40
Anal. Chem. 1088, 60,40-47
Adsorption/Thermal Desorption with Small Cartridges for the Determination of Trace Aqueous Semivolatile Organic Compounds James F. Pankow,* Mary P. Ligocki,’ Michael E. Rosen, Lorne M. Isabelle, and Kenneth M. Hart Water Research Laboratory, Department of Environmental Science and Engineering, Oregon Graduate Center, Beaverton, Oregon 97006
Adsorptlon/thermal desorption (ATD) wlth small bed volume (0.68 cm’) Tenax cartridges and direct desorption to a fused-silica capllary gas chromatography column has been studled for the determlnatlon of a wide range of semlvolatlie compounds In water. Recoveries were determlned for a 5 mln, 250 OC desorptlon step alone, as well as for the complete ATD method. Desorption recoveries near 100% were found for a varlety of phenols and other monocyclk aromatics, polycycllc aromailc hydrocarbons (PAHs) up to 1methylphenanthrene,pestkldes up io dieldrin, phthalates up to diethyl phthalate, and alkanes up to trlcosane (Cs). Most phenols were not recovered weH In the full ATD method studies. Virtually all other compounds studled, Including nonphenollc monocycllc aromaUCs, PAHs, pesticides, and several alkanes were recovered well. Even compounds such as pyrene, butyl benzyl phthalate, and p,p’-DDE (1,l’-(dlchloroethenylldene)bls[4-chlorobenzene]), which were not 100% recovered In the short 5-mln desorption step alone, gave full ATD method recoveries near 100%; during the aqueous-phase sampling step, these compounds sorbed near the sample Inlet end of the bed, facllltailng recovery by thermal desorption.
Work in our laboratory (1-5) has demonstrated the advantages of adsorption/thermal desorption (ATD) with the sorbent Tenax in the direct sampling and analysis of water for nonpolar trace organic compounds. (“Tenax=will be used here to refer to both Tenax-GC and Tenax-TA; the latter is manufactured so as to have lower blank levels.) These advantages include a high inherent sensitivity and a demonstrated applicability ranging from l,l,l-trichloroethane ( 3 , 4 ) for sample volumes of the order of 50-100 mL, to polycyclic aromatic hydrocarbons (5)for sample volumes of several liters. Even low volatility compounds such as benzo[ghi]perylene (6) and decachlorobiphenyl(7) may be desorbed from Tenax. The sensitivity of ATD is due to the fact that the organics sorbed from the entire sample may be desorbed to a gas chromatography (GC) column. As it is employed in our laboratory for semivolatile compounds, the ATD method involves (1) passage of a sample through a sorbent cartridge, (2) two-step, centrifugation and vacuum desiccation of the cartridge, (3) addition of an internal standard (IS), (4) carrier gas purge for the removal of O2 and IS-related solvent, (5) desorption to a capillary column under whole column cryotrapping (WCC) conditions, and (6) GC temperature program run. With WCC at -80 “C, even rather volatile analytes may be trapped on a thin film (0.25 pm) column (8). Present address: EnvironmentalQuality Laboratory, California Institute of Technology, Pasadena, CA 91125. 0003-2700/88/0380-0040$01.50/0
Our first use of ATD with Tenax-GC (1,2) involved cartridges with bed volumes of -5 cm3. At that time, the general perception was that such large cartridges could not easily be desorbed directly and with no splitting to a capillary column; the “mismatch” between the desorption flow rate and the GC column flow rate was perceived as being too large. An appropriate interface between the two was therefore developed (9). That interface utilized a second, small bed volume (0.63 cm3) Tenax-GC cartridge. After the desorption of the sampling cartridge to the interface cartridge, the latter was desorbed at a lower flow rate (no splitting) to a capillary column under WCC conditions. Due to the inconvenience and possible losses involved in using intermediate trapping, we then began to investigate the extent to which small cartridges with bed volumes of 1 cm3 or less can themselves be used for water sampling with subsequent direct desorption to a capillary column (3-5). Despite the fact that we have recently demonstrated that cartridges with bed volumes of - 5 cm3 may be desorbed directly to capillary columns (IO),our interest in small sample cartridges has remained strong. Indeed, when the sample volume is of the order of a few liters or less, a small bed volume may often suffice. Also, small cartridges will alleviate the need for the comparatively long and/or hot desorptions required for large cartridges (6, 7). Finally, while supercritical fluid desorption has been discussed as a means to reduce the harshness of desorption conditions ( I I ) , the ease of using thermal means to recover target analytes from small cartridges will allow ATD to remain attractive in many situations. As the size of a sorbent bed volume decreases, increasing attention must be paid to the sampling conditions if the fractional sorption recovery (R) is to remain close to 1. For strongly sorbed compounds, a semiempirical relationship is available for R which considers the residence time in the cartridge as well as the film diffusion mass transfer aspects of sorption onto 35/60 mesh Tenax-GC (2). R values close to 1 can be obtained for many nonpolar compounds with cartridges of small bed volume by adjusting the sample volume flow rate. This paper describes the development of aqueous ATD with Tenax for semivolatile compounds with 0.68-cm3bed volume cartridges and direct desorption to a fused-silica capillary column with WCC. I t focuses on the determination of the compound-dependent recoveries for the desorption step and for the complete ATD method. This study supplements earlier work in our laboratory, which has relied in part on estimates of breakthrough (4, 12) and comparisons of analytical results from a second method (12) to validate aqueous ATD.
EXPERIMENTAL SECTION Study Compounds and Cartridge Preparation. Most of the study compounds were from Chem Service (West Chester, PA). The halogenated aliphatics were from Supelco (Bellefonte, PA). The cartridges were of Pyrex glass with a bed length, inside, 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
41
Table I. Desorption Step Recovery Results Relative to Naphthalene for Four Desorption Replicates and Three On-Column Injection Replicates
W4
Di/Zi
Monocyclic Aromatics toluene chlorobenzene ethylbenzene m- + p-xylene o-xylene 1,3,5-trimethylbenzene (mesitylene) n-propylbenzene nitrobenzene 1,2,4-trimethylbenzene 1,4-dichlorobenzene 1,2,4,5-tetramethylbenzene(durene)
Phenols 0.87 f 0.11 0.86 f 0.04 0.92 f 0.04 1.05 0.04 0.96 f 0.01 0.96 f 0.04 0.99 f 0.04 1.06 f 0.02 1.01 f 0.02 1.01 f 0.02 1.01 f 0.03
*
2-methylphenol 3+4-methylphenol 2-methoxyphenol 2,6-dimethylphenol 2-nitrophenol 2,4-dimethylphenol 2,4-dichlorophenol 3,5-dimethylphenol 2,6-dichlorophenol 2,4,6-trimethylphenol 2,4,6-trichlorophenol 2,3,4,6-tetrachlorophenol
Heterocyclic Aromatics dibenzofuran dibenzothiophene
pentachlorophenol 1.02 f 0.04 1.15 f 0.05
OXO-PAHs 9-fluorenone xanthone 9,lO-anthracenedione
1.18 f 0.07
1.02 f 0.07 0.85 0.12
*
Pesticides wHCH T-HCH heptachlor aldrin dieldrin p,p'-DDE p,p'-DDD p,p'-DDT
1.11 f 0.07
1.07 f 0.07 0.98 f 0.06 1.01 f 0.01 0.86 f 0.05 0.59 f 0.24 0.34 f 0.30 0.14 f 0.12 Phthalates
diethyl phthalate dibutyl phthalate butyl benzyl phthalate bis(2-ethylhexyl) phthalate
1.19 f 0.04 1.14 f 0.05 0.18 f 0.17 0.25 f 0.12
and outside diameters of 5.4,0.40, and 0.64 cm, respectively. The overall cartridge length was 9.5 cm. Each cartridge was packed with -0.11 g of resieved 35/60 or 60/80 mesh Tenax-GC or Tenax-TA (Alltech Associates, Inc., Los Altos, CA). Other preparation detaiIs are available (3). Desorption Step Recovery Studies. Desorption step recoveries were determined for the compounds in Table I. A 1.0-pL portion of an 8 ng/pL per component standard in methanol (predominately) was injected into the center of the sorbent bed of a cartridge. Desorption was then carried out as described below onto a 0.32 mm i.d., (1.22 pm film thickness, 25 m long, CP-SilgCB fused-silica capillary column (Chrompack, Bridgewater, NJ). During the desorption, the compounds moved through half of the sorbent bed and onto the column. In the full ATD method, the sample inlet end of the cartridge is the desorption outlet end; the desorption is carried out in a backflush mode. Thus, since strongly retained compounds will collect near the inlet of a cartridge during sampling, these recovery experiments were not biased toward good recoveries. For comparison, 1.0 pL of an 8 ng/pL standard was injected on-column at 35 "C onto the same column described above. The standard was prepared from the same stock solutions but was composed predominately of methylene chloride since that solvent is better suited than methanol for on-column injection. The analyses were carried out with temperature-programming on a Hewlett-Packard 5790A GC (Palo Alto, CA) interfaced (13) to a Finnigan 4000 mass spectrometer/data system (MS/DS) (Sunnyvale, CA). The MS was scanned from 50 to 450 amu in
1.15 f 0.03 1.07 f 0.03 1.07 f 0.04 1.09 f 0.06 1.06 f 0.01 1.05 f 0.04 1.12 f 0.07 1.09 f 0.05 1.00 f 0.06 1.07 f 0.02 1.21 f 0.11 1.12 f 0.10 0.97 f 0.15
PAHs and Derivatives naphthalene 2-methylnaphthalene 1-methylnaphthalene 2,6-dimethylnaphthalene 1,3- + 1,6-dimethylnaphthalene 1,4- + 1,5- + 2,3-dimethylnaphthalene 1,2-dimethylnaphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene 2-methylphenanthrene 1-methylphenanthrene fluoranthene pyrene benzo[a]fluorene benzo[blfluorene
1.00 f 0.00 1.02 f 0.03 0.98 f 0.04
1.01 f 0.02 1.02 f 0.04 1.02 f 0.04 0.98 f 0.04 1.01 f 0.05 0.99 f 0.02 0.99 f 0.08 1.13 f 0.07 1.11 f 0.11 1.04 f 0.06 1.01 f 0.06 0.72 f 0.21 0.61 f 0.25 0.30 f 0.25 0.24 f 0.20
Alkanes eicosane (C,) heneicosane (Czl) docosane (Czz) tricosane (Cza) tetracosane (Cz4) pentacosane ((2%) hexacosane (C,)
1.03 f 0.03 1.02 f 0.02 0.94 f 0.13 0.80 f 0.25 0.53 f 0.26 0.31 f 0.21 0.21 f 0.16
0.5-5 intervals. The standard m / z values were used for quantitation. Full ATD Method Recovery Studies. The apparatus used to spike an aqueous solution and pass that solution through a cartridge included a reservoir, an injection port, a stirred mixing chamber, and one or more cartridges connected in series with Teflon Swagelok 0.64-cm tubing unions. The rationale of using this approach is discussed elsewhere (1). Purified water was obtained from a Millipore Super Q (Bedford, MA) ion exchange/activated carbon system. Experiments were carried out both with nonprewetted and forcibly prewetted cartridges. When used, forced prewetting was carried out as described elsewhere (14). After assembly, the flow rate was set at 0.15 mL/s, and 2.0 pL of a standard solution (predominately methanol, at 10 ng/pL per component) was injected into the flowing water. A total of 1L of water was then passed through the cartridges in each test. For the cartridge dimensions employed, a fractional recovery of 0.82 may be predicted for each cartridge for 35/60 mesh sorbent (2). A water blank was prepared by passing 1 L of the system water through a cartridge with no injection of standard. Prior to desorption, residual water was removed by the two-step centrifugation/vacuum desiccation procedure (14). The vacuum step was carried out in a multiport vacuum chamber. The duration of the vacuum was 20 min. A liquid nitrogen cold trap protected the cartridges from contaminants in the vacuum line and the pump from water. The cartridges were desorbed and analyzed as described below. Cartridge Desorption. The device in Figure 1 was used for
42
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
Vespei-Graphite Disk
/
&
Vespel-Graphite Ferrule To Power Controller Upper Carrier Line, 30 PSI
Coo Iing Water Out
f
UCL Snap Valve
(In
Tenax-GC
100 Watt Cartridge Heater
To power L o w e r Drive Tube
GC Oven Door
Cooling Water In
Ill
&!L
-
\ 114'
I
F u s e d Silica Capillary Column
//
/
Thermocouple
r /
*
Controller
.
1 '3
100 Watt Cartridge Heater
,Sweep
Line
+Vent Needle Valve Lower Carrler
VesDei GraDhlte
cm long, 0.16 c m Stainless S t e e l Tube Scale:
,
O.D. 2 cm
,
Figure 1. Device for thermally desorbing ATD cartridges.
desorbing the cartridges. Although presented vertically, it is most conveniently used in a horizontal configuration. It was mounted on the door of the GC. The device is composed of an outer heater block, a desorption chamber, and a cap. To change a cartridge, the internal screw in the cap is loosened, and the cap is removed. After the upper drive tube is taken out, the cartridge is removed by using a special extraction tool. The pin on the inside desorber wall retains the lower drive tube in place via a right-angle keyway so that the cartridge may be pulled free of the standard 0.64cm-i.d. Vespel/graphite ferrule (Alltech). After the next cartridge is inserted, the process is reversed the cap is tightened only moderately on the large Vespel/graphite ferrule 90 that the upper drive tube may still be moved axially through the large ferrule. The large ferrule may be machined from Vespel/graphite rod stock obtained as SP-22 Polyimide from Du Pont (Wilmington, DE). The internal screw in the cap is then tightened on the drive tubes to compress and seal the ferrule at the bottom of the cartridge. The lubricatingcharahr of the Vespel/graphite disk on the upper drive tube permits the rotation and compression of the internal screw against the upper drive tube. Since there is little pressure differential across the cartridge, little or no leakage occurs around the bottom ferrule: all desorbing analytes move directly onto the column. (Some carrier gas leakage out of the top of the device is not problematic as long as it does not decrease the effective desorption pressure, cause excessive waste, or permit excessive flow during the carrier gas purge step.) The fitting on the column end of the desorber was cut from a stainleas steel Swagelok union for 0.16-cm-0.d. tubing, and silver soldered in place. Exposure to hot metal surfaces is minimized by bringing the column up into the cartridge through a piece of 0.16-cm-0.d. stainless steel tubing. The column end of the desorber is situated inside the GC oven. An auxiliary heater is provided so that the WCC temperatures do not cool that end during the desorption. At the conclusion of the desorption, the column end of the desorber cools and preheating the carrier gas during the GC run is avoided.
Three gas lines with snap valves (SVs) are used. An upper carrier line (UCL) supplies the desorption carrier gas (helium). A lower carrier line (LCL) provides carrier (helium) for the chromatographic run. A sweep line (SL) vents O2and solvent from the cartridge during the gas purge step, as well as contaminants on the exterior of the cartridge from the chamber during the desorption step. When open, the UCL is operated at -30 psi; when open, the LCL is operated at 6-10 psi. A cartridge is desorbed in the Figure 1 device as follows: (1)Internal or Full Standard A d d e d A 2.0-pL portion of the IS or full standard solution in methanol is added the to sorbent bed -1 cm from the sample inlet end. (2) Cartridge Inserted with sample inlet end @ward the column: (a) The UCL-SV is closed. (b) If the column is connected directly to the MS, the LCL-SV is opened to keep carrier gas flowing to the column inlet. (c) The cap is opened and the cartridge is inserted; the LCL-SV is closed prior to inserting the cartridge (this prevents uncontrolled flow through cartridge). (d) The cap is replaced; ( 3 ) Gas F'urge: (a) With the SL needle valve set at -15 mL/min for the LCL pressure, the SL-SV is opened. (b) The LCL-SV is opened for 10 min to purge O2and solvent, and simultaneously, (c) the GC is brought to WCC temperature. (In this work, -80 "C.) (4) Cartridge Desorbed to Column: (a) The LCL-SV is closed, (b) the UCL-SV is opened, and (c) the desorber is heated within 2 min to the desorption temperature. Contaminants on the cartridge exterior are removed by passage out of the SL at 210 mL/min. (In this work, the desorption time, temperature, and carrier flow (for desorber at 250 O C , 30 psig) were 5 min at 250 OC, and -10 mL/min, respectively.) ( 5 ) GC Run: (a) The SL-SV is closed to save carrier gas (b) the UCL-SV is closed, (c) the LCGSV is opened, and (d) the GC is programmed up at 5-10 OC/min. (In this work, the columns used were: 25 m, 0.22 fim film, CP-Si18-CB (Chrompack), and a 30-m, 0.25-pm film,DB-5 (J&W Scientific, Folsom, CA). Other
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
GC/MS conditions were the same as those given above.) (6) Desorber Coolecl: (a) Water is passed through the heater block to retum the desorber to ambient temperature, and (b) water is removed from the block by opening compressed air connected to the water line. For each desorption step recovery run, the cartridge was redesorbed for 15 min prior to reuse and the column was taken to 300 O C to ensure that no carry-over would occur for any compounds for which desorption was difficult.
RESULTS Desorption Step Recoveries. The results of the recovery experiments for the 5 min, 250 "C desorptions are given in Table I. Compounds given as groups were not resolved on the column used, and could also not be distinguished on the MS. Therefore, the results given should be taken as averages for those compounds. Blank values are expressed as percents of the amounts added to allow direct comparison with the recoveries. The behaviors of the compounds were examined relative to naphthalene since that compound is 100% retained in the gas purge step, and since it survives the desorption step well; the desorption and on-column injection peak areas for naphthalene were never statistically different. For the four replicate desorption runs, the quantity Di= (peak area i)/ (peak area naphthalene) was determined. The analogous quantity (Ii)was determined for the three replicate on-column injection runs. If it is assumed that the injections provided a 100% response for each compound, and also that all losses after the desorption (e.g., those in the column) were either negligible or occurred in a linear manner, then Dilli = 1.0 will indicate a desorption efficiency of 100% for compound i. No trends were observed in the replicate desorption results, and so neither increased nor decreased desorbability from the cartridge was observed. All of the phenols and other monocylic aromatics gave Dilli values that were close to 1.0. The values of somewhat less than 1.0 for toluene, chlorobenzene, and ethylbenzene are not believed to be due to less than 100% desorption efficiencies for these compounds, but rather slight artifacts in the oncolumn injection determinations of these comparatively volatile compounds. Also, although the source of the slight bias toward Dilli values slightly greater than 1.0 for the phenols is not known, we believe that these compounds may also be desorbed with 100% efficiency. For the polycyclic aromatic hydrocarbons (PAHs) and PAH-derivatives studied, Di/li values close to 1.0 were found for all compounds up to and including 1-methylphenanthrene. Incomplete recoveries were observed for the PAH compounds beyond 1-methylphenanthrene, pesticides beyond dieldrin, phthalates beyond diethyl phthalate, and alkanes beyond tricosane (CZ3). These results are consistent with those obtained by Pankow et al. (6, 7) in other studies. As they have discussed, improved recoveries can often be obtained by increasing the desorption time, carrier gas flow rate, and/or temperature. Increasing the carrier gas flow rate helps to minimize degradation on the cartridge since it reduces the residence time there. Considering that the desorption time used in this work was fairly short (5 min), and that desorption temperatures of greater than 250 OC are possible with Tenax (6, 7),substantial improvements are undoubtedly possible for the heavy compounds in Table I that exhibited recoveries of
