Advances
nalytical-scalesupercritical fluid extraction (SFE)is undergoing a transition from laboratories dedicated to innovative research to those conducting routine method d ment as well as analysis of real samples. Although researchers who have been working in the field for nearly a decade might view the adoption of analytical supercritical technology as slow, in reality it has been quite rapid. The prospect of decreased analysis and operator time via automation, together with lower waste generation and disposal costs, has driven academic, regulatory, government, and industrial laboratories to bring SFE to the forefront of available techniques. Because of these advantages and the anticipated high return on investment, manufacturers have moved quickly to de-
Mary Ellen P. McNally E. 1. du Pont de Nemours and Co.
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SFE ofers researchers the chan examine extraction parameters that were too dificult to determine with classical 1iquid extraction velop commercial instruments for SFE. Movement to multiple sample and multiple extraction apparatus occurred in less than five years, possibly because SFE is one of the few sample preparation technologies that can be automated. Indeed, ease of use is one of the factors that has fueled widespread interest in the applicab i b of the technique to fields as diverse
300 A Analytical Chemistry, Vol. 67,No. 9,May 1, 1995
as pharmaceutical, polymer, food, and environmental science. The focus of current research in environmental and agricultural SFE applications is addressing the theoretical understanding of the supercritical extraction process, which takes place inside the extraction cell. In this article I will present an overview of the current theoretical understanding of SFE and attempt to predict the directions of future work. The experiments described here were selected to show the basis of current hypotheses. Because developments in this area are being actively pursued, it should be considered as a “snapshot in time.” Current theory and experiments
The fact that viable analytical results are being obtained via SFE and used to answer real-world problems indicates that it is not necessary to understand all theo0003- 2700/95/0367-308A/$09.00/0 0 1995 American Chemical Society
retical mechanisms for effective,routine case and result in quantitativeextractions use of SFE. However, a better understand- will also be considered non-ideal. When ing of the theory governing the SFE procombined, examples and theory from both cess should reduce the number of experi- ideal and non-ideal systems aid in dements in which SFE is unsuccessful, exscribing the entire mechanistic picture. tend the base of applications, and perhaps make a rational, a priori approach to The extraction process such extractions feasible. Removing an analyte from a matrix reTheoretical modeling processes gener- quires knowledge about the solubility of ally try to describe an ideal case. The rapid the solute, the rate of transfer of the solgrowth of SFE has already extended its ute from the solid to the solvent phase, and use beyond that range-non-ideal saminteractions of the solvent phase with the ples and analytes have been successfully matrix. These processes have frequently extracted. The mechanisms to be studied been illustrated with the extraction trianare diverse. Theoretical predictive mecha- gle shown in Figure 1 (I). Collectively, these factors control the effectiveness of the SFE process, if not extraction prooped to a level that permits characteri- cesses in general. However, to obtain a zation of all potential interactions. Ideal better understanding of their contribuas well as non-ideal sample-analyte systions, hypotheses for solubility, matrix, tems have been demonstrated, and experi- and kinetic interactions must be considmental results that illustrate the limitaered individually. tions of the current theoretical principles Solubility considerations. Solubilhave been obtained. Therefore, a division ity has been predicted based on SFC (2), of the theory to address ideal and noncorrelations with molecular structure (3), ideal systems is necessary. theoretical calculations (4), and view-cell These terms, coined here for SFE, are experiments of a few analytes (5).In any taken from basic solvation theory and reextraction process, solubility of the anaflect the types of experimental systems lyte of interest in the extraction solvent is that have been examined. The ideal sysrequired. In classical liquid extractions, tem is defined as one that encompasses solvent polarity is typically varied to inthe extraction of nonpolar materials from crease analyte solubility. polar and moderately polar matrices by a nonpolar solvent such as C02.
The use of supercritical fluids allows solubility to be adjusted for choice of solvent and variations of the pressure and temperature (i.e., density) of the solvent used. The key is to understand how solvent polarity changes with these adjusted pressures and temperatures. A commercially available software program (Isco, Lincoln, NE) equates the solvent poty (strength) of a supercritical fluid via Hildebrand solubility parameter and allows the user to calculate this parameter for mixtures of modifiers with CO, or other common SFE phases. Such an approach gives analysts a starting point to begin SFE analyses based on procedures using liquid solvents with which they are more familiar. In 1989,J. W. King of the U S . Department of Agriculture (USDA) proposed four basic parameters necessary to understand solute/analyte behavior in a supercritical fluid (4). These parameters (the ere analyte solubility starts, the pressure at which maximum solubility of the analyte is achieved, the pressure range for fractionation of the sample, and the relevant physical parameters of the solute) permit predictions of analyte solubility and serve, in essence, as a basis for establishing conditions in method develop ment.
the desired analyte(s) is less soluble in
non-ideal systems, which have been successfully analyzed with SFE, present a variety of choices in terms of modifier and conditions.These options offer different theoretical insights from the ideal or classical SFE experiments. The third case, polar analytes to be extracted from polar matrices using a nonpolar solvent, would not be considered possible in classical liquid extraction systems. For the purposes of classifying SFE processes, examples that illustrate this
Sol int
Figure 1. Factors to consider in the extraction process. (Adapted with permission from Reference 1.)
Analytical Chemistry, Vol. 67,No. 9,May 1, 1995 309 A
The first of these parameters, the threshold or miscibility pressure, is defined as the pressure at which the solute starts to dissolve in the supercritical fluid. This parameter can be technique dependent and depends on the sensitivity of the method chosen to monitor the solute concentration in the supercritical fluid phase. However, even an approximate knowledge of this pressure or corresponding density is beneficial. This threshold pressure can be used as a starting pressure for SFE method development. The second parameter is the pressure at which the solute/analyte achieves maximum solubilityin the supercritical fluid. As is the case in many extraction systems, maximum solubilityis achieved when the solubility parameter of the solvent (supercritical fluid) is equal to that of a given solute. The solubility maximum can be determined experimentally with an equation that relates the interaction parameter due to enthalpy, the entropy interaction parameter, the solubilityparameters of the gas as a function of temperature and pressure, the solubility parameters of the analyte as a function of temperature and pressure, the molar volume of the gas, the gas constant, and the temperature. Solubility parameters for supercritical fluids can be calculated via an expression that relates the solubility parameter of the gas to its critical and reduced-state properties. Most of the values required to make this calculation,however, are not available to the analytical chemist developing a method. Experimentally varying pressure over the range of interest will determine whether there is a maximum solubility for the analyte of interest. The third parameter is the fractionation pressure range, where an analyte's solubility will vary between zero and the maximum value in the supercritical fluid. In this range, the solubility of one solute relative to another can theoretically be controlled in the supercritical fluid. However, as King points out, it is extremely rare to be able to control an SFE experiment so well as to isolate one component from another without further cleanup. The isolation of components, of course, is more easily optimized in this fractionation pressure range when there are large differences in physical properties of the target solutes.
Figure 2. Mole fraction versus Pressure for DDT-SuPercritica' fluid Cogsystem at 60 O C . (Adapted with permissionfrom Reference 4.)
Thus the fourth basic parameter, a knowledge of the physical properties of the solute, can be extremely important. The melting point is particularly important because most solutes dissolve more readily in supercritical fluids if they are in the liquid state. The analyte solubility parameters can be calculated from group contribution methods for known molecular structures but should be corrected for temperature to prevent large errors in the Hildebrand solubility parameter. The Flory critical interaction parameter, xc,can be calculated from the ratio of the molar volume of the solute to the molar volume of the gases (4). I€the analytical problem is removal of large amounts of an analyte from a matrix, maximum solute solubility in the supercritical extraction fluid is desirable. However, removal of trace levels of solute from a matrix need not require the maximum pressure or the maximum solubility of the analyte in the supercritical fluid. For example, Figure 2 shows a plot of the onset solute solubility in the supercriticalfluid as a function of pressure for the DDTsupercritical CO, system. The intersection pressure of x and xc corresponds to the miscibilitypressure for the DDT-supercritical CO, system. The fractionation of one solute from another based on differential threshold pressures requires precise pressure control. As King reported (4,optimum extraction of DDT from fat (75.2%) was achieved when SFE was conducted at 204 atm, which is close to the miscibilitypressure for DDT (Figure 2). Therefore, extraction at higher pressures may yield optimum extraction efficiencies of a particular ana-
310 A Analytical Chemistry, Vol. 67,No. 9,May 1, 7995
lyte but at the price of selectivity.This situation also holds for classical extractions using liquids. Another way to predict analyte solubility in a supercritical fluid is via elution in an SFC system. Illustrations of these correlations as applied to environmentalmolecules have been reported by McNally and Wheeler (6, 7) and by McNally, Wheeler, and Melander. Chromatographic retention times, however, do not reflect all the parameters that must be evaluated in an extraction process. Instead, SFC data confirm the solubility of the solute in the mobile phase. In the case of SFC, where density and temperature programming are conducted, a known analyte can be injected into the chromatographic system to view solute migration under specific pressure and temperature conditions and determine where analyte solubility exists. These reports presented a systematic study by functional group of retention time in capillary and packed SFC systems. The effect of adding polar modifiers (THF and a series of alcohols, including methanol, ethanol, isopropanol,and hexanol) to the mobile phase was also measured. Figure 3 shows an example of the effects of position and functional group on capacity factor (7)for two sulfonyl chlorides and a sulfonyl fluoride molecule, which are precursors to herbicides. Dramatic increases in capacity were seen when sulfonylfluoride was compared with the sulfonyl chlorides. This was because of the strong hydrogen bonding interaction of the fluorine moiety with the surface silanols of the silica stationary phase. The interaction of the surface was not easily overcome, even with a highly polar mobilephase modifier such as methanol. As a result, high k' values were obtained. Similar retention could be expected for this molecule from a matrix that had the ability to hydrogen bond with the fluorine moiety. The engineering literature is full of solubility measurements of solutes in supercritical fluids. One frequently used method involves the use of a supercritical fluid view cell. These devices can be made with a high-pressure stainless steel cylinder equipped to hold a sapphire window, whose thickness determines the amount of pressure it can withstand. This is a time consuming and tedious process, but it is
informative, especially if scaleup is anticipated (8, 9). In most analytical laboratories, this elaborate method of determining solubility is not available. Matrix considerations. In SFE, as well as in most extraction procedures, the effects of the sample matrix are the least understood. Variability of matrix type is wide, and the physical and chemical complexity of matrices make extractions difficult. Detailed studies are sometimes needed to optimize SFE for the same analyte from different matrices. In environmental analysis the most common matrix is soil, which in itself constitutes a s e p arate science. With classical sample preparation techniques, the effects of individual matrix constituents are difficult to determine. By using the SFE process and the higher precision results it provides, scientists can now experimentally interpret the effects of the individual matrix components. This capability opens a complex arena of understanding and control that has not been previously investigated in depth. In method development, controlling parame ters are determined first, then conditions are optimized to enhance their effects. Better knowledge of the individual matrix contributions will allow optimal control of the process. To illustrate this concept, examples have been chosen to show different approaches to understanding matrix interactions in SFE. Extraction solvent polarity. Hawthorne and co-workers have progressively extended SFE by examining the extraction of nonpolar analytes from solid matrices of all types (10-15). Basically, these can be considered ideal systems for SFE in the proposed model. In 1992, they conducted a comparison of supercritical N,O, CO,, and Freon-22 (CHCIF,) for the extraction of PCBs and PAHs from sediment, waste sludge, and railroad bed soils (15).They reported that the average percent recovery of PCBs from a standard reference river sediment (SRM 1939) obtained using SFE compared with Soxhlet extraction varied with the fluid chosen for the extraction. The best overall recoveries were achieved with Freon-22. These results were generated at the same pressure (400 atm) for all the fluids examined, but at two separate temperatures to stay above the critical temperature.
The CO, and N,O extractions were conducted at 50 " C; the critical temperatures are 32 "C and 37 "C, respectively. The Freon-22 extractions were performed at 100 " C; the critical temperature is 96 " C. Because the temperature differences resulted in different densities during extractions, additional extractions were conducted at equivalent densities. Of the three fluids used, the dipole moment, and thus the polarity, is highest for Freon-22 (1.4 D) . The dipole moments of N,O and CO, are significantly less, 0.2 and 0.0 D, respectively. Higher extraction efficiencies were observed with supercritical Freon-22. Hence, higher polarity of the Freon-22 solvent phase was considered to be the feature that produced these higher recoveries. From a theoretical viewpoint, if the pcr larity of the analyte is considered the principal characteristic of the molecule in chooa ing an extraction solvent, choosing Freon-22
as the most appropriateextractionfluid for the nonpolar PCBs and PAHs would not have been obvious. Rather, the most nonpolar solvent would have been selected to remove the nonpolar analytes from the polar matrix. From the dipole moment comparison, this would have been CO,. Based on analyte solubility alone, Freon-22 would not have been selected over N,O and C02, even if classical liquid extraction strategies were being conducted. In actuality, one must also consider analyte-matrix interactions in the selection of the extraction fluid. For these interactions the polarity of the sediment, waste sludge, or railroad bed soil is variable and the dipole moment of the Freon-22 makes it the best extraction solvent choice. It has the advantage of minimizing or overcoming any interactions that have taken place between the solute molecule and the soil matrix, thereby leading to more efficient extractions.
Methanol Ethanol Isopropanol
Figure 3. Effects of position and functional group on capacity factor in SFC. (Adapted with permission from Reference 7.)
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 311 A
Co-additives in the extraction fluid. Another way to reduce analytematrix interactions is to incorporate a co-additive in the extraction fluid. This technique was illustrated in the extraction of primary aromatic amines from soil by SFE (16).The amhes can be classified as polar analytes using the model presented here. The soil, depending on the type and its constituents, is likely a mixture of polar and nonpolar portions, so this example can be considered a non-ideal case. For the most effective extraction, low concentrations of amines in methanol-modified CO, removed the primary aromatic amines from soils. The analyte amines were 1,4phenylenediamine, 2,4 diaminotoluene, benzidine, 4,4‘-methylene-bis (2-chloroaniline), 3,3’-dimethylbenzidine, and 3 3 dichlorobenzidine. The addition of polar modifiers, most commonly methanol, for the extraction of polar solutes has been shown to be effective by several researchers (16-19). However, in certain cases, the addition of a methanol modifier is not sufficient for the quantitative removal of analytes. Retention of the amines by the soil matrix was hypothesized to result from interactions with silanol groups in the soils. Increasing basicity of the amines led to an increase in analyte retention (or silanol
Figure 4. Potential interaction of amine modifiers with surface silanols on a silica particle. (Adapted with Dermission from Reference 16.), . .
interaction) by the soil. Addition of an amine to the methanol-modifiedCO, improved extraction efficiencies, and using additives with a pK, value greater than any of the amines of interest was the most efficient. Of the three additives examined (l,&hexanediamine, 2,4diaminotoluene, and 1,4-phenylenediamine), 1,6hexanediamine, whose straightchain hydrocarbon structure with the amine groups on the ends minimized steric hindrances encountered with the aromatic amine additives, yielded the highest extraction efficiencies.This effect is graphically illustrated in Figure 4 (16).The 1,Ghexanediaminealso had the highest pK, value (11.9) compared with the analytes of interest, whose pK, values ranged from 3 to 6. Moisture content. Conflicting theories on the effects of water or moisture on extraction recoveries appear in the literature. For nonpolar analytes,the presence of water appears to be detrimental, whereas for polar analytes, the effect seen thus far is beneficial. In the first two of the following examples, moisture content was reduced to effectively control the extraction efficiency in the SFE process. The nonpolar analytes being extracted in these instances were highly soluble in pure CO, and thus fit into the ideal category. The other examples illustrate that moisture may need to be added to the sample matrix to achieve adequate extraction. An example of an ideal system by our classification scheme is the interlaboratory study that was conducted to determine the accuracy and precision of proposed EPA methods for the extraction of petroleum hydrocarbons from solid matrices with supercritical CO, (Method 3560) and subsequent analysis of the extracts by IR spectrometry (Method 8440) (20).Each of the 14 laboratories that participated in the study extracted (in triplicate) four solid matrices spiked with petroleum hydrocarbons (“PHs) at concentrations of 614 to 32,600 mg/kg. The matrices investigated were three standard reference soils and a certified clay soil spiked with TPHs. The overall accuracy of this collaborative study was 82.9%.Precision appeared to be matrix dependent for interlaboratory comparisons but less so for intralaboratory comparisons. Moisture content of the samples
312 A Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
influenced recovery; high moisture yielded lower recoveries. A drying agent was recommended to improve the recovery, and anhydrous magnesium sulfate was reported to be successful at a wide variety of moisture levels. Moisture content was also a controlling factor in the extraction of > 30 types of pesticides at levels ranging from 0.005 to 2 ppm in a variety of food matrices (21). Carrots, lettuce, peanut butter, hamburger, and fortified butterfat and potatoes were extracted by USDMFDA scientists using supercritical CO,. The system was developed by King et al. and allowed extraction to be used for trace (2 0.001 ppm or 1ppb) pesticide residue analysis (22).For example, pesticide residues could be extracted from large samples (i.e., 26 g of a nonfatty item having a 95% moisture level or 50-g samples of a fatty item containing 50%moisture). To control the moisture content in these experiments, pelletized diatomaceous earth was used as an enhancer and mixed with fatty and nonfatty samples prior to loading into the extraction vessel. If this example is considered in terms of polarity and ideal/non-ideal matrix analyte interactions, the following correlations can be drawn. The pesticides are nonpolar and are sufficientlysoluble in supercritical CO,. The food matrix with the high water content can be assumed to be aqueous and therefore polar until the water is removed. The presence of the fatty or nonfatty solid or semisolid food in this aqueous matrix tends to trap the analytes of interest via micelles. However, after the water is removed, the system can be considered ideal and the SFE process with CO, proceeds easily. Moderately polar molecules have been extracted from plants and soils with modified supercritical fluids using multivessel SFE (23).Methanol and water were the modifiers of choice in these experiments, in which the solid matrix phase is saturated with the modifier and then pressurized in the extraction cell along with the supercritical fluid. For the extraction of the fungicide Nustar from wheat, the sample size was 0.1 g. Nustar is a moderately polar pesticide with a molecular weight of 315.4, a water solubility of 900 g/L, and a solubility of > 2 kg/L in many organic solvents. The 0.1 g sample
was placed inside a 5-mL extraction vessel, and residual volume inside the vessel was filled with reagent-grade sand. Variability in sample size had an inconsequential effect on analyte recovery as long as the remaining volume inside the vessel was filled with an inert material. For the extractions conducted at 75 "C, the average overall recovery was 91%.In this work, aged samples sometimes required repeated extractions for complete recovery. The analogous classical extraction comparison would be the introduction of a second liquid phase in a separatory funnelextraction. Automation required the ability to introduce modifier and supercritical fluid separately because the modifier could, for some matrices, act as a swelling agent. This example illustrates a nonideal SFE matrix/analyte system. The matrix is polar, as is the analyte. The choice of CO, as the solvent phase would not be experimentally or theoretically predicted as optimum without modifier. Fahmy et al. investigated the effects of polar modifiers on the matrix swelling of environmental samples under supercritical fluid conditions (24).Plant materials, soils, and clays were examined via a highpressure sapphire view cell, and the extent of swelling was measured. A correlation between extractability and swelling of the matrix is illustrated in Figure 5 (24). Interestingly, swelling reaches a maximum and then drops at higher pressures. Optimum pressures for both extraction and swelling were illustrated in the systems examined. It was hypothesized that water acted as a swelling agent for the matrix while other modifiers such as methanol were acting as solubilizing agents for the analyte of interest once the matrix had swollen. Several different modifiers were reported to impart different degrees of swelling on the matrices. Physical changes. Scanning electron micrographs are useful for examining the surface of a soil before and after SFE. Research conducted by Simon and coworkers at the U.S. Geological Survey suggests that physical changes of the sur-
Schulten and Schnitzer have described the components of soils that are typically extracted during SFE. They include alkanes, alkenes, alcohols, tt-fatty acids, unsaturated fatty acids, dioic acids, aliphatic ketones, n-alkyl monoesters, dioic acid dimer, and dioic acid trimer (26). These components, if present at high percentages, can act as additional modifiers, additives, or entrainers in the extraction of soils and should not be overlooked during the extraction process. Matrix components have the potential to aid or hinder the extraction of the analyte of interest. Although this area has not been critically investigated, any matrix has the potential to contribute components that will influence extractability of the analyte of interest by altering the properties of the extraction fluid.
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Figure 5. Effects of polar modifiers on the matrix swelling of environmental samples under supercritical fluid conditions. System: Water-CO, at 45 O C ; closed circles: % recovery; open circles: % swelling. (Adapted from Reference 24.)
zero, and the analytes move through the matrix by a process similar to diffusion. The authors comment that the model performs reasonably well despite the nonideal nature of the samples that were compared. Variances from model predicthe first three tions are probab assumptions. Fo olute mass transfer is not totally a result of diffusion from a homogeneous medium; it also involves diffusion out of pores, migration from one adsorption site to another, and displacement of analyte molecules on adsorption sites by the supercritical fluid. Pawliszyn also described a kinetic model for the SFE process that was designed to aid in the optimization of SFE Kinetic considerations. Kinetic models have been proposed to explain ex- conditions with any media (28).It correlates mass transfer kinetics with the chrotraction rates in SFE processes, but exmatographic elution process and convoperimental results supporting these theories are scattered throughout a variety of lution theorem. Matrix particles are assumed to consist of an organic layer on papers. Bartle and co-workers described an impermeable core; the analyte is adSFE extraction kinetics using the hot-ball model, which assumes that the solute dif- sorbed onto the core surface. The process suggested to extract the fuses out of a homogeneous spherical analyte is as follows. First, the analyte particle (a matrix particle) into a medium (the supercritical fluid) in which the ex- must be removed from the surface, diffuse through the organic part of the matrix to tracted species is infinitely dilute (27). This model assumes that the matrix parti- the matrix- fluid interface, and then undergo solvation by the supercritical solvent cles are all of the same size, these particles are spherical, the analyte of interest is phase. Additional transport mechanisms accounted for by Pawliszyn's model consist uniformly distributed throughout the matrix before the extraction is conducted, the of difEusion through the fluid that is stagrate of flow is so rapid that the concentra- nant and transport of the fluid contained in tion of the analyte remains at or close to the interstitial pores of the matrix.
Matrix componelzts have the potential to aid or hinder the extraction of the analyte of interest.
Analytical Chemistry, Val. 67, No. 9, May 1, 1995 313 A
The relationship between the amount of analyte removed from the vessel versus time can be obtained by convoluting a function describing the rate of mass transfer between the phases with a model chromatographic elution time profile. This calculated rate includes the matrixcomplex dissociation rate cone diffusion rate constant, and the time constant that describes swelling of ch will facilitate removal or some combination of these rates. Results obtained with this that varying the geometry be extractor is an efficient means of removing and collecting desorbed species from the matrix. To accomplish rapid extraction,the model outlines the optimizationprocess as consisting of two discrete steps. First, the solubility of the analyte in the extraction fluid over the matrix should be improved. This is accomplished using pressure, temperature, or co-solvents to decrease the retention of analytes on the matrix.
Second,to ensure rapid mass transfer of the analytes from the matrix to the fluid, experimental conditions should be optimized, perhaps not to the same point that achieves maximum solubility. The physicochemical phenomenon responsible for slow mass transfer is different from the elution process of a solubilized analyte in the extraction fluid. The possibility of ensuring rapid mass transfer could necessitate a static extraction step. Understanding molecular influences. As early as 1954, investigations of the solubilities of more than 250 substances in liquid CO, were reported by Francis (a). These experiments, conducted at 26 "C (subcritical),showed that partially miscible liquids became homoge neous when up to 40% (w) COzwas added. The solutes investigated ranged widely in polarity and included acids, aldehydes, halides, alcohols, amines, alkanes, vegetable and fuel oils, kerosene, and ted than benzenes. Ternary dmgrams 400 mixtures were determined, includw
several having binodal and island phase diagrams. Solubilitiesof these types of compounds in supercriticalfluids can be extrap olated from examples in this report. However, no report investigating as many solutes under supercriticalconditions for CO, exists. Stahl and co-workersin 1978 investigated the ability of supercritical CO, and N,O to extract a variety of molecules where polar functional moieties were a systematic fashion (30).The in-
e theoretical basis of the supercritical n process is just starting to be unthrough the routine investigaess, although complipportunity to exam-
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314 A Analytical Chemistry, Vol. 67, No. 9, May 1, 7995
difticult to determine with classical liquid extraction. In SFE as well as other sample preparation method development schemes, a variety of chemical and physical interactions must be considered.
(26) Schulten, H. R; Schnitzer, M. Soil Sci. SOC. Mary Ellen P. McNally, a member of AnaAm. J. 1991,55,1603-11. lytical Chemistry's A o a g e Advisory Panel, (27) Bartle, K. D.; Clifford, k A; Hawthorne, is a research associate at DuPont, where S. B.; h g e n f e l d , J.J.; Miller, D. J.; Robinherprimary research interests include SFE, son, R 1. Sufieycrit.Fluids 1990.3.143-
I thank David M. Johnson of DuPont Agricultural Products for his careful review of this manuscript and his helpful suggestions.
(29) Francis, A. W.J. Phys. Chem. 1954,58, 1099-1114. (30) Stahl, E.; Schilz, W.; Schutz, E.; Willing, E. Angew. Chem. Znf.Ed. Engl. 1978,1?, 731-38.
Pont de Nemours and Co., Agricultural Products Department, Experimental Station, Wilmin@On, DE E402/3328BJ
1988M402.
References (1) Tehrani, J. Am. Lab. 1993,2,40 HH. (2) McNally, M. E.; Wheeler, J. R; Melander, W. R LC-GC 1988,6(9), 816-30. (3) King, J. W.; Friedrich, J. P. J. Chromafog. 1990,517,449-58. (4) King, J. W.J. Chromafogr.Sci. 1989,27, 355-64. (5) LemerZ, R M.; Johnston, K. P. Fluid Phase Equilib. 1990,59(1), 31-55. (6) Wheeler, J. R; McNally, M. E. Fres. 2. Anal. Chem. 1988,330,237-42. (7) McNally, M. E.; Wheeler, J. R J. Chromafogr. 1988,447,53-63. (8) McHugh, M. A; Guckes, T. L. Macromolecules 1985,18,674. (9) Seckner, A. J.; McClellan, A. IC;McHugh, M. A. J. AlChE 1988,34,9-16. (10) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59,1705-8. (11) Hawthorne, S. B.; Miller, D. J. J. Chromafogr. 1987,403,63-76. (12) Hawthorne, S. B.; Miller, D. J.; Krieger, M. S. Fres. 2.Anal. Chem. 1988,330(3), 2 11-15. (13) Hawthorne, S. B.; Krieger, M. S.; Miller, D. J. Anal. Chem. 1989,61,73640. (14) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J. J. Chromafogr.Sci. 1990,28(1), 2-8. (15) Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992, 64,1614-22. (16) Oostdyk, T. S.;Grob, R L;Snyder, J. L.; McNally, M. E.J. Chromafogy.Sci. 1993, 31 (5), 177-82. (17) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59,1705. (18) Lopez-Avila, V.; Dodhiwala, N. S.; Beckert, W. F. J. Chromafog.Sci. 1990,28, 468. (19) Richards, M.; Campbell, R M. LC-GC 1991,9(5), 358. (20) Lopez-Avila, V.; Beckert, W. F.; Young, R; Kim,R J. Assoc. Oft:Anal. Chem. 1993, 76,555-64. (21) Hopper, M. L.; King, J. W.J. h o c . Oft: Anal. Chem. 1991, 74(4), 661-66. (22) King, J. W.; Johnson, J. H.; Friedrich, J. P. J. Agric. Food Chem. 1989,37,951-54. (23) McNally, M.E.P.; Deardorff, C. M.; Fahmy, T. M. Supercritical Fluid Extrac-
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tion: New Directions and Understandings; Symposium Series 488; American Chemical Society: Washington, DC, 1992. (24) Fahmy, T. M.; Paulaitis, M. E.; Johnson, D. M.; McNally, M. E. Anal. Chem. 1993, 65,1462. (25) Simon, N. U. S. Geological Survey, Water Resources Division, National Center, R e ston, VA; personal communication. CIRCLE 1 ON READER SERVICE CARD
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