Environ. Sci. Technol. 1002, 26, 1226-1234
Bopp, L. H.; Ehrlich, H. L. Arch. Microbiol. 1988, 150, 426-43 1. Hortisu, H.; Futo, S.; Miyazowa, Y.; Ogai, S.;Kawai, K. Agric. Biol. Chem. 1987,51, 2417-2420. Sposito, G. In Geochemical processes at mineral surfaces; Davis, J. A., Hayes, K. F., E&; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; pp 217-228. Eary, L.E.; Rai, D. Environ. Sei. Technol. 1988,22,972-977.
(55) Balistrieri, L. S.; Chao, T. T. Geochim. Cosmochim. Acta 1990,54, 739-751. Received for review October 7,1991. Revised manuscript received February 6,1992. Accepted February 12,1992. This research was funded by Cooperative Agreement CR816805-01 from the U.S.Environmental Protection Agency, Environmental Research Laboratory, Duluth, M N .
Comparative Evaluation of Diffusive and Active Sampling Systems for Determining Airborne Nicotine and 3-Ethenylpyridine Mlchael W. Ogden' and Katherine C. Maioio
Research and Development, R. J. Reynolds Tobacco Company, Winston-Salem, North Carolina 27102
A diffusive (or passive) sampling device (PSD) was constructed and evaluated for determining nicotine and 3-ethenylpyridine in environmental tobacco smoke (ETS). The PSD, capable of sampling in either a single-face or dual-face mode, employs a Teflon membrane filter as windscreen and a sodium bisulfate-treated glass-fiber filter as collection medium. Analysis of the bisulfate-treated filter is by GC with thermionic-specific detection after extraction in ethanol/water/sodium hydroxide solution and back-extraction in ammoniated heptane using N ethylnornicotine as internal standard. Recovery of both analytes was found to be dependent on the extractant volume and extracting time. PSD sampling rate was determined to be 31.5 mL/min for nicotine and 27.8 mL/min for 3-ethenylpyridine (single-face exposure). Nicotine results were equivalent among this passive sampler, active sampling employing sodium bisulfate-treated filters, and active sampling employing collection on XAD-4 resin (AOAC and ASTM official methods). However, active sampling with bisulfate-treated filters was found to be unsuitable for 3-ethenylpyridine determination. Precision among sampler types was 4-5% for active sampling and 10-12% for passive sampling. Detection limits were comparable between the active sampling methods. Although higher for passive sampling, due to lower sampling rates, detection limits are compatible with sample collection times of at least 8 h in indoor environments. Passive sampler exposure times of up to 5 days have been validated.
Introduction Environmental tobacco smoke (ETS) exposure is a topic of considerable current research activity. A complex mixture in high dilution, ETS is best characterized by the quantitation of unique or highly selective tobacco smoke components in a concentrated sample of the air matrix. Nicotine is currently used by many researchers as the marker of choice in indoor air quality investigations due to its high selectivity for tobacco smoke and the relative ease with which it can be determined using modern analytical instrumentation. Nicotine is almost exclusively in the vapor phase of ETS aerosol (1-3), and a variety of sampling techniques have been developed and implemented for its determination. These methods have been reviewed previously and a
* Correspondence address: Research and Development, R. J. Reynolds Tobacco Co., P.O.Box 2959, Winston-Salem, NC 27102. 1226
Environ. Sci. Technol., Vol.
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number of the most widely used techniques have been compared (4-7). The predominant methods currently in use for determining ETS nicotine are the following: sorbent bed sampling systems that collect vapor-phase nicotine on either XAD-4 resin for solvent extraction ( 4 9 )or Tenax for thermal desorption (5,10); filter sampling systems that collect total ETS nicotine on acid-treated filters (sometimes in Combination with an untreated prefilter) (3, 4, 11);diffusion denuder sampling systems (sometimes followed by acid-treated filters) for definitive separation of vapor- and particulate-phase nicotine (3,4,12-14);and diffusive (or passive) sampling systems for collection of vapor-phase nicotine (13,15,16). Of these methods, the XAD-4 sorbent collection with extraction in triethylaminemodified ethyl acetate (8,9) is by far the most widely used, validated, and accepted method. Having been formally subjected to ruggedness testing (8) and two international collaborative studies (8, 17),the XAD-4 procedure has been adopted by the Association of Official Analytical Chemists (AOAC) (18) and the American Society for Testing and Materials (ASTM) (19) as official methods, and by the US. Environmental Protection Agency (20)as one of two recommended methods [the other being an acid-treated filter collection method (21)]. Although nicotine is the most widely used tracer of ETS in indoor air, it may not be the ideal tracer once thought, due to unpredictable decay kinetics (12). While some researchers have suggested that nicotine can underestimate exposure to ETS in some instances based on this decay (14,22),others have demonstrated that nicotine overestimates exposure to ETS in many other situations (23-25). On the basis of these potential discrepancies, 3-ethenylpyridine (a combustion product of nicotine) may be a more suitable tracer of ETS vapor. This compound has been shown to follow first-order decay kinetics (12), and methods exist for simultaneous determination with nicotine (14,26,27). This latter method (26,27)utilizes the XAD-4 sorbent tube collection and is a minor modification to the official methods described earlier. Until the potential discrepancies surrounding the use of nicotine as tracer are better understood, indoor air quality investigations should include both nicotine and 3-ethenylpyridine determinations in their protocol. For practical considerations, this dual determination should be possible from a single sample using a collection system that is portable, lightweight, unobtrusive, and compatible with personal and area monitoring, uses readily available materials and instrumentation, and is, above all else, reliable.
0013-936X/92/0926-1226$03.00/0
0 1992 American Chemlcal Soclety
Based on these criteria, the XAD-4 and acid-treated filter samplers are the two systems which are used by most researchers in the field and are also the ones likely to be adopted by newcomers. To date, there has not been a comprehensivecomparison made by a single laboratory of these two sampling system types and the associated samplepreparation and analysis. Several cursory comparisons have been reported for v ~ i aspects o ~ of the methodologies incorporating XAD-4 andacid-treated filter collection media and also for active (i-e., with a pump) and passive samplers incorporating these media (for nicotine only). m e most comprehensive of these evaluations (7) showed overdl fair agreement between sampler types; however, abnormalities were observed in the data which could not be explained, and some limitations of the experimental design prevented valid comparisons. These included the use of only high ETS concentrations, insufficient control samples, and untested modifications to some systems prior to and during the experiments. An earlier comparison between annular denuder and m-4sorbent collection showed excellent agreement for nicotine (4) although the "ETS" concentrations sampled were again extremely high (100-1000 pg/m3), and the number of samples was limited. Comparison of both systems with a stainless steel passive sampler (with acidtreated filter as collection medium) was poor due to adsorptive losses of nicotine on the steel sampler components. A later study of this same passive sampler with XAD-4 as collection medium (16) confirmed the adsorption problem and reported ways to minimize its impact. However, even under ideal conditions, results with the XAD-4 passive sampler were significantly less precise than results with the active XAD-4 sorbent sampler. A sodium bisulfate-treated filter has been reported as collection medium in both an active (11)and passive (15) sampler. From its encouraging performance in comparison with other methods (7), this type of passive sampler appears to be a useful design warranting further evaluation. One other group has reported attempts to implement the collection and analysis procedure for these treated filters and noted some difficulties and discrepancies regarding recovery and comparison with other methods (28,29)when following the published procedures. Based on our own need for a long-term personal and area monitoring device, we undertook development of a passive sampler employing sodium bisulfate-treated filters as the collection medium and the associated analytical method. Although similar in design to the one described previously (151, this sampler can be operated in either single- or dual-faceexposure modes and the analysis is capable of determining both nicotine and 3-ethenylpyridine. RePorted herein is a full validation of this passive sampling device in both sampling modes. Comparison with active sampling using both sodium bisulfate-treated filter and m-4sorbent tube collection is also shown. In addition, the first full comparison of these two active sampling techniques for determining nicotine and 3-ethenylpyridine reported. Experimental Section Chemicals. The following chemicals (highest purity available) were purchased and used as received: nicotine from Eastman Kodak, Rochester, NY; sodium bisulfate monObdrate,4-ethenylpyridine, quinoline, triethylamine, and heptane from Aldrich Chemical Co., St. Louis, MO; *'-ethylnornicotine from Dr. Georg Neurath, Hamburg, Gemax absolute ethanol from Quantum Chemical Corp., 'uscola, IL; 10 N sodium hydroxide solution from Fisher
Flgure 1. Photographs of passive sampling devices for single-face (a, top) and dual-face (b, bottom) exposure.
Scientific, Fair Lawn, NJ; anhydrous, research grade ammonia from Scott Specialty Gases, Plumsteadville, PA; ethyl acetate from Mallinckrodt, Inc., Paris, KY; and methanol from Burdick & Jackson, Muskegon, MI. [methyZ-D3]Nicotinewas custom synthesized with purity assayed at 99+ % . Distilled water was further purified with a Barnstead NANOPure I1 water system (Barnstead Co., Dubuque, IA). Passive Monitor. Passive sampler housings were constructed from three-piece 37-mm polystyrene filter cassettes (part no. MOO0 037 AO, Millipore Corp., Bedford, MA). The windscreen for single-face exposure was made by cutting the larger diameter monitor ring section (i.e., the middle spacer) even with the inner lip and solvent welding a Schleicher & Schuell (Keene, NH) 37-mm TE39 poly(tetrafluoroethy1ene)filter (15-pm pore size) onto the cut face. Solvent welding was performed by placing the windscreen filter on a clean sheet of aluminum foil, saturating this filter with methylene chloride so that a small excess was visible, and pressing the modified cassette section down onto the filter. Filters (oriented with the windscreen up) dried in place at room temperature within a few minutes. A g/s4-in.hole was drilled into the side lip of the back cassette section, and a spring clip (part no. 1120, John F. Maguire Co, Pawtucket, RI) was attached with a 1/8-in.-diameter aluminum pop-rivet. The singleface passive monitor is shown in Figure la. Environ. Sci. Technol., Vol. 26, No. 6, 1992
1227
For dual-face monitors, extra monitor rings are required (part no. M000037RS, Miltipore Corp.) The front half was the same as described above. The back half was constructed by solvent welding (as above) a windscreen onto the smaller diameter side of the unmodified monitor ring. Strap clips (part no. 1130 panduit cable tie, John F. Maguire Co.) were placed around this back half and cinched tight. A few drops of methylene chloride were placed into the seam formed at the union of the strap and monitor ring to weld the strap to the sampler housing. The dual-face monitor is shown in Figure lb. Collection media were prepared by saturating 37-mm glass-fiber filters (part no. TX4OHI2OWW, Pallflex Co., Putnam, CT) in 4% sodium bisulfate solution (prepared by dissolving 40 g of sodium bisulfate monohydrate in 1 L of distilled, deionized water). Filters were placed Teflon side up into a Petri dish containing the solution and allowed to stand ca. 30 s. Filters were then placed fiber side up onto a clean tray and dried under vacuum in a vacuum desiccator for at least 1h. Treated filters retained ca. 10 mg of sodium bisulfate. For single-face sampling, treated filters were placed fiber side up into the cassette bottom half and the windscreen assembly was pressed into place. No support pad was used under the treated filter. For dual-face sampling, two configurations were evaluated. In one, a single treated filter was held in place between the two cassette halves. In the other configuration, two treated filters were placed together with Teflon backing touching and held in place between the two cassette halves. Prior to and immediately following sampling, monitors were stored in 4-ounce polypropylene specimen jars with screw-cap closures. For method development work, some samplers were stored under a variety of conditions (actual conditions described with results). For experiments validating the uptake rate of the passive monitor, all samples were analyzed within 24 h of sampling. Active Monitors. For active monitoring with treated filters, filters were prepared as described above and placed into unadulterated 37-mm cassette housings on top of a gasket (part no. A31, Sloan Valve Co., Franklin Park, IL) to ensure a leak-tight fit. During sample collection, air was drawn through the filters at 50-1000 mL/min using either personal sampling pumps, electronic mass flow controllers, or critical orifices. All air flows were calibrated with a Gilibrator bubble flowmeter (Gilian Instrument Corp., W. Caldwell, NJ). Active monitoring with XAD-4 sorbent tubes was as described previously (8,18-20) using tubes from SKC, Inc. (part no. 226-30-11-04; Eighty Four, PA). Air flows were controlled over the same range with the same devices used for treated filters. Test Atmospheres. Simulated ETS was generated in a 35-m3furnished room by smoldering cigarettes at predetermined intervals. The number of cigarettes smoldered ranged from 2 to 8/h for exposures up to 1 day and from 2 to 30/day for exposures up to 5 days. Total exposure times ranged from 40 min to 122 h. Room ventilation (estimated by building engineers to be ca. 6 air changes/h), temperature, and relative humidity were neither controlled nor measured. Additional room air mixing was achieved by two oscillating fans located in diagonal corners of the room. Sample Preparation. Samples collected on sodium bisulfate-treated filters were prepared for analysis by placing filters in 15-mL polypropylene centrifuge tubes and adding 100 pL of internal standard solution [internal standard solution prepared by adding 5 pL of quinoline 1228 Envlron. Scl. Technol., Vol. 26, No. 6, 1992
and 2.5 pL of N-ethylnornicotine (NEN) to 100 mL of ethanol]. Distilled, deionized water (2 mL) was added, and the tube was vortexed for ca. 15 s. This was followed by addition of 2 mL of 10 N sodium hydroxide and vortex mixing for ca. 15 s and addition of ammoniated heptane (0.25-2 mL) and vortex mixing for 1-10 min. (Ammoni. ated heptane was prepared by bubbling ammonia for 2-3 min through a glass impinger containing heptane.) iuter method development, 10-min vortex extractions were performed on a Big Vortexer (sample capacity of 20 tubes) from Glas-Col Apparatus Co., Terre Haute, IN. A portion of the heptane layer (top) was transferred to an autosam. pler vial for analysis, Preparation of XAD-4 sorbent tubes for analysis wm a described previously (8, 9, 18-20) using 1 mL of ethyl acetate containing 0.01% triethylamine as extrection solvent. A 50-pL aliquot of internal standard solutio11w a used (internal standard solution prepared by adding 10 p~ of quinoline and 5 pL of NEN to 100 mL of triethyla. mine-modified ethyl acetate). Analysis. Routine analyses of all samples were per. formed on a Hewlett-Packard (Avondale, PA) Model 5880 gas chromatograph with splitless injection (2-pL autosampler injection with 30-s splitless period) and thermionicspecific (nitrogen) detection (8,18-20,27). The column used was a 30 m X 0.32 mm i.d. fused-silica capillary coated with a 1.0-pm film of DB-5 (a 5% phenyl methylsilicone) from J&W Scientific, Rancho Cordova, CA. The oven temperature was programmed from 50 (l-min hold) to 250 "C at 10 OC/min. Carrier gas was helium at 15 psig. Injector and detector temperatures were 225 and 300 O C , respectively. Under these conditions, approximate retention times were as follows: 7.9 (3-ethenylpyridine), 12.8 (quinoline), 14.4 (nicotine), and 15.6 min (NEN) (27). Quantitation was by the method of internal standards using NEN as internal standard for treated-filter samples and quinoline as internal standard for XAD-4 samples. Calibration standards for treated-filter samplw were prepared from a stock solution of nicotine (0.04 lug/&) and 4-ethenylpyridine (0.02 pg/pL) in methanol. For instrument calibration, two sets of working standards (five standards/set) were prepared fresh daily by mixing appropriate stock volumes (typically 2.5-50 pL) with 100 ILL of internal standard and 1 mL of ammoniated heptane. Typical concentration range of standards was 0.1-2.0 pg/mL for nicotine and 0.05-1.0 pg/mL for 4-ethenylpyridine. [NOTE: Methanol stock volumes greater than 50 pL should not be added to 1mL of heptane containing 100 pL of ethanol (internal standard). If higher concentration standards are needed, use a more concentrated stock solution.] Calibration for 3-ethenylpyridine W@ performed with the 4-ethenyl isomer since the S-ethed isomer is not available commercially (27). The two sets of standards were loaded into an autosampler tray bracketing the samples for analysis. Quantitation for the XAD-4 procedure followd essentially the same pattern except all solutions were prepsed in the triethylamine-modified ethyl acetate (8, 4, 18, 19)* Limits of detection (LOD) for all methods w m deter. mined as 3s0 (where so represents the standard of the blank response) by established procedures (30731!' For nicotine, where a blank value was always detected, tw entailed repeated blank determinations (31). For eth? nylpyridine, where a blank value was never detected, tw entailed extrapolation to so from low-level s9mPles Of known concentration (30). Analysis by GC-MS was employed during method de' velopment for confirming peak identity and a!so for In.
I
suitability studies. For these experiments,
tstpal thyl-J)3]nicotine was added to the internal standard
(4 pL to ethyl acetate; 2 pL to ethanol) along with o u e and NEN. Chromatographic conditions were as '@ bed above on a Hewlett-Packard Model 5890-Series Full-scan spectra ( m / z 40-250) were recorded on n i e l mass spectrometer (VG Instruments, Danvers, after YO-eV electron-impact ionization. Selected-ion Juornabgrms of the base peak for each analyte were for quantitation and were as follows: m / z 105 (e&enylpyridine), 129 (quinoline), 84 (nicotine), 87 ([methyl-D3]nicotine), and 98 (NEW. Sampler Cleanup. Passive samplers were cleaned m n exposures by either soaking in methanol for 5 min followedby air-drying or by detergent washing in Micro (International Products Corp., Trenton, NJ; dilution of 75 &/gallon of warm water) followed by water and methanol and air-drying.
110
t
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A
i
110
$.
Results and Discussion Treated Filter Analysis. Attempts were made to implement a previously reported analytical procedure (11) for extracting sodium bisulfate-treated filters. However, considerable experience with nicotine and inherent sensitivity fluctuations associated with thermionic-specific det&,orsmandated inclusion of an internal standard in &e analysis. Quinoline (used in the XAD-4 method) and NEN were chosen initially for comparison. NEN was &mn on the basis of the realization that quinoline might not be suitable for the treated-filter extraction process (involvingsodium hydroxide/ heptane partitioning) due to solubility differences with nicotine. The only other modification made was employment of the same temperature-programmed capillary column gas chromatographic method already in use for quantitation of 3-ethenylpyridine and nicotine (27) instead of isothermal packed column separation (11). The initial experiments were disappointing, with ethenylpyridine and nicotine recoveries (from spiked filters) routinely varying between 50% and 150%. Similar irW h i t i e s in nicotine recovery were noted by others (28, a).Since we realized that the 0.25-mL extractant volume WBB unnecessary for retaining sufficient sensitivity with &e instrumentation being used (and it added complexity to the analysis), the first modification was increasing expactant volume to 1mL and, after several experiments, "wing vortex extraction time to 10 min. Results were ?Proved, but differences were still noted between the two mternal standards. GC-MS analysis with deuterated nicotine as internal standard was employed to resolve the brepancies. The average results from three separate experiments comparing the three internal standards are ("F in Figure 2a (for ethenylpyridine) and Figure 2b (for mmtme)as a function of extractant volume. With the use of Volumes and 10-min extraction times, recovery was cornistent and quantitative, and results were comparable among the three internal standards. For routine work, W a s selected as the most suitable internal standard [Or determining both 3-ethenylpyridine and nicotine from bated filters. Additional recovery experiments were performed in combination with study of analfie stability during the Prolonged sampling that would occur with passive samp h . XAD-4 sorbent tubes and treated filters were each with ethenylpyridine (2.4 pg) and nicotine (5.1 pg). Matched pairs of the two samplers were continuously exe d (in triplicate) to ca. 50 mL/min flow of clean air, mtrOgen, and oxygen for a period of 4 days. Results are in Table I, showing good recovery of nicotine a-ized
??
/
60c
li 50
/
IS=QUINOLINE IS ED, -NIC OTINE v IS=NEN 0
t
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EXTRACTANT VOLUME (mL) (ammoniated heptane) I 110
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EXTRACTANT VOLUME (mL) (ammoniated heptane) Flgure 2. Percent recovery as a function of extractant volume for kthenylpyridine (a) and nicotine (b).
Table I. Nicotine and 4-Ethenylpyridine Recovery from Spiked Filters and XAD-4 Sorbent" % recovery, mean f SD
nicotine condition spike control air nitrogen oxygen pooled
XAD-4 98.8 f 1.2 95.6 f 2.1 100.6 f 3.5 94.6 f 1.1 97.6 f 2.9
4-ethenylpyridine
Pad XAD-4 Pad 90.6 f 2.2 100.8 f 0.8 66.2 f 1.4 93.9 f 0.8 96.4 f 0.5 48.8 f 1.5 94.4 f 1.6 99.8 f 0.2 34.1 f 1.8 93.4 f 1.5 96.8 f 0.4 63.2 f 0.0 93.1 f 2.0 98.7 f 2.1 53.2 t 14.8
"Each medium was exposed to a flow rate of approximately 50 mL f min of each gas for 4 days.
from both devices and good recovery of ethenylpyridine from the XAD-4 sorbent only. Ethenylpyridine, being less basic than nicotine, is not as strongly retained as nicotine on the acid-treated filters, resulting in lms during sampling and poor recovery. There is no apparent difference in recovery among the various gases for either analyte, demonstrating resistance to oxidation. Preparation time necessary for 50 samples is ca. 6 h for treated filters and 4 h for XAD-4 sorbent tubes (5 h if front and back resin sections are analyzed separately). Environ. Sci. Technol., Vol. 26, No. 6, 1992
1229
DF, (3.213k0.033)
'
DF, (3.310k0.018)
FRONT m
Table 11. Theoretical (20 OC, 1 atm) and Empirical Sampling Rates for Single- and Dual-Face PSD Exposure Modesa
'
sampler face DB1 (3.261+0.008)
front back total (dual face)
BACK
Schematic representations of front (a) and back @) diffusion Windscreens. Dimensions (mean f SD; n = 20) are in centimeters.
The sampler uptake rate, D A / L , is dimensionally equivalent to a volumetric flow rate and is the theoretical sampling rate of the device. Some confusion exists over which passive monitor dimensions should be used for A and L. Cross-sectional area and length have been defined as both the area and length of open diffusion channels (33) (e.g., the open area and thickness of the windscreen filter described here; definition A) and the area of the collection surface and length of the diffusion gap (32) (e.g., the exposed area of the treated filter and distance from windscreen to treated filter; definition B). Both seta of definitions apply to any diffusive sampler, and both must be calculated. The relevant parameters are those that give rise to the rate-limiting mass transport and, in some designs, could be diffusion through the windscreen and, in others, could be diffusion across the gap between windscreen and collection medium. All relevant parameters for the present design are illustrated schematically in Figure 3 for both the front and back windscreens. Calculation by definition A yields an approximate value for AIL (front windscreen) of 17 cm, using an estimate for the open fraction of the exposed windscreen area (2.5% of 8.11 cm2)and known filter thickness (0.012 cm). Calculation by definition B yields an AIL value of 9.152 cm (8.603 cm2/0.940 cm). For this sampler, rate-limiting diffusion is governed by transport of analyte to the collection surface and not diffusion through the windscreen membrane, and definition B is appropriate in this case. Assuming this situation also held for the windscreen reported previously (15),the approach taken by those authors appears correct even though an incorrect area was apparently used. (Since the cassette holder sections used to construct windscreens are tapered, the diameter near the windscreen is not the same as the diameter next to the treated filter.) The average distance from windscreen to treated filter in the monitors reported here (0.94 cm, Figure 3) differs from the value of 1.17 cm reported previously (15) due to apparent differences in cutting the monitor rings. It is imperative that any laboratory utilizing this methodology employ the services of a machine shop to reproducibly cut the monitor rings and to also accurately determine the resulting dimension (LF in Figure 3). This parameter significantly impacts the sampler uptake rate and will introduce serious bias into the theoretical sampling rate if not determined accurately on the actual samplers in use. 1230 Envlron. Sci. Technol., Vol. 26, No. 6, 1992
3-ethenylpyridine, mL/min theor empir
34.0 30.7 64.7
41.7 37.7 79.4
31.5 na 67.3
27.8 na 64.1
na, not applicable.
e 3.
Passive Monitor Design. The theoretical sampling rate of diffusion-based samplers is calculated from Fick's law (32),which relates the mass (M, in pg) collected by the sampler to the diffusion coefficient (D, in cm2/s) and ambient concentration (C, in pg/m3) of the analyte, sampling time duration ( t , in s), and sampler dimensions for cross-sectional area (A, in cm2) and length (L, in cm) through the equation M = (DA/L)Ct
nicotine, mL/min theor empir
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075
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0
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400
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600
700
ACTIVE SAMPLER CONC. x TIME (pg*hr/m3)
Flgure 4. Plot for determlnlng passive sampler (single-face) uptake rate for nicotine. Llne shown is linear regresslon for X A D 4 acthre sampler. Slope Is dimensionally equivalent to volumetric flow rate.
The diffusion coefficient for nicotine (25 "C, 1 atm) has been reported as 0.059-0.065cm2/s (3) based on calculation and experimentation. No diffusion coefficient values have been reported for ethenylpyridine. A recent review of methods for estimating diffusion coefficients for passive samplers (34)and Perry's Chemical Engineers' Handbook (35)recommend the Fuller, Schettler, and Giddings (FSG) equation for calculating diffusion coefficients. For nicotine and ethenylpyridine at 20 "C and 1 atm, these values are 0.062 and 0.076 cm2/s, respectively. The value for nicotine is in excellent agreement with literature values, and for consistency, the values calculated by the FSG method are used here to estimate the passive monitor sampling rates at 20 "C. These rates, for both single- and dual-face exposure, are summarized in Table I1 along with rates determined experimentally. Sampling rates were determined empirically by exposing passive (n = 6 1 2 ) and active monitors (both treated-filter and XAD-4 methods, n = 3 of each) to the same ETS atmosphere. This experiment was repeated across a wide range of ETS concentrations and exposure times (n = 13). A plot of nicotine mass determined on the passive monitor against concentration X time as determined from both active samplers is shown in Figwe 4. The rate determined against the XAD-4 sorbent method (31.5 mL/min; 95% confidence interval, 30.1-32.9 mL/min) is no different from the rate determined against the active treated-filter method (31.3 mL/min; 95% confidence interval, 27.1-35.5 mL/min). Strictly speaking, the value determined against the XAD-4 sampler is the more appropriate rate since the XAD-4 sorbent and the passive monitor collect vaporphase nicotine while the active treated-filter method collects total nicotine (vapor and particulate phases) (7). Fluctuations in the contribution of particulate-phase nicotine may be partly accountable for the wider confidence interval associated with the active treated-filter method. Due to the minimal fraction of ETS nicotine residing in the particulate phase, the rates determined by
3w
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0 ACTIVE SAYPLER=XAD-4: slope=27.8
mt/rnin (+2.9): R2=0.977
0’3
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/I N , , , , , , , , , , , , , , , , ] 20
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ACTIVE SAMPLER CONC. x TIME (pg*hr/rn3)
mure 5. Plot for determining passive sampler (single-face) uptake rate for 3-ethenylpyridine. Line shown is linear regression for XAD-4 &e sampler. Slope is dimensionally equivalent to volumetric flow rate.
the two active systems are not significantly different at a 95% confidence level. Also, both values are in good agreement with theoretical values (see Table 11). For 3-ethenylpyridine, the rate determined against the XAD-4 method was 27.8 mL/min (95% confidence interval, 24.9-30.7 ml/min) and the plot is shown in Figure 5. The rate determined against the actively pumped treated-filter method is in serious error due to loss of ethenylpyridine during sampling (noted previously). The difference between theoretical and empirical sampling rates is attributed to the sodium bisulfate-treated filter not being a perfect sink for ethenylpyridine as assumed by Fick’s law (32). With the correct uptake rate, the passive monitors are capable of precise 3-ethenylpyridine determination, as evidenced by the good repeatability for replicate determinations shown in Figure 5. The error bars in Figures 4 and 5 represent standard deviations of the respective measurements. Expressed as relative standard deviations (RSD), the average precision for nicotine is 4.0% for both the active treated-filter and XAD-4 methods and is 11.9% for the passive sampler. For 3-ethenylpyridine,average RSDs are 4.4% for the XAD-4 method and 10.1% for the passive sampler. Precision for the active treated-filter sampler was not calculated due to the inadequacy of this sampler for ethenylpyridine collection. The precision obtained with these passive samplers is notably better than the 24.9% RSD found previOWly for nicotine with commercially avadable stainless steel passive samplers (16). When operated in the dual-face exposure mode, the W i v e monitors have different theoretical sampling rates between the two halves (Table 11) due to the differing dimensions, as noted in Figure 3. Analysis of variance mdicates a significant difference between halves in nicotine mas collected (p < 0.0001) with the back half collecting 20% less nicotine than the front. Although the same difference is predicted for ethenylpyridine, no difference observed (p = 0.60), presumably due to the reversible adsorption on the treated filter postulated previously. An additional caveat in dual-face sampling is that the two sides of the Pallflex filter used as collection medium different. The filter front is a glass-fiber mat; the back le Teflon-coated. It was expected that the different sides would themselves impart a difference to the mass uptake Ofmalyte,and this was verified experimentally. In several dual-face monitors were loaded with a single Pad or two pads with the glass-fiber sides facing the windscreens. Nicotine uptake was increased by 4.1 ‘70 (p
PAD/PAD
PAD/XAD-4
xAD-4/PAE
xAD-4IxAD-4
COLLECTION MEDIUM Figure 6. Active sampler collection efficiency for ETS nicotine with samplers connected in series. Designation of PADIXAD-4, for exam ple, indicates an XAD-4 sorbent tube connected in series behind a treated filter pad.
= 0.003) using dual pads while ethenylpyridine uptake increased 9.3% (p = 0.0001). This is of no practical consequence as long as samplers are validated and utilized routinely in the same configuration. For single-face sampling, the glass-fiber mat should be oriented toward the windscreen. For practical considerations in dual-face sampling, a single filter is recommended with the glassfiber mat oriented toward the windscreen used in the single-face mode. Previously exposed passive samplers were observed to increase the background nicotine level on clean filters when the holders were reused in subsequent experiments as blanks, thus indicating the need for sampler cleanup between uses. To test the effectivenessof two cleaning agents (methanol and Micro detergent), 24 samplers were exposed to a high-ETS environment (50 pg/m3 nicotine) for 5 h. Half of the samplers were cleaned with a methanol rinse, and half were subjected to a more rigorous cleanup using detergent and a final methanol rinse. All samplers were then reloaded with new treated filters, sealed with the cleaned windscreens, and placed in jars (cleaned in the same fashion as the samplers) for 2-week storage. Six samplers from each cleanup procedure were stored in the dark at room temperature, and the other six were stored at -10 “C in a freezer until analyzed. No 3-ethenylpyridine was detected in blanks stored under any of the four conditions; however, sampler cleanup and storage had a significant impact on nicotine levels. Micro cleanup reduced backgrounds by an average of 25% below corresponding samplers rinsed with methanol and stored at room temperature (p = 0.005) and reduced backgrounds by an average of 19% compared to methanol cleanup and freezer storage (p = 0.01). These results demonstrate that nicotine adsorbed onto the sampler housing continues to be “sampled” by the monitor on storage at room temperature and emphasizes the need for rigorous sampler cleanup between exposures and the need for blanks to be prepared, stored, and analyzed concurrently with samples. Furthermore, samples should be stored in a freezer until analyzed, and if possible, the treated filter should be removed from the monitor housing as soon as possible after sampling and stored frozen in a clean centrifuge tube (or other container used for further sample preparation). Comparison of Active Filter and XAD-4 Sorbent Methods. These two active sampling techniques were compared for collection efficiency in trapping ETS nicoEnviron. Sci. Technol., Vol. 26, No. 6, 1992
1231
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Table 111. Method Detection Limits for Nicotine (NIC) and 3-Ethenylpyridine (3-EP) in Samplers Evaluated"
rg/sample 3-EP NIC
sampler treated filter (passive; single face) treated filter (passive; dual face) treated filter (active at 1.5 L/min) XAD-4 (active a t 1.5 L/min)
0.007 0.007 na 0.005
0.013 0.013 0.013 0.010
pg/m3 (1h) 3-EP NIC 4.20 1.82 na 0.06
limits of detection pg/m3 (8 h) 3-EP NIC
6.88 3.22 0.14
0.52
0.11
0.01
0.23
na
0.86 0.40 0.02 0.01
pg/m3 (24 h) 3-EP NIC 0.29 0.14 0.006 0.005
0.17
0.07 na 0.002
pg/m3 (120.) 3-EP NIC 0.04 0.02
0.06
na na
na na
-
ana, not applicable. 28
=PAD
,
=XAD I
I
T
I
24 20 \
3
16
W-
E
12
0
Y 2
0.03
8 4
0
B
A
C
INTERNAL STANDARD Figure 7. Comparison of ETS nicotine results between XAD-4 and treated filter pad samplers with GC-MS analysis and three internal standards: A, quinoline; B, NEN; C, [D,]nicotine.
tine, recovery during sample preparation, and overall equivalency. Collection efficiency was determined by connecting samplers in series in all possible configurations (i.e., pad followed by pad, pad followed by XAD-4, XAD-4 followed by XAD-4, and XAD-4 followed by pad). Results from one experiment are shown in Figure 6. A single filter pad traps 99+% of total ETS nicotine. Backup filters were found to collect 0.4%, and backup XAD-4 sorbent tubes were found to collect
