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Chapter 5 Analytical Supercritical Fluid Extraction for Food Applications Tracy Doane-Weideman and Phillip B. Liescheski Isco Incorporated, Lincoln, NE 68504 Abstract In this review, we explore the fundamental concepts of supercritical fluids and supercritical fluid extractions. Carbon dioxide and other solvents are discussed; the solubility theory is introduced together with the calculation of the density of carbon dioxide. The state-of-the-art instrumentation is presented in terms of fundamental components. The most widely used application of analytical SFE is in the food industry and this review includes fats, oils, vitamins, and pesticides in research and routine applications. Introduction Supercritical fluid extraction (SFE) is becoming an important sample preparation method in the chemical analysis of food products, especially for fats and fatty oils. SFE has been used successfully for over a decade in analyses of food samples (1,2). The most popular SFE solvent is carbon dioxide (CO 2 ). Triglycerides, cho- lesterol, waxes, and free fatty acids are quite soluble in supercritical CO 2 . The sol- ubility of polar lipids, such as phospholipids, can be improved by augmenting the supercritical CO 2 with a small addition of ethanol or other polar modifier solvent. Even though CO 2 is considered a “green-house” gas, it is ubiquitous in nature and can be retrieved from the environment and returned clean (3). As a result, SFE can still contribute positively to “Green Chemistry.” CO 2 has the additional advantage of being nonflammable and less toxic than most organic solvents. For example, petroleum ether, which is commonly used in fat extractions, can be easily detonat- ed by static electricity, and diethyl ether can form explosive peroxides. On the other hand, some fire extinguishers use CO 2 , which is also commonly found in foods and drinks such as bread and carbonated drinks. Finally, several common chlorinated solvents are banned by law, and supercritical CO 2 can be an alternative to these solvents. All of these factors make SFE attractive. What Is a Supercritical Fluid? A supercritical fluid is a dense gas (4). It is compressible and thus expands to com- pletely fill its container. A liquid, on the other hand, takes the shape of its container Copyright © 2004 AOCS Press but does not expand to fill the container. Instead it settles at the bottom. Supercritical fluids, unlike the air we breathe, have densities comparable to liquids. As a result, these fluids have solvating power. A supercritical fluid can be defined as a form of matter in which the liquid and gaseous phases are indistinguishable (5). The three most common phases of matter on earth are solid, liquid, and gas. The phase of a pure simple substance depends on the temperature and pressure. A plot showing a substance's phase for a given temperature and pressure is called a phase diagram. Figure 5.1 is a phase diagram for CO 2 . In a phase diagram, the solid, liquid, and gas regions are divided by branches or equilibrium curves. These curves represent valuable information concerning the substance's melting, boiling, or sublimation temperatures at given pressures. These curves characterize the tem- peratures and pressures at which two phases coexist in equilibrium. For example, the liquid-gas equilibrium curve divides the liquid and gaseous phase regions. On this curve, the substance coexists as both a liquid and gas (vapor) in equilibrium. When the temperature and pressure change so that the substance leaves the liquid phase region and crosses the equilibrium curve into the gas phase region, the sub- stance boils. As its state crosses this curve, there is an entropy change. In the case of boiling, its entropy increases and absorbs heat, known as heat (enthalpy) of vaporization. In the case of condensation, its entropy decreases and liberates heat, known as heat of condensation. An obvious physical change is seen in the sub- stance as its state crosses one of these curves. Fig. 5.1. Phase diagram of carbon dioxide. Temperature (°C) Pressure (atm) Copyright © 2004 AOCS Press The three equilibrium curves (solid-gas, solid-liquid, liquid-gas) intersect at a common point, called the triple point. At this point, the substance coexists in equi- librium with all three phases. Each substance has only one triple point. The solid- liquid equilibrium curve radiates from the triple point to infinity. The solid-gas equilibrium curve radiates from the triple point and eventually terminates at absolute zero and vacuum. The liquid-gas equilibrium curve does not radiate indef- initely from the triple point but terminates at another important point, called the critical point. This point is the critical temperature and critical pressure of the sub- stance. Beyond the critical point, there is no longer an equilibrium curve to divide the liquid and gaseous regions; thus, the liquid and gas phases are no longer distin- guishable. There are no physical changes observed as the substance's state crosses over this region. This region of the phase diagram is sometimes called the super- critical fluid region. The critical temperature is the temperature above which the substance can no longer be condensed into a liquid. Increasing the pressure will not induce conden- sation. For a liquid that partially fills a tube, the liquid's meniscus disappears when it is heated above the critical temperature. The critical pressure is the vapor pres- sure of the substance at its critical temperature. It is also the maximum vapor pres- sure of the substance because at a higher temperature, the liquid phase cannot be distinguished from its vapor. Based on the critical point, a supercritical fluid can also be defined as the state beyond the critical temperature and critical pressure of the substance. The critical temperature for CO 2 is 31.1°C, and its critical pressure is 72.84 atm. Supercritical fluids can be considered on the molecular level. Molecules have both kinetic and potential energies. The kinetic energy is related to the motion of the molecules, which depends on the temperature. The potential energy is related to the Van der Waal force, the close proximity attractive interaction between mole- cules. The potential energy of molecules depends on how close they are to each other. This attractive force between solvent molecules and solute molecules allows for solvation, the dissolving process. It also allows solvent molecules to “stick” together into clusters, thus forming a liquid. The molecules aggregate locally but there is no long-range order as observed in solid crystals. In the liquid phase, exter- nal pressure is not required to keep the molecules close together because they already “stick” together. However, these “sticky” molecules also give rise to high- er surface tension, viscosity, and slower diffusion. Such properties can hinder an extraction process. In supercritical fluids, the temperature is above the solvent's critical temperature. At these higher temperatures, molecules move more quickly and thus have a higher kinetic energy. This higher kinetic energy reduces the sig- nificance of the potential energy to a point at which the molecules no longer “stick” together. As a consequence, lowered surface tension, viscosity, and faster diffusion allow supercritical fluids to perform better during extraction. Lower sur- face tension allows the fluid to “wet” surfaces better and to penetrate more deeply into small pores and features. However, higher pressure is required to keep the Copyright © 2004 AOCS Press molecules close together to maintain the molecular attraction necessary for solva- tion. The pressure must be at least above the critical pressure. Higher pressure yields higher density at a constant temperature. In turn, higher density yields greater solvating power. Advantages and Disadvantages The most popular SFE solvent is carbon dioxide. There are several reasons for its popularity. First, CO 2 is inexpensive and commercially available even at high puri- ty. Second, it is nonflammable, unlike many organic solvents, and is used in some fire extinguishers. It also does not support combustion, except in the extraordinary case of burning magnesium. Third, CO 2 is relatively nontoxic, especially in com- parison to many organic solvents; it is actually present in air, foods, and drinks. Some caution must be followed with the use of CO 2 . Because CO 2 does not sup- port combustion or human respiration, it can be an asphyxiant at high concentra- tions. Fourth, its critical temperature is low, allowing it to be used to extract ther- mally liable analytes. Its critical temperature and pressure are easily attainable. As a comparison, the critical temperature of water, 374.0°C, is a challenge for many materials. Finally, CO 2 is environmentally compatible. Even though it is consid- ered a “green-house” gas, it is ubiquitous in nature. Other solvents have been used in SFE, but they have serious drawbacks. Nitrous oxide has been used in extracting environmental samples, but it is a strong oxidizing agent. An explosion was reported due to its use with organic materials (6). Nitrous oxide, also known as “laughing gas,” has narcotic properties. Propane has been used in the extraction of fats from food samples, but it is highly flamma- ble and a small leak in the extractor plumbing could be a disaster. Ammonia is polar and has a practical critical point, but it is a strong, corrosive base and is toxic and obnoxious. Fluoroform is also polar and has a practical critical point, but it is expensive and not readily available (7). It may also damage the environment. Freons have proven to be excellent SFE solvents, but they are suspect in damaging the environment, especially the ozone layer. As a result, they are being banned. Water, which is environmentally friendly, has too high a critical temperature to be practical. Also, its pK w approaches 1 at this temperature, making it quite corrosive to steels and other metals. Carbon dioxide does have a few disadvantages. First, it is practically the only solvent for SFE, as shown in the previous paragraph. Even though supercritical flu- ids offer flexible solubility depending on pressure, CO 2 still has limited solvating power. As a rule of thumb, its solvent strength is comparable to that of hexanes. Because it is nonpolar, extracting polar analytes can be a challenge. Fortunately, the solvent strength of CO 2 can be enhanced by the addition of a small amount of polar modifier solvent or a surfactant agent. The extraction of polar analytes can be improved by the addition of a small amount of ethanol. In the area of supercritical fluid cleaning, DeSimone and colleagues (8,9) developed fluorinated dendritic sur- Copyright © 2004 AOCS Press factants that significantly enhanced the cleaning performance of CO 2 when added in small amounts. Second, the high pressure necessary for SFE is a concern because some people are still uncomfortable with such pressures. Current technol- ogy makes such pressure safe to use in the laboratory; however, it renders the equipment expensive. To reduce the cost of the equipment, sample size is restricted because smaller high-pressure vessels are safer and less expensive. Smaller sam- pling size can be a disadvantage though for nonhomogeneous sample matrices. Smaller sampling sizes can also reduce the detection sensitivity of the analytical method and increase measurement error. These disadvantages must be addressed to develop a successful SFE application method. Giddings-Hildebrand Solubility Theory The solubility of analytes in a supercritical fluid can be treated by thermodynamics. The Gibbs free energy of the mixing process can be described by the Gibbs- Helmholtz equation: ∆G mix = ∆H mix – T∆S mix For the process of solvation to proceed spontaneously, the value of the Gibbs free energy of mixing ∆G mix must be negative. Because the solvation (dissolution) process increases the disorder of the analyte-solvent system, the entropy of mixing ∆S mix is expected to have a large positive value. The spontaneity of the solvation process ultimately depends on the heat of mixing ∆H mix . A smaller value predicts greater solubility (10). Using the earlier work of van Laar and Lorenz on the vapor pressure of binary liquid mixtures according to the Van der Waal equation, Hildebrand and Scott showed that the heat of mixing ∆H mix can be expressed as follows: ∆H mix = ϕ s ϕ n (a s 1/2 /V s – a n 1/2 /V n ) 2 where ϕ s ϕ n is the partial volume factor, V s and V n are the molar volumes for the solvent and analyte, and a s and a n are the Van der Waal intermolecular attraction parameters for the solvent and analyte. They further showed that a 1/2 /V = (∆E v /V) 1/2 where ∆E v is the energy of vaporization of either the liquid solvent or liquid analyte. The heat of mixing ∆H mix is produced from the breaking and reformation of attractive forces between solvent-solvent, analyte-analyte, and solvent-analyte molecules. Intuitively, it should be related to their energy of vaporization. Seeing the physical sig- nificance of this formula in reference to solvation, they defined it as the solubility parameter δ, also known as the Hildebrand solubility parameter: Copyright © 2004 AOCS Press δ = a 1/2 /V = (E v /V) 1/2 so that ∆H mix = ϕ s ϕ n (δ s – δ n ) 2 where δ s and δ n are the Hildebrand solubility parameters for solvent and analyte (11). To keep the heat of mixing as small as possible, the solubility parameter of the solvent should be similar in value to the solubility parameter of the analytes. A smaller difference gives higher solubility. The solubility parameter for a supercritical fluid cannot be determined from the energy of vaporization as with liquids because the liquid and vapor phases cannot be distinguished (12). Giddings and colleagues assumed that a supercritical fluid sol- vent can be described qualitatively by the Van der Waal state equation. The intermole- cular attraction parameter can be related to the critical values of the solvent as a = 3P c V c 2 where P c and V c are the critical pressure and critical molar volume, respectively (13). Upon substitution, they obtained δ s = (3P c ) 1/2 V c /V The volumetric ratio can be written in terms of the reduced density as V c /V = ρ/ρ c = ρ r giving δ s = (3P c ) 1/2 ρ r From experimental data, they were led to the better estimate δ s = 0.47 P c 1/2 ρ r where δ s is in units of (cal/cm 3 ) 1/2 , and P c is in atmospheres (14). King and Friedrich showed that the reduced solubility parameter ∆, defined as ∆ = δ n /δ s is a good indicator of analyte solubility in a supercritical fluid. Solubility improves as the reduced solubility parameter approaches 1. They correlated solubility data using solubility parameters for analytes estimated by Fedors’ method (15). One advantage of the reduced solubility parameter is that it is unitless. Figure 5.2 relates the density of supercritical CO 2 with its corresponding Hildebrand solubility parameter based on Giddings’ formula. The solubility para- Copyright © 2004 AOCS Press meters for a few common solvents are also included for comparison. Table 5.1 contains the Hildebrand solubility and reduced solubility parameters for a few common lipid analytes as reported in the literature and estimated by Fedors’ method (16). Fedors’ group contribution method estimates an analyte’s solubility parameter solely from information about its molecular structure (17). Table 5.2 contains an example of a calculation for estimating the Hildebrand solubility para- meter by Fedors’ method. It can be incorporated into a spreadsheet for calculating other fatty acids and their corresponding glycerides. Even though higher temperatures at a given pressure would lower the density of the supercritical fluid, the overall extraction performance should be enhanced. First, supercritical fluids can solvate liquids better than solids. Performing an extraction at a temperature above the melting point of the analyte should improve the recovery. Second, temperature affects the solubility parameter of the analytes. The values pre- sented in Tables 5.1 and 5.2 are for 25°C; higher temperatures tend to reduce the ana- lyte’s solubility parameter. According to King, a temperature increase of 60°C can reduce an analyte’s solubility parameter by 1.0–1.5 cal 1/2 /cm 3/2 (18). As a final point, CO 2 Density (g/mL) Solubility Parameter (cal/cc) 1/2 Fig. 5.2. Carbon dioxide density vs. Hildebrand solubility. Copyright © 2004 AOCS Press solubility is less of a concern for trace-level analytes, such as pesticides in foods. The above discussion applies mainly to analytes present at high levels in the sample where solubility saturation could be an issue. Much lower pressures may actually be suffi- cient for analytes on the ppb level. CO 2 Density Calculations The solvent strength of supercritical CO 2 can be determined from its density as shown previously. Its density is related to its pressure and temperature. Unfortunately, the ideal gas law is useless because a supercritical fluid is far from being an ideal gas. The Van der Waals equation predicts certain qualitative properties of a supercritical fluid, but it is not quantitatively accurate. A better state equation is required for dense gases. TABLE 5.1 Hildebrand Solubility and Reduced Solubility Parameters for Lipid Analytes a Hildebrand solubility parameter Reduced solubility parameter Reported Fedors’ method in CO 2 at 80°C and Analyte MPa 1/2 MPa 1/2 (cal/cm 3 ) 1/2 200 atm 400 atm 600 atm Pentane 14.5 14.5 7.1 0.71 0.99 1.11 Hexane 14.9 14.9 7.3 0.69 0.96 1.08 Heptane 15.3 15.2 7.4 0.68 0.95 1.07 1-Butanol 23.1 23.2 11.3 0.45 0.62 0.70 1-Octanol 20.9 21 10.3 0.49 0.68 0.77 Stearyl alcohol 19.3 9.4 0.54 0.75 0.84 Hexyl acetate 17.3 17.7 8.7 0.58 0.81 0.91 Methyl oleate 17.7 8.6 0.59 0.82 0.92 Stearyl stearate 17.6 8.6 0.59 0.82 0.92 Tripalmitin 18.6 18.3 9.0 0.56 0.78 0.88 Triolein 1 8.5 18.3 8.9 0.57 0.79 0.89 Tristearin 17.9 18.2 8.9 0.57 0.79 0.89 Distearin 19.3 9.5 0.53 0.74 0.83 Monostearin 22 10.8 0.47 0.65 0.73 Glycerol 36.1 40.9 20 0.25 0.35 0.39 Acetic acid 21.4 22.8 11.2 0.45 0.63 0.70 Butyric acid 18.8 21.2 10.3 0.49 0.68 0.77 Palmitic acid 18.8 9.2 0.55 0.77 0.86 Oleic acid 18.7 9.1 0.55 0.77 0.87 Stearic acid 18.7 9.1 0.55 0.77 0.87 Cholesterol 20.7 19.6 9.6 0.53 0.73 0.82 a Source: References 15,16. Copyright © 2004 AOCS Press According to the Law of Corresponding States, two gases with the same reduced temperature and reduced pressure are in corresponding states. Both gases should have the same reduced density. A reduced state parameter is the state para- meter divided by its corresponding critical value. For example, the reduced tem- perature is the temperature divided by the critical temperature of the gas. Even though quantitatively inaccurate, the Van der Waal equation predicts the Law of Corresponding States (19). For dense gases, the deviations from the ideal gas law can be treated by the compressibility factor Z which is defined as Z = M w P/(ρRT) where, M w , P, T and ρ are the molecular weight, pressure, temperature, and weight density of the gas, and R is the ideal gas constant. For ideal gases, Z = 1. The com- pressibility factor can be expressed as a function of reduced temperature and reduced pressure: Z(T r ,P r ). According to the Law of Corresponding States, gases in corresponding states should have the same compressibility factor Z value. In the engineering literature, there are isotherm plots of Z vs. reduced pressure at various reduced temperatures, which are universal for all gases (20). Using these graphs, along with knowledge of the critical point for a gas, the weight density of any dense gas can be determined as follows: ρ = M w P/[RT Z(T/T c , P/P c )] Even though the Law of Corresponding States is not exact, it is sufficiently accu- rate for practical engineering calculations. TABLE 5.2 Example for Estimating the Hildebrand Solubility Parameter by Fedors’ Method Diolein: CH 2 –COO–(CH 2 ) 7 –HC=CH–(CH 2 ) 7 –CH 3 CH–COO–(CH 2 ) 7 –HC=CH–(CH 2 ) 7 – CH 3 CH 2 –OH in i U i (cal/mol) n i ⋅U i V i (cm 3 /mol) n i ⋅V i CH 3 2 1125 2250 33.5 67 CH 2 30 1180 35400 16.1 483 CH 1 820 820 –1 –1 HC= 4 1030 4120 13.5 54 OH 1 7120 7120 10 10 COO 2 4300 8600 18 36 COOH 0 6600 0 28.5 0 Σ n i ⋅U i = 58310 Σ n i ⋅V i = 649 δ = (Σ n i ⋅U i /Σ n i ⋅V i ) 1/2 9.48 (cal/cm 3 ) 1/2 | | Copyright © 2004 AOCS Press Pitzer and colleagues improved upon the Law of Corresponding States by intro- ducing an acentric factor ω for each gas. The acentric factor is a measure of the devia- tion of the entropy of vaporization from that of a simple fluid (21). Pitzer's work is based on a virial expansion treatment of dense gases (22). Using the acentric factor ω, the compressibility factor Z can be determined more accurately by Z(T r ,P r ) = Z 0 (T r ,P r ) + ω Z 1 (T r ,P r ) where Z 0 and Z 1 are obtained from their published tables through linear interpola- tion. Using this improved compressibility factor Z, a more accurate value for the density of a gas can be calculated using the formula in the previous paragraph. Figure 5.3 is a contour plot of the density of CO 2 for various temperatures and pressures based on Pitzer’s work. As noted earlier, supercritical CO 2 can be augmented with another modifying solvent to enhance its solubility for challenging analytes. These binary solvent mixtures have different critical values and solubility parameters. Their new values can be calculated using the modified Handinson-Brobst-Thomson equations: V b = 1/4 [(x s V s + x m V m ) + 3(x s V s 2/3 + x m V m 2/3 )(x s V s 1/3 + x m V m 1/3 )] T cb = [x s 2 x s T cs + 2x s x m (V s V m T cs T cm ) 1/2 + x m 2 V m T cm ]/V b Fig. 5.3. Contour plot of CO 2 density. Temperature (°C) Pressure (atm) Copyright © 2004 AOCS Press [...]... Reverchon, E., and Stassi, A (1998) Almond Oil Extraction by Supercritical CO2 Experiments and Modeling, Chem Eng Sci 21: 3711–3718 41 del Valle, J.M., and Uquiche, E.L (2002) Particle Size Effects on Supercritical CO2 Extraction of Oil- Containing Seeds, J Am Oil Chem Soc 79: 1261–1266 42 Official Methods and Recommended Practices of the American Oil Chemists’ Society, 5th ed (1996) Am3-96, Oil in Oilseeds:... 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Copyright © 2004. n i ⋅V i CH 3 2 11 25 2 250 33 .5 67 CH 2 30 1180 354 00 16.1 483 CH 1 820 820 –1 –1 HC= 4 1030 4120 13 .5 54 OH 1 7120 7120 10 10 COO 2 4300 8600 18 36 COOH 0 6600 0 28 .5 0 Σ n i ⋅U i = 58 310 Σ n i ⋅V i =