oil extraction and analysis phần 5 pps

A supercritical fluid can be defined as a form ofmatter in which the liquid and gaseous phases are indistinguishable 5.The three most common phases of matter on earth are solid, liquid,

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 andsupercritical fluid extractions Carbon dioxide and other solvents are discussed; thesolubility theory is introduced together with the calculation of the density of carbondioxide The state-of-the-art instrumentation is presented in terms of fundamentalcomponents The most widely used application of analytical SFE is in the foodindustry and this review includes fats, oils, vitamins, and pesticides in research androutine applications

Introduction

Supercritical fluid extraction (SFE) is becoming an important sample preparationmethod 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 (CO2) Triglycerides, cho-lesterol, waxes, and free fatty acids are quite soluble in supercritical CO2 The sol-ubility of polar lipids, such as phospholipids, can be improved by augmenting thesupercritical CO2with a small addition of ethanol or other polar modifier solvent.Even though CO2is considered a “green-house” gas, it is ubiquitous in nature andcan be retrieved from the environment and returned clean (3) As a result, SFE canstill contribute positively to “Green Chemistry.” CO2has 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 theother hand, some fire extinguishers use CO2, which is also commonly found infoods and drinks such as bread and carbonated drinks Finally, several commonchlorinated solvents are banned by law, and supercritical CO2can 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 pletely fill its container A liquid, on the other hand, takes the shape of its container

com-but does not expand to fill the container Instead it settles at the bottom Supercriticalfluids, 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 ofmatter 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 Aplot showing a substance's phase for a given temperature and pressure is called aphase diagram Figure 5.1 is a phase diagram for CO2 In a phase diagram, thesolid, liquid, and gas regions are divided by branches or equilibrium curves Thesecurves represent valuable information concerning the substance's melting, boiling,

or sublimation temperatures at given pressures These curves characterize the peratures and pressures at which two phases coexist in equilibrium For example,the liquid-gas equilibrium curve divides the liquid and gaseous phase regions Onthis 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 liquidphase 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

tem-of boiling, its entropy increases and absorbs heat, known as heat (enthalpy) tem-ofvaporization 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)

The three equilibrium curves (solid-gas, solid-liquid, liquid-gas) intersect at acommon 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-gasequilibrium curve radiates from the triple point and eventually terminates atabsolute zero and vacuum The liquid-gas equilibrium curve does not radiate indef-initely from the triple point but terminates at another important point, called thecritical 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 dividethe 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 crossesover 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 nolonger 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 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 bedistinguished from its vapor Based on the critical point, a supercritical fluid canalso be defined as the state beyond the critical temperature and critical pressure ofthe substance The critical temperature for CO2is 31.1°C, and its critical pressure

pres-is 72.84 atm

Supercritical fluids can be considered on the molecular level Molecules haveboth kinetic and potential energies The kinetic energy is related to the motion ofthe molecules, which depends on the temperature The potential energy is related

to the Van der Waal force, the close proximity attractive interaction between cules The potential energy of molecules depends on how close they are to eachother This attractive force between solvent molecules and solute molecules allowsfor solvation, the dissolving process It also allows solvent molecules to “stick”together into clusters, thus forming a liquid The molecules aggregate locally butthere 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 theyalready “stick” together However, these “sticky” molecules also give rise to high-

mole-er surface tension, viscosity, and slowmole-er diffusion Such propmole-erties can hindmole-er anextraction process In supercritical fluids, the temperature is above the solvent'scritical temperature At these higher temperatures, molecules move more quicklyand 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 fasterdiffusion allow supercritical fluids to perform better during extraction Lower sur-face tension allows the fluid to “wet” surfaces better and to penetrate more deeplyinto small pores and features However, higher pressure is required to keep the

molecules close together to maintain the molecular attraction necessary for tion The pressure must be at least above the critical pressure Higher pressureyields higher density at a constant temperature In turn, higher density yieldsgreater solvating power.

solva-Advantages and Disadvantages

The most popular SFE solvent is carbon dioxide There are several reasons for itspopularity First, CO2is inexpensive and commercially available even at high puri-

ty Second, it is nonflammable, unlike many organic solvents, and is used in somefire extinguishers It also does not support combustion, except in the extraordinarycase of burning magnesium Third, CO2is 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 CO2 Because CO2does 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 manymaterials Finally, CO2 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 strongoxidizing agent An explosion was reported due to its use with organic materials(6) Nitrous oxide, also known as “laughing gas,” has narcotic properties Propanehas 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 ispolar and has a practical critical point, but it is a strong, corrosive base and is toxicand obnoxious Fluoroform is also polar and has a practical critical point, but it isexpensive and not readily available (7) It may also damage the environment.Freons have proven to be excellent SFE solvents, but they are suspect in damagingthe 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 wapproaches 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 onlysolvent for SFE, as shown in the previous paragraph Even though supercritical flu-ids offer flexible solubility depending on pressure, CO2 still has limited solvatingpower 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 CO2can be enhanced by the addition of a small amount ofpolar modifier solvent or a surfactant agent The extraction of polar analytes can beimproved by the addition of a small amount of ethanol In the area of supercriticalfluid cleaning, DeSimone and colleagues (8,9) developed fluorinated dendritic sur-

factants that significantly enhanced the cleaning performance of CO2when added

in small amounts Second, the high pressure necessary for SFE is a concernbecause some people are still uncomfortable with such pressures Current technol-ogy makes such pressure safe to use in the laboratory; however, it renders theequipment expensive To reduce the cost of the equipment, sample size is restrictedbecause 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 analyticalmethod and increase measurement error These disadvantages must be addressed todevelop 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:

∆Gmix= ∆Hmix– T ∆SmixFor the process of solvation to proceed spontaneously, the value of the Gibbs freeenergy of mixing ∆Gmixmust be negative Because the solvation (dissolution)process increases the disorder of the analyte-solvent system, the entropy of mixing

∆Smixis expected to have a large positive value The spontaneity of the solvationprocess ultimately depends on the heat of mixing ∆Hmix A smaller value predictsgreater solubility (10)

Using the earlier work of van Laar and Lorenz on the vapor pressure of binaryliquid mixtures according to the Van der Waal equation, Hildebrand and Scottshowed that the heat of mixing ∆Hmixcan be expressed as follows:

∆Hmix= ϕsϕn(a s1/2/V s – a n1/2/V n)2where ϕsϕnis the partial volume factor, V s and V nare the molar volumes for the

solvent and analyte, and a s and a nare the Van der Waal intermolecular attractionparameters for the solvent and analyte They further showed that

a1/2/V = ( ∆Ev /V)1/2where ∆Evis the energy of vaporization of either the liquid solvent or liquid analyte.The heat of mixing ∆Hmixis produced from the breaking and reformation of attractiveforces 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 solubilityparameter δ, also known as the Hildebrand solubility parameter:

δ = a1/2/V = (E v /V)1/2

so that

∆Hmix = ϕsϕn (δs – δn)2where δs and δn are the Hildebrand solubility parameters for solvent and analyte(11) To keep the heat of mixing as small as possible, the solubility parameter ofthe solvent should be similar in value to the solubility parameter of the analytes Asmaller difference gives higher solubility

The solubility parameter for a supercritical fluid cannot be determined fromthe energy of vaporization as with liquids because the liquid and vapor phases cannot

be distinguished (12) Giddings and colleagues assumed that a supercritical fluid 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

sol-a = 3P c V c2where P c and V c are the critical pressure and critical molar volume, respectively(13) Upon substitution, they obtained

δs = (3P c)1/2 V c /V

The volumetric ratio can be written in terms of the reduced density as

V c /V = ρ/ρc = ρrgiving

δs = (3P c)1/2 ρrFrom experimental data, they were led to the better estimate

δs = 0.47 P c1/2 ρrwhere δs is in units of (cal/cm3)1/2, and P c is in atmospheres (14) King andFriedrich showed that the reduced solubility parameter ∆, defined as

∆ = δn /δs

is a good indicator of analyte solubility in a supercritical fluid Solubility improves

as the reduced solubility parameter approaches 1 They correlated solubility datausing solubility parameters for analytes estimated by Fedors’ method (15) Oneadvantage of the reduced solubility parameter is that it is unitless

Figure 5.2 relates the density of supercritical CO2 with its correspondingHildebrand solubility parameter based on Giddings’ formula The solubility para-

meters for a few common solvents are also included for comparison Table 5.1contains the Hildebrand solubility and reduced solubility parameters for a fewcommon lipid analytes as reported in the literature and estimated by Fedors’method (16) Fedors’ group contribution method estimates an analyte’s solubilityparameter solely from information about its molecular structure (17) Table 5.2contains an example of a calculation for estimating the Hildebrand solubility para-meter by Fedors’ method It can be incorporated into a spreadsheet for calculatingother fatty acids and their corresponding glycerides

Even though higher temperatures at a given pressure would lower the density ofthe supercritical fluid, the overall extraction performance should be enhanced First,supercritical fluids can solvate liquids better than solids Performing an extraction at atemperature 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 canreduce an analyte’s solubility parameter by 1.0–1.5 cal1/2/cm3/2 (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.

solubility is less of a concern for trace-level analytes, such as pesticides in foods Theabove discussion applies mainly to analytes present at high levels in the sample wheresolubility 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 CO2can be determined from its density asshown 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 supercriticalfluid, but it is not quantitatively accurate A better state equation is required for densegases

TABLE 5.1

Hildebrand Solubility and Reduced Solubility Parameters for Lipid Analytesa

Hildebrand solubility parameter

Reduced solubility parameter Reported Fedors’ method in CO2 at 80°C and Analyte MPa 1/2 MPa 1/2 (cal/cm 3 ) 1/2 200 atm 400 atm 600 atm

According to the Law of Corresponding States, two gases with the samereduced temperature and reduced pressure are in corresponding states Both gasesshould 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 Eventhough quantitatively inaccurate, the Van der Waal equation predicts the Law ofCorresponding 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 anydense gas can be determined as follows:

Pitzer and colleagues improved upon the Law of Corresponding States by 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 isbased on a virial expansion treatment of dense gases (22) Using the acentric factor ω,

intro-the compressibility factor Z can be determined more accurately by

Z(T r ,P r ) = Z0(T r ,P r) + ω Z1(T r ,P r)

where Z0and Z1are 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 CO2for various temperatures andpressures based on Pitzer’s work

As noted earlier, supercritical CO2can be augmented with another modifyingsolvent to enhance its solubility for challenging analytes These binary solventmixtures have different critical values and solubility parameters Their new valuescan be calculated using the modified Handinson-Brobst-Thomson equations:

where V b , V s , and V mare the characteristic molar volumes for the binary mixture,

principal solvent, and modifying solvent, respectively; x s and x mare the mole

frac-tions for the solvent and modifier; and T cb , T cs , and T cm are the critical tures of the binary mixture, solvent, and modifier, respectively The acentric factor

tempera-ωband the molecular weight M wb of the binary mixture are treated as weightedaverages using the mole fractions as weights

ωb= x sωs + x mωm 00

M wb = x s M ws + x m M wm

where ωs and ωm are the acentric factors for the solvent and modifier, and M ws and M wmare the molecular weights of the solvent and modifier The critical pres-

sure P cbof the binary mixture is first calculated by determining the value of the

compressibility factor Z cbat the critical point using Pitzer’s tables:

Z cb= 0.291 – 0.08ωb

The critical pressure is then determined using the definition of the compressibility

factor Z:

P cb = Z cb RT cb /V b Using these critical values for the binary mixture, the compressibility factor Z can

be calculated from Pitzer’s tables for a given temperature and pressure (23) The sity and Hildebrand solubility parameter can then be calculated for the new mixture.Table 5.3 lists the critical values for a few binary mixtures Isco Incorporated offers aMicrosoft Windows–based program that calculates the state parameters and solubilityparameters for CO2and a few binary mixtures As a final note, this theory does notadequately treat polarity and hydrogen bonding This treatment is only an approxima-tion but has been successfully applied to a wide range of solvent systems

The instrumentation required to perform a successful SFE is commercially able The process begins with a clean source of fluid, which in most cases is ahigh-pressure cylinder of CO2 A pump is used to increase the pressure of the fluidabove its critical pressure The working extraction pressure is determined by thedensity required to dissolve the target analytes from the sample The sample is con-tained in the extraction chamber, which is heated to the desired extraction tempera-ture above the critical point The pressurized fluid is brought to temperature by thechamber and allowed to flow through the sample matrix to extract the analytes.After the sample, the analyte-laden fluid flows to a restrictor, which controls theflow rate of the fluid The restrictor maintains the high pressure of the fluid in thechamber At the restrictor, the supercritical fluid loses its solvating strength as itspressure drops to atmosphere After the restrictor, the analytes can be collected foranalysis Figure 5.4 shows a block diagram of a complete SFE system

avail-Fluid Source

In SFE, an organic extraction solvent is replaced with CO2 Just as with organicsolvents, the CO2source must be clean enough so as not to contaminate the samplebeing analyzed As with most solvents, high-pressure CO2is commercially avail-able in different grades of purity For trace-level analyses, such as extracting pesti-cides at ppb levels, the CO2must be at its highest grade of purity Most gas ven-dors can supply an SFE-grade that is intended for such applications, generally in

Fig 5.4 Block diagram of a typical SFE system.

either stainless steel or aluminium cylinders The next level of purity is SFC-grade,which is slightly less pristine, but also less expensive This grade should be suffi-cient for extracting fat from food products intended for fatty acid profile (FAME)analysis In less demanding applications, such as a simple gravimetric fat determi-nation, welding-grade CO2is satisfactory In our experience, welding-grade CO2has been used successfully in more demanding applications; however, this grade isnot carefully controlled for purity and there may be variation of purity from cylin-der to cylinder If in doubt, always include blanks in the analysis to check the puri-

ty of the fluid supply This is good laboratory practice

CO2is supplied in high-pressure cylinders The large cylinders generally tain 40 pounds (18 kg) of liquid CO2 At room temperature, the head (vapor) pres-sure is ~900 psi or 60 atm For the pumps to operate efficiently, liquid CO2must

con-be supplied Cylinders must con-be equipped with an eductor or dip tucon-be to deliver theliquid fraction at the bottom instead of the gas head at the top SFE-grade cylindersshould automatically be supplied with dip tubes CO2cylinders with helium-headpressure are also available to improve the performance of HPLC-style reciprocat-ing piston pumps The helium-head pressure is usually charged between 130 and

200 atm However, there are concerns about the helium diluting the solventstrength of the CO2because a significant amount of helium dissolves in liquid

CO2 It has been reported that helium-tainted CO2can reduce extraction mance, but the extraction had to be performed well below optimal conditions todetect a difference (24) Helium-charged cylinders are not necessary for syringepumps or head-cooled reciprocating pumps especially designed for SFE Most SFEvendors supply pumps that do not require helium-charged cylinders

perfor-The solvating power of CO2can be enhanced by the addition of a smallamount of modifying solvent The purity of the modifier solvent must be ensured.For trace analyses, GC-grade solvent is satisfactory, whereas HPLC-grade should

be sufficient for most other applications If cost is an important issue, then blanks

Fig 5.5 Block diagram of an online SFE-FTIR system.

should be included in the analysis to ensure purity In the past, bottled gas vendorshave supplied CO2 mix cylinders containing a modifier solvent There have beenconcerns about delivered mixture consistency throughout the life of the cylinder.Most SFE vendors now supply equipment with special modifier pumps that pre-cisely meter and mix the modifier solvent with the CO2 Modifier pumps shouldrender CO2 mix cylinders unnecessary.

Pumps

The vapor pressure of CO2 at room temperature is below its critical pressure Evenwith cylinders charged with helium, the pressure is still too low for most applica-tions, and a pump is required to increase and control the pressure of the CO2 Asshown earlier, higher pressure produces higher density fluid, which offers greatersolvent strength The solvent strength of a supercritical fluid can be tuned by itspressure (see Table 5.1)

Both syringe pumps and reciprocating piston pumps have been used in SFEapplications Syringe pumps offer smoother pressure control, i.e., no pulsation,higher flow rates, and more precise volumetric delivery Syringe pumps, on theother hand, may have to interrupt an extraction to be refilled if the extraction time

is lengthy They generally are refilled before the start of an extraction to reducesuch interruptions Reciprocating pumps are smaller and less expensive thansyringe pumps but can be limited in maximum flow rate The pump head must becooled to prevent vapor lock at higher flow rates Reciprocating pumps are alsoless precise in volumetric delivery due to variable fill efficiency The controllingsoftware for a reciprocating pump must be sophisticated enough to determine andcompensate for refill efficiency for each stroke of the piston Delivered volumebecomes an important issue in modifier solvent addition

Most SFE vendors offer pumps that can deliver up to 680 atm In most cations, 500 atm pressure is sufficient The pumps must also maintain these pres-sures with both accuracy and precision at maximum flow rate Most extractionscan be performed in a reasonable time with pressurized fluid flows of >4 mL/min

appli-If several extractions are performed in parallel, then this number must be ered in determining the maximum flow rate required Performing four extractions

consid-in parallel may demand up to 16 mL/mconsid-in of 680 atm CO2 At this flow rate, thepump must maintain the programmed extraction pressure in all chambers contain-ing samples

Most SFE vendors also offer separate pumps to meter and mix modifier vents volumetrically Volumetric control is essential in this application to ensure aconsistent mix Both the volumes of the CO2and modifier solvent must be accu-rately known to maintain the correct percentage Modified CO2can be delivered bytwo syringe pumps, in which one delivers the high pressure CO2and the otherdelivers the pressurized organic solvent In this scheme, the two fluids are volu-metrically metered and mixed at the final pressure With syringe pumps, the vol-

sol-umes of both the CO2and organic solvent are exactly known at the final pressure.

As a result, there is less concern about the compressibility (degree of volumechange due to a pressure change) of the organic solvent, which can be volumetri-cally significant at 500 atm Two reciprocating pumps can also deliver modified

CO2 With reciprocating pumps, the two fluids can be easily mixed at the lowercylinder pressure This scheme puts lower performance demands on the pumphardware, thus allowing for a less expensive modifier pump Unfortunately, thevolumetric metering calculations become more complicated The pump-controllingsoftware must be sophisticated enough to compensate for the compressibility of the

CO2and the organic solvent at the final SFE pressures To further complicate thesituation, the main pump delivers a mixture instead of pure CO2 This can affectthe refill efficiency of the pump, which in turn affects the total volume delivered.The software supplied by most SFE vendors takes these factors into consideration.Finally, during modifier mixing, most pumps report a volume percent (%vol)instead of a mole fraction (%mol)

Extraction Chamber

The extraction chamber serves three functions First, it controls the temperature ofthe extraction by maintaining the temperature of the fluid and sample Second, itcontains the high pressure of the supercritical fluid This function places highdemands on the material and design of the chamber Third, it contains the sample

to be analyzed and allows the supercritical fluid to flow through the matrix withoutallowing the sample to extrude

Because the critical temperature of most fluids is above room temperature, theextraction chamber for SFE must be heated This can be accomplished with electri-cal heating elements embedded in the heating block surrounding the extractionchamber These heating elements are powered and controlled by a proportionaltemperature controller The chamber temperature is usually measured with a ther-mocouple, which serves as feedback for the proportional controller The heatingblock typically includes heat-exchanging coils, which serve to equilibrate the tem-perature of the supercritical fluid before it comes in contact with the sample Thetypical temperature range for an SFE system should be between 40 and 150°C with

an accuracy and precision of ± 2°C

The chamber should also be constructed to withstand the maximum allowableextraction pressure with a factor of four safety margin In other words, if the maxi-mum operating pressure is 680 atm, the chamber should be designed to withstand

2720 atm at the maximum operating temperature (typically 150°C) This marginensures that SFE instruments are extremely safe The chamber should also beequipped with safety devices such as a rupture disk or a pressure relief valve thatwill gracefully relieve the pressure if it should rise above 150% of the maximumoperating pressure Although obvious, it should be noted that a supercritical fluidunder high pressure is more dangerous than a liquid at the same pressure The high