Modifier Effects on Supercritical CO2

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Anal. Chem. 1995,67,641-646

Combined Temperature/Modifier Effects on Supercritical COn Extraction Efficiencies of Polycyclic Aromatic Hydrocarbons from Environmental Samples Yu Yang, Ahmad Gharaibeh, Steven B. Hawthome,* and David J. Miller Energy and Environmental Research Center, University of North Dakota, Grand Forks, North Dakota 58202-9018

Marine sediment, diesel soot, and air particulate matter were extracted with 400 atm of pure C02 or modified CO2 (10 vol % methanol, diethylamine, or toluene added directly to the sample) at conventional (80 "C) and high (200 "C) temperatures for 15 min of static followed by 15 min of dynamic SFE. Pure C02 extractions were also performed for 30 min in the dynamic mode and showed no significant increase in recoveries over pure CO2 extractions with the static/dynamic procedure. An increase in PAH recoveries was observed from all three samples by raising the temperature of either the pure or moditled CO2 from 80 to 200 "C,which demonstrates that the temperature enhancement is independent of the sample matrix. In contrast, the modifier effects at both 80 and 200 "Cwere dependent on the sample matrix and modifier identity. In general, methanol was the poorest modifier for all three samples at either temperature and frequently yielded no increase in recoveries compared to pure C02. Both toluene and diethylamine yielded increased recoveries at both temperatures from the air particulate matter, but only diethylaminegave significantly enhanced recoveries from all three samples. The enhancement in recoveries with high temperature and modifier were additive, indicating that temperature and modifiers have Merent mechanisms of improving PAH recoveries. Extractions at 200 "C with diethylamine modifier yielded the highest recoveries, which agreed well with recoveries determined by 14-48-hSoxhlet extractions. Supercritical fluid extraction (SFE) has recently received attention as an alternative to conventional liquid solvent extraction~.'-~Because of its low critical point, low toxicity, and cost, supercritical COz has been the most common fluid for SFE applications. However, pure COZ frequently fails to efficiently extract many organics from environmental samples such as sediments, soils, diesel soot, and air particulate matter at conventional extraction conditions (e.g., 200-400 atm at 40-80 "C), which demonstrates that COZ may have insufficient ability either (1) Hawthorne, S. B. Anal. Chem. 1990,62,633A (2) Vannoort, R W.; Chervet, J.-P.; Lingeman, H.; DeJong, G. J.; Brinkman, U. A Th. J. Chromatogr. 1990,505, 45. (3) Chester, T. L.; Pinkston, J. D.; Raynie, D. E. Anal. Chem. 1992,64,153R (4) Camel, V.; Tambute, A; Caude, M. J. Chromatogr. 1993,642,263. (5) Janda, V.; Bartle, IC D.; Cliord, A A J. Chromatogr. 1993,642,283. (6) Alexandrou, N.; Pawliszyn, J. Anal. Chem. 1989,61,2770. 0003-2700/95/0367-0641$9.00/0 0 1995 American Chemical Society

to solvate some organics or to interact with the analyte/matrix complex to remove the analytes into the bulk supercritical COZ6-l6 To improve recoveries of environmental pollutants with pure COz, modified fluids have been used to increase extraction e f f i c i e n c i e ~ , ~because J ~ ~ ~ ~ -modifiers ~~ can either increase the solubility of the target analyte or interact with active sites on the sample matrix, which can help COZ to efficiently extract the analyte. The most common modifier used in SFE has been methanol because of its high solvent polarity parameter, which can greatly increase the polarity of C02.zo-22Numerous other modifiers such as water, organic amines and acids, aromatic compounds, and other organic solvents have also been used to enhance recoveries in SFE.23-27Recently, the influence of modifier identity has been investigated for supercritical COZ extraction at 80'C and 400 atm.15 In general, the extraction efficiencies were improved compared to pure COZextraction. However, the effect of modifier depended on modifier identity, target analyte, and sample matrix,15and different modifiers were the most effective for various analyte/matrix combinations. More recently, a different approach to increase SFE efficiencies has been based on (7) Alexandrou, N.; Lawrence, M. J.; Pawliszyn, J. Anal. Chem. 1992,64,301. (8) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem.

1993,65,338. (9) Paschke, T.; Hawthorne, S. B.; Miller, D. J.; Wenclawiak, B.J. Chromatogr,

1992,609,333. (10) Hawthorne, S. B.; Langenfeld,J. J.; Miller, D. J.; Burford, M. D. Anal. Chem. 1992,64,1614. (11) Onuska, F. I.; Terry, K. A J High Resolut. Chromatogr. 1989,12,357. (12) Hawthorne, S. B.; Miller, D. J. Anal. Chem. 1987,59,1705. (13) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. J. J Chromatogr. Sci. 1990, 28,2. (14) Sauvage, E.;Rocca, J.-L.; Toussaint, G.J. High Resolut. Chromatogr. 1993, 16,234. (15) Langenfeld, J. J.; Hawthorne, S. B.; Miller, D. J.; Pawliszyn, J. Anal. Chem. 1994,66,909. (16) Pawliszyn, J. J. Chromatogr. Sci. 1993,31,31. (17) Onuska, F. I.; Terry, K A J High Resolut. Chromatogr. 1989,12,357. (18) Wright, B. W.; Wright, C. W.; Gale, R W.; Smith, R D. Anal. Chem. 1987, 59,38. (19)Wheeler, J. R; McNally, M. E. J. Chromatogr. Sci. 1989,27,534. (20) Janssen, J. G. M.; Schoenmakers, P. J.; Cramers, C. A J. High Resolut. Chromatogr. 1989, 12,645. (21) Lochmuller, C. H.; Mink, L. P. J. Chromatogr. 1989,471,357. (22) Lochmuller, C. H.; Mink, L. P. J. Chromatogr. 1987,409,55. (23) Schtifer, IC; Baumann, W. Fresenius J. Anal. Chem. 1989,332,884. (24) Morrison, J. F.; MacCrehan, W. A Proceedings of the 5th International Symposium on Supercritical Fluid Chromatography and Extraction, Baltimore, MD, January 1994. (25) Levy, J. M.; Dolata, L.; Ravey, R M.; Storozynsky, E. Holowczak, IC A J. High Resolut. Chromatogr. 1993,16,368. (26) Oudsema, J. W.; Poole, C. F. J. High Resolut. Chromatogr, 1993,16,198. (27) Via, J. C.; Braue, C. L.; Taylor, L. T. Anal. Chem. 1994,66, 603. (28) Eckert-Tilotta, S. E.; Hawthorne, S. B.; Miller, D. J. Fuel 1993,72,1015. Analytical Chemistry, Vol. 67,No. 3,February 1, 1995 641

Table 1. Comparison of Combined TemperatureModlfier Effects on Extraction Efficiencies of PAHs from Marine Sediment (SRM 1941) % recovery (% RSD based on triplicate extractions)

cert conc:

(ug/g, % RSD)

phenanthrene fluoranthene Pyrene benz[a]anthracene benzo [b+k]fluoranthene* benzo [alpyrene perylene indeno[ 1,2,bcd]pyrene benzo [ghi]perylene

0.58 (10) 1.22(20) 1.08(19) 0.55(14) 1.22(20) 0.67(19) 0.42(8) 0.52(16) 0.57(7)

phenanthrene fluoranthene pyrene benz[a]anthracene benzo [ b+k]fluoranthene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene benzo [ghilperylene

0.58(10) 1.22(20) 1.08(19) 0.55(14) 1.22(20) 0.67(19) 0.42(8) 0.52(16) 0.570

phenanthrene fluoranthene pyrene benz[alanthracene benzo [b+k] fluoranthene benzo[a]pyrene perylene indeno[ 1,2,Scdlpyrene benzo [ghilperylene

0.58(10) 1.22(20) 1.08(19) 0.55(14) 1.22(20) 0.67(19) 0.42 (8) 0.52(16) 0.57(7)

phenanthrene fluoranthene Pyrene benz[a]anthracene benzo[b+k] fluoranthene benzo[alpyrene perylene indeno[ 1,2,3-cd]pyrene benzo [ghilperylene

0.58(10) 1.22(20) 1.08(19) 0.55(14) 1.22(20) 0.67(19) 0.42(8) 0.52(16) 0.57(7)

pure C02 80 "C (Operator 1) 63(13) 80(4) 73(5) 81(3) 82 (20) 27(9) 22(29) 13(14) 15(23) 80 "C (Operator 2) 79(13) 83(9) 85(11) 77(16) 64(33) 23 (41) C

C

C

21(22) 22 (25)

toluene/COz

diethylamine/C02

85(3) 94(11) 100(9) llO(14) 105(17) 35(23) 33(22) 25(25) 25(23)

82(11) 90 (4) 91(1) 107(7) 102(17) 34 (1) 27(14) 22 (8) 31(16)

96(17) 80(8) 90 (9) 125(15) 131(6) 54(14) 52 (6) 64(16) 57(17)

102(16) 86(12) 87(12) 84 (8) 76(11) 29(17)

79 (4) 76 (9) 76(5) 86(7) 75m 37(11)

69(16) 81(16) 86(16) 85(15) 103(20) 49(19)

C

lO(59) ll(43) 200 "C (Operator 1) 109(5) lOO(15) 105(6) 94(14) 76 (9) 28(28) 23 (29) 19(27) 17(29) 200 "C (Operator 2) 108(8) 107(8) loo(11) 80(12) 32(16)

methanol/COz

C

C

19(19) 19(15)

28(22) 31(17)

51(24) 48(23)

106(20) 111(15) 102(17) 111(22) 98(6) 40(12) 3907) 52(15) 52(14)

123(9) 121(7) 118(5) 135(4) 137(11) 43 (28) 37(28) 41(30) 45(22)

116(17) 108(16) 121(3) 147(16) 154(16) 83(25) 72(19) 806) 75(13)

C

100(10) 91(9) 103(10) 93(10) 30(14) C

33(16) 35(11)

C

120(4) 119(3) 129(4) 121(5) 60(7) C

55(10) 59(6)

C

110(3) 116(5) 151(20) 151(6) 75m C

86(9)

NIST certified concentrations based on two sequential l&h Soxhlet extractions. The sum of benzo[b]-and benzo[klfluorantheneis reported because these two species were not resolved with the chromatographic conditions used. Not determined.

raising the extraction t e m p e r a t ~ r e , 8 $using ~ ~ - ~pure ~ COZ,despite the fact that the majority of SFE studies have traditionally been done at temperatures lower than 100 "C, apparently for two reasons. First, the upper extraction temperature has been limited by commercial SFE instrumentation. Second, there often exists a belief that analyte solubility will be decreased by lowering the COz density in the process of increasing the temperature at constant pressure. However, the solubility is not only influenced by the density of COZbut also by the vapor pressure of the target a n a l ~ t e . For ~ ~ ,many ~ ~ organics of environmental interest, the (29) Lee, H.-B.; Peart, T. E.; Hong-You, R L.; Gere, D.R]. Chromatogr. A 1993, 653,83. (30) Meyer, A;Kleibohmer, W.; C a m " , IC/. High Resolilt. Chromatog. 1993, 16, 491. (31) Furton, K G.; Jolly, E.; Rein, J. J. Chromatogr. 1993,629, 3. (32) Levy, 1.M.; Dolata, L. A; Ravey, R M. ]. Chromatog. Sci. 1993,31, 349. (33) Tang, P. H.; Ho, J. S.; Eichelberger, J. W. J.-Asoc. Ofl Anal. Chem. 1993, 76, 72. (34) Hawthorne, S. B.; Miller D. J. Anal. Chem. 1994,66, 4005. (35) Krunik, R T.; Holla, S. J.; Reid, R C. J. Chem. Eng. Data 1981,26, 47. 460. (36) Quirin, R T. Fette, Seven, Anstn'chm 1982,84,

642 Analytical Chemistry, Vol. 67,No. 3, February 1, 1995

influence of the analyte vapor pressure on solubility is much stronger than the influence of the COz density. Therefore, the solubility of organics can be greatly increased at higher temperat u r e ~ . It ~ ~has , ~also ~ been suggested that raising extraction temperature can increase in the kinetics of the extraction desorption process.8J6 While it has been reported that SFE efficiencies have been substantially improved with pure COz by raising the temperature to 200 O C , 8 , 3 the recoveries of some organics (e.g., high molecular weight PAHs such as benzolghilperylene and indeno[l,2,%cd]pyrene) are still below those obtained with conventional liquid solvent (e.g., Soxhlet) methods. To date, the combination of moditiers and elevated temperatures (e.g., 200 OC) to increase SFE recoveries has not been reported. The purpose of the present study is to investigate the effects of combined temperature and modifiers on supercritical C02 extraction efficiencies of PAHs from environmental solids (37) Miller, D. J; Hawthorne, S. B. Anal. Chem., in press. (38) Bartle, K. D.; Clifford, A A; Jafar, S. A; Shilstone, G. F.J. Phys. Chem. Ref Data 1991,20, 713.

Table 2. Comparison of Combined Temperatum/ModWior Effwtr on Extraction Efflciencler of PAHr from Dlerei soot % recovery (% ~~

Soxhlet &g/g,% EDa) naphthalene acenaphthalene

fluorene phenanthrene anthracene fluoranthene PVene benz[a]anthracene chrysene + triphenyleneb benzo[ b f k ]fluoranthenee benzo[elpyrene naphthalene acenaphthalene fluorene phenanthrene anthracene fluoranthene PVene benz[alanthracene chrysene triphenylene benzo[b+klfluoranthene benzo[elpyrene

+

pure COz

RSD based on triplicate extractions) toluene/COz diethylamine/COz

methanol/COz

80 "C

95(20) 69 (30) 82(16) 80(6) 28(11) 73 (9) 55(12) 27(8) 28(17) 16(21) 20(25) 200 "C 120(22) 54 (7) 85(16) 105(8) 54(19) 77(20) W17) 38(20) 37(21) 35 (7l 41(28)

lOl(21) 67(30) 96 (8) 85 (4) 31(10) 866) 66(12) 35(18) 39(12) 15(18) 21(25)

116(14) 115(29) 99(12) 105(2) 33 (8)

122(22) 73 (27) 95(1) 115(1) 77(21) 91 (3) 91 (6) 56@) 53(10) 43 (24) 53 (5)

114(18) 122(23) 110(6) 115(3) 85(26) 105(8) lOO(9) 71(26) 74(17) f33(21) 74(12)

82(10)

73(11) 32(16) 36 (9) 22(23) 19@)

% RSD based on triplicate extractions. * The sum of chrysene and tri henylene is reported because these two species were not resolved with the chromatographic conditions used. e The sum of benzo[b]-and benzoPk1fluoranthene is reported because these two species were not resolved with the chromatographic conditions used.

including marine sediment, diesel soot, and air particulate matter. Supercritical COZ extractions were performed at two different temperatures (80 and 200 "C) , chosen because reference data can be found at these temperature~!J~~~ Methanol, diethylamine, and toluene were used as modifiers (10% v/v) in this study, because they represent acid, base, and aromaticity, respectively, and have previously been reported to increase recoveries from certain environmental samples.15 The extraction efficiencies of PAHs from marine sediment and air particulates were evaluated by comparing them to the certified values, while the recoveries of PAHs from diesel soot were calculated based on the values obtained by 1 4 h Soxhlet extractions that were performed in our laboratory. EXPERIMENTAL SECTION

Samples. Two of the three samples used in this study were certified reference materials that contained PAHs at low microgram per gram concentrations. One was marine sediment (SRM 1941) and another was urban air particulate matter (SRM 1649). Both samples were obtained from the National Institute of Standards and Technology (NIST,Gaithersburg, MD). The third sample was diesel soot that was collected from the tailpipes of several buses at the University of North Dakota. Supercritical Fluid Extractions. SFC grade carbon dioxide (Scott Specialty Gases, Plumsteadville, PA) was pressurized by an Isco Model 260D syringe pump (Isco, Lincoln, NE). A 0.5, 0.1-, or 0.2-g sample of marine sediment, diesel soot, or air particulates, respectively, was placed inside a 0.8mL SFE cell (50 mm x 4.6 mm i.d., Keystone ScienWc, Bellefonte, PA). Before extraction, a modifier was added to the extraction cell by pipeting 80 pL (10% (v/v) compared to cell volume) onto the sample in

the extraction cell. The extraction temperature (80 or 200 "C) was controlled by placing both the extraction cell and a 1-m-long preheating coil made from l/l&.-o.d. (0.020-i.-i.d.) stainless steel tubing inside a Hewlett-Packard 5890 Series I1 gas chromatograph. To avoid high-pressure buildup during the heating step, the cell was first pressurized (with the outlet valve closed) to ca. 50 atm of COZ,the inlet was closed, and the cell was then heated to the extraction temperature. Once the extraction temperature was reached, the extraction cell was pressurized to 400 atm by opening the inlet valve of the cell, while the outlet valve remained closed. Static extractions were performed by closing the inlet valve of the cell after the pressure achieved equilibrium inside the extraction cell. After a 15min static extraction, first the outlet valve and then the inlet valve of the cell was opened and the dynamic extraction was performed. The flow rate of the supercritical fluid (0.6-1 mL/min measured as liquid COZat the pump) was controlled by a lkm-long piece of fused-silica tubing with 30-4bpm i.d. (Polymicro Technologies, Phoenix, AZ). For the 3bmin dynamic extractions (no static step), the outlet and inlet valves of the cell were immediately opened after the equilibration of temperature and pressure in the extraction cell (ca. 1 min for 80 "C, and 2 min for 200 "C). Occasionally the restrictor plugged during the dynamic extractions. In this case, a heat gun was used to warm the restrictor for a couple of seconds in order to unplug the restrictor. Extracted analytes were collected by placing the outlet end of the restrictor into a 7.4-mL vial containing 5 mL of methylene chloride (Supelco, Bellefonte, PA). Collection solvent volume was maintained at ca. 5 mL during the dynamic extraction by small additions of methylene chloride. Internal standards (deuterated PAHs, discussed below) were added after extraction but prior to GC/MS analysis. Analytical Chemistry, Vol. 67, No. 3, February 1, 1995

643

Table 3. Comparison of Combined TemperatureiModlfler Effects on Recoveries of PAHs from Air Partlculate Matter (SRM 1649) % recovery vs NIST (%

cert cone (ug/g, % E D ) phenanthrene fluoranthene* Pyrene benz[a]anthracene* benzo [b+k]fluorantheneb benzo[alpyrene* indeno[12Scd]pyrene* benzo [ghi]perylene*

4.5(7) 7.~7) 7.2(7) 2.6(12) 8.2(5) 2.9(17) 3.3(15) 4.5(24)

phenanthrene fluoranthene Pyrene benz [ a] anthracene benzo[b+k] fluoranthene benzo[a]pyrene indeno[ 12Scdlpyrene benzo [ghilperylene

4.50) 7.1(7) 7.2(7) 2.6(12) 8.2(5) 2.9(17) 3.3(15) 4.5(24)

phenanthrene fluoranthene Pyrene benz[alanthracene benzo[b+klfluoranthene benzo[a]pyrene indeno[ 123cdIpyrene benzo [ghi]perylene

4.5(7) 737) 7.2(7) 2.6(12) 8.2(5) 2.9(17) 3.3(15) 4.5(24)

pure COz 80 "C (Operator 1) 58(22) 59(22) 52 (22) 63(22)

200 "C (Operator 1) 87(12) 82(9) 77 (8) 74(10) 105(6) 65(6) 56(14) 59(2) 200 "C (Operator 2) 111(5) loo(12) 95(16) 89(13) 109(3) 62(15) 50(20) 49(6)

methanol/COz

RSD based on triplicate extractions) toluene/COz diethylamine/COz

79(11) 75(11) 66(11) 72(15) 68(17) 42 (20) 20(14) 21(28)

72(16) 74(18) 67(18) 94(18) lOl(19) 76(20) 64 (23) 42(25)

81(1) 83(2) 76U) 104(3) 109(5) 68(27) 57(26) 41(16)

98G') 89G) 85(1) 94(10) 106(3) 57(5) 52(21) 50 (7)

lOO(1) 92 (3) 86(1) 89(5) 122(2) 75(9) 75(9) 69 (9)

91(14) 91(3) 82(2) 89(5) 112(3) 70(14) 52(13) 54(14)

106(6) 98(12) 86(4) 86(3) 103(11) 47 (24) 44(1 1) 49 (6)

122(7) 103(4) 96 (4) 123(7) 132(5) 75(8) 77 (8) 77(10)

115(5) 104(19) 98(20) lOO(5) 124(6) 68(9) 60 (8) 66(15)

NIST concentrations based on 48h Soxhlet extractions. Concentrations of species marked with an asterisk are ceMed by NIST. Other concentrations are provided as informational values by NIST. * The sum of benzo[bl- and benzo[klfluoranthene is reported because these two species were not resolved with the chromatographic conditions used.

Soxhlet Extractions. Approximately 0.1 g of diesel soot was weighed into a cellulose extraction thimble, and the sample was extracted with 150 mL of methylene chloride for 14 h at the rate of ca. 20 cycles/h. M e r the Soxhlet extractions were completed, internal standards (deuterated PAHs) were added to the extracts, and the volume of solvent was reduced under a gentle stream of clean nitrogen to ca. 5 mL. Extract Analysis. All extracts were analyzed by GC/MS (Hewlett Packard 5988) using a 25-m-long x 0.32-mm4.d. (0.17pm film thickness) HP-5 column (Hewlett Packard,Avondale, PA). Autosampler injections were performed in the splitless mode for 0.3 min. The oven temperature was 80 "C ramped to 320 "C at 8 "C/min. PAH quantitation was based on the addition of deuterated internal standards representing each mass of PAHs including phenanthrene-dlo, pyrene-dlz, benz[alanthracene-d~~, benzo[blfluoranthene-dlz, and benzo [ghil perylene-dlz, as well as standard curves generated from appropriate dilutions of NIST SRh4 164% (PAHs in acetonitrile). The GC/MS was operated in the selected ion monitoring mode (SIM) for the molecular ion of each PAH and deuterated PAH. RESULTS AND DISCUSSION

The PAH recoveries from marine sediment, diesel soot, and air particulate matter are shown in Tables 1-3. Some of the recoverieswere separately obtained by two independent operators (marked in the tables), using independent extraction systems and calibration standards, in order to confirm the data. In nearly all cases, the results of the two operators were within the standard deviations for the triplicate extractions and analyses done by each operator. 644 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995

Since the purpose of this investigation was to determine the intluence of the temperature and modiiier on extraction efficiencies of PAHs, relatively short extraction times (15min static/l5min dynamic) were utilized so that the recovery enhancementsby high temperature and modifiers could be observed. In general, increasing the temperature from 80 to 200 "C yielded higher recoveries for the majority of PAHs for each fluid tested (Le., pure COZand each of the three modiiier/COz mixtures), demonstrating that the temperature enhancement is independent of the matrix. In contrast, the addition of modifiers did not always increase recoveries (compared to pure COZ)as described below. Static vs Dynamic Extraction. Since our simple modifier method (adding organic modifiers onto samples inside the extraction cell) requires a static (no-flow) step followed by a dynamic (flowing) step to recover the analytes, we first determined whether the 15min static/l5min dynamic procedure yielded lower recoveries than 3@mindynamic extraction with pure COZ. If the extraction rates are limited by solubility considerations (including chromatographic retention), the 3@min dynamic extraction should yield substantially higher recoveries.I6 However, if the kinetics of the desorption of the analytes into the supercritical fluid limit the extraction rate, the static/dynamic and the dynamic procedures should yield similar recoveries since the sample is exposed to the extraction fluid for the same total time. As shown in Figure 1for the marine sediment, and Figure 2 for the diesel soot, 30 min of dynamic extraction yielded little (id any) increase in recoveries over 15 min of static extraction followed by 15min dynamic extraction. The two methods generally yielded recoveries within one standard deviation unit (as shown by the

rtatk/dynamlo dynamk .

_..

~

..

I--"

Phonmnt hreno

Fluormnthene

-

Bonxolmlpyrono 8.nxolghllporylono

120 I

a8

t9

a0

Phenanthrene

Fluorantheno

Benrolelpyrene

Naphthalene

Phenanthrene

Flouranthene

Benrolelpyrene

120

-

p loo 8

80 I

60 40

Naphthalene

I

100 -

s

1

I

a

t~

-

a0

40 20 -

20

-

0-

-

Phonmnthrono

Fluormntheno

Benxotm~pyrono Bonxolghllporylono

0

Figure 1. Recoveries of representative PAHs from marine sediment using 15-min statid15-min dynamic extractions vs 30-min dynamic extractions, both at 80 (top) and 200 "C (bottom). Error bars represent one standard deviation determined from triplicate extractions at each condition.

Figure 2 Recoveries of representative PAHs from diesel soot using 15-min statidl5-min dynamic extractions vs 30-min dynamic extractions, both at 80 (top) and 200 "C(bottom). Error bars represent one standard deviation determined from triplicate extractions at each condition.

error bars on the figures) whether the extractionswere performed at 80 or 200 "C. These results demonstrate that the extraction rates for PAHs from these samples are controlled more by the kinetics of the desorption process than by solubility-related phenomena (including chromatographic elution).16 In practical terms, the similarity in results using the static/dynamic and the dynamic procedures indicatesthat a single addition of modifier to the cell (with a static extraction step) should be sufficient to allow the modifier to sufficiently enhance the extraction rates without the need for premixed CO2/modifier fluids or dual pumping systems.

typically were lower than Soxhlet recoveries, SFE at 200 "C generally gave higher recoveries than those obtained at 50 "C. Further increases in SFE temperature to 350 "C did not yield increased recoveries but did show evidence of causing thermal degradation of some species (particularly aromatic amines) and the possible production of some low molecular weight PAHS.~ The results shown in both the present investigation and the literature8v3 indicate that 200 "C is a reasonable upper temperature to efficiently extract PAHs from environmental samples. The increased recoveries of the high-temperature extractions could be caused by two factors. Fit, high temperature can result in higher vapor pressure of the analyte, thus increasingthe analyte solubility despite the decrease in the density of C02 at high temperatures (at constant pressure). For example, the solubility of anthracene in C02 at 400 bar and 200 "C was reported to be 52 times greater than that obtained at 50 "C, although the density of C02 at 200 "C is only half of that at 50 oC.37Second, since analytes can be strongly bound to real-world samples, a certain amount of energy is required to desorb them from the sample matrix. Increasing the extraction temperature can provide more energy to the extraction system, greatly increasing the rate of the desorption process at high temperatures.16 In the present study, increasing the temperature of pure C02 extraction generally yielded quantitativerecovery of low molecular weight PAHs,but the recovery of high molecular weight PAHs were still ~ 6 0 % . Therefore, a modifier was required to improve the extraction efficiencies of those high molecular weight PAHs. Modifier Effect at Conventional Extraction Temperature (80 OC). The results shown in Tables 1-3 demonstrate that extractionswith modified C02 at 80 "C did not increase extraction efficiencies compared to pure C02 extraction at 200 "C in most cases. However, unlike the effect of temperature, the modifier influence on SFE efficiencies is matrixdependent. For example,

Temperature Effect on Pure C02 Extraction Efficiencies (80 vs 200 OC). As shown in Tables 1-3, increasing the extraction temperature from 80 to 200 "C generally increased the recoveries of PAHs from all three samples with pure C02, which demonstrates that the temperature influence on SFE efficiencies is independent of the sample matrix. (Note that all extractions in Tables 1 to 3,including the pure C02 extractions used the 15 min static/l5min dynamic procedure.) The recoveries of most low molecular weight PAHs (from naphthalene, MW = 128, to pyrene, MW = 202) were improved from ca. 50 to 80%at 80 "C to nearly quantitative at 200 "C for the sediment and air particulates, although the improvement was not so s i m c a n t for the diesel soot. The yield of high molecular weight PAHs were increased 2-4-fold by raising temperature from 80 to 200 "C for the diesel soot and air particulates, but only small increases were seen from the sediment. Similar results were reported in ref: where raising the extraction temperature to 200 "C (350 atm) resulted in PAH recoveries that were 2-6 times greater than those obtained at 50 "C and 350 atm from air particulates.8 Most recently, Hawthorne and MilleP reported the comparison of Soxhlet and low- and hightemperature supercritical C02 extraction efficiencies of organics from environmental solids. While SFE efficiencies at 50 "C

Analytical Chemistry, Vol. 67, No. 3, February 7, 7995

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at 80 "C, none of the modifiers tested showed significant increases in recoveries of any of the PAHs from the diesel soot. Diethylamine gave substantial increases in recoveries (particularly of the high molecular weight PAHs) from the marine sediment, and both toluene and diethylamine greatly increased the recoveries of the high molecular weight PAHs from the air particulate matter. Even though methanol has been widely used as a modifier in previous SFE studies, it was the poorest modifier tested for all of the samples when the extractions were performed at 80 "C. In fact, methanol yielded little, if any, increases in PAH recoveries for any of the test samples. Since the analytes were the same from all three samples, and since they were present at similar concentrations (low microgram per gram), the results in Tables 1-3 clearly demonstrate that the ability of a modifer to interact with the matrix is more important than its ability to interact with the PAH solutes. Based on these results, it seems unlikely that (for these samples) modifiers act by increasing the solubility of the a n a l y t e ~ . ~ ~A- ~more l likely mechanism is that the modifier covers active sites and prevents readsorption or partitioning of the analyte back onto the matrix active sites.15 Additionally, it was proposed recently that modifiers can interact with the analyte/matrix complex and lower the activation energy barrier of de~orption.~ Combined Temperature/Modifier Effect. Although modifiers at 80 "C gave fairly comparable recoveries to pure COZ extraction at 200 "C, a distinct increase in the recoveries appeared by raising the temperature to 200 "C with modified COZ. The temperature influence on the modifer effect was most clearly demonstrated by the extractions from diesel soot (Table 2). While no improved recoveries of high molecular weight PAHs were observed by all three modifiers at 80 "C, the extraction yield was significantly increased by the three modifiers when the temperature was raised to 200 "C. Increased recoveries at 200 "C were found for all three samples with all three modifiers (Tables 1-3), which indicates that, just like the temperature enhancements using pure COZ, the temperature enhancement of PAH recoveries using modified COz is independent of the sample matrix. However, the recovery enhancements by modifiers at 200 vs 80 "C are not the same for the three samples used in this study. The highest recovery enhancement was found by the extractions of diesel soot, and the recoveries of high molecular weight PAHs were increased ca. 3-4 times. For marine sediment and air particulates,the PAH recoveries were enhanced up to 2-fold by modifiers at 200 "C compared to those obtained by modifiers at 80 "C. (However, higher enhancements were possible with the diesel soot since recoveries at 80 "C were lower than the other two samples.) It should be noted that these significant increases in the recoveries were observed with all three modifiers, which indicates that the effect of elevated temperature does not depend on modfier identity. However, it should also be noted that the addition of modifer sometimes yielded no increase in recoveries in addition (39) Yonker, C. R; Smith, R D.J. Phys. Chem. 1988,92, 235. (40) Levy, J. M.; Ritchey, W. M. HRC CC, J High Resolut. Chromatogr. Chromatogr. Commun. 1987,10, 493. (41) Yonker, C. R; Gale, R W.; Smith, R D.J Chromatogr. 1986,371, 83.

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to that gained by raising the extraction temperature. For example, adding methanol modifier at 200 "C yielded no increase in recoveries over those obtained using pure COz from the air particulates (Table 3). At both 80 and 200 "C, methanol was the poorest modifier for the extraction of PAHs. Both toluene and diethylamine gave similar recoveries when extractions at either 80 or 200 "C are compared for the two matrices with high aromatic content (diesel soot, air particulates). However, diethylamine was superior for the marine sediment. Using diethylamine as a modifer at 200 "C, the recoveries of most PAHs were quantitative after 30 min of SFE (15min static/l5min dynamic) when compared to 1448h Soxhlet extractions. It should also be noted that these efficiencies were obtained without the need for modifier addition pumps or premixed fluids. The extraction mechanism of combined high temperature and organic modifier is unknown. Based on our results, the temperature and modifier appear not to act by the same mechanism, because the temperature enhancement is independent of the sample matrix and modifier identity, while the modifier enhancement does depend on the sample matrix and modifier identity. In addition,high temperature and organic modifiers have an additive effect for improving the SFE efficiencies of PAHs from environmental solids. As discussed above, the highest recoveries were obtained by modified COz at 200 "C for all three samples used in this study. CONCLUSIONS

Raising the extraction temperature from 80 to 200 "C increased SFE recoveries of PAHs from marine sediment, air particulates, and diesel soot using either pure or modifed COZ,demonstrating that the effect of temperature is independent of the sample matrix. Addition of organic modifiers to supercritical COZcan enhance the recoveries compared to those obtained using pure COZat both conventional (80 "C) and high (200 "C) temperatures. The combination of modifier and elevated temperature was most effective; however, the choice of modifer was highly matrixdependent. In general, diethylamine modifier at 200 "C gave the highest PAH recoveries and yielded good agreement with 1448h Soxhlet extractions. The results shown in this paper demonstrate that extraction using supercriticalCOZwith modifiers at high temperature could be the most efficient SFE method for extracting PAHs from environmental samples and indicate that this approach should be useful for other analytes provided that they have sufficient thermal stability. ACKNOWLEDGMENT

The financial support of the US. Environmental Protection Agency (EMSL, Las Vegas, NV), and instrument loans from ISCO are gratefully acknowledged. Received for review August 24, 1994. Accepted November 28, 1994.@ AC940844Z @Abstractpublished in Aduonce ACS Abstracts, January 1, 1995.