solid sorbent material.

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Supercritical Fluid Extraction

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Extractions can be performed in dynamic, static, or combination modes.

In a dynamic extraction the supercritical fluid continuously flows through the sample in the extraction vessel and out the restrictor to the trapping vessel.

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In static mode the supercritical fluid circulates in a loop containing the extraction vessel for some period of time before being released through the restrictor to the trapping vessel.

In the combination mode, a static extraction is performed for some period of time, followed by a dynamic extraction.

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Modifier

For the purpose of increasing the polarity of the supercritical CO2 (and thus its extracting efficiency for more polar components), additives such as methanol is added to the fluid. This is known as modifier.

Other examples of modifier include ethanol, 1-propanol and acetonitrile

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Some applications of SFE

• Removal of grease and other fouling material • Removal of impurities from chemical

products

• Breaking of azeotropes • Fractionation and purification of polymers

e.g. removal of unchanged monomers from polymers

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In the food and pharmaceutical industries, SFE is used in

• Decaffeinating of coffee and tea • Extraction of essential oils (vegetable and fish oils) • Extraction of flavors from natural resources

(nutraceuticals) • Extraction of ingredients from spices and red peppers • Extraction of fat from food products • Fractionation of polymeric materials • Extraction from natural products • Photo–resist cleaning • Precision part cleaning

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SUPERCRITICAL FLUID CHROMATOGRAPHY (SFC)

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Introduction

Supercritical fluid chromatography (SFC) is a hybrid of gas and liquid chromatography that combines some of the best features of each.

Supercritical fluid chromatography is of importance because it permits the separation and determination of a group of compounds that are not conveniently handled by either gas liquid or liquid chromatography.

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Supercritical fluid chromatography (SFC)

a. SFC is a chromatographic technique in which the mobile phase is a supercritical fluid.

a. The use of a supercritical fluid mobile phase in chromatography was first proposed in 1958 by J. Lovelock. The first actual report use of this in a chromatographic system was in 1962 by Klesper et al, who used it to separate thermally-labile porphyrins.

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Supercritical fluid chromatography (SFC)

c. SFC is of importance because it permits the separation and determination of a group of compounds that are not conveniently handled by either GC or LC.

These compounds (1) are either non-volatile or thermally labile so that the GC are in-applicable, and (2) contain no functional groups that make impossible to be detected by means of spectroscopic or electrochemical techniques employed in LC.

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Supercritical fluid chromatography

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Theory of SFC

Since supercritical fluids have properties between those of gases and liquid, their use as a mobile phase offers several advantages.

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a. One advantage is that supercritical fluid have lower densities and viscosities than liquids. This results in larger diffusion coefficients for solutes in SFC than LC.

This results in better efficiencies and higher optimum linear velocities in SFC than LC.

Advantages of SFC:

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The plate height of a SFC System is given by the van Deemter equation.

H = A + B/u + Cu

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b. SFs have higher densities than gas, so that mobile phase has a greater chance of interacting with the solute than that in GC (i.e., carrier gas). This makes the mobile phase important in determining the retention of solutes on the system and give more flexibility in optimizing the separation.

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For example, retention of solutes in SFC can be changed by using a different column (i.e. different stationary phases) as in GC, or by changing the mobile phase strength as in LC.

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isobaric Pressure programmingFlow-rate programming

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c. One major advantage of SFC is its ability to use detector available for either GC or LC, such as FID, UV-Vis, and Fluorescence detectors. This gives it a wide range of both universal and selective detections for use in either analytical or preparative-scale work.

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LC detectors:

Thermal conductivity detector (TCD): 10-7 M (103-fold range)

Flame Ionization detector (FID): 10-10 M (a 105-fold range)

Nitrogen-phosporus detector (NPD): 10-10 M (a 106-fold range)

Electron capture detector (ECD): 10-14 ~ 10-16 M (a 103- 104 fold range)

Flame photometric detector(FPD):10-14 M (P, S)

Electrochemical detector (S, halogen,nitrogen-)

Refractive Index Detector(10-5 to 10-6 M)

Absorption Detector (UV/Vis)

(10-8 M)

Fluorescence Detector(10-10 M)

Conductivity Detector

(10-6 M)

Electrochemical Detector

(10-11 M)

GC detectors:

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Schematic of a gas chromatograph

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Schematic of a liquid chromatograph

Pump

Injector

ColumnDetector

Mobile Phases

Gradient Controller

•

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Instrumentation for SFC can be obtained commercially or adapting system used for either LC and GC.

The chromatograph is generally consists of :

1. Gas supply, usually CO2, 2. Pump, 3. Injector4. Oven5. Column in a thermostat-controlled oven, 6. Detector7. Recorder/computer

(A restrictor is also required to maintain the high pressure in the column, placed after the detector)

Instrumentation for SFC

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The main difference of a SFC than a LC or GC system is the need to control both temperature and pressure of mobile phase. This must be done to keep the mobile phase as a supercritical fluid.

Control of the pressure (density) of the supercritical fluid can also used to vary strength of mobile phase during the gradient elution in SFC .

The column is usually a capillary GC column, but packed LC columns can also be used. The FID is the most common detector, but other GC or LC detectors can also be used.

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Supercritical fluid chromatograph

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Depending on which supercritical fluid is used, it is also possible to use SFC at lower T than GC. This makes it more useful in the separation of thermally unstable compounds.

The stationary phases used in SFC can be similar to those in LC as well as GC. Either packed or open-tubular columns may be used.

Because of these advantages, SFC is commonly viewed as a technique which is complementary to both LC and GC.

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There are two types of analytical columns used in SFC, packed and capillary.

Packed columns contain small deactivated particles to which the stationary phases adhear. The columns are conventionally stainless steel. Capillary columns are open tubular columns of narrow internal diameter made of fused silica, with the stationary phase bonded to the wall of the column.

Stationary Phases / Columns

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Mobile Phases

• The most widely used mobile phase for SFC 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.

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Effects of Pressure

• Pressure changes in supercritical chromatography have a pronounced effect on the capacity factor k’. This effect is a consequence of the increase in density of mobile phase with increase in density of the mobile phase with increases in pressure.

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Detectors

SFC is compatible with both HPLC and GC detectors. As a result, optical detectors, flame detectors, and spectroscopic detectors can be used. However, the mobile phase composition, column type, and flow rate must be taken into account when the detector is selected as they will determine which detector is able to be used. Some care must also be taken such that the detector components are capable of withstanding the high pressures of SFC.

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Detectors

• A major advantage of SFC over HPLC is that the flame ionization detector of gas chromatography can be employed. Mass spectrometers are also more easily adapted as detectors for SFC than HPLC.

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Applications

SFC has been applied to a wide variety of materials, including •Natural products, •Drugs, •Foods, •Pesticides and herbicides, •Surfactants, •Polymers and polymer additives, •Fossil fuels, and •Explosives and propellants.

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Dimethylpolysiloxane: non-volatile and special function groups

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