Anal. Chem. 1989, 61,1333-1343
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Measurement of Natural Trace Dissolved Hydrocarbons by in Situ Column Extraction: An Intercomparison of Two Adsorption Resins Mark B. Yunker,* Fiona A. McLaughlin, Robie W. Macdonald, and Walter J. Cretney Ocean Chemistry Division, Institute of Ocean Sciences, P.O.Box 6000, Sidney, British Columbia V8L 4B2, Canada
Brian R. Fowler Seakem Oceanography Limited, 2045 Mills Road, Sidney, British Columbia V8L 3S1, Canada
Trevor A. Smyth
C. B. Research International Corporation, P.O.Box 2010, Sidney, British Columbia V8L 3S3, Canada
Chromosorb T and XAD2 resins are compared for the In sltu extraction of alkane and polycycllc aromatic hydrocarbons (PAHs) from fresh- and seawater. In column efficiency experiments, Chromosorb T yielded higher recoverles than XAD-2 for n-alkanes at 3 and 0.6 ng/L concentratlons per component. Chromosorb T columns gave good recoveries for PAHs of three and more rings (0.4 ng/L per component) and XAD-2 for PAHs of four and more rlngs (0.06 ng/L per component). Lower molecular weight PAHs were recovered poorly by Chromosorb T and contamlnated by XAD-2. Prlnclpal component analysls (PCA) dlscrlmlnated well between Chromosorb T and XAD-2 dissolved hydrocarbon In sltu samples and their respective blanks. The PCA models could also dlstlngulsh between the groups of samples collected with each resln. Between-resln difference was more Important than sampllng locatlon for hydrocarbon composltlon; thls dlfference In resln adsorption characterlstlcs shows up dramatically In the mean sample and blank plots for the hydrocarbons. The majorlty of blank-corrected XAD-2 alkane concentrations were below the llmlt of detectlon. I n contrast, the majority of the alkanes below trlacontane were quantlflable for the samples on Chromosorb T. PAHs In the phenanthrene to chrysene range gave comparable results for the two reslns. The Chromosorb T In sltu methodology provides the first dlssolved hydrocarbon measurements that are unquestlonably above the measured mean blank. With this technlque IndMdual alkanes and PAHs at pg/L concentratlons In natural waters can be quantified.
INTRODUCTION One of the most challenging tasks in analytical chemistry is the unambiguous determination of trace constituents in natural waters. For hydrocarbons, the classical technique has been the collection and extraction of bulk water samples. However, the level of contamination that may result from such bulk collections can overwhelm the natural concentrations of hydrocarbons to the extent that ”many of the analytical values to be found in the older literature merely immortalize the extent of shipboard contamination” (1). A more effective sampling method for dissolved trace constituents is the in situ pumping method (1-4). Large volumes of water are pumped through an adsorption column by a noncontaminating pumping system. The sampling apparatus can be passed through the surface microlayer before starting, and sample manipulations on the concentrated sample can take place under clean-room conditions either
onboard ship or in a shore-based laboratory. The technique is limited only by the capacity of the prefilter, the backpressure of the adsorption column, and the battery supply for the pump. To use the in situ pumping method, one must select an appropriate adsorption resin with known extraction efficiency for hydrocarbons and know the minimum volume of water required to produce a sample statistically greater than the mean blank. Polystyrene-divinylbenzene copolymer resin (XAD-2) has been widely used for the extraction of hydrocarbons from water ( 4 , 5 ) ,and the long-term stability of hydrocarbon samples on this resin has been demonstrated (4). However, XAD-2 does not perform well at seawater pH, and it is extremely difficult to clean and keep clean ( 1 ) . Many aromatic impurities have been identified in the XAD-2 resin (6, 7); after contact with water the cleaned resin continues to release aromatic hydrocarbons, which may then be extracted with ether (6). These impurities raise the detection limits and can invalidate quantitation of these compounds in water, even after thorough cleaning of the resin. Chromosorb T porous poly(tetrafluoroethy1ene) (TFE) Teflon has been recommended (8)for the adsorption of PAHs from water. Slauenwhite et al. ( 1 )found no measurable hydrocarbon blanks with it (using thin-layer chromatography with flame ionization detection) and therefore chose Chromosorb T over XAD-2 and CI8 bonded silica gel as the preferred packing for hydrocarbons. In view of the above, we judged Chromosorb T resin to have better potential for sampling dissolved trace hydrocarbons than its leading rival, the more widely used XAD-2 resin. G6mez-BelinchBn et al. (9) have recently reported an intercomparison study of XAD-2 with polyurethane foam and liquid-liquid extraction for the extraction of hydrocarbons from seawater. This work is part of the hydrocarbon method development and technique validation performed prior to carrying out a large sampling program in the Canadian Beaufort Sea and Mackenzie Estuary during 1987. When we began, little was known about the natural levels of dissolved hydrocarbons we were likely to encounter. Erickson and Fowler (10) found wintertime dissolved hydrocarbon concentrations in the Mackenzie Delta to be less than detectable either by routine in situ sampling on XAD-2 resin (volumes up to 100 L) or by conventional water sampling and batch extraction with 20-L samples. Accordingly, we set our preliminary target sample volumes at 200 L of water by in situ pumping.
EXPERIMENTAL SECTION General Procedures. Analytical procedures were carried out in a class-100 clean room dedicated to hydrocarbon analyses with access restricted to trained analytical staff wearing one-piece clean
0003-2700/89/0361-1333$01.50/00 1989 American Chemical Society
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which was delivered at 150 mL/min to the test and backup extraction columns. The volume of standard delivered was reI 1 1 I corded hourly with flow rates being adjusted as necessary. The feed water was flushed through for 1 h before installing the columns. Column experiments were conducted in duplicate with approximately 500 L of water pumped through each column. Sampling. Hydrocarbons in water were extracted in situ with XAD-2 HYDROCARBON Seastar water samplers (2). The sampler is a microprocessorL CLEANING SPIKE PUMP COLUMNS STANDARD controlled battery-powered pump in a pressure case which draws Figure 1. Schematic of the experimental system for the resin column water at a preset flow rate through a filter unit and extraction efficiency tests. column and measures the volume pumped. A flow rate of 150 mL/min was used. Before each field trip, the Teflon fiter holders and connecting tubing were cleaned with 2% RBS detergent, room suits. Solvents (BDH Omnisolv) were redistilled through distilled water, acetone, and dichloromethane. Filters were loaded burle packed columns. Hydrocarbon-free water was prepared from before each deployment by using clean tweezers with GF/F on glass-distilled water refluxed overnight with alkaline potassium the bottom (outlet end) and GF/D on top. The Teflon column permanganate, redistilled, and extracted with dichloromethane. was then attached to the sampler, and all tubing connections were Sodium hydroxide (10 M, Baker Analysed) was extracted with tightened. During operation, water passed sequentially through 7:3 dichloromethane/hexane (6 X 100 mL). Saturated sodium the GF/D and GF/F filters, the resin column, and the pump. chloride solution (BDH assured) was solvent extracted before Samples from the Fraser River and Captain Passage, Saltspring dilution to a 3% solution. Glassware was soaked in 2% RBS Island, were collected from the CSS Vector. Four in situ water detergent (Pierce Chemical), baked overnight (a 350 "C forced-air samplers and in situ pumps were bolted to a submersible frame oven was used for all baking), and rinsed with dichloromethane to obtain simultaneous samples of the dissolved, particulate, and before use. Sodium sulfate (BDH assured) and silica gel (BDH, colloidalphases for hydrocarbon analysis (11). The in situ pumps 60-120 mesh) were baked overnight. Teflon fittings and film were sampled through a common port (upstream end of the frame) with soaked in 2 % RBS and Soxhlet extracted overnight with dia 500-pm stainless steel screen prefilter. Teflon tubing connected chloromethane. Glass fiber filter papers (Whatman, 142 mm), the sampling port to the sampler filter pack inlets. Just before both GF/D and GF/F (nominal pore size 2.7 and 0.7 pm, resampling (frame-mounted deployments only) these inlet hoses spectively), were baked overnight and stored in baked aluminum were filled with methanol to prevent air locks. The samplers were foil pouches. started with a 10-min delay and lowered over the side to the In Situ Water Sampler Columns. Chromosorb T Teflon desired depth. This delay allowed the samplers to be submerged TFE resin (Manville Corp., approximately 30-45 mesh, special before pumping, thus minimizing contact between the sample and order) was sieved to 30-45 mesh size (375-500 pm). The resin the surface microlayer. To terminate sampling, the frame was was slurried in acetone and packed, with tapping, into Teflon raised until the inlet port was just under the surface and the in columns (Seastar Instruments, 37 ern long, 2.5 cm o.d., 1.9 cm situ samplers were turned off. i.d.) while a flow (30-50 mL/min) of acetone was maintained. For deployments in Patricia Bay, Saanich Inlet, each sampler Each column contained 55 g of resin retained at each end by FEP was lowered individually over the side of a barge tied to the dock. (fluorinated ethylene polypropylene) Teflon mesh (Micromesh, A 1.0-min delay was used, and after 5 min at the selected depth 297 pm) secured between two Teflon collars. For Norman Wells each sampler was raised to just under the surface to verify that (Mackenzie River) Chromosorb T resin samples, the 0.5-in. (1.25 it was still operating (the samplers are designed to shut off after cm) pipe thread Teflon end plugs for the water sampler columns 4 min if the flow is too low). In a few cases the pumps had to (supplied by the manufacturer) were replaced with Swagelok be restarted. stainless steel 0.5411. (1.25 cm) pipe thread to 0.375-in. (0.95 cm) For through-ice deployments in the Mackenzie River, a hottube male connectors and corresponding end caps. This modiwater hole melter was used. Teflon lines and the filter pack on fication provided a more positive seal at low temperatures and the in situ sampler were protected from freezing before deployreduced the risk of contamination during connection of the ment. A 0.1-min delay was used and operation of the samplers columns to the sampler. The XAD-2 resin columns (20-60 mesh) verified after 5 min as above. and their cleaning have been described by Green and Le Pape After sampling, the columns were detached and capped. Filter (4). Identical Teflon columns were used for both resins. papers were folded one-quarter round and stored in labeled baked Water sample columns were cleaned in batches of four or eight aluminum foil pouches. All samples were stored frozen until by using freshly distilled solvent produced by a 5-L Soxhlet still. extraction. Methanol was pumped (Micropump, Teflon gears) at 75 mL/min Analytical Procedures. Column Elution Procedure. The per column for 24 h, dichloromethane for a further 24 h, and elution apparatus, with an empty stub column in place of the methanol for a final rinse until there was no dichloromethane in the effluent. Columns were sealed as described above and stored extraction column, was rinsed with methanol and dichloromethane with the contents under methanol until use. (150 mL each) and methanol again (100 mL). Fittings were Column Efficiency Tests. A 1250-L aluminum tank (1.14 installed with care to avoid contamination. For column blanks, methanol was displaced from unused columns with 100 mL of m diameter, 1.22 m deep) and lid were cleaned by recirculating RBS detergent solution for several hours and thoroughly flushing hydrocarbon-free water. with filtered (Filtrite polyester cartridge) tap water for 24 h. The For a single-column extraction, a 1.00-mL aliquot of the working internal standard was added directly to a dichloromethane-wetted tank was filled with filtered tap water, which was used as a clean water supply. The experimental system is shown schematically 1-L separatory funnel. The working internal standard contained in Figure 1. All components in contact with the feed water were known amounts of the 13 perdeuterated hydrocarbons described thoroughly cleaned sequentially with acetone, dichloromethane, below. The column was eluted upward into the separatory funnel methanol, and water prior to assembly. with methanol (150 mL) and then dichloromethane (250 mL) at The tank supplied 1000 L of water, which was continuously a flow rate of 2-5 mL/min. Hydrocarbon-free water (200 mL) spiked with dissolved hydrocarbon standards to produce conwas added and the separatory funnel shaken vigorously for 1min. centrations of approximately 1-2 and 0.1-0.2 ng/L of individual If phase separation was poor, hydrocarbon-free water saturated n-alkane and PAH components, respectively, delivered to each with sodium chloride (50 mL) was added; this step was seldom extraction column. Water from the tank was pumped (Microrequired. The dichloromethane layer was drawn off into a 500-mL pump, graphite gears, variable speed) through a glass fiber filter flask and the aqueous methanol extracted twice more with di(Gelman AE) in an all-Teflon housing and then through two chloromethane (50 mL). The combined dichloromethane extracts XAD-2 cleanup columns in series. A standard, containing even were back-washed twice with 3% hydrocarbon-free aqueous son-alkanes and selected PAHs dissolved in acetonitrile, was disdium chloride (50 mL) and dried over sodium sulfate ( 5 g). The pensed continuously from a nitrogen-pressurized (0.5-0.7 atm) extract was transferred in portions to a 250-mL Kuderna-Danish reservoir using two f i e metering valves (Nupro, SS-SS2) in series. concentrator, 1mL of carbon tetrachloride was added, and solvent The spike was added at 16-20 pL/min to the clean feed water, volume was reduced to approximately 0.5 mL in a water bath at CHROMOSORB I OR XAD-2 TEST COLUMN
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50-55 "C. The extract was quantitatively transferred with dichloromethane (2 mL) to a silica gel filter column (60 X 5 mm, 1 g of 5% water deactivated silica gel) and eluted with dichloromethane (10 mL). In subsequent Beaufort Sea field studies (12),dual (parallel) Chromosorb T column deployments were used for sampling due to the high resistance to flow of the Teflon resin (see part 2 of Results and Discussion). The column blanks used with those samples are included as part of this comparison. The double-column extraction required a change in procedure to guard against loss of internal standard from the larger 2-L separatory funnel. Hydrocarbon-free water (50 mL) was added to the 2-L separatory funnel, both columns were eluted simultaneously into it with methanol (150 mL each) and then dichloromethane (250 mL each), and the internal standard (1.00 mL) was added, followed by hydrocarbon-free water (350 mL). The remainder of the extraction procedure was identical with the single-column procedure, except that amounts of material and sizes of glassware were doubled. Analysis. Samples were analyzed by using a Finnigan 9600/33003 GC/MS with an Incos 2300 data system running SuperIncos software, revision 5.5. A 1-m uncoated fused silica retention gap was installed in conjunction with a 30-m DB-5, 0.25-pm film capillary column (J & W Scientific) inserted directly into the ion source. The mass spectrometer was tuned and mass calibrated daily with perfluorotributylamine (FC43). MS scans were acquired from 41 to 500 amu in 1.00 s with a 0.01-s settling time and with storage to disk of mass peaks greater than 50 counts. Samples (0.5-1.0 pL) were introduced by using a 1-min splitless Grob injection at room temperature. At 2 min the oven was heated ballistically to 80 "C, and at 4 min the MS source and detector were turned on. At 4.5 min the oven temperature was programmed at 6 "C/min to 300 "C. Data were acquired from the beginning of the temperature program for 38.3 min (2300 scans). A GC calibration standard containing 47 hydrocarbons, 13 perdeuterated internal standards, and a fragmentation standard (decafluorotriphenylphosphine, DFTPP) was run daily to determine retention times, response factors (relative and absolute), and system performance. The MS fragmentation performance was determined periodically with DFTPP and met the accepted ion abundance criteria for this compound (13). The n-alkanes from Cll to CBBplus seven isoprenoids were quantified relative to [2Hso]tetracosane. [*Hz6]Dodecaneand [2H74] hexatriacontane were used to monitor volatility losses and high-temperature GC behavior, respectively, and were also quantified relative to [2H,]tetracosane. The 21 PAHs measured from naphthalene to benzo[ghi]perylene were quantified relative to [2H8]naphthalene, l-methyl[2Hlo]naphthalene,[2Hs]acenaphthylene, [2Hlolacenaphthene,[2Hlo]anthracene,[2Hlo]pyrene, [2H121chrysene,[2H12]benzo[klfluoranthene,i2H121with the apbenzo[a]pyrene, and [*H14]dibenz[a,h]anthracene propriate deuterated standard being used for each class of PAH. In all cases, target compounds were located and quantified with automated procedures using relative retention times and mass chromatogram peak maxima for characteristic ions. Where possible, the relative responses of the PAH to the perdeuterated standards were calibrated with the National Bureau of Standards (NBS) SRM-1647 mixture of PAH in acetonitrile. Principal Components Analysis. Unsupervised principal components analysis (PCA) was used to examine the interrelationship among samples in the various data sets (14,15). Since a hydrocarbon data set can exhibit large differences in absolute magnitude between, for example, the alkanes and PAHs in a given sample, data were initially log-transformed,as recommended by Kvalheim (15). This minimizes the influence of large differences in the absolute size of GC peaks and makes the distribution of each variable more normal. To eliminate zero values in the data sets, half of the minimum peak area (50 counts) was substituted into the calibration routine and adjusted for the peak area and relative response of the appropriate deuterated standard. Incos procedures require that mass peaks of greater than the specified storage limit of 50 counts be present in a minimum of two adjacent scans before a peak is detected, and hence the actual instrument detection limit is 100 area counts. The PCA program used the nonlinear iterative partial leastsquares (NIPALS) algorithm (14, 16,17)and allowed us to ex-
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amine both scores and loadings. The scores and loadings represent, respectively, the contributions of each sample and each variable to each principal component. The log-transformed variables were mean centered (by subtracting the mean from each variable on a variable by variable basis) before PCA analysis. Mean centering was chosen to avoid closure (In,which could arise from the normalization technique recommended by Kvalheim (15).
RESULTS AND DISCUSSION Hydrocarbons in natural water are distributed among the dissolved, colloidal, and particulate phases: Assignment to phase is operationally defiied by the sampling techniques used (18, 19). Companion papers (11) will discuss the intercomparison of hydrocarbon sampling techniques for particulates and make an assessment of the importance of each phase to the overall hydrocarbon budget. Here, we define dissolved hydrocarbon phase as the fraction that passes through stacked GF/D and GF/F glass fiber filters (nominal pore size 2.7 and 0.7 pm, respectively) and is adsorbed onto the resin column. The measurements reported here provide no information on colloidal hydrocarbons, which pass through both filter and resin column. The performance of the Chromosorb T and XAD-2 resins was compared in three ways: (1)column efficiency obtained by using 350-500 L of water spiked with dissolved hydrocarbons at anticipated "natural" levels, (2) hydrocarbon blank levels from resin columns (both in part I),and (3) comparison of resin columns attached to in situ samplers deployed simultaneously (part 2). 1. Comparison of Resin Sorption Efficiencies and Blank Characteristics. Resin Column Efficiency Tests. Many tests on spike recovery and column efficiency have been performed previously with XAD-2 resin. None have utilized more than a few aliphatic and aromatic hydrocarbons nor have they approximated the hydrocarbon concentrations found in natural water samples. From winter studies in the Mackenzie River (10) we expected dissolved hydrocarbon concentrations to be a maximum of approximately 1-2 ng/L per alkane and 0.1-0.2 ng/L per individual PAH component. By contrast, for example, Osterroht (20) performed recovery tests in seawater using pristane, n-hexadecane, and phenanthrene at concentrations greater than or equal to 1 pg/L, and James et al. (21) used perdeuterated p-xylene, naphthalene, hexadecane, and phenanthrene spiked into drinking water a t 100 ng/L. For Chromosorb T, the only hydrocarbon spike recovery tests we are aware of utilized phenanthrene at concentrations of 500 pg/L (1). T o simulate natural hydrocarbon concentrations in our column efficiency tests (3-4 orders of magnitude below those previously attempted), prefiltered water from a reservoir tank was pumped through two XAD-2 precolumns to remove dissolved hydrocarbons, spiked with even n-alkanes and selected PAHs, and then pumped through the Chromosorb T or XAD-2 test column (Figure 1). Two Chromosorb T and two XAD-2 columns were evaluated (Table I). The measured concentrations of the even n-alkane spikes were blank corrected for each column by using an average of the two adjacent odd n-alkanes. For PAH, column blanks from the same time period as that of the tank tests were used for blank correction; for Chromosorb T only naphthalene required correction. Table I gives the effective aqueous concentration of the hydrocarbon spike for each component and the percent recovered from each column after elution. For n-alkanes, Chromosorb T gave comparable recoveries that were in general higher than the XAD-2 recoveries (also comparable) for most of the alkanes. The general fall-off in recovery for triacontane and above is likely linked to the extremely low solubility of these n-alkanes. Chromosorb T showed poor recovery of PAHs with molecular weight less than that of phenanthrene. Recoveries were
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Table I. Results of the Column Efficiency Determination Chromosorb T 402-L extracted vol
effective compound
dodecane tetradecane hexadecane octadecane eicosane docosane tetracosane hexacosane octacosane triacontane dotriacontane tetratriacontane hexatriacontane total n-alkanes (n-C12-n-C3B) naphthalene 2-methylnaphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benz [ a ]anthracene chrysene benzo[k]fluoranthene benzo[a]pyrene total PAH
concn,” ng/L
spike recov, %
2.6 2.5 2.6 2.7 2.7 2.8 3.0 4.4 2.7 2.4 3.2 2.9 3.0
67 55 47 60 72 76 71 79 40 24 13 10 4
37.6 0.88 0.75 0.82 0.19 0.20 0.13 0.40 0.39 0.20 0.18 0.40 0.21 4.73
XAD-2
355-L extracted vol effective spike concn,” ng/L recov, % 2.7 2.6 2.7 2.8 2.9 2.9 3.2 4.6 2.9 2.5 3.3 3.0 3.2
59 49 57 70 83 82 89 98 65 55 43 36 30
39.2 3
