Anal. Chem. 1002, 6 4 , 301-3 1
30 1
Cleanup of Complex Organic Mixtures Using Supercritical Fluids and Selective Adsorbents Nick Alexandrou,' Michael J. Lawrence: and Janusz Pawliszyn* The Guelph- Waterloo Centre for Graduate Work in Chemistry, Uniuers y of Waterloo, Waterloo, Ontario, Canada N2L 3Gl The selecllvky of various adsorbents toward the fractlonatlon of complex organlc mlxtures consisting of compounds havlng slmllar solublllty parameters and contalnlng polychlorlnated dlbenzo-p-dloxlns (PCDDs) as target analytes was lnvestlgated using supercrltlcal fluids. Tenax, Florlsll, alumlna, carbon, and chemically modlfled slllca (C,8 and CN) were used as adsorbents. On the bask of the results, a simple fractionatkn scheme was proposed. A 15mln extractkn wlth supercrltkal carbon dioxide (CO,) at 3000 psl removed over 75 % of the chkrlnated benzenes and PCBs from Fbrlsil. Full recoverles of the PCDDs were subsequently obtalned by extracting wlth nltrous oxMe (N20) for 90 mln at 6000 psi. The fractlonatlon process was used to clean-up munlclpal Inclnerator fly ash extracts before quantltatlon of PCDDs. Dltferences In the basic properties between C02 and N,O are used to explakr the differences In extractkn recoverles from Florisll. The afflnlty of Florlsll toward PCDDs Is associated with the presence of magnesium Ions fused Into the slllca lattice. Heavler metals In the lattice, whlch are present In many common environmental matrkes such as fly ash, also result In chemisorption of PCDDs, but the adsorptbn process Is very slow because of hlgh energy barrler of adsorption. Good fractknatkn between PCDDs and PCBs can be also achteved by uslng actlvated carbon or Porous Graphltlc Carbon: however, poor recoveries of target analyles (accuracy) and poor preclslon makes thls method unsuitable for practlcal appllcatkns. The resuns provlde a better understandlng about the processes whlch control the extraclbn rates of analytes from envlronmental samples.
INTRODUCTION The first step in environmental or biological sample analysis involves separating the organic compounds of interest from the matrix. This matrix can be soil, water, fly ash, biological tissue, or other material. This process, at the present time, is achieved by using liquid extractions. These methods are time consuming and are very expensive since they require high-purity organic solvents. Liquid extractions also generate a significant amount of toxic solvent waste. An attractive alternative is supercritical fluid extraction (SFE).This approach has been explored for some time by chemical engineers (1-3) and recently attracted the attention of analytical chemists (4-6). SFE is slowly being recognized as the preferred alternative to the commonly used Soxhlet method of extraction in many analytical applications involving environmental materials. Replacement of organic solvents with supercritical fluids resulted in much faster and more convenient extractions. However, after extraction the extract is still a complex organic mixture which, in most cases,requires Present address: Environment Canada, Atmospheric Environment Service, 4905 Dufferin St, Downsview, Ontario, Canada M3H 5T4. Present address: Ontario Ministry of the Environment, Laboratory Services Branch, 125 Resources Rd, Rexdale, Ontario, Canada M9W 5L1.
the application of clean-up procedures to remove interferences for proper quantitation of target compounds. Partial fractionation of organic contaminants can be achieved by performing SFE at various pressures (7-9) or using different fluids (10) or modifiers (11-13). However, in the majority of important complex environmental samples such as soil, pulp or fish tissue, this simple procedure is not satisfactory due to the poor partitioning selectivity and the presence of a large number of coextractives. More sophisticated techniques, developed by chemical engineers, that are based on solubility diagrams of solids in pure fluids (such as, for example, the crossover region method (14),have limited use in analytical applications. Firstly, it is very difficult to saturate the fluid with the target compounds which are often present in parts per trillion (ppt) levels in the matrix. Secondly,contamination from interferences can vary significantly from sample to sample with the absolute level, in most cases, being much higher than the target compounds. This will lead to unpredictable solubility properties of the fluid mixture since the impurities will act as modifiers. Thirdly, interfering compounds often have solubility properties similar to the target analytes, and therefore it is very difficult to differentially solvate them. Alternative clean-up procedures with supercritical fluids, need to be developed. At the present time the methods of choice use large amounts of organic solvents, are labor intensive, and are difficult to automate. For example, current clean-up procedures for the determination by polychlorinated dibenzo-p-dioxins and dibenzofurans present in complex samples involve several chromatographic column separations that require days to complete (15). The main objective of this research was to develop a simple alternative clean-up procedure similar to the solid-phase extraction technique (16). In our approach the organic solvents have been replaced with supercritical fluids. The principle of this separation scheme is based on varying the partitioning of organic compounds between the stationary phase and the mobile fluid in such a way that the interferences are eluted fiist, followed by removal of target compounds. This process is facilitated by applying adsorbents specific to the compounds of interest and using selective fluids. The application of a similar scheme has already been discussed by King (8) and Voorhees (17) for the separation of pesticides from fats. The solubility parameters of these groups of compounds differ substantially. In this work, a more difficult problem was tackled, as the solubility parameters corresponding to the target analytes and interferences are similar. This method was applied to the analysis of complex organic mixtures involving PCDDs as target analytes.
THEORETICAL CONSIDERATIONS The extraction of analfrom adsorbing matrices proceeds by three basic steps (10): the removal of compounds from the surface of the matrix, the solvation of the analytes in the fluid, and the mass transport of the solubilized molecules to the bulk of the fluid to allow their removal from the extraction vessel. To optimize analytical SFE,it is necessary to define which of these steps is the rate-determining step. These processes
0003-2700/92/03640301$03.00/00 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
will be unique to each analyte because of the chemical differences of each molecule, the matrix,and the matrix/analyte interaction. SF extraction conditions can be selected to emphasize the differences which exist between interferences and the target analytea present in the complex mixture to achieve effective fractionation. The mass transport step is the least promising step for introducing selectivity. The primary driving force in this step, within small size pores or in the solid matrix, is molecular diffusion (18). The rate of this process is defined by the diffusion coefficient. This constant is inversely proportional to the square root of the molecular weight of the solute at constant density of the fluid (19).Therefore, separation could only be accomplished between species which have substantially different molecular weights. These differences can be emphasized by adjusting the porosity of the matrix (20). In addition, separation between molecules varying substantially in molecular size, may be achieved through size exclusion chromatography by utilizing a matrix that has a wide range of pore size (21).The clean-up method based on this principle can be useful in monitoring for the presence of monomers or low molecular weight contaminates in polymer extracts. However, in most cases, the interferences do not differ substantially in size or molecular weight from the target analyta. The second step, solvation of the analytes in the fluid, has been investigated extensively (1-3). To ensure the dissolution of the analyte in the fluid, the cohesive energy, which holds the crystal of the substance together, must be overcome by the interaction of the molecules with the fluid. When these conditions are fulfilled, the solubility properties of the solutes are defined by the solubility (6) parameter (22):
where AE, is the energy of vaporization, Vis the molar volume, AH, is the heat of vaporization, M is the molecular weight of the solute or the fluids, and D is the density. The solubility parameter, for the supercritical fluid, is calculated more conveniently from the equation proposed by Giddings (23):
6 = 1.25p,05(p/pl)
(2)
where P, is the critical pressure of the fluid, p is the density of the supercritical fluid, and p1 is the density of the fluid in the liquid state. The solubility of the analytes in the fluid is related to the difference between the solubility parameters corresponding to the fluid and solute. The smaller the difference the better the solubility (22).Therefore, it is feasible to design experiments where the analytes of interest are soluble in the fluid while the interferences are not. This is only possible in cases where the analytes and interferences have substantially different solubility parameters. The most promising separation p r m can be achieved by the presence of an additional phase in the system. A distribution constant, K, that is related to the differences in solubility properties of two phases, a stationary liquid phase and the supercritical fluid, may be defined (8).There must be a substantial difference in K and therefore between the solubility parameters corresponding to the analytes and interferences to ensure a complete separation based on a lowefficiency system such as a packed extraction vessel. Highefficiency multistep separation techniques, such as chromatography, can be very successful even for small differences in K, but they are very expensive and require good control of such experimental conditions as flow rate, pressure, and time of fraction collection. Another approach which was found to be very successful for fractionation was the use of the solid surface adsorbents. Successful separation is based on partitioning differences
I
t
A C T " ENERGY Ea) OF DESORPTDN
-EW
L-AE
REACTlON C O O R D ~
Flgure 1. Free energy vs reactlon coordinate indicating energy requirements for desorption of chemisorbed specles.
caused by the difference in interactions between target compounds and interferences with the surface. The interaction between the surface and the analyte can be in the form of a weak adsorption through dispersive interactions but can also involve the sharing of electrons between the adsorbent and the analyte (chemisorption)(24). Interaction with the surface is much more specific compared to a liquid. Small differences in geometry, basicity, or acidity of the molecules can substantially differentiate between individual compounds. For example, it is well-known that isomers, in many cases, are successfully separated by solid-liquid chromatography. Therefore, molecules which have very similar solubility parameters but differ in chemical or steric properties can be successfully separated by using selective adsorbents. The other advantage is that the elution process can be designed to be selective since it proceeds through the formation of the activation complex. Figure 1 shows the energy requirements for the desorption process. If partitioning is involved, with liquids as the stationary phase, the equilibrium is established immediately according to the partition coefficient (if we neglect the adsorption-desorption kinetics at the interface). However, the desorption process, from a solid surface, is kinetically limited, as any chemical reaction, because of the energy barrier of desorption. Thermal energy easily overcomes the low energy barriers associated with weakly adsorbed species. For strongly adsorbed species, the activation energy must be reduced by selective interaction of the solvent molecules with the matrix-solute complex in order to break the bond. Again in this case, the solubility parameters of different eluting solvents are not the most important parameter, but rather their more detailed chemical and/or structural properties. For example, the extraction of native PCDDs and PCDFs was recently reported from fly ash, where COPand NzO,two structurally similar molecules with almost identical solubility parameters produced entirely different results. COz could not remove the native PCDDs from the matrix while NzO produced quantitative recoveries. It was then possible to clean-up the mixture using this selective interaction, by extracting the mixture T i t with COzto remove interferences and then with NzO to remove the target analytes (10).In this paper, the objective was to find a selective adsorbent to fa-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
cilitate a similar successful fractionation between PCDDs and the interferences present in supercritical fluid extracts of real environmental samples.
EXPERIMENTAL SECTION Distilled in glass grade hexane, toluene, benzene, methanol, ethanol, acetonitrile, diethylamine, acetone, and pyridine were purchased from Fisher Scientific, Nepean, ON. The liquified bone-dry carbon dioxide (CO,), nitrous oxide (N20),helium, ECD grade helium, and nitrogen were acquired from Inter City Welding, Kitchener, ON. Florisil,mesh size sO/lOo, XAD, Tenax, cyanopropyl (CN),and octadecyl (CIS) were obtained from Supelco Canada (Oakville, ON). The Porous Graphitic Carbon, alumina, and silica were purchased from Mandel Scientific, Guelph, ON. The 1,2,4,5tetrachlorobenzene (T4CB),hexachlorobenzene (H&B), 2,2',5trichlorobiphenyl(P3CB),2,2',3,4,5'-pentachlorobiphenyl (P,CB), (P&B) were purchased and 2,2',3,3',4,4',5,5'-octachlorobiphenyl from Ultra Scientific, North Kingstown, RI, while the T4CDD, 08CDD,and [3C]-2,3,7,&T4CDDwere obtained from Cambridge Isotope Laboratories, Woburn, MA. All vials and remaining glassware were washed for 30 min in an ultrasonic bath with Sparkleen detergent followed by rinsing with copious amounts of deionized water. The vials and glassware were then dried at 100 "C for 30 min followed by 250 "C for 2 h. The glassware was rinsed thrice with the appropriate solvent prior to its use. The stainless steel extraction vessels were sonicated with water and detergent for 30 min at 30 "C. After rinsing with deionized water, the vessels were sonicated in hexane for 30 min at 10 "C, followed by methanol for 30 min at 10 *C. The vessels were dried at 100 "C for 30 min. A 10-gsample of each adeorbent (except for Tenax) was cleaned by the Soxhlet method of extraction, with 250 mL of hexane for 20 h with a solvent cycle of 15 min. Coarse porosity glass fritted extraction thimbles were used in the extractions. Tenax was found to swell and dissolve when it was cleaned with this method, and thus supercritical NzOwas used in cleaning this adsorbent. The adsorbents were air-dried and stored for future use. An initial 100-mL standard solution consisting of T4CB (455 ppbillion), &CB (8 ppb), P3CB (160 ppb), PsCB (160 ppm), T4CDD(94 ppb), and O&DD (61 ppb) was prepared in hexane. P8CBwas later added to the mixture at a concentration of 290 ppb. These compounds were chosen since they are found in environmental samples and all the compounds contain the aromatic ring with several chlorines. If the PCDDs are fractionated from the PCBs and chlorinated benzenes, then it should be poasible to fractionate the PCDDs from other similar interfering compounds. PCDDs are suspected carcinogens. The personnel handling these species must wear a laboratory coat or coveralls, repiratory protection, and neoprene gloves. A 250-mL solution of methoxychlor[l,l,l-trichloro-2,2-bis(4methoxyphenyl)ethane], at a concentration of 50 ppb, was used as the intemal standard to compensate for any discrepancies in concentration volumes. Methoxychlor was chosen because it is very stable and does not coelute with any of the compounds present in the standard solution. The off-line SFE apparatus consists of a source of SF (the thermal expansion pump (,E)),an extraction vessel, a restrictor, and a vial containing hexane. The extraction vessel was constructed out of 1/4-in X 3,25411 stainless steel tubing with a '/4-'/& reducing union (Swagelok Canada, Hamilton, ON) at either end. Stainless steel frits (2 pm) (Chromatographic Specialties Inc., Brockville, ON) were used to contain the matrix in the extraction vessel. To avoid leaks or explosions caused by undertightening or overtightening of the 1/4-1/16-in. reducing unions, it is recommended that when first constructing the extraction vessel, the 1/4-in.nut be tightened one and one-quarter turns after fiigertight. With any subsequent use, a one-quarter turn is sufficient to obtain an adequate seal. A graphite vespel ferrule (40% graphite) (Chromatographic Specialities Inc.) was used to attach the restrictor to the extraction veaseL The restrictor (20 cm X 25 pm i.d.) from Polymicro Technologies Inc., Phoenix, AZ, is a fused-silica capillary that was used to maintain the pressure in the system and to deposit the extracted compounds in a 1.8-mL vial. The vial contained 0.5 mL of hexane and 0.1 mL of the internal standard (methoxychlor). The flow rate of
303
the expanded fluid was monitored using a ball flow meter (Brooks, Markham, ON) to ensure the stability of extraction conditions. In order to optimize the extraction parameters, many experiments involving all the adsorbents were performed. Enough adsorbent was placed in the extraction vessel with the aid of a funnel to leave a 0.5-cm clearance at the top of the extraction vessel. To avoid cross contamination,a separate extraction vessel was prepared for each adsorbent. A 100-pL Hamilton syringe was used to deliver 100 pL of standard mixture directly on top of the adsorbent. The extraction vessel was reconnected to the system after the solvent had evaporated. The adsorbent was then extracted with the appropriate SF and SF conditions. Following the extraction, each extract was concentrated to 200 pL by directing a gentle stream of nitrogen into the vial and was then stored in a refrigerator for future GC analysis. Time fractionations, different SFs, pressure variations, and temperatures were investigated to determine the optimum conditions. All experiments were performed in triplicate. To determine the various times of extraction, fractions were collected after 2,4, 6, 8, 10, 15, 25, 45, 60, and 90 min. These results allowed us to approximate the length of the extraction. The experiments were then repeated collecting 1-min fractions around the approximate time interval to obtain the actual extraction profile over time. A PID (proportional integrator derivative) controller from Omega Engineering Inc. (Stanford, CT) was used for the pressure variation studies. The pressure was varied from 2000 to 6000 psi at 1000 psi intervals. The time of the extraction, at each pressure, was 15 min. Two extraction procedures were developed to fractionate the PCDD compounds from the other species adsorbed on Florisil. In the first procedure, the spiked Florisilwith the complexmixture was extracted with COz at 3000 psi for 15 min and 40 "C to selectivelyremove the interferences. The extraction vessel was then connected to a N20-filledpump and extracted for 90 min at 6OOO psi and 40 "C to recover the PCDD congeners. The second method involved extracting the spiked Florisil with C02at 3000 psi to isolate the chlorobenzenes and PCBs. A pressure of 6000 psi and an extraction time of 15 min were then used to remove T4CDDwith COP Finally, N20at 6OOO psi and an extraction time of 90 min was then utilized to extract O&DD. These investigations were repeated seven times to provide a good estimation of the standard deviation of the process. PGC, 1g, was dispersed onto glass wool. This was then packed into an extractionvessel and spiked with 100 pL of standard. The PCBs and chlorinated benzenes were isolated with COzat 10000 psi for 60 min. The orientation of the extraction vessel was changed, and 1% toluene was used as a modifier with COz at loo00 psi and 1h to extract the PCDDs. The modifier was placed in an extraction vessel to prevent contamination of the pumps and placed upstream from an extraction vessel containing the carbon. The two other columns often used in clean-up of environmental solids are alumina and silica/base silica/acid silica columns. An alumina column was prepared by packing the alumina into the extraction vessel. The silica column was prepared by adding 0.1 g of silica into the bottom of the extraction vessel, 0.2 g of 33% 1M NaOH/silica, 0.1 g of silica, 0.2 g of 44% H2S04/silica,and 0.1 g of silica Each column was spiked with the standard solution and extracted with N20. The municipal incinerator fly ash (1.0 g) was extracted as previously described (IO)with NzO for 60 min at 6000 psi and an extraction temperature of 40 "C. The extract was concentrated to 100 p L and stored for future SFE clean-up. The fly ash extracts were cleaned by spiking onto Florisil and extracting for 15 min with COzat 3000 psi. The PCDDs/PCDFs were then extracted at 6000 psi with N20 for 90 min. Gas chromatographic analysis was performed using a Varian 3400 gas chromatograph (GC) equipped with an electron capture detector (ECD), flame ionization detector (FID), septum-programmable injector (SPI) and a 30-m X 250-pm fused-silicacapillary column (0.25-pm film thickness) (DB-5) (J&W Scientific, CA). Helium at a flow rate of 1.0 mL/min was used as the carrier gas, with nitrogen at 29.0 mL/min serving as the makeup gas. The standard solution and extracts were analyzed by using the following temperature program. The initial oven temperature was 150 "C for 1 min, programmed to 300 "C at 10 "C/min and
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table I. Molecular Weight, Molar Volume, Density, and Solubility Parameters of the Various Compounds Used in the Standard Solution V,
compd
MW
cm3/mol
g/cm3
iw,, cal/mol
6(H)
T4CB H&B P3CB P5CB PBCB T4CDD O&DD
215.9 284.8 257.5 326.4 429.8 320.0 456.0
NA" NA 247.3 289.1 351.8 275.6 359.2
1.86 1.57 1.04 1.13 1.22 1.17 1.28
12828.8 15199.1 NA NA NA NA NA
10.3 9.0 9.3 9.7 10.4 9.8 10.6
P,
NA denotes not available.
held for 14 min. The SPI program consisted of an initial temperature of 150 "C for 0.1 min, programmed to 300 "C at 150 "C/min and held there for 5 min. The temperature program used in the fly ash extract consisted of an initial oven temperature of 50 "C for 1min, programmed to 230 "C at 15 "C/min and held there for 1min; the column was then programmed to 300 "C at 3 "C/min and held there for 20 min. The SPI program consisted of an initial temperature of 60 "C for 1min, programmed to 300 "C at 150 "C/min and held there for 20 min. The ECD temperature in both cases was 325 "C. A Varian 8100 autosampler was used to inject 1-pL samples. A slow injection rate of 0.2 pL/s was used. This was necessary to avoid any back-flushing of the sample into the SPI and contaminating the injector body. Further identificationand quantitation of the compounds was achieved with a Varian Saturn GC/MS system equipped with a SPI and a 30-m X 250-pm Rtxl (Restek Corporation, PA) column with a film thickness of 0.1 pm. During the fractionation procedure N20 was one of the supercritical fluids used. Caution must be taken with this fluid, as it is a strong oxidant and an explosion may occur if excesa heat is applied or organic modifiers are used. Respiratory protection, a fumehood, or a well-ventilated laboratory should be utilized when this fluid is handled.
RESULTS AND DISCUSSION The main focus of the investigations was the development of a clean-up procedure for the analysis of samples containing PCDDs using simple in-line and modifier-free process shown in Figure 2. In most of the cases, the analysis of PCDDs is hindered by the presence of much higher concentrations of polychlorinated biphenyls (PCBs) and chlorinated benzenes. It is quite difficult to separate PCDDs from these interferences because the solubility parameters of PCDDs, PCBs, and chlorinated benzenes are very similar. The values for the chlorinated benzenes were calculated using eq 1. For the other compounds, the AHvvalues were not available and Small's
I
Figure 2. On-line SFE system involving clean-up.
molar attraction coefficients (26,27)were used. The solubility parameter is calculated by summing Small's constants (EG) a t 25 "C:
6 = DCG/M
(3)
This method was used in calculating b for the PCBs and PCDDs. Their solubility parameters can be calculated using eq 3 and are between 9.3 and 10.6, as shown in Table I. Molar volumes, calculated by using the Le Bas method (28-30),were used to estimate the densities. To accomplish fractionation, stationary phases, often used in reversed-phase chromatography, and solid adsorbents which have demonstrated selectivity toward the analytes of interest were investigated. The extraction recoveries of PCBs, PCDDs, and chlorinated benzenes were tested with C02and N20,the two most common supercritical fluids. Both of these fluids have similar (23) solubility parameters, about 9 at 6000 psi. In the initial experiments, the extraction feasibility was tested by spiking an empty extraction vessel with the standard mixture and performing extractions at 6000 psi with both C02 and N20. Full recoveries were achieved within the f i i t 5 min, indicating that the compounds are soluble in both SFs and that the system will not interfere in the isolation of these species from the adsorbents. The first parameter that was investigated was the time necessary to quantitatively extract the compounds at 6OOO psi from the adsorbents. A pressure of 6000 psi was chosen since it is often the maximum operating pressure of many pumps and therefore yields the highest attainable density. It was necessary to determine if full recoveries of the standard mixture can be obtained at 6000 psi from the various adsorbents before proceeding to optimize the extraction parameters. The results indicate that full recoveries of the compounds in the standard mixture are possible with some adsorbents at these conditions (Table 11). In most cases, the time required for full recoveries was substantially higher than the 5 min required to isolate the analytes from the empty extraction vessel. This indicates some selectivity of the adsorbents toward the analytes. In general N20 was a better
Table 11. Percent Recoveries of the Standard Mixture from Various Adsorbents Using Supercritical C 0 2 and NzOo time, sorbent c18
CN XAD
Tenax Alumina Florisil PGC
SF
coz
NZO COZ N20 COZ NZO COZ NZO COZ NZO COZ
NZO COZ NZO
min
5 5 5 5 60 45 60 50 15 15 15 90 90 90
T4CB
H&B
P3CB
P5CB
T4CDD
OSCDD
101 f 2 103 f 2 83 f 17 108 f 6 96 f 5 100 f 4 85 f 7 87 f 2 104 f 6 99 f 5 95 f 1 97 f 6 41 f 19 43 f 19
104 f 4 103 f 6 101 f 2 110 f 5 97 f 14 108 f 11 72 f 3 71 f 5 58 f 9 76 f 1 71 f 7 78 f 2 30 f 14 32 f 16
89 f 7 109 f 9 103 f 2 115 f 6 73 f 11 86 f 11 75 f 4 78 f 4 50 f 7 67 f 7 77 A 9 81 f 9 41 f 16 41 f 15
97 f 5 115 f 13 100 f 14 112 f 3 62 f 6 72 f 1 79 f 4 82 f 1 93 f 16 72 f 9 81 f 4 74 f 2 26 f 10 27 f 9
94 f 12 107 f 1 100 f 14 108 f 11 59 A 13 72 f 1 84 f 5 92 f 1 62 f 8 77 f 6 82 f 7 81 f 4 3f1 4f1
99 A 6 104 f 2 100f3 107 f 2 62 f 10 70 f 10 88 f 4 97 f 5 32 f 5 100 f 12 ND 100f7 ND ND
"A pressure of 6000 psi waa used. ND denotes not detected.
ANALYTICAL CHEMISTRY, VOL. 04, NO. 3, FEBRUARY 1, 1992
choice of fluid since it gave higher recoveries. In most cases, it was found that it was possible to quantitatively extract the chlorinated benzenes and PCBs with these extraction conditions. PGC was the exception, as the recoveries were very low. Low recoveries from the carbon matrices were previously reported for clean-up procedures using organic solvents (15). Also, the recoveries of PCBs from XAD resin with COz were only approximately 70%. Recoveries of PCDDs varied depending on the adsorbent and the fluid. Generally, NzO ensured complete extractions from all adsorbents except from PGC where the recoveries for TICDD were low and no O8CDD was ever extracted. The interesting result was observed for Florisil. Here, COz did not extract 08CDD but full recoveries were obtained with NzO. Less dramatic results were observed for alumina when extractions with COz resulted in only 30% recoveries while N20 produced quantitative results. Full recoveries of all compounds present in the standard mixture were obtained when both C18 and CN liquid phases were used. The solubility parameters of these stationary phases can be estimated by assigning them values correspondingtoodane(7.6) and pentanitrile (9.5)(26). CN should exhibit better solubility toward the compounds of the standard mixture since their solubility parameter (9-10) is much closer to its value. To emphasize the differences between the phases, pressure fractionation experiments were performed where 15-min fractions were colleded at pressures ranging from 2000 to 6000 psi in 1000 psi intervals (Figure 3). The difference between CN and C18 is illustrated here. About 50% of the analytes were extracted in the first 15 min a t 2000 psi, and the other half at 3000 psi when C18was used as the adsorbent and NzO as the fluid (Figure 3A). When CN was utilized as the adsorbent,NzO at 2000 psi extracted only 30% of the spike and 70% a t 3000 psi (Figure 3C). This difference is to be expected as the solubility parameter of CN (9.5) is closer to the value corresponding to the compounds in the mixture (about 10) compared to cl8 (7.6). This results in higher partitioning into the CN stationary phase and stronger retention. All compounds in the standard mixture are eluting to the same degree, as expected, since their solubility parameters are similar. Less efficient elution resulted when NzO was replaced by COz. It was necessary to use an additional 15 min at 4000 psi to fully recover all the compounds (Figure 3B). Figure 3B shows the data for CI8only, but similar observations were made for CN. The above data strongly suggeat that N20 is a better fluid than COz although their solubility parameters estimated using eq 2 are similar. It also appears that it is impossible to fractionate the PCDDs from the PCBs and chlorinated benzenes by using the simple method outlined in Figure 2 and liquid stationary phases. XAD resin and Tenax were also used in our investigations. XAD resin has a structure of singlemember rings (polystyrene divinylbenzene), and Tenax has four rings in one polymeric unit. XAD resin produced excellent recoveries for chlorinated benzenes, but for PCBs and PCDDs they were about 60% when COZ was used and 70% for NzO (Table 11). The recoveries from Tenax were significantly better, quantitative for both COzand NzO. Some selectivity was observed for both XAD and Tenax, as shown on the time fractionation diagram for &CB, P5CBand 08CDD (Figure 4). However, the overlap between the target analytes and interferences was quite high. The affiity of the compounds was higher for Tenax (Figure 4A) than for XAD resin (Figure 4C) since they were retained to a larger degree on this adsorbent. The separation is particularly good when COzis used as the fluid and Tenax as the sorbent (Figure 4B). A total of 98% of P5CB and only 10% of 08CDD were removed after the first 15 min of extraction. This separation can be significantly enhanced by lowering the
coo
3 000
2 000
3 000
2
4 000 PRESSURE (PSI )
4 000
5 000
6 000
5 000
6 000
305
PRESSURE (PSI 1
Figure 3. Percent recoveries obtained from C18by varying the pressure of the supercritical N,O (A) and supercriticalCOz (B). (C) Resutts obtained from CN with N,O. The time of the extraction at each pressure was 15 min.
pressure during collection of the first fraction. This approach to clean-up has been applied in solid-phase extraction procedures used in analyzing PCDDs in an aqueous matrix (31). The last three adsorbents, PGC (26,32),alumina (33),and Florisil (34),are well-known to exhibit selectivity toward the PCDDs and PCBs and have been used extensively in clean-up of organic mixtures involving these andytes. From Table 11 it appears that only Florisil and alumina are suitable for the supercritical fluid extraction clean-up. The recoveries from these matrices are quantitative for all the compounds in the standard mixture. The most surprising result was the extraction efficiency of 08cDD from Florisil. No removal of this compound was obtained with COz, and full recoveries were obtained with NzO. This was not expected considering the similar solubility parameters and molecular structures of both fluids. A similar case was reported for the extraction of fly ash (IO), which was discussed previously in the theoretical part of this paper.
308
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992 Table 111. Fractionation of the PCDDs from the Chlorobenzenes and PCBsa
.
HsCB P,CB * 0,COO *
80.
conditions T4CB HBCB P,CB
COz
P&B
PsCB
T4CDD OsCDD
9 6 + 1 7 3 + 3 7 7 + 10 8 3 f 9 9 4 f 11 ND
ND
3000 psi 15 min
40 "C N20
ND
ND
ND
ND
ND
83i10 9 9 + 5
6000 psi 90 min
40 OC PsCB waa included in the mixture to illustrate the fractionation is not based on the molecular weight of the compounds. Florisil was utilized as the adsorbent. Units are in percent recoveries. ND denotes not detected. 0
B
10
20
30
40
50
TIME IMIN)
Table IV. Isolation of T&DD from O,CDD, Chlorobenzenes,
1001
/I/
and PCBs'
H,CB P,CB * 0,coo *
80
conditions
COZ 3000 psi 15 min 40 OC COS 6000 psi 15 min 40 OC NzO 6000 psi 90 min 40 OC
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10
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30
40
50
60
TIME ( M I N )
_-
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E
P,CB * 0,coo
50 40
30
0
HsCB
PSCB
P&B
T&DD
88 f 8 74 f 9 81 f 9 71 f 14 ND
OsCDD ND
ND
ND
ND
ND
8 1 + 1 0 ND
ND
ND
ND
ND
ND
100+14
"Florisil was the adsorbent. Units are in percent recoveries. ND denotes not detected.
* HC ,B
110
60 c z
TICB
10
30
20
40
50
TIME MINI
Flgve 4. Time fractionation studies from Tenax wlth N,O (A) and CO, (B). The resutts for XAD wlth N20as the supercritical fluid are shown in (C). Pressure = 6000 psi.
These results indicate a high selectivity of Florisil toward the PCDDs. This is very evident when extraction time profiles are exadned (Figure 5). The chlorinated benzenes and PCBs were extracted quantitatively with COz in the first 15 min of extraction, but T4CDD did not appear until after 20 min (Figure 5A). Similarly, the interferences eluted in the first few minutes when NzOwas used as the fluid with O&DD did not appear in the collection vessel until after 25 min of extraction (Figure 5B). Additional selectivity can be obtained in experimenting with pressure variations (Figure 6). The chlorinated benzenes and PCBs were extracted with COz at 3000 psi from Florisil (Figure 6A). Only a few percent of T4CDD were extracted a t 3000 psi with COz, but this compound was not extracted fully until the pressure of the COz was increased to 6000 psi. Some O&DD was extracted beginning at 5OOO psi. When the fluid was switched to NzO for the pressure variation study, different results were obtained (Figure 6B). NzOextracted T4CDD and 08CDD beginning at 3000 psi, with T4CDDbeing fully extracted at 4OOO psi, and O&DD was still being extracted a t 6000 psi. From Figure
6B, the total recovery of 08CDD was approximately 50%. Florisil was extracted for 15 min at each pressure, hence the low recovery of 08CDD. However, subsequent time studies indicate that 08CDD is fully recovered after 90 min at 6000 psi with N20. COz at 3000 psi can thus be utilized to remove the first four compounds, followed by a NzOextraction at 6OOO psi to remove both PCDD congeners. The GC-ECD chromatograms corresponding to this fractionation scheme are shown in Figure 7 , and the percent recoveries are s u m m a h d in Table 111. By slightly changing the extraction conditions, it is also possible to fractionate T4CDDfrom 08CDD,as shown in Figure 7 and Table IV. After the chlorinated benzenes and the PCBs were eluted, the COz pressure was increased to 6000 psi, and T4CDDwas removed from Florisil. O&DD was then extracted by NzO at 6000 psi. The chromatograms for this procedure are illustrated in Figure 7C,D. The chromatogram in Figure 7E is that of a blank extraction of Florisil with NzO. Time fractionation studies were performed to determine the optimum length of the extraction. The PCBs, chlorinated benzenes, and T4CDDwere extracted after 15 min with N20 as the extraction medium. Traces of 08CDD appeared after 25 min, but it was not fully extracted until 90min. This time fractionation was also carried out with COa at 3000 psi. The chlorinated benzenes and PCBs were fully isolated after 15 min with no trace of the PCDD congeners. However, if the extraction was carried out for a longer length of time, a trace of T4CDD was detected. P&B was included in the mixture to demonstrate that fractionation of the six compounds from Florisil is not based on the size (molecular weight, boiling point) of the compounds. The molecular weight and size of P8CB is intermediate between T4CDD and 08CDD. P8CB was extracted at 3000 psi with COz along with the four lower molecular weight compounds (Figure 7A). T4CDDand O&DD were then isolated with N20 (Figure 7B). The percent recoveries are listed in Table 111. This result indicates that the fractionation is due
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1902
307
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Figure 5. Thne fractknatbn experhnents from Florisil wtth C02 (A) and N20 (e). (C) Results for alumina with N20. A pressure of 6000 psi was used.
to different c b m of compounds and not the molecular weight of the compounds. Them resulta may be rationalized by looking at the structure of Florisil (35). Florisil is obtained by fusing MgO with SiOz at high temperatures. Its structure and composition are represented by MgO-Si02;Na&304 (15.5% MgO, 84.0% SOz, 0.5% NaaOJ. The replacement of Si4+by Mg2' in the silicate matrix results in a localized positive charge and a diffused negative charge in the system (35). Electronegative oxygens, chlorines, and the planar aromatic molecules of the PCDDs interact strongly with these charges, and as a result, the PCDDs are more strongly retained on Florisil than the other oxygen-free PCBs and chlorinated benzenes, resulting in the fractionation. The more basic 0,CDD interacts more strongly than T4CDD. Bases are known to be chemisorbed on the surface of Florisil (35). To explain the significant difference between the fluids,the dipole momenta and basicity of NzO and COz should be compared. Both NzO and COz produced similar recoveries of the chlorinated benzenes and PCBs. However, only N20
10 n
2 000
3 000
4 000 5 000 PRESSURE IPS I 1
6 000
Flgurr 6. Percent recoveries obtained from Florisil by varying the pressure of the supercritical N 2 0 (A) and COP (B). The results for alumina with N 2 0 are Illustrated in (C). A pressure of 6000 psl was Used.
yielded full recoveries of the PCDDs. NzO is a stronger base than COz (36). Therefore these gas molecules will more effectively compete for the absorption site with PCDDs. They will interact with the bonding between the PCDDs and Florisil, producing a low-energy activation complex which will allow a rapid dissociation of the bond and releasing the PCDDs. The basic NzO might also interact with the aromatic ring to form ?r complexes and therefore weaken the bond. T his situation was observed for amines and an anthraquinone derivative adsorbed onto the glass surface (37). Further experiments c o n f i i e d this explanation. Extractions (15 min) were performed using COz modified with methanol, a solvent which possesses a high dipole moment and proton-donating properties, and with diethylamine, a solvent which has basic properties. The results shown in Table V indicate that the presence of basic amine in the system
308
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992 Table V. Percent Recoveries of the Standard Mixture from Florisil‘
A
conditions
coz 91f9 3000 psi 15 min 1%methanol 40 OC COZ 89f3 3000 psi 15 min 1%acetone 40 OC coz 104f 11 3000 psi 15 min 1%pyridine 40 OC COZ 89f10 3000 psi 15 min 1% diethylamine 40 O C
B i
1
I
.
H6CB P3CB
P&B
T,CDD OsCDD
75f9 8 1 f 9 75f8
ND
ND
7 5 i 9 7 4 f 3 72f5
21f8
7f2
76 f 8 83 i 4 71 f 44 44f 3
18f 2
74f6 76f6 75f7
52+6
58f9
“COz at 3000 psi with a number of modifiers (order of increasing basicity) served as the extraction mediums. ND denotes not detected.
C
I D
T&B
INTERNAL STANDARC \
E
evidence of this conclusion was the data obtained when pyridine and acetone were used as modifier. Theae compounds have intermediate basicity between diethylamine and methanol. The recoveries were closely related to the base strength of the modifier (36)(Table V). In addition, full extraction recoveries of PCDDs were also obtained with COz when pure unmodified silica was used as the adsorbent. This clearly indicates that chemisorption sites are associated with the presence of metals fused into the silica lattice. There are two challenges which face an analytical chemist in dealing with real samples. Firstly, there is a wide spectrum of matrices to be extracted, and secondly, the concentration of the species varies from one sample to the next. Thus any procedure must be validated over a range of concentrations that might exist in different samples. The above procedure was carefully investigating by spiking Florisil with three different concentrations of the standard mixture. Three solutions were prepared at half, three-quarters, and twice the concentration of the compounds listed previously. These were then spiked onto Florisil and extracted for 15 min with COz at 3000 psi followed by NzOfor 90 min at 6OOO psi. After each extraction Florisil was spiked with 100 p L of hexane and extracted to see if there was any carry over of any compounds from one extraction to the next. No such carry over was observed. Each experiment was performed in triplicate, with the recoveries being similar to those in Table 111. To validate the accuracy of the method, surrogate standard was used. 13C-T4CDDwas spiked onto Florisil and extracted as described previously. An 80.2 f 5.4% recovery was obtained with C02at 6OOO psi, and an 83.6 4.2% recovery with N 2 0 at 6000 psi for 15-min extractions. The variance in the extraction efficiencies may be due to degradation of the Florisil packing. It was observed that the recoveries of the various compounds began to decrease after the sixth set of extractions, resulting in an increase in the sample standard deviations. Additionally, impurities would begin to appear in the extract. This problem w a eliminated ~ by replacing the Florisil packing after each fifth set of extractions. As discussed in the Introduction, the method described here is one which resembles that of solid-phase extraction with the organic solvents being replaced by SFs. The method described above for the fractionation of PCDDs from other compounds present in solution was applied to the clean up of complex
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Figure 7. GCICD chromatogramsillustrating the fractionation of the standard mixture into (A) PCBs and chlorinated benzenes and (B)
polychlorinated dibenzepdioxins and then fractlonation between (C) and (D)08CDD. (E) Blank extraction.
T,CDD
allows extraction of PCDDs with carbon dioxide. The interaction of the amine with the analyte-matrix complex lowers the energy barrier of desorption and facilitates release of PCDDs. Methanol, as the modifier, does not change the extraction rates compared to pure COz,although the solubility parameter of the mixture is closer to the solubility parameter of the analytes compared to the amine mixture. Additional
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
309
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Flgure 8. GC-ECD chromatogram(A) of a SFE of fly ash with N,O. The fly ash was extracted for 60 min at 6000 psi with N20. This sample was then spiked onto Florisil and extracted with COPat 3000 psi for 15 min to remove interferences, followed by N20at 6000 psi for 90 min (B). The chromatographic conditions consisted of an Initial oven temperature of 50 OC for 1 min, programmed to 230 OC at 15 OC/mln and held at 230 OC for 1 min, programmed to 300 OC at 3 OC/mIn and held there for 20 min. The SPI program consisted of an hitiel temperatwe of 60 OC for 0.1 min, programmedto 300 OC at 150 OC/min and held there for 20 min.
organic mixtures produced after extraction of fly ash samples (IO). The extract obtained by the N20 extraction of fly ash was spiked onto Florisil and cleaned by extracting with COz for 15min at 3000 psi followed by N20 for 90 min at 6OOO psi. The clean-up of this matrix is clearly illustrated in the GCECD chromatograms in Figure 8. The clean-up step successfully removed most of the chlorinated interfering compounds that elute prior to the PCDDs. Furthermore, most of the speciesthat were present between the various congeners in the chromatogram in Figure 8A were either eliminated or reduced, as indicated by the cleaner regions between the PCDD congeners in the chromatogram in Figure 8B. This clean-up process is also evident within the various congeners themselves as fewer and sharper peaks are present. Lastly, the removal of interfering compounds from PCDDs is shown in the single-ion chromatograms of the raw fly ash and cleaned-up fly ash extracts in Figure 9. The level of interference decreased substantially after the clean-up step. This can be illustrated by analyzing single-ion chromatograms for mle = 424 corresponding to the heptachlorodibenzo-pdioxins (H&DD). Three peaks are shown in the raw extract of the fly ash. However the first peak in the chromatogram in Figure 9A corresponding to an interference has dwappeared in the cleaned sample (Figure 9B). Furthermore, the abun-
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Flgure 9. (A) Single-ion chromatograms of a raw supercritical fluid extract of fly ash with N20 at 6000 psi and 60 min. (B) Single-ion chromatograms of the extract after Florisil clean-up. The chromatographic conditions consisted of an initial oven temperature of 50 'C for 0.10 mln, programmed to 120 OC at 40 OC/min, programmed to 240 OC at 10 OC/mIn, programmed to 280 OC at 30 OC/mIn and held there for 6.80 min. The SPI program consisted of an initial temperature of 100 OC for 0.10 min, programmedto 300 OC at 200 OC/mIn and held at 300 OC for 20 min.
dance of H,CDD has increased from 0.17 '70 in the raw extract data to 0.49% after the clean-up procedure, which indicates that a larger proportion of the target compound is in the injection mixture. The entire extraction/clean-up/GC/MS process can be simplified by performing it on-line. To do so, an extraction vessel with the sample matrix can be joined in series upstream of another containingan adsorbent (Figure 2). A SF can then be used to extract analytes from the sample matrix, depositing them on Florisil. Florisil would then be extracted by the above method in order to separate target anal* from interferences. This simple system was evaluated for the analysis of spiked sand. Quantitative recoveries and full fractionation were obtained. In this situation, PCDDs and interferences were extracted quantitatively from sand at 3000 psi and with carbon dioxide. The interferences passed through Florisil while the PCDDs were adsorbed. Then the application of N20 at 6000 psi allowed the removal of the target analytes. This simple system will work only in situations where the PCDDs can be extracted quantitatively from the matrix at 3000 psi. If higher pressures need to be used to ensure complete removal of analytes from the sample, then a more complex system, involving valves, needs to be applied. The first step in such a process is the extraction at the required conditions with the expansion of the fluid mixture and deposition of target analytes onto the adsorbent followed by the clean-up step. The desorption rate of the PCDDs from the surface of Florisil is limited by the kinetics of the desorption step. Therefore, extraction times need to be quite long, for example
310
ANALYTICAL CHEMISTRY, VOL. 64, NO. 3, FEBRUARY 1, 1992
Table VI. Percent Recoveries of the Standard M i x t u r e from Porous Graphitic Carbon’
conditions
TICB
H&B
PBCB
P&B
T&DD
O&DD
coz
40 f 19 30f 14 41 f 16 28f 1
ND
ND
coz
50f 11 61 f 16 49f 10 34f 17 ND
ND
6000 psi 60 min 40 ‘C 10000 psi 60 min 40 ‘C
co2
78 f 9
18000 psi 60 min 40 “C
coz
10 000 psi 1% toluene 120 min static 60 min dynamic 40 ‘C
61
* 10
66 f 14 61 f 15 42 f 27 38f 19
ND
59 f 13 52 f 8
ND
64 f 9
19 f 7
a C02 at various conditions was used as the extraction medium. ND denotes not detected.
90 min for 08CDD. Thus, to reduce the amount of fluid, the static mode of extraction should be considered. Recovery of the PCDDs was quantitative when a 120-min extraction with N20 was followed by a 15-min dynamic sample collection. During this process, 75% less fluid was used. This is important in considering lowering the cost of the method and reducing the use of fluids in field applications or in poorly ventilated laboratories. The results obtained for Florisil are analogous to the results obtained for fly ash (IO). It could be expected that the surface structures of these matrices are very similar since both were produced during high-temperature fusion of silica with metal impurities. The results tend to support this theory. Application of diethylamine as a modifier with C02 facilitated a complete removal of native PCDDs from fly ash. In addition, recent studies by Ross and Karasek (38)indicate that the use of amines prevents the formation of PCDDs during the incinertion process. These results indicate that the amines block the surface adsorption sites. However, the surface of fly ash is not identical to that of Florisil since the extracted fly ash cannot easily reabsorb the PCDDs. Spiked PCDDs can be extracted quantitatively with C02from fly ash. This is most likely the effect of the activation energy barrier of adsorption and might be associated with the presence of heavy metals in the lattice of the fly ash compared to the light metal, magnesium, in Florisil. In the case of fly ash the adsorption is very slow. This example emphasizes the difficulties in producing a repreeentative spike in a real matrix, since aging and chemisorption on original sites should be considered. Similar effects can be expected from other environmental materials, such as sand, soils, and sediments, since metals containing silica constitute significant components of these matrices. It is also expected that other supercritical fluids which posses stronger basic properties, such as chlorofluorohydrocarbons or ammonia (36),can be used effectively in the place of NzO. A carbon matrix interacts differently with PCDDs since there was no difference in recoveries when C02 or N20 was used. Also, the use of amine or methanol did not change the recoveries. The experimental condition which had a significant effect on recoveries was the increase of pressure, as shown in Table VI. Extraction efficiencies for chlorinated benzenes and PCBs increased from about 40% to 50% when the pressure was increased from 6000 to 10000 psi and then close to 70% when the pressure was increased to 18OOO psi. At that
preasure, a significant amount of T4CDD (38%) was extracted, but no 08CDD was removed. Enhanced recoveries, by about lo%, were also observed when a small amount of toluene was added to the extraction fluid. Similarly, benzene and toluene were found to be the best solvents in liquid clean-up using carbon sorbents (15). This would indicate that strong dispersive interactions between the delocalized ?r bonds present in the analyte and in the graphite are responsible for poor recoveries. The above results indicate that carbon matrix is not a suitable adsorbent for supercritical fluid clean-up, unless very high pressures are utilized. Research results indicate that it is impossible to quantitatively extract any large aromatic molecules from matrixes which contain some carbon with supercritical C02 or N20 (39,40). Therefore, such samples must be considered with extra caution. On the other hand the activated carbon can be used as an effective trap to remove traces of large organic compounds from supercritical fluids and therefore purify them prior to use in extraction.
ACKNOWLEDGMENT We thank Professor T. McMahon and I. Chatzis for their helpful discussions. The Ontario Ministry of the Environment kindly supplied the municipal incinerator fly ash from the Commissioner Street incinerator in Toronto, ON. Registry No. T4CB,95-94-3; H&B, 118-74-1;P&B, 3768065-2; P&B, 38380-02-8; TdCDD, 1746-01-6;OBCDD, 3268-87-9; P&DD, 3608822-9; H&DD, 3446546-8;H&DD, 37871-00-4; COZ, 124-38-9; NOz, 10024-97-2; C, 7440-44-0; alumina, 1344-28-1; Tenax, 24938-68-9; Florisil, 1343-88-0.
REFERENCES Wilke, G. Angew. Chem., Int. Ed. Engl. 1978, 77,701. Williams, D. Chem. €ng. Sci. 1981, 36, 1769. PauWs, M.; et at. Rev. Chem. €ng. 1982, 7 , 179. Hawthorne, S. AMI. Chem. 1990, 62,633A. Vannoort, R.; Chervet, J.; Llngerman, H.; DeJong, 0.;Brinkman. U. J . chromerod*. 1990,505,45. (6) Lopez-Avlla, V.; Dodohiwala, N.; Beckert. W. J . Chromatogr. Scl. 1990. 28,468. (7) Hawthorne, S.; Miller, D. J . Chromatogr. Scl. 1988, 24,258. (8) King, J. W. J . Chrometogr. Scl. 1989, 27,355. (9) Levy, J. M.; Cavalier. R. A.; Bosch, T. N.; Rynaski, A. M.; Huhak, W. E. J . Chrometogr. Scl. 1989, 27,341. (10) Alexandrou, N.: Pawlkyn. J. AMI. Chem. 1989, 67, 2770. (11) Onuska, F. I.: Teny. K. A. J . Hlgh Resduf. Chromtogr. 1989, 72, 357. (12) Wrlght, B.; Wrlght, C.; Gale. R.; Smlth, R. A M I . Chem. 1987. 56’, 38. (13) Hawthorne, S. 8.; Miller, D. J.; Langenfeld, J. J. J . Chromatogr. Scl. 1990, 28,2. (14) Chlmowk, E.; Pennlsl, K. A I C M J . 1988, 32, 1665. (15) Smith, L.; Stalling, D.: Johnson, J. AM/. Chem. 1984, 56, 1830. (16) Junk, G.; Rlchard, J. A M I . Chem. 1988, 60,451. s , J . Mlcmcdomn Sep. 1991, 3, 11. (17) Muugavel, 8.; V ~ ~ t l w eK. (18) M l e n , F. A. Porous Medle; Academlc Press: London, 1979. (19) Fuller, E.: Schettler, P.; W i n g s , J. Ind. Eng. Chem. 1988, 58, 19. (20) Rasher, B. D.; Ma, Y. H. Am. Inst. Chem. fng.J . 1977, 23,303. (21) Hopper. M. L.; Orlffilt, K. R. J . Assoc. Off. AMI. Chem. 1987, 70, 724. (22) SMnoda, K. prlndpks Of Solution end Sdublllny; Marcell Dekker, Inc.: New York, 1974. (23) W l n g s , J.: Myers, M.; Keller, R. Sclence 1988, 762, 67. (24) W a r , G. Chedsorpflon: An €xpsrW”elApproech; Butterworths: London 1970. (25) Lamence,M. J. The Development of the Thermal Expansbn Pump for Superattical Fluid Extractlon. MS Thesis, Unhferslty of Waterloo, 1991. (26) CRC H a m Of m b b y end phvslcs, 67m 4.;CRC Press: Boce Raton, FL, 1986. (27) Small, P. A. Appl. Chem. 1953. 3. 71. (28) Reid, R. C.; Rausnk, J. M.; Shemood, T. K. The properties of Qeses 8ndLh7u&, 3rd ed.;McGraw-Hill: New York, 1977. (29) Shlu, W. Y.; Doucette, W.; Gobas. F.; Andren, A.; Mackay, D. Environ. Scl. Technol. 1988, 22,651. (30) Shlu, W. Y.; Mackay. D. J . Phys. Chem. Ref. Defa 1988, 15, 911. (31) Lawrence, M.; Cdquhoun, R. M.; Pawllsryn, J. Indirect Superatticel Fluid Extractlon of Polychlorinated Dibenzo-p-Dloxins from Ralnwater and other Aqueous Matrlces. In Proceedlngs of the 1990 Internatlona1 Symposlum on Measurements of Toxic and Related Pollutants, Raleigh, NC, 1990 p 123 ( U S EPA Report No. EPA/600/9-90/026). (32) Creaser, C.: ACHadded, A. AMI. Chem. 1989, 61, 1300. (33) Clement, R.; Toslne, H. M. Mass Spectrom. Rev. 1988, 7. 593. (34) a m , C. S.; Chan, H. S.; Neff, 0. S. Anel. Chem. 1975. 47,2319. (35) Adams, J.; a m , C. S. J . Chromtogr. 1984, 285,89. (1) (2) (3) (4) (5)
Anal. Chem. 1902, 64, 311-315
(36) Lias, S. G.; Bartmess, J. E.; Liebman. J. F.; Holmes, J. L.; Levln, R. D.; ~ a ~ l a r w. d , G. J. phys. chem. Ref. ~ a t awas, suppi pi. 1). (37) Pawliszyn. J.; PhilliDs, J. J . photo&" 1982, 19, 357. (38) Ross, B.; Lacombe', D.; Naikwadl, K. P.; Karasek. F. W. Chemosphere (39)
(40)
ieeo, 20, 1967. Alexandrou, N.; Lawrence, M.; Pawliszyn, J. "Supercritical Fluid Extractkn of Fly Ash Samples for Repld Determinationof Polychbrinated Dibenzo-D-Dioxinsand Dibenzofurans". in Proceedinas of the 1990 htematbnat Symposium on h4easwem6nts of Toxic &d Related POIlutents, Ralelgh, NC, 1990; p 94 U.S. EPA Report NO. EPA1600/9-90/ 026). Alexandrou, N. P. Ciean-up of Complex Environmental Mixtures Using
311
Selecthe Adsorbents and Supercrltlcal Fluids. M.S. Thesis, University of Waterloo, 1991.
R E C for~review July 12,1991. Accepted Odober 24,1991. Financial from the National sciences and Research COunCil of Canada, Imperial oil of Canada, varian and Varian Canada was greatly appreciated. This paper was presented,in during 1991 International symposium on Supercritical Fluid Chromatography in Park city, UT.
Separation of Metal Ions with Sodium Bis(trifluoroethy1)dithiocarbamate Chelation and Supercritical Fluid Chromatography K.E. Laintz, Jya-Jyun Yu,and C. M. Wai* Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Bis(trlfluoroethyl)dithbcarbamate(FDDC) forms stable complexes wlth arsenk (As3+) and other metal ions (Bi3+, Co3+, Fe3+, Hg", NP+, Sb3+, and Zn2+) which can be separated by capillary supercrttlcal fluid chromatography (SFC) using CO, as a mobile phase. The fluorinated ligand Is superlor to its hydrogenated form, dlethyldlthlocarbamate (DDC), with respect to thermal staMNty and sdublllty In supercritlcal CO,. Substokhlometrlc solvent extraction studies showed that the stabllity constants of metal-FDDC complexes were greater than those of their nonffuorlnated analogues. Using this FDDC extraction and SFC analysis, separatlon and detectlon of arsenlc specks In the presence of other metals can be achieved with the detectlon limit In the ppb range.
INTRODUCTION A widely used preconcentrationtechnique for trace elements is complexation with the derivatives of dithiocarbamic acid, such as sodium diethyldithiocarbamate (Na(DDC)), followed by a variety of separation and detection methods (1,2).This technique has limited chromatographic use due to the chemical instability and thermal lability of many metal complexes. Since capillary supercritical fluid chromatography (SFC) has been shown to be a method of analysis for thermally labile compounds (3), it is logical to apply this technique to the analysis of metal dithiocarbamates. In prbceeding with this application, we found that many of the complexes formed with metals and Na(DDC) were only partially soluble in supercritical carbon dioxide, as evidenced by poor peak shape due to band broadening, also evidenced by poor reproducibility and chromatographic memory. Neeb and co-workers have shown that substitution of fluorine for hydrogen in DDC, as in the case of sodium bis(trifluoroethy1)dithiocarbamate (NaFDDC), can generally enhance the volatility and thermal stability of the resulting metal chelates (4-8).Substitution of fluorine for hydrogen in some surfactants has been shown to enhance ita solubility in supercriticalcarbon dioxide (9). Taylor and ceworkers (IO) have investigated the SFC separation of some metal j3-diketonates by using methanol-modified C02as a mobile phase. The fluorinated acetylacetone chromium chelate was eluted with a retention time lower than that of ita nonfluorinated 0003-2700/92/0364-0311$03.00/0
analogue. Therefore, these fadors should favor the separation of fluorinated metal chelates in supercritical C02. The behaviors of metal-FDDC complexes in SFC have not been reported in the literature. This paper describes the resulta of our recent study concerning the separation of arsenic and other metal-DDC and -FDDC complexes in SFC using COz as a mobile phase. The application of FDDC extraction followed by SFC as a technique for analyzing metal ions and arsenic species in interstitial water samples is described. A careful survey of the literature, however, shows a lack of information on the stability constants of metal-FDDC complexes. The stability constants of some metal-FDDC complexes determined by this study are also included. EXPERIMENTAL SECTION Instrumentation. A Lee Scientific Model 602 supercritical fluid chromatography system with a Neslab RTE-110 constanttemperature bath was used for all analyses reported in this work. This system was equipped with a timed-split rotary injection valve and a flame ionization detector (FID). All chromatogramswere run using supercritical C02 (Matheson) as the mobile phase and a 5 m 100 pm i.d. by 195 pm 0.d. SB-methyl-100 Superbond capillary column (Lee Scientific). The chromatographicsignals were recorded and processed using a HP-3390A integrator. The temperature and density conditions for the analyses were computer controlled and are reported in the Results and Discussion. Reagents. The stock solutions (As,Bi, Co, Fe, Hg, Ni, Sb, and Zn) used in this study were Atomic Spectral Standard Baker Analyzed Reagents from the J. T. Baker Chemical Co. Sodium diethyldithiocarbamate (Na(DDC))was purchased from the Fisher Scientific Co. Other chemicals including chloroform and ethanol were purchased from EM Science. Ammonium acetate buffer was prepared by mixing 120 g of glacial acetic acid (J. T. Baker Ultrapure Reagent) and 134 g of concentrated NHIOH (Aldrich ACS Reagent) and diluting to 1L. The pH value was adjusted by dropwise addition of HN03 and/or NH40H. Deionized water was prepared by passing distilled water through an ion-exchange column (Barnstead ultrapure water purification cartridge) and a 0.2-pm filter assembly (Pall Corp., Ultipor DFA). Sodium bis(trifluoroethy1)dithiocarbamate (Na(FDDC))was syntheaized in our lab according to the procedures outlined in the literature (6). The starting material,bis(Muoroethyl)amine,waa purchased from PCR Research Chemicals. The standard metal-DDC and -FDDC complexes used for calibration were prepared by adding an excess amount of ligand to the metal solutions at the appropriate pH (7). The resulting precipitates were extracted into chloroform, and the organic phase was washed with deionized 0 1992 American Chemlcal Society
