Report
Advancesin ENVIRONMENTAL
A
nalytical-scale supercritical fluid extraction (SFE) is undergoing a transition from laboratories dedicated to innovative research to those conducting routine method development 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. I. du Pont de Nemours and Co. 308 A
as pharmaceutical, polymer, food, and enSFE offers researchersvironmental science. The focus of current research in envithe chance to examine ronmental and agricultural SFE applicais addressing the theoretical underextraction parameters tions standing of the supercritical extraction which takes place inside the exthat were too difficult process, traction cell. I will present an overto determine with viewInofthisthe article current theoretical understanding of SFE and attempt to predict the diclassical liquid rections of future work. The experiments described here were selected to show the extraction basis of current hypotheses. Because develop 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 applicability of the technique tofieldsas diverse
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
velopments 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 © 1995 American Chemical Society
retical mechanisms for effective, routine use of SFE. However, a better understanding of the theory governing the SFE process should reduce the number of experiments in which SFE is unsuccessful, extend the base of applications, and perhaps make a rational, a priori approach to such extractions feasible. Theoretical modeling processes generally try to describe an ideal case. The rapid growth of SFE has already extended its use beyond that range—non-ideal samples and analytes have been successfully extracted. The mechanisms to be studied are diverse. Theoretical predictive mechanisms in SFE, although capable of providing some guidance, have not been developed to a level that permits characterization of all potential interactions. Ideal as well as non-ideal sample-analyte systems have been demonstrated, and experimental results that illustrate the limitations of the current theoretical principles have been obtained. Therefore, a division of the theory to address ideal and nonideal systems is necessary. These terms, coined here for SFE, are taken from basic solvation theory and reflect the types of experimental systems that have been examined. The ideal system is defined as one that encompasses the extraction of nonpolar materials from polar and moderately polar matrices by a nonpolar solvent such as C02. Conversely, the non-ideal system is illustrated by the analyte-matrix pair in which the polarities are reversed. That is, the desired analyte(s) is less soluble in pure C02 (more hydrophilic), whereas the matrices are more hydrophobic. These 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
case and result in quantitative extractions will also be considered non-ideal. When combined, examples and theory from both ideal and non-ideal systems aid in describing the entire mechanistic picture. The extraction process
Removing an analyte from a matrix requires knowledge about the solubility of the solute, the rate of transfer of the solute from the solid to the solvent phase, and interactions of the solvent phase with the matrix. These processes have frequently been illustrated with the extraction triangle shown in Figure 1 (i). Collectively, these factors control the effectiveness of the SFE process, if not extraction processes in general. However, to obtain a better understanding of their contributions, hypotheses for solubility, matrix, and kinetic interactions must be considered individually. Solubility considerations. Solubility has been predicted based on SFC (2), correlations with molecular structure (3), theoretical calculations (4), and view-cell experiments of a few analytes (5). In any extraction process, solubility of the analyte of interest in the extraction solvent is required. In classical liquid extractions, solvent polarity is typically varied to increase analyte solubility.
The use of supercriticalfluidsallows 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 polarity (strength) of a supercritical fluid via the Hildebrand solubility parameter and allows the user to calculate this parameter for mixtures of modifiers with C0 2 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 pressure where 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 development.
Analytes
Kinetics
Analyte physical parameters
Solufëçsolvent interactions
Physic; cherafel interactions
Solubility Physical and chemical interactions
Matrix
Kinetics Swelling
Solvent
Figure 1 . Factors to consider in t h e extraction process. (Adapted with permission from Reference 1.) Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 309 A
Report The first of these parameters, the threshold or miscibility pressure, is de fined as the pressure at which the solute starts to dissolve in the supercritical fluid. This parameter can be technique depen dent 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 correspond ing density is beneficial. This threshold pressure can be used as a starting pres sure for SFE method development. The second parameter is the pressure at which the solute/analyte achieves maxi mum solubility in the supercritical fluid. As is the case in many extraction systems, maximum solubility is achieved when the solubility parameter of the solvent (super critical fluid) is equal to that of a given solute. The solubility maximum can be de termined experimentally with an equa tion that relates the interaction parameter due to enthalpy, the entropy interaction parameter, the solubility parameters of the gas as a function of temperature and pres sure, 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. Solu bility parameters for supercritical fluids can be calculated via an expression that relates the solubility parameter of the gas to its critical and reduced-state proper ties. 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 fraction ation 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 ex tremely rare to be able to control an SFE experiment so well as to isolate one com ponent 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 proper ties of the target solutes. 310 A
lyte but at the price of selectivity. This situ ation also holds for classical extractions , using liquids. Another way to predict analyte solubil \ λ \ ity in a supercritical fluid is via elution in V an SFC system. Illustrations of these cor K ^ ^ Zc relations as applied to environmental mole 100 200 300 cules have been reported by McNally and () Wheeler (6, 7) and by McNally, Wheeler, Pressure (atm) and Melander. Chromatographic reten Figure 2. Mole fraction versus tion times, however, do not reflect all the pressure for DDT-supercritical fluid parameters that must be evaluated in an C 0 2 system at 6 0 °C. extraction process. Instead, SFC data (Adapted with permission from Reference 4.) 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 in Thus the fourth basic parameter, a jected into the chromatographic system knowledge of the physical properties of to view solute migration under specific the solute, can be extremely important. The melting point is particularly important pressure and temperature conditions and determine where analyte solubility ex because most solutes dissolve more readily in supercritical fluids if they are in ists. the liquid state. The analyte solubility These reports presented a systematic parameters can be calculated from group study by functional group of retention time contribution methods for known molecu in capillary and packed SFC systems. The lar structures but should be corrected for effect of adding polar modifiers (THF and temperature to prevent large errors in the a series of alcohols, including methanol, Hildebrand solubility parameter. The ethanol, isopropanol, and hexanol) to the Flory critical interaction parameter, %c, can mobile phase was also measured. Figure 3 be calculated from the ratio of the molar shows an example of the effects of posi volume of the solute to the molar volume tion and functional group on capacity fac of the gases (4). tor (7) for two sulfonyl chlorides and a sulfonyl fluoride molecule, which are pre If the analytical problem is removal of large amounts of an analyte from a matrix, cursors to herbicides. Dramatic increases in capacity were seen when sulfonyl fluo maximum solute solubility in the super ride was compared with the sulfonyl chlo critical extraction fluid is desirable. How ever, removal of trace levels of solute from rides. This was because of the strong hy drogen bonding interaction of the fluorine a matrix need not require the maximum pressure or the maximum solubility of the moiety with the surface silanols of the sil ica stationary phase. The interaction of the analyte in the supercritical fluid. For ex surface was not easily overcome, even ample, Figure 2 shows a plot of the onset solute solubility in the supercritical fluid as with a highly polar mobile-phase modifier such as methanol. As a result, high k' a function of pressure for the DDTsupercritical C02 system. The intersection values were obtained. Similar retention could be expected for this molecule from pressure of χ and xc corresponds to the a matrix that had the ability to hydrogen miscibility pressure for the DDT-super bond with the fluorine moiety. critical C02 system. The fractionation of one solute from an The engineering literature is full of sol other based on differential threshold pres ubility measurements of solutes in super sures requires precise pressure control. critical fluids. One frequently used method As King reported (4), optimum extraction involves the use of a supercritical fluid of DDT from fat (75.2%) was achieved view cell. These devices can be made with when SFE was conducted at 204 atm, a high-pressure stainless steel cylinder which is close to the miscibility pressure equipped to hold a sapphire window, for DDT (Figure 2). Therefore, extraction whose thickness determines the amount at higher pressures may yield optimum of pressure it can withstand. This is a timeextraction efficiencies of a particular ana consuming and tedious process, but it is 6.0 5.0 4.0 x3.0 2.0 1.0 0.0
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
informative, especially if scale-up 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 separate 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 parameters 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 N20, C02, and Freon-22 (CHC1F2) 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 C02 and N20 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 N20 and C0 2 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 polarity of the analyte is considered the principal characteristic of the molecule in choosing an extraction solvent, choosing Freon-22
36 34 32 30 28
as the most appropriate extraction fluid 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 C02. Based on analyte solubility alone, Freon-22 would not have been selected over N20 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
26 24 22 20 18 16 H 14 12
10 8
6 4 2 o-Nitrobenzenesulfonyl chloride
p-Nitrobenzenesulfonyl chloride
o-Nitrobenzenesulfonyl fluoride
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
Report Co-additives in the extraction fluid. Another way to reduce analyte-matrix 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 amines 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 C0 2 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 resultfrominteractions with silanol groups in the soils. Increasing basicity of the amines led to an increase in analyte retention (or silanol
Surface of silica particle
Figure 4. Potential interaction of amine modifiers w i t h surface silanols on a silica particle. (Adapted with permission from Reference 16.) 312 A
interaction) by the soil. Addition of an amine to the methanol-modified C0 2 improved extraction efficiencies, and using additives with a pKa value greater than any of the amines of interest was the most efficient. Of the three additives examined (1,6-hexanediamine, 2,4-diaminotoluene, and 1,4-phenylenediamine), 1,6hexanediamine, whose straight-chain 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 1,6-hexanediamine also had the highest pKa value (11.9) compared with the analytes of interest, whose pKa 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 thefirsttwo 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 C02 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 C0 2 (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 (TPHs) 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
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 USDA/FDA scientists using supercritical C02. The system was developed by King et al. and allowed extraction to be used for trace (> 0.001 ppm or 1 ppb) 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 sufficiently soluble in supercritical C02. 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 C0 2 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 wasfilledwith reagent-grade sand. Variability in sample size had an inconsequential effect on analyte recovery as long as the remaining volume inside the vessel wasfilledwith 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 funnel^xtraction. 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 C0 2 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 surface take place during SFE (25). Whether these changes result from the pressures and temperatures used or the influence of the fluid is unknown, but the matrix surface clearly undergoes changes during the supercritical extraction process.
Schulten and Schnitzer have described the components of soils that are typically extracted during SFE. They include alkanes, alkenes, alcohols, «-fatty acids, unsaturated fatty acids, dioic acids, aliphatic ketones, «-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.
Matrix components have the potential to aid or hinder the extraction of the analyte of interest. Kinetic considerations. Kinetic models have been proposed to explain extraction rates in SFE processes, but experimental results supporting these theories are scattered throughout a variety of papers. Bartle and co-workers described SFE extraction kinetics using the hot-ball model, which assumes that the solute diffuses out of a homogeneous spherical particle (a matrix particle) into a medium (the supercritical fluid) in which the extracted species is infinitely dilute (27). This model assumes that the matrix particles are all of the same size, these particles are spherical, the analyte of interest is uniformly distributed throughout the matrix before the extraction is conducted, the rate of flow is so rapid that the concentration of the analyte remains at or close to
Pressure (psi) Figure 5. Effects of polar modifiers on the matrix swelling of environmental samples under supercritical fluid conditions. System: Water-C0 2 at 45 °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. Variancesfrommodel predictions are probably related to thefirstthree assumptions. For example, solute 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 conditions with any media (28). It correlates mass transfer kinetics with the chromatographic elution process and convolution theorem. Matrix particles are assumed to consist of an organic layer on an impermeable core; the analyte is adsorbed onto the core surface. The process suggested to extract the analyte is as follows. First, the analyte must be removedfromthe surface, diffuse through the organic part of the matrix to the matrix- fluid interface, and then undergo solvation by the supercritical solvent phase. Additional transport mechanisms accounted for by Pawliszyn's model consist of diffusion through the fluid that is stagnant and transport of the fluid contained in the interstitial pores of the matrix.
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 3 1 3 A
Report 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 elutiontimeprofile. This calculated rate includes the matrixanalyte complex dissociation rate constant, the diffusion rate constant, and the time constant that describes swelling of the matrix (which will facilitate removal of the analyte) or some combination of these rates. Results obtained with this model indicate that varying the geometry of a packed-tube extractor is an efficient means of removing and collecting desorbed species from the matrix. To accomplish rapid extraction, the model outlines the optimization process 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 C0 2 were reported by Francis {29). These experiments, conducted at 26 °C (subcritical), showed that partially miscible liquids became homogeneous when up to 40% (w) C02 was added. The solutes investigated ranged widely in polarity and included acids, aldehydes, halides, alcohols, amines, alkanes, vegetable and fuel oils, kerosene, and substituted benzenes. Ternary diagrams for more than 400 mixtures were determined, including
several having binodal and island phase diagrams. Solubilities of these types of compounds in supercritical fluids can be extrapolated from examples in this report. However, no report investigating as many solutes under supercritical conditions for C02 exists. Stahl and co-workers in 1978 investigated the ability of supercritical C02 and N 2 0 to extract a variety of molecules where polar functional moieties were varied in a systematic fashion (30). The information in this landmark study illustrates that SFE can be applied to solutes having diverse molecular polarities and functional groups. The future
The theoretical basis of the supercritical extraction process is just starting to be understood through the routine investigations that have been and continue to be conducted. The process, although complicated, offers us the opportunity to examine extraction parameters that were too
Just Published!
Career Transitions for Chemists T oday's chemist must be prepared to respond to an ever-changing array of opportunities and obstacles in his or her career. This book is an essential guide for chemists who may be facing a job change or career transition. It includes chapters on personal assessment, salaries and trends, personal data formats, networking, and, importantly, deciding on your career move.
Career Transitions for Chemists details careers in laboratories, including analytical, physical, inorganic, organic, biochemistry, polymer, and ecological chemistry. It also describes nonlaboratory careers, including regulatory work, law, human resources, marketing and sales, and chemical information. One section describes other careers, including computers, education, and selfemployment, focusing on small business and consulting. Whether you're looking for interviewing tips or a new direction for your career, this book has something to offer you. Dorothy Rodmann, Donald D. Bly, Fred Owens, and Essential Resources for the Ann-Claire Anderson
ACS
240 pages (1995) Clothbound: ISBN 0-8412-3052-8 $29.95
Paperbound: ISBN 0-8412-3038-2 $14.95
314 A
CAREER TRANSITIONS for CHEMIST
DOROTHY RODMAN DONALD D. BLY FRED OWENS ANN-CLAIRE ANDERSON
PUBLICATIONS Chemical Sciences
Order from: American Chemical Society, Distribution Office Dept. 74, 1155 Sixteenth Street, NW, Washington, DC 20036 Or CALL TOLL FREE 1 -800-227-5558 (in Washington, DC 872-4363) and use your credit card! FAX: 202-872-6067. ACS Publications Catalog now available on internet: gopher acsinfo.acs.org
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995
difficult to determine with classical liquid extraction. In SFE as well as other sample preparation method development schemes, a variety of chemical and physi cal interactions must be considered. I thank David M. Johnson of DuPont Agricul tural Products for his careful review of this manuscript and his helpful suggestions.
References (1) Tehrani, J. Am. Lab. 1993,2,40 HH. (2) McNally, M. E.; Wheeler, J. R; Melander, W. R LC-GC1988, 6(9), 816-30. (3) King, J. W.; Friedrich, J. P.J. Chromatogr. 1990,517,449-58. (4) King,J. W.J. Chromatogr. Sci. 1989,27, 355-64. (5) Lemert, R M.; Johnston, K. P. Fluid Phase Equilib. 1990,59(1), 31-55. (6) Wheeler, J. R; McNally, M. E. Fres. Z. Anal. Chem. 1988,330,237-42. (7) McNally, M. E.; Wheeler, J. R.J. Chro matogr. 1988,447, 53-63. (8) McHugh, M. A; Guckes, T. L. Macromolecules 1985,18,674. (9) Seckner, A J.; McClellan, A K.; McHugh, M.A.J. AIChE 1988,34,9-16. (10) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59,1705-8. (11) Hawthorne, S. B.; Miller, D.J./. Chro matogr. 1987,403,63-76. (12) Hawthorne, S. B.; Miller, D. J.; Krieger, M. S. Fres. Z. Anal. Chem. 1988,330(3), 211-15. (13) Hawthorne, S. B.; Krieger, M. S.; Miller, D. ].Anal. Chem. 1989, 61, 736-10. (14) Hawthorne, S. B.; Miller, D. J.; Langen feld, J. J./. Chromatogr. Sci. 1990,28(1), 2-8. (15) Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. O.Anal. Chem. 1992, 64,1614-22. (16) Oostdyk, T. S.; Grob, R L.; Snyder, J. L; McNally, M. E.J. Chromatogr. 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. Chromatogr. 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 /. Assoc. Off. Anal. Chem. 1993, 76, 555-64. (21) Hopper, M. L.; King, J. W.J. Assoc. Off. Anal. Chem. 1991, 74(4), 661-66. (22) King, J. W.; Johnson, J. H.; Friedrich, J. P. /. Ague. Food Chem. 1989,37,951-54. (23) McNally, M.E.P.; Deardorff, C. M.; Fahmy, T. M. Supercritical Fluid Extrac tion: New Directions and Understandings; Symposium Series 488; American Chemi cal 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, Reston, VA; personal communication.
(26) Schulten, H. R; Schnitzer, M. Soil Set. Soc. Am. J. 1991,55,1603-11. (27) Battle, K. D.; Clifford, Α. Α.; Hawthorne, S. B.; Langenfeld, J. J.; Miller, D. J.; Robin son, R.J. Supercrit. Fluids 1990,3,14349. (28) PawliszynJ./. Chromatogr. Set. 1993,31, 31-37. (29) Francis,A. W.J. Phys. Chetn. 1954,58, 1099-1114. (30) Stahl, E.; Schilz, W.; Schutz, E.; Willing, E. Angew. Chem. Int. Ed. Engl. 1978,17, 731-38.
Mary Ellen P. McNally, a memberofAna lytical Chemistry's A-page Advisory Panel, ÎS a research associate at DuPont, where her primary research interests include SFE, LC, SFC, and CZE. Address correspondence about this article to her at E. I. du Pont de Nemours and Co., Agricultural Products Department, Experimental Station, E402/3328B, Wilmington, DE 19880Ό402.
Characterize materials. Optimize quality. Improve products. With Thermal Analysis Systems from METTLER TOLEDO. The new TA Station TAS811 : Window-based software with built-in sophisticated database. Unique method concept for automated unattended operation. The new High Performance Measuring Cell DSC820: Revolutionary ceramic sensor giving highest sensitivity and best resolution. Uncomplicated robotic motion leads to reliable sam ple changing. For more information, please call or write. Mettler-Toledo Inc., PO Box 71, Hightstown, Ν J 08520 Phone 1-800 METTLER, Fax 609 426 0121
METTLER
TOLEDO
CIRCLE 1 ON READER SERVICE CARD
Analytical Chemistry, Vol. 67, No. 9, May 1, 1995 315 A
