Anal. Chem. 1994,66,909-916
Role of Modifiers for Analytical-Scale Supercritical Fluid Extraction of Environmental Samples John J. Langenteld,tp$Steven B. Hawthorne,’*t Davld J. IWHler,t and Jamsr Pawlbzyn* Energy and Envlronmental Research Center, Unlverslty of North Dakota, Grand Forks, North Dakota 58202, and Department of Chemlstry, Unlverslty of Waterloo, Waterloo, Ontario, &mda N2L 3Gl
Supercritical fluid extraction (SFE)using eight different C02 + organic modifier mixtures and one ternary mixture (C02 + methanol/toluene) at two different concentrations (1and 10% v/v) was performed on two certified reference materials including polychlorinatedbiphenyls (PCBs)from river sediment and polycyclic aromatic hydrocarbons (PAHs) from urban air particulate matter. The modifier identity was more important than modifier concentration for increasing extraction efficiencies. Acidic/bask modifiers including methanol, acetic acid, and aniline greatly enhanced the extraction of PCBs. Low molecularweight PAHswere best extracted with modifiers including aniline, acetic acid, acetonitrile, methanol/toluene, hexane, and diethylamine. In contrast, modifiers capable of dipole-induced dipole interactions and T-T interactions such as toluene, diethylamine, and methylene chloride were the best modifiers to use for SFE of high molecular weight PAHs from air particulates. In general, increasing the modifier concentration from 1 to 10% (v/v) had Uttle effect on PCB and low molecular weight PAH recoveries, although the recoveries of high molecular weight PAHs from urban air particulate matter were enhanced significantly at the higher modifier concentration. Although there is no definite theory that explains modifier selection for SFE,it appears that modifiers should be selectedon the basis of matrix characteristicsand the target analytes. Supercritical fluids are becoming an acceptablealternative to conventional liquid solvents for the rapid analytical-scale extraction of environmental samples.’-3 The most common fluid to date has been supercritical C02 because of its reasonable critical properties, low toxicity, and chemical inertness. In addition, supercritical C02 has Lewis base characteristics, induced dipole interactions, and quadrupole interactions that allow it to solvate numorous compounds ranging in polarity from nonpolar to moderately polar. However, poor recoveries associated with certain analyte/ matrix combinations (even at low analyte concentrations) in analytical-scale supercritical fluid extraction (SFE) of real environmental samples indicate that a suitable supercritical fluid must not only be able to solvate analytes of interest, but must possess properties that allow it to interact with the analyte and the matrix to efficiently partition the analyte into the t University 8 University
of North Dakota. of Waterloo. (1) Hawthorne, S. B. Anal. Chcm. 1990, 62, 633A. (2) Vannoort, R. W.; Chervet, J.-P.;Lingeman, H.;DcJong, 0.J.; Brinkman, U. A. Th. J . Chromatogr. 1990, 505, 45. (3) Cheater, T. L.;Pinkston, J. D.; Raynie, D.E. Anal. Chcm. 1992,64, 153R. 0003-2700/94/030bQ00O~Q4.5Q/O Q 1004 Amorloan Chemlcal Soolety
bulk supercritical fluid.ll4-6 While C02 is a relatively good solvent, these solvating interactions are weak and it has been shown that pure C02 often cannot extract environmentally persistent pollutants such as chlorinateddioxins from fly ash: polycyclic aromatic hydrocarbons (PAHs) from urban air particulate matter,7 PAHs and nitroaromatics from diesel exhaust particulate matter? and polychlorinated biphenyls (PCBs) from river ~ediment.~*g Therefore, it appears that stronger fluids are required. Previous studies have shown that other pure supercritical fluids such asN2O and CHClF2 have theability tosignificantly improve extraction rates and efficiencies of a wide variety of analyte/matrix combination~.~*”3While these pure fluids may not be acceptable to use on a routine basis (CHClF2 can contribute to ozone depletion, and N2O is an explosion hazard14 and does not always increase extraction efficiencies9), the choice for other pure fluids is limited by the need for reasonable critical parameters. However, if the chemical properties (e& dipole moment, acid/base properties, polarizability, etc.) of these fluids are examined, modified supercritical fluids can be identified which possess properties similar to N20 and CHClF2. Modified fluids have been used extensivelyin SFC to reduce chromatographicretention and to introduce selectivity to the separation’s20as well as to increase extraction efficiencies in SFE.’OJ1J2 To date, the most common modifier employed in SFE has been methanol, chosen because of its high solvent polarity parameter, and its ability to deactivate the active (4) Alexandrou, N.; hwli8zyn, J. Anal. Chcm. 1989, 61, 2770. (5) Alexandrou, N.; hwlLzyn, J. Anal. Chcm. 1992,64, 301. (6) Hawthome,S. 8.; Miller, D.J.; Laqenfeld, J. J. Prauedfngojrhrlnternarfonal Symparfum on SuprcrftfcalN u f d Chromatography and Extractfon;h r k City, UT, January 1991; p 91. (7) Langenfeld, J. J.; Hawthorne, S.Bo;Miller, D. J.; Pawlirzyn, J. Anal. Chcm. 1993, 65, 338. ( 8 ) Parchkc, T.; Hawthorne,S.B.; Miller, D. J.; Wenclawiak, B. J. Chromatogr. 1992, 609, 333. (9) Hawthorne,S. B.; Langenfeld, J. J.; Miller, D. J.; Burford, M. D. Anal. Chcm. 1992, 64, 1614. (IO) Onuaka, F. I.; Terry, K.A. J. Hfgh Rcsolut, Chromatogr. 1989,12, 357. (1 1) Hawthome, S. B.; Miller, D.J. Anal. Chcm. 1987, 59, 1705. (12) Hawthorne, S. B.; Miller, D. J.; Langenfeld, J. 1. J. Chromatogr. Scf. 1990, 28, 2. (13) Sauvage, E.; ROWS, J.-L.;Touuaint, G. J. Hfgh Rcsolut. Chromatogr. 1993, 16, 234. (14) Siwers, R. E.; Hamen, B. Chcm. Eng. News 1991,69, No. 29, 2. (IS) Jansrren. J. G. M.; Schocnmaken, P. J.; Cramen, C. A. J. Hfgh R c d u t . Chromatogr. 1989,12,645. (16) Lochmuller, C. H.;Mink, L. P. J. Chromotogr. 1989, 471, 357. (17) Lochmullcr, C. H.;Mink, L. P. J. Chromatogr. 1987, 409, 55. (18) Yonker, C. R.; Smith, R. D. J. Phys. Chcm. 1988,92,235. (19) Levy, J. M.; Ritchey, W. M. HRC CC, J. Hfgh Rcsolut. Chromatogr. Chromatogr. Commun. 1987, 10, 493. (20) Yonker, C. R.; Gale, R. W.; Smith, R. D. J . Chromatogr. 1986, 371, 83. (21) Wright, B. W.; Wright, C. W.; Gale, R. W.; Smith, R.D. Anal. Chcm. 1987, 59, 38. (22) Wheeler, J. R.; McNally, M. E. J . Chromatogr. Scl. 1989, 27, 534.
Ana!~tlcaIChomI8t1y,Vola66,No. 6,Mrch 15, 1994 BOB
sites on column supports in SFC.l5-l7 In addition, numerous other modifiers with different chemical characteristics have been used to enhance recoveries in SFE.' Unfortunately,since the role that the modifier plays in analytical-scale SFE of heterogeneous environmental samples is not well understood, choosing a modifier for an application has been highly empirical. Furthermore, other factors such as the effects of modifier concentration, the sample matrix, and the analyte type can complicate the choice. Unfortunately, very few investigations have focused on these parameters in modified SFE applications. This report describes a rapid and simple method to survey a group of modifiers to obtain maximum extraction efficiencies. Eight different modifiers with different chemical properties and one ternary mixture (COz + 50/50 volume ratio of methanol and toluene) were examined for the extraction of native (not spiked) analytes from certified reference materials, including PAHs from urban air particulate matter (SRM 1649) and PCBs from river sediment (SRM 1939). Two modifier concentrations were used (1 and 10% v/v) to determine the effect of the modifier concentration in SFE. The 1% modifier concentration was chosen because it does not significantlyincrease the solubilizing power of supercritical C02 as determined by solubility experiments based on solvatochromic probes.23 Therefore, at this concentration, the modifier only has the ability to disrupt matrix/analyte interactions. In contrast, the 10% modifier concentration has been shown to enhance the bulk solubility properties of supercritical C02.23 It should be noted that the purpose of this investigation was to determine the role of the modifier on extraction efficiencies and not necessarily to obtain quantitative recoveries. Therefore, short extraction times were utilized so that the extraction efficiency enhancements over pure COZat each modifier condition could be observed.
EXPERIMENTAL SECTION Samples. The five samples that were used in this study contained native (not spiked) PAHsor PCBs at low microgram per gram concentrations. The PCB-contaminated sediment (SRM 1939) was obtained from the National Institute of Standards and Technology (NIST, Gaithersburg, MD) and used as received. The sample contained ca.3 wt % water and ca. 10 wt 8 organic content, as determined by thermal gravimetric analysis, The PAH-contaminated urban air particulate matter sample (SRM 1649) and the PAHcontaminated marine sediment (SRM 1941) were also obtained from NIST. The water content was ca. 4% for each sample, while the organic content was determined to be ca. 38 and 1196, respectively, The railroad bed soil was obtained near a set of railroad ties in Hastings, MN, and was sieved through a 2-mm sieve to remove any stich or other debris. Thesamplecontainedca. 1%water andca. 896organiccontent. The diesel soot was obtained from the tailpipe of a school bus and contained ca. 4% water and ca. 82% organic content. Supercritical Fluid Extraction#. An Ism Model 260D syringe pump (Isco, Lincoln, NE) was filled with SFC-grade carbon dioxide (Scott Specialty Gases, Plumsteadville, PA), which was then pressurized to 400 atm. A 0.3-g sample of urban air particulate matter or a 0.5-g sampleof river sediment
was placed inside a 2.5-mL extraction cell that was supplied with an Isco Model SFX 2-10 extractor. Prior to extraction, a modifier was added to the extraction cell by pipeting 25 (1% v/v) or 250 pL (10% v/v) directly onto the sample in the extraction cell. (It should be noted that the modifier concentrations are based on thevolume of the empty extraction cell and are altered slightly by the presence of the sample. However the effect of the sample volume on the modifier concentration is relatively small. For example, the 0.5-g sediment sample only occupies ca. 0.2 mL, making the actual concentrations of the modifier ca. 1.1 and 11%. Since there are minor differencesin the volumes occupied by the different samples used in this study, the values of 1 and 10% are used throughout for convenience.) The cell was then placed into the extractor and pressurized by opening the inlet valve. After a 5-min static (nonflowing) extraction period, the outlet valve was opened and the extraction cell was flushed with pure CO2 for an additional 10 min (dynamic extraction). The flow rate of the supercritical fluid (ca. 1 mL/min liquid COZmeasured at the pump) during the dynamic extraction step was controlled by a 10-cm-long piece of fused silica tubing with an internal diameter of 32 pm (Polymicro Technologies, Phoenix, AZ). The extraction cell temperature was regulated by the extractor at 80 OC due to the elevated critical temperatures of some of the modified COZmixtures. (The miscibility of modifiers in COz is reviewed in ref 24.) Extracted analytes were collected by placing the outlet end of the restrictor into a 7.4-mL vial containing 5 mL of Fisher Optima Grade acetone (PCBs from river sediment) or methylene chloride (PAHs from all other samples). Internal standards (860 ng of 2,3/,4,5/,6-pentachlorobiphenylZ5for the PCB extracts and 1 pg of chrysenedlz for the PAH extracts) were added after SFE but prior to gas chromatographic analysis. SFE extracts were then concentrated under a gentle stream of clean N2 to a volume of approximately 2 mL. GM Chromatographic Analysis. PCB quantitations were performed using a Hewlett-Pachrd 5890 gas chromatograph (Wilmington, DE) equipped with an electron-capturedetector with hydrogen as the carrier gas and nitrogen as the detector makeup gas. Autosampler injections were performed in the splitless mode (0.Zmin splitless period) into a 60 m long X 0.25 mm i.d. (0.25-pm film thickness) DB-5column (J&W Scientific, Folsom, CA). The injector and detector temperatures were maintained at 300 OC, The oven temperature was held at 150 OC for 40 min, ramped at 1 OC/min to 220 OC, and ramped to 330 OC at 3 OC/min.zS Calibration standards of PCB congeners in isooctane (2.08 pg/mL each congener) were obtained from Supelco (Bellefonte, PA) and diluted appropriately. PAH analyses were performed using a Hewlett-Paclcard 5988 OC/MS, operating in the selected ion mode for the molecular ion of each PAH. Autosampler injections were performed in the splitless mode for 0.2 min into a 25 m long X 0.32 mm i.d, (0.17-pm film thickness) HP-5 capillary column, The oven temperature was 80 OC ramped to 330 O C at 8 OC/min. PAH quantitations were based on the injection of standard mixtures of PAHs in acetonitrile obtained from NIST (SRM 1647b). (24) Pago, S. H.;Sumptor, S.R.; Loo, M.L. JQMfcmwlvmnSup. 1992, 4, 91. (25) Robbrt, R.E,;Chcrlor, S.N.;Ouonthor, F.R.;Koltor, B. 1.;Paw,R. M.; Schmtz, M.M.; who, S.A. Frrsrnlua' J. Anal. Chrm. 1992,342,30.
Table 1. Supercrltkal Fluid Extraction Efflciencles d Polychkrtmld Blphenyk from Rlver SdhmM Ualq DII1.nnt Madykn and Concentratlons % recovery (% RSDP
congenerb
PCB no.'
cert conc, p g / g (% RSD)d
2,2',5 2,4,4' 2,2',5,5' 2,2',3,5' 2,3',4,4' 2,2'4,5,5' 2,3',4,4',5 2,2'3,4,4',5' 2,2'3,4,4',5,5'
18 28 52 44 66 101 118 138 180
3.46 (2) 2.21 (5) 4.48 (1) 1.07 (11) 0.93 (1) 0.82 (1) 0.51 (2) 0.57 (2) 0.16 (6)
congenerb 2,2',5 2,4,4' 2,2',5,5' 2,2',3,5' 2,3',4,4' 2,2'4,5,5' 2,3',4,4',5 2,2'3,4,4',5' 2,2'3,4,4',5,5'
COz
1%methanol
10% methgnol
1%CHzClz
PCB no.'
10% toluene
10% hexane
18 28 52 44 66 101 118 138 180
37 (2) 48 (3) 49 (3) 58 (3) 47 (12) 67 (3) 67 (3) 65 (5) 83 (7)
29 (9) 35 (15) 41 (9) 43 (16) 37 (18) 55 (10) 54 (11) 55 (10) 69 (11)
10% CHZC12
1%toluene
% recovery (% RSD)
congener* 2,2',5 2,4,4' 2,2',5,5' 2,2',3,5' 2,3',4,4' 2,2'4,5,5' 2,3',4,4',5 2,2'3,4,4',5' 2,2'3,4,4',5,5'
PCB no.' 18 28 52 44 66 101 118 138 180
1% aniline
10% aniline
1% diethylamine
10% diethylamine
1% acetic acid
10% acetic acid
52 (12) 55 (11) 69 (4) 77 (8) 61 (6) 80 (4) 87 (3) 108 (7) 115 (9)
a Percent recoveries based on NIST certified concentrations. Percent relative standard deviations are based on triplicate SFE extractions (5-min static followed by 10-min d amic) at each condition. Modifier concentrations in supercritical COS are based on volume/volume percent. b PCB congeners indicatefiy their chlorine substitution pattern. PCB congener numbers taken from ref 37. d NIST certified concentrations baaed on twos uential16-h Soxhlet extractions. e Methanol and toluene were mixed at a 50/50 volume ratio and the appropriate volume of the mixture was a a e d to the extraction cell.
RESULTS AND DISCUSSION Effect of Modifiers on Extraction Efficiencies. The percent recoveries (versus NIST certified concentrations based on 3248-h Soxhlet extractions) of PCBs from river sediment and PAHs from urban air particulate matter are shown in Tables 1 and 2. As mentioned earlier, the short extraction times (5-min static followed by 10-min dynamic extraction) were purposely utilized so that subquantitative recoveries were achieved, thereby allowing one to directly compare the effect of each modifier on the extraction efficiencies of the target analytes. Triplicate SFE extractions (5-min static followed by 10-min dynamic) were performed at each condition, and the percent relative standard deviations (9% RSDs) were generally
