Chapter 33

Supercritical Fluid Chromatography

Dong-Sun Lee/ CAT-Lab / SWU

Supercritical Fluid Chromatography (SFC)

In supercritical fluid chromatography (SCF) the mobile phase is a supercritical gas or a near critical liquid. Compared to gas chromatography, where gas is under ambient pressure, and liquid chromatography, where liquid is used as mobile phase, the solvent power of the fluid mobile phase in SFC can be varied by density, e.g., by pressure changes at constant temperature. Solubility generally increases with pressure under supercritical conditions of the mobile phase. Since temperature is near the critical temperature of the mobile phase, temperature sensitive compounds can be processed. Chromatographic separation can be carried out at constant pressure (isobaric operation) or with increasing pressure (pressure programmed). In addition, temperature can be varied. Temperature directly determines the vapour pressure of the feed components and the density of the mobile phase and, indirectly, adsorption equilibrium. With higher temperature, vapour pressures of the feed components increase exponentially. Density decreases proportionally to temperature if conditions are far from critical, but in the region of the critical point of the mobile phase, which is the main area of application of supercritical fluid chromatography, density varies dramatically with temperature. In this region, the solvent power of the mobile phase, which increases with density, is substantially changed.

Introduction of the physico-chemical properties of the supercritical fluids

A pure supercritical fluid (SCF) is any compound at a temperature and pressure above the critical values (above critical point). Above the critical temperature of a compound the pure, gaseous component cannot be liquefied regardless of the pressure applied. The critical pressure is the vapor pressure of the gas at the critical temperature. In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described as intermediate to the two extremes. This phase retains solvent power approximating liquids as well as the transport properties common to gases.

Supercritical fluid, as its called, is heavy like liquid but with penetration power of gas. These qualities make supercritical fluids effective and selective solvents.

Property Density (kg/m3 ) Viscosity (cP) Diffusivity (mm2 /s)

Gas  1  0.01  1-10

SCF 100-800 0.05-0.1  0.01-0.1

Liquid  1000  0.5-1.0  0.001

A comparison of typical values for density, viscosity and diffusivity of gases, liquids, and SCFs

Some of the advantages and disadvantages of SCFs compared to conventional liquid solvents for separations:

Advantages

• Dissolving power of the SCF is controlled by pressure and/or temperature

• SCF is easily recoverable from the extract due to its volatility

• Non-toxic solvents leave no harmful residue

• High boiling components are extracted at relatively low temperatures

• Separations not possible by more traditional processes can sometimes be effected

• Thermally labile compounds can be extracted with minimal damage as low temperatures can be employed by the extraction

 Disadvantages

• Elevated pressure required

• Compression of solvent requires elaborate recycling measures to reduce energy costs

• High capital investment for equipment

Solvents of supercritical fluid extraction

The choice of the SFE solvent is similar to the regular extraction. Principle considerations are the followings.

• Good solving property

• Inert to the product

• Easy separation from the product

• Cheap

• Low PC because of economic reasons

Carbon dioxide is the most commonly used SCF, due primarily to its low critical parameters (31.1°C, 73.8 bar), low cost and non-toxicity. However, several other SCFs have been used in both commercial and development processes.

Fluid Critical Temperature (K) Critical Pressure (bar)

Carbon dioxide 304.1 (31.3oC)  73.8Ethane 305.4 48.8Ethylene 282.4 50.4Propane 369.8 42.5Propylene 364.9 46.0Trifluoromethane (Fluoroform) 299.3 48.6Chlorotrifluoromethane 302.0 38.7Trichlorofluoromethane 471.2 44.1Ammonia 405.5 113.5Water 647.3 (374.1 oC) 221.2Cyclohexane 553.5 40.7n-Pentane 469.7 33.7Toluene 591.8 41.0

Compound Tcº C Pc atm d*

CO2 31.3 72.9 0.96

C2H4 9.9 50.5 ---

N2O 36.5 72.5 0.94

NH3 132.5 112.5 0.40

n-C5 196.6 33.3 0.51

n-C4 152.0 37.5 0.50

CCl2F2 111.8 40.7 1.12

CHF3 25.9 46.9 ---

H2O 374.1 218.3 ---

Typical Supercritical Solvents

* Density in g/ml at 400 atm.

chromatography-liquid (LC); flow programming; isothermal supercritical fluid chromatography; programmed density/pressure/temperature supercritical fluid chromatography;

The coal tar separation shown here using three methods indicates the type of differences one encounters using the three methods. If we select gas chromatography, with a temperature gradient from 80C to 250C over the 50-minute retention time, we end up with a reasonable separation. Cooling the column would have to be added to the time of separation.

Using a Polarity gradient in HPLC going from 50 to 100% acrylonitrile, an acceptable separation can be effected in forty minutes.

Turning finally to SFC, a pressure ramp is used in the solvent, such that solvent density rose from 0.225g/L to 0.7 g/ml over the retention period of 120 minutes. Certainly the last is not the best analytical solution, however, it does eliminate the need for acrylonitrile recover and recycle. The recompression of the supercritical solvent would seem to be simpler. However to the man in the lab, liquid chromatography is the clear winner.

The art and scientific of adding cosolvents or modifiers in SFC is one that has had a great deal of academic and industrial attention. Generally the cosolvent or modifier is a material added to the supercritical solvent at low percentage, which makes a significant change to the solvent properties. For example the selectivity of 2-naphthol with respect to anthracene can be enhanced greatly by adding methanol to supercritical CO2. Unmodified CO2 will give a selectivity of abo

ut 7. It rises to 30 it 3.5% methanol is added as a cosolvent.

Two good solvents for non-polar mixtures are CO2 and ethylene. Predicting the

effect of cosolvents is a tricky business, since one is dealing with exchange between solid surfaces and a mixed solvent. However it is important that we make progress in understanding this frontier of phase equilibria. One major area where limited progress has been achieves is in ionic materials. A great deal of work has focused on proton acceptors or donors. There has been interest in dipolar molecules, surfactants and other more complex molecules. Progress in the last groups has been the result of clever detective work, Edison experimentation, and in a few cases dumb luck. Hydrogen bonding is an area where significant work is still required. The concept of producing gradients in cosolvents promises to be a powerful weapon.

From a prediction standpoint the prediction of the behavior of cosolvents in chromatography is far more complex than the equivalent extraction case. In the latter there is a physical equilibrium of the solutes between a liquid or solid phase and the solute-cosolvent mixture. The foundations for such predictions are firmly laid. Dealing with unknown solids, such as soil can greatly increase the complexity and quickly lead to intractable situations. The case of chromatography involves exchange of multiple solutes between a supercritical mixture and solutes adsorbed in often-unresolved way on the surfaces and pores of an active solid. This latter situation is currently theoretically intractable. The result is that one must rely almost totally upon the experimentation to develop chromatographic cosolvent system.

The figure to the left shows the result on solubility of modifying cosolvents in the in a chromatographic separation or ortho benzoic acid. Note that both cosolvents have a dramatic effect upon solubility - almost a factor of a 50 at the lower temperatures. As the temperature rises, the methanol proves to be the better cosolvent. It is an interesting exercise to theorize why this trend occurs.

The use of gradients in cosolvents are not a viable proposition in industrial applications, because of the additional separation required at the end of the process to separate the mixed solvent.

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Supercritical Extraction

For over a quarter of a century supercritical fluids, primarily carbon dioxide, have been used as a solvent in extraction processes performed under supercritical conditions. Carbon Dioxide has several properties that recommend it for this duty. Its critical temperature is 31.3C, making near room temperature operations possible. It is non-toxic, non-flammable and approved by FDA for use in food and pharmaceutical plants. Its critical pressure is 72.9 atm., which is considered moderate. Its properties have been exhaustively studied, so it continues to be the extraction solvent of choice. Caffeine removal is an excellent example of the application of this technology.

Most supercritical Fluid Extraction processes are quite simple and follow the scheme shown in the following Fig. A sample is placed in the Sample thimble, and supercritical fluid is pumped through the thimble. The extraction of the soluble compounds is allowed to take place as the supercritical fluid passes into a collection trap through a restricting nozzle. The fluid is vented in the collection trap, allowing the solvent to either escape or be recompressed for future use. The material left behind in the collection trap is the product of the extraction. Obviously this is a batch process. This is acceptable for the analytical purpose to which the method is applied, but could not be considered commercially, unless there was some extreme purpose to which the process was being applied.

Applications of SFC

Chiral separations

Fatty acid separations

Vitamin separations

Purification of pharmaceuticals

Nutraceutical products extractions

Essential oils from plant

Photoresist cleaning

Fractionation of polymers

Impregnations and reactions

Industrial applications

The special properties of supercritical fluids bring certain advantages to chemical separation processes. Several applications have been fully developed and commercialized.

Food and flavouring

SFE is applied in food and flavouring industry as the residual solvent could be easily removed from the product no matter whether it is the extract or the extracted matrix. The biggest application is the decaffeinication of tea and coffee. Other important areas are the extraction of essential oils and aroma materials from spices. Brewery industry uses SFE for the extraction of hop. The method is used in extracting some edible oils and producing cholesterine-free egg powder.

Petrolchemistry

The destillation residue of the crude oil is handeled with SFE as a custom large-scale procedure (ROSE Residum Oil Supercritical Extraction). The method is applied in regeneration procedures of used oils and lubricants.

Pharmaceutical industy

Producing of active ingradients from herbal plants for avoiding thermo or chemical degradation. Elimination of residual solvents from the products.

Other plant extractions

Production of denicotined tobacco.

Environmental protection

Elimination of residual solvents from wastes. Purification of contaminated soil.