2518
Anal. Chem. 1984, 56,2518-2522
Tenax GC Cartridges in Adsorption6olvent Extraction of Aqueous Organic Compounds Christian Leuenberger a n d J a m e s F.Pankow* Department of Chemical, Biological, and Environmental Sciences, Oregon Graduate Center, 19600 N. W. Walker Road, Beaverton, Oregon 97006 An adsorptlon/solvent extractlon (ASE) preconcentratlon procedure based on cartrldges (bed volume 4.5 mL) of the sorbent Tenax GC Is described for nonpolar compounds. After sample loading, cartrldges were desiccated and extracted. At equlvalent aqueous concentrations of 80 ng/L (80 ppt) wRh extract analyses by capillary gas chromatography, examples of the % recoverles obtained for the first cartridge and the sum of two cartridges In serles were as follows: 1,3-dlchlorobenzene, 74 %, 85 %; hexachiorobutadlene, 65 %, 77 %; LY-HCH,59 %, 75 %; 7-HCH, 58 %, 79 %; p p‘-DDE, 73 % , 84 %; and p ,p’-DDD, 76 %, 84%. Virtually all of the material that reached the cartridges was captured on two cartridges. Some wall losses occurred before the analytes reached the cartridges. The 15 % breakthrough from the first to the second cartridges Is In agreement wlth values predlcted on the basls of dlffuslon-llmlted sorptlon.
-
Many trace aqueous organic analytical methods involve the preconcentration of dissolved analytes by means of a solid sorbent. The use of a cartridge-type sorbent bed facilitates field sampling and/or general laboratory handling. The expedient passage of the sample through the cartridge can help to reduce volatilization losses as we11 as losses to the vessel walls. If a portion of an analyte of interest is associated with other material in the water (e.g., humic materials, soil particles, sediment particles, detritus, etc. ( I ) ) ,that portion may pass unretained through the bed. If a whole sample analysis is all that is required, such associations would be disadvantageous for sampling with sorbent cartridges, However, there are situations in which one desires to differentiate between the dissolved and associated components. For example, the environmental fates of organic compounds are strongly affected by such dissolved/sorbed distributions, It is therefore of increasing interest to provide a distinction between the amounts of a given compound which are (1) dissolved and unassociated with other materials, (2) associated with other dissolved materials (e.g., low molecular weight “fulvic” acids (Z),and (3) associated with suspended (e.g., colloidal) particulate materials, (The degree of such associations may be predicted by using the compound’s octanol/water partition coefficient (K,) and the organic carbon content of the sorbing materials (1, 2).) If the amount of nonfilterable, sorbing organic carbon is minimal, the distinction between truly dissolved and particulate-associated materials may be achieved by pressure filtering through a silver membrane followed by a sorbent cartridge. (Pulling the sample through with vacuum risks volatilization losses.) The recovery of the organic compounds sorbed on the cartridge may proceed by either thermal desorption (3-8) or solvent extraction (9-13). The sequences “adsorption/thermal desorption” and “adsorption/solvent extraction” may be abbreviated “ATD” and “ASE”, respectively. An obvious advantage of the sorbent Tenax GC in ATD is its high thermal stability. This stability may also help to provide ASE with this sorbent with low blank levels. Pankow et al. (3-6) have investigated the types of compounds which may be sorbed by Tenax GC in aqueous sam0003-2700/84/0358-2518$01.50/0
pling, as well as the sampling conditions under which this sorbent will perform efficiently. In ATD, recoveries of 180% have been obtained at the 0.020-1.0 Fg/L level for a broad range of nonpolar compounds including chlorinated hydrocarbons, chlorinated monocyclic aromatics, and polycyclic aromatic hydrocarbons (PAHs). Recently, in a study of contaminated groundwater, Pankow et al. (8) observed breakthrough losses of less than 7% for l,l,l-trichloroethane and several chlorobenzenes from 0.68-mL bed volume cartridges packed with 60/80 mesh Tenax GC. Thus, although the low specific surface area of Tenax GC (19-21 m2/g (14, 15)) places restrictions on the sampling conditions (51, such compounds sorb strongly on this material. As a result, reasonably fast sample volume flow rates of 0.15-1.0 mL/s and relatively low cartridge bed volumes of 0.68-4.5 mL are simultaneously possible (3-8), and cartridge sampling with this sorbent for nonpolar compounds is attractive. (Very polar compounds are less well retained in aqueous sampling with this sorbent.) ATD with Tenax GC holds many advantages: high sensitivity and precision (8), interfaceability with capillary GC (4,16), suitability for field work (7,8,17), and desorbability of large compounds such as benzo[ghi]perylene (18). A disadvantage of ATD, however, is that it does not allow for any sample cleanup prior to the analysis. In determinations such as those involving polychlorinated biphenyls (PCBs) and gas chromatography with electron capture detection (GC/ECD), there is often a need for such cleanup, and ASE with subsequent cleanup would be more attractive. The macroreticular XAD resins have received the most attention as sorbents in ASE (12,13). However, carefully cleaned resin may still be subject to blank problems due to the release of impurities from interior sections of the beads (13). Previous studies of ASE with Tenax GC (9-11) have involved the following steps: (1) water sample in a glass vessel spiked with analytes, (2) sample passed through sorbent bed, (3) bed extracted, (4)vessel walls washed with solvent, (5) vessel washings combined with sorbent bed extract, and (6) analysis by GC. It is unfortunate that the extracts produced in steps 3 and 4 were not analyzed separately. Had that been done, distinctions could have been made between the amounts of compounds sorbed on the bed and the amounts which had been sorbed on the vessel walls. Therefore, although the overall recoveries of PCBs, several PAHs, and several pesticides were reported to be in the 90-100% range (Sll),those studies say less than they could have about the suitability of Tenax GC in ASE. On the basis of (1)the blank problems which can occur with the XAD resins, (2) our familiarity with Tenax GC and the good recoveries and low blank levels we have obtained with it in ATD (3-5,16-18), and (3)the deficiencies of the existing ASE/Tenax GC literature, the goal of the work described in this paper has been to provide an investigation of ASE/Tenax GC. EXPERIMENTAL SECTION Cartridge Preparation. The bed length, id., volume, sorbent (Tenax GC, Alltech Assoc., Deerfield, IL) mass, and sorbent mesh were 7.0 cm, 0.90 cm, 4.5 mL, 0.75 g, and 35/60, respectively. Each 0 1984 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
n
Liquid petroleurn ether Glass contraction/ expansion bellows
114' Teflon unions (Swagelok)
T e n a x GC adsorbent cartridge vapor side arm
Rlslng vapor
Liquid p a t r o l e u m athar
1 9 / 2 2 joint
PSmLflask with graduated conical bottom -10mL
petroleum ether
Scala:
2om
Clean boiling chip
H
Flgure 1. Apparatus for extracting sorbent cartridges.
cartridge was equipped with a 1.5-cm piece of 0.64-cm-0.d.glass tubing on each end of the cartridge to allow connections to 0.64-cm-0.d. tubing via Teflon Swagelok (Crawford Fitting Co., Solon, OH) union fittings. After being packed, each cartridge was conditioned by solvent extracting twice for 3 h with refluxing petroleum ether and then thermally desorbing for 3 h at 280 OC with precleaned helium gas at 60 mL/min. (Another satisfactory conditioning procedure involves the passage of 0.5 L of 50:50 hexane/acetone through a cartridge followed by thermal desorption (649.)The solvent extraction was carried out by using the apparatus shown in Figure 1. The vacuum jacket on the vapor arm of this apparatus prevents premature condensation of the solvent. Solvent flow is upward through the cartridge. This mode helps to displace air in the cartridge. Recovery Experiments. Recovery experiments were conducted by injecting a standard solution of the test compounds in acetone into a stream of water which flowed from a pressurized reservoir through the following elements: (1) injection port, (2) mixing chamber, and (3) Tenax GC cartridges connected in series. After injection, a volume of water was then passed through the system so as to simulate the passage of sample water through the cartridges. The advantage of carrying out recovery studies with this method is that wall and volatilization losses are minimized (3). The solubility of the analytes must, however, be great enough that no compounds precipitate. Increasing solubility, water flow rate, and solvent solubility, and decreasing concentration and injection rate all minimize precipitation. Decreasing solubility of the coanalytes may promote precipitation of a more soluble compound due to the solubility reduction which accompanies coprecipitation. If precipitation does not occur,then the recoveries obtained should be lower bounds on the recoveries possible with the overall water volume since the analyte goes on as an initial slug and is then washed with analyte-free water (3). Prior to injection of the standard solution, a valve was closed on the injection side of the cartridges. The cartridges were then evacwted at the outlet end to 155torr. Water (Millipore (Bedford, MA) Super-Q purification system) was then allowed to enter and forcibly wet the cartridges. (Recent work in our laboratory has
2519
indicated that such forced wetting may not be needed (6, a).) The flow rate was set at 1.0 mL/s, and 0.5 L was passed through the cartridges. With the water still flowing, the standard (20 pL, 4 ng/pL per component in acetone) was injected at 0.1 p L / s . (If the analytes were completely dispersed into the flowing water during the injection, the concentration going onto the cartridges would have been 0.4 pg/L. All of the study compounds have solubilities greater than this value (19). The least soluble is p,p'-DDE at 14 pg/L (20).)After the injection, an additional 1.0 L of water a a s passed through the system to simulate an overall sample volume of that size. Initial experiments were carried out by using a cartridge packod with glass wool as the mixing chamber. (The dimensions of the cartridge were the same as for a Tenax GC cartridge.) When it became apparent that some of the less soluble compounds were being sorbed to the glass wool, a second mixing chamber was devised and used in otherwise identical recovery experiments. It was constructed of glass and was composed of a 1.4-cm-i.d.,5.0 cm high mixing chamber, a short inlet leading to the injection port, and a short outlet leading to the cartridges. It was mixed with a small Teflon-coated magnetic stirbar. After passage of the 1 L of water, the wet cartridges were desiccated (10-min centrifugation at 3500 rpm in a Pyrex culture tube, 30-min evacuation at 40 pmHg (3)). They were spiked with an overall recovery standard (2 pL, 40 ng/pL butylbenzene in acetone) and then extracted for 3 h with 15 mL of petroleum ether (Figure 1). (Tenax GC is also stable with respect to hexane and acetone (21).Hexane was not used due to its relatively high boiling point (69 "C). Acetone contains impurities due to self-condensation, though it is useful in ASE/Tenax GC (17) providing that the analytes elute from the GC column at temperatures higher than the acetone dimer, etc.) The extract was concentrated by Kuderna-Danish (K-D) to 2 mL, spiked with external standard I (ES-I) (2 pL of 40 ng/pL 1,2,4-trichlorobenzeneand 6-HCH in acetone), and then analyzed at this stage for the chlorinated compounds by GC/ECD. Absolute recovery quantitation took place based on the responses of the ES-I compounds. ("External standard" =I standard external to sample during workup steps, but internal to sample at the time of analysis; "internal standard" = standard internal to sample during both workup and analysis (22).) In order to increase the concentrations in the extracts to values more readily quantifiable by GC/MS/DS, an additional volume reduction by Nzblowdown (35 "C) to 0.2 mL followed. Before analysis by GC/MS/DS, a second external standard (ES-11, 2 pL of 40 ng/pL isopropylbenzene, 1-methylnaphthalene, and fluoranthene in acetone) was added for quantitation purposes. The responses of the ES-I compounds relative to the ES-I1 compounds provided a measure of the losses which occurred during the Nzblowdown step. GC/ECD Analyses. A 25 m X 0.25 mm i.d., 0.25-pm film thickness DB-5 fused silica capillary column (J&W Scientific, Rancho Cordova, CA) was mounted in a Varian Aerograph (Palo Alto, CA) 2400 GC. The latter was equipped with an on-column injector (J&W Scientific) and a Varian 83NiECD. The injector He purge flow rate was 10 mL/min. The GC operating conditions were as follows: He linear carrier gas velocity, 40 cm/s (ambient temperature); Nz make-up gas flow rate at detector, 30 mL/min; and detector temperature, 330 O C . After the injection at 50 "C, the oven was programmed at 8 "C/min to 270 "C. Chromatograms were recorded and peaks integrated with a Model 4100 Spectra-Physics (Santa Clara, CA) integrator. GC/MS/DS Analyses. The same types of injector and column used in the GC/ECD analyses were installed on a Finnigan 4000 GC/MS/DS (Sunnyvale, CA). The column was passed directly through the transfer line to within 5 mm of the source. The analyses were carried out under the following conditions: carrier gas linear velocity, 40 cm/s (at 50 "C); GC temperature program, 50-270 "C at 8 "C/min; transfer line, source, and MS manifold temperatures, 240, 250, and 100 "C, respectively; and electron multiplier, 1800 V. MS data acquisition was performed by using the multiple ion detection (MID or "selected ion monitoring") technique with 0.5-1.0 amu wide windows centered on the mass-defect-corrected m/z values. R E S U L T S AND DISCUSSION The recoveries obtained here with ASE/Tenax GC were
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table I. Absolute Recoveries During Concentration from 15 mL (5.3 ng/pL per Component) to 2.0 mL by K-D and then to 0.2 mL by NzBlowdown”
compounds
ref compd (external std 11)
recovery f 1 s, % (three replicates)
Analyte Compounds o-xylene 1,3-dichlorobenzene 1,2-dichlorobenzene 2-bromo-rn-xylene hexachlorobutadiene 2-chloronaphthalene 2,6-dimethylnaphthalene acenaphthene fluorene CY-HCH Y-HCH heptachlor 9,lO-anthracenedione p,p’-DDE p,p ’-DDD
77 f 4 87 f 6 85 f 6 79 f a 72 f 6 71 f 3 74 f 5 92 f 8 85 f 5 92 f 7 92 f 8 93 f 13 98 f 11 97 f 11 85 f 16
1 1 1 2 2 2 2
3 3 3 3 3 3 3 3
Overall Recovery Standard Compound (Added before K-D Concentration) butylbenzene
83 f 4
1
External Standard I Compounds (Added after K-D Concentration, before N2 Blowdown) 1,2,4-trichlorobenzene 8-HCH
73 f 8 96 f 8
1
3
“Analyses by GC/MS/DS. All recoveries based on external standard I1 compounds (added after N2 blowdown): 1 = isopropylbenzene; 2 = 1-methylnaphthalene; and 3 = fluoranthene. influenced by whatever losses occurred during (1)the cartridge loading step (sorption on the glassware preceding the cartridges and breakthrough), (2) the extraction step, and (3) the concentration steps (K-D and N2 blowdown). Absolute recovery data obtained €or the concentration steps alone are presented in Table I. Since the ES-I compounds were added
after K-D concentration but before the Nz blowdown, their recoveries allow estimates of the losses incurred during the Nz blowdown step. The recovery of 1,2,4-trichlorobenzene (73%) is similar to the recoveries obtained for the similar compounds o-xylene through 2,6-dimethylnaphthalene (average recovery, 78%). This indicates that a large share of the losses of the latter group was due to the Nz blowdown. This is not surprising since volume reduction through Nz blowdown is known to be capable of causing substantial losses of volatile compounds (23-25). The losses are small for the larger, less volatile compounds. Table I1 presents the absolute recoveries obtained with (1) cartridge loading through the glass-wool mixing chamber, (2) extraction, and (3) K-D concentration and Nz blowdown. For the compounds 1,3-dichlorobenzene through fluorene, the average absolute recoveries obtained on cartridge 1, cartridge 2, and cartridges 1 + 2 (“sum”) were 64%, 13%, and 77%, respectively. The fact that losses occurred during K-D/N2 blowdown concentration steps (Table I) implies that the absolute recoveries in Table I1 underestimate the recoveries achieved during the passage of the sample through the cartridge. Dividing the absolute recoveries by the fractional equivalent of the Table I recoveries will allow the correction for the K-D/Nz blowdown concentration losses (though not for the loading step or extraction step losses). The concentration-step-corrected recoveries are presented in parentheses next to the absolute recoveries in Table 11. With this correction, the average cartridge 1, 2, and sum recoveries for 1,3-dichlorobenzenethrough fluorene were quite good, 79%, 17%, and 96%, respectively. For the compounds a-HCH, 7-HCH, p,p’-DDE, and p,p’-DDD, however, the average absolute recovery on the first cartridge was 25%, substantially less than the 64 % average observed for 1,3-dichlorobenzene through fluorene. Since the K-D/Nz blowdown recoveries were high for the HCHs, p , p ’-DDE,p , p ’-DDD,their recoveries remain poor even after the correction for the concentration step losses. We interpret the poor recoveries for the latter compounds in terms of adsorption losses within the high surface area glass-wool mixing chamber. Hexachloro-l,3-butadiene may also have been subject to such losses.
Table 11. Absolute and Concentration-Loss-Corrected Recoveries (the Latter in Parentheses) Obtained with ASE with Tenax GC and the Glass-Wool Mixing Chamber with K-D and NzBlowdown to 0.2 mLR
compounds
ref compd (external std 11)
cartridge 1
recovery f Is, % (three replicates) cartridge 2
sum
Analyte Compounds 1,3-dichlorobenzene 1,2-dichlorobenzene 2-bromo-rn-xylene hexachlorobutadiene 2-chloronaphthalene 2,6-dimethylnaphthalene acenaphthene fluorene CY-HCH Y-HCH heptachlor 9,lO-anthracenedione p,p’-DDE p,p’-DDD butylbenzene
1 1
1
1 2 2
2
3 3 3 3 3 3 3
57 f 11 (66 f 14) 61 f 4 (72 f 7) 56 f 3 (71 f 5) 46 f 15 (64 f 22) 68 f 7 (95 f 10) 75 f 7 (101 f 12) 72 f 4 (78 f 8) 74 f 6 (87 f 9) 36 f 25 (39 f 27) 25 f 29 (27 f 31) 53 f 39 (57 f 43) 57 f 14 (58 f 16) 17 f 6 (18 f 7) 21 f 27 (25 f 33)
12 f 1 (14 f 2) 15 f 2 (18 f 3) 10 f 2 (13 f 3) 8 f 3 (11f 4) 14 f 2 (20 f 3) 16 f 3 (22 f 4) 16 f 3 (17 f 4) 16 f 3 (19 f 4) 9 f 11 (10 f 12) 14 f 9 (15 f 10) 11 f 2 (12 f 3) 18 f 9 (18 f 9) 3 f 2 (3 f 2) 12 f 11 (14 f 13)
Overall Recovery Standard Compound (Added before Extraction) 86 f 2 (104 f 6) 85 f 3 (102 f 6) 1
69 f 11 (80 f 14) 66 f 5 (90 f 8) 66 f 4 (84 f 6) 54 f 15 (75 f 22) 82 f 7 (115 f 10) 91 f 8 (123 f 13) 88 f 5 (95 f 9) 90 f 7 (106 k 10) 45 f 27 (49 f 30) 39 f 30 (42 f 33) 64 f 39 (69 f 43) 75 f 17 (76 18) 20 f 6 (21 f 7) 33 f 29 (39 f 36)
*
NA (NA)
External Standard I Compound (Added after K-D Concentration, before N2 Blowdown) NA (NA) 90 f 3 (123 f 14) 88 f 7 (120 f 16) 1,2,4-trichlorobenzene 3 I1 compounds (added after N2 blowdown): 1 = isoAll recoveries calculated based on external standard “Analyses by GC/MS/DS. propylbenzene; 2 = 1-methylnaphthalene;and 3 = fluoranthene. NA = not applicable since standards added to all cartridges.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Table 111. Absolute Recoveries Obtained with ASE with Tenax GC Cartridges with Glass/Stirbar Mixing Chamber and K-Dto 2.0 mLa recovery f Is, % ref compd (external cartridge cartridge cartridge 3d I* 2c std I) compounds 1,3-dichlorobenzene 1,2-dichlorobenzene hexachloro1,3butadiene CX-HCH Y-HCH heptachlor p,p'-DDE p,p'-DDD
sum
1
74f12
13f4
2fl
89f13
1
72f11
13f4
2 f 1
87f12
1
65f7
12f2
4f1
81f7
2
59f11 58f16 73f12 73f9 76f5
16f4
4fl 5fl
79f12 84f18 79f12 87flO 85f6
2 2 2
2
21f8
5f2 11f5 8f2
1f1
3 f 1 If1
"Analysis by GC/ECD. All recoveries calculated based on External Standard Icompounds (added after K-D concentration): 1 = 1,2,4-trichlorobenzene; 2 = 8-HCH. *Five replicates. cThree replicates. d T ~replicates. o Table I11 presents absolute recovery data obtained by using the lower surface area glass/stirbar mixing chamber. No corrections for K-D/N2 concentration losses are presented here since these extracts were analyzed by GC/ECD after K-D concentration but before N2 blowdown. The absolute recoveries were quite satisfactory, particularly in view of the fact that the analyses were carried out a t the equivalent low concentration of 80 ng/L. p,p'-DDE and p,p'-DDD were now recovered as well as the other compounds. For all compounds, the cartridge 1,2,3, and sum recoveries (absolute) averaged 69%, 12%, 3%, and 84%. (The special sensitivity of the ECD allowed a third cartridge to be analyzed.) These data indicate that little material escaped onto the third cartridge. The -16% which remains unaccounted for was therefore lost either before the material reached the cartridges (e.g., sorption to the walls of the tubing and mixing chamber) or during the
2521
cartridge extraction. On this basis, the average amount of material which reached the first cartridge and then broke through that cartridge was 18%. After the Table I11 extracts were analyzed by GC/ECD, they were concentrated by N2 blowdown to 0.2 mL and reanalyzed by GCIMSJDS (Table IV). The number of compounds in the extract which could be determined by GC/MS/DS was greater than the number determinable by GC/ECD. For the compounds 1,3-dichlorobenzenethrough fluorene, the average cartridge 1,2, and sum recoveries (absolute) were 63%, 7%, and To%, respectively. These data are similar to those obtained with the glass-wool mixing chamber (Table 11). However, as was observed in Table 111, the cartridge 1 , 2 and sum recoveries (absolute) for the HCHs, p,p'-DDE, and p,p'-DDD were much improved over those in Table 11. For all compounds in Table IV,the average cartridge 1,2, and s u m rec6veries corrected for concentration losses were 72%, 8%, and 80%, respectively. The amount of material lost during loading or extraction averaged -20%. The amount of material which passed unretained through the second cartridge was probably less than 1%. These recovery data are very similar to those in Table 111. This is as expected since the Table IV data have been corrected for the N2 blowdown losses and Table I11 data were not subject to those losses. For the Table 1 V data, though, the average cartridge 1 to 2 breakthrough was 10%. The above data indicate that the losses incurred during the K-D/N2 concentration step may be compensated for either by separate recovery experiments as described here (Table I) or through the use of carefully chosen internal standards. The latter could be added at the beginning of the extraction and would allow the compensation for extraction losses as well as the concentration step losses (the former were not corrected for in this study). The losses associated with sorption on the glass before the analytes reach the cartridges do not represent limitations of the sorbent itself, though they do indicate the need to transfer waterborne analytes to the cartridges without losses if the resultant data are to reflect true aqueous-phase concentrations.
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Table IV. Absolute and Concehtration-Loss-Corrected Recoveries (the Latter in Parentheses) Obtained with ASE with Tenax GC Cartridges with G l a d s t i r b a r Mixing Chamber and K-D/N2 Blowdown Concentration to 0.2 mLn compounds
ref compd (external std 11)
cartridge 1
recovery f Is, % (three replicates) cartridge 2
sum
Analyte Compounds o-xylene 1,3-dichlorobenzene 1,2-dichlorobenzene 2-bromo-rn-xylene hexachlorobutadiene 2-chloronaphthalene 2,6-dimethylnaphthalene acenaphthene fluorene CX-HCH T-HCH heptachlor 9,lO-anthracenedione p,p'-DDE p,p'-DDD butylbenzene
62 f 4 (81 f 7) 10 k 4 (13 f 4) 59 f 6 (68 f 8) 8 f 3 (9 f 3) 59 f 6 (69 f 9) 8 f 3 (9 f 3) 2 61 f 13 (77 f 17) 4 f 3 (5 f 4) 52 f 8 (72 f 13) 2 5 f 3 (7 f 4) 2 62 f 7 (87 f 11) 7 f 5 (10 f 7) 2 70 f 9 (95 f 14) 10 f 6 (14 f 9) 2 83 f 12 (90 i 15) 3 f 2 (3 f 2) 3 57 f 12 (67 f 15) 7 f 4 (8 f 6) 51 f 13 (56 f 15) 3 7 f 6 (8 f 7) 3 48 f 10 (52 f 12) 14 f 7 (15 f 8) 3 65 f 14 (70 i 18) 5 f 2 (6 f 3) 3 62 f 12 (63 f 14) 8 f 2 (8 f 2) 3 61 f 8 (63 f 11) 2 f l ( 2 f 1) 62 f 12 (73 f 20) 3 3 f 2 (4 f 3) Overall Recovery Standard Compound (Added before Extraction) 1 1 1
72 f 6 (94 f 8) 67 f 7 (77 f 9) 67 f 7 (78 i 10) 65 f 13 (82 f 18) 57 f 9 (79 f 14) 69 f 9 (97 f 13) 80 f 11 (109 f 17) 86 f 12 (93 f 15) 64 f 13 (75 f 16) 58 f 14 (64 f 17) 62 f 12 (67 f 14) 70 f 14 (76 f 18) 70 f 12 (71 f 14) 63 f 8 (65 f 11) 65 f 12 (77 f 20)
1 80 f 1 (96 f 5) 78 f 4 (94 f 7) NA (NA) External Standard I Compounds (Added after K-D Concentration, before N2 Blowdown)
1,2,4-trichlorobenzene 1 82 f 4 (112 f 13) 79 f 5 (108 f 14) NA (NA) 6-HCH 3 84 f 7 (88 f 10) 80 f 4 (83 f 8) NA (NA) "Analyses by GC/MS/DS. All recoveries calculated based on external standard 11 compounds (added after N2 blowdown): 1 = isopropylbenzene; 2 = 1-methylnaphthalene;and 3 = fluoranthene. NA = not applicable since standards added to all cartridges.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 13, NOVEMBER 1984
Taken together, the Table I11 and IV data indicate that during the passage of the sample through the cartridge at 1 mL/s, sorption on the first 4.5-mL bed volume cartridge occurred with approximately 82-90 % efficiency (10-18 % breakthrough). This is in very good agreement with the 86% recovery (or 14% breakthrough) predicted at the outset of this work based on the model of Pankow et al. (5) for the experimental conditions used. In the development of this model for 35/60 mesh Tenax GC for the types of strongly retained compounds studied here, Pankow et al. (5) have shown that this breakthrough process is not caused by the exhaustion of a fixed amount of sorbing surface area. Rather, it is caused by the diffusional limitation of the transfer of analyte through the sample fluid to the sorbing beads. By modifyng the cartridge dimensions, mesh size, and/or the sample volume flow rate, this recovery may be raised to near quantitative levels. The experiments carried out here, however, were designed with expediency as well as recovery in mind to allow a sample volume flow rate as large as 1 mL/s, recoveries in the 80-90% range were tolerated. For the cartridge dimensions and mesh size used here, the recovery efficiency can be raised to 95% (5% breakthrough) by lowering the flow rate to 0.4 mL/s (5). In a study of groundwater contamination with ATD/Tenax GC, Pankow et al. (8) recently obtained breakthroughs of less than 7% for compounds such as l,l,ltrichloroethane and several chlorobenzenes. Since the sorption process is diffusion limited and not sorption site limited at concentrations a t least as high as 5 pg/L, to a first approximation the extent of breakthrough remains constant as the sample volume is increased to many liters (3). This is an obvious advantage from the sensitivity viewpoint. The work presented in this paper was carried out with purified water. Some statements are therefore required concerning the applicability to real-world samples. As discussed above, one factor which could decrease the recoveries would be analyte association with dissolved organic matter. This problem is not specific to the use of Tenax GC but applies to other sorbents as well. When such organic matter and/or the KO,of the analytes of interest are low, this problem will not be significant (2, 2). The issue of the degree to which Tenax GC will become “fouled” and therefore less retentive through the sorption of such organic matter is a separate matter that requires further attention. The recent work by Pankow et al. (8) involving ATD/Tenax GC and 50-mL groundwater samples indicated little loss of retentive power toward the compounds l,l,l-trichloroethane, chlorobenzene, and l,4-dichlorobenzene in the presence of dissolved organic carbon (DOC) values of several milligrams/liter. On this basis, and on the basis of the data presented in this paper, the potential of Tenax GC in ASE for nonpolar compounds appears confirmed.
ACKNOWLEDGMENT The technical assistance of Lorne M. Isabelle is gratefully acknowledged. Registry No. a-HCH, 319-84-6; 7-HCH, 58-89-9;p,p’-DDE, 72-55-9; p,p’-DDD, 72-54-8; Tenax GC, 24938-68-9; o-xylene, 95-47-6;1,3-dichlorobenzene,541-73-1; 1,2-dichlorobenzene,958750-1; 2-bromo-m-xylene,576-22-7;hexachloro-1,3-butadiene, 68-3; 2-~hloronaphthalene,91-58-7; 2,6-dimethylnaphthalene, 581-42-0; acenaphthene, 83-32-9;fluorene, 86-73-7;heptachlor, 76-44-8; 9,10-anthracenedione, 84-65-1.
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RECEIVED for review April 23, 1984. Accepted June 18, 1984. We express our appreciation to the Swiss National Science Foundation for financial support to Christian Leuenberger during this study.
