214
Anal. Chem. 1985, 57,214-219
Thermally Desorbable Passive Sampling Device for Volatile Organic Chemicals in Ambient Air Robert G. Lewis* and James D. Mulik
Environmental Monitoring Systems Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 27711 Robert W. Coutant
Battelle Columbus Laboratories, Columbus, Ohio 43201 George W. Wooten and Carl R. McMillin’
Monsanto Company, Dayton, Ohio 45418
A passlve sampler was developed for short-term, low-level air monltorlng applications. The small, Stainless steel devlce Is simply designed and Inexpenslve. I t has a hlgh equlvalent sampling rate, Is reusable and rechargeable, and Is designed for thermal desorptlon. Laboratory and field tests wlth Tenax GC as the sorbent have shown that the device compares very favorably wlth active (pump-based) samplers and has much better sensltlvlty than commercial passive monltors which utlllze actlvated charcoal. Performance was examlned under controlled test chamber atmospheres and In actual outdoor and Indoor sltuatlons. Sampling rates were calculated for several volatlle organlc chemlcals. An extensive evaluation of the effects of alr veloclty on performance also was undertaken.
Most passive sampling devices (PSDs) function on the basis of molecular diffusion. Ideally, the sampling rate follows Fick’s first law of diffusion
A m = D+2m - C,) where m is the mass flow rate, D is the diffusion coefficient of the chemical of interest, A is the area of the diffusion channels, 1 is the length of the diffusion channels, C, is the external (ambient) concentration of the chemical, and Co is the gas-phase concentration of the chemical at the surface of the collector or sorbent. Since PSDs require no pump or flow regulation system, they are considerably smaller, less costly, and less obtrustive than active sampling devices. Provided that the collected chemicals are tightly bound to the sorbent (Co 0 ), they may be left unattended for prolonged periods, since they do not depend on power sources. These characteristics make PSDs ideally suited not only for personal exposure monitoring but also for microenvironment applications and for use at remote sites where electrical power is unavailable. Most commercial devices use activated carbon as the collector; therefore, for most organic chemicals sorption is thermally irreversible in a practical sense and Co is essentially zero. Solvents such as carbon disulfide or a mixture of CS2 in methanol must be used to desorb the chemicals for analysis. Concentration by evaporation of the solvent extract is impractical for the analysis of volatile organic compounds. Consequently, carbon-based commercial dosimeters generally do not have adequate sensitivity for ambient (ppbv level) air
-
Present address: University of Akron, Akron, OH 44325. 0003-2700/85/0357-0214$01.50/0
monitoring. In this paper, we report on the development and evaluation of a high-efficiency PSD designed to be thermally desorbed. While the device is capable of utilizing any granular sorbent, Tenax GC was selected for the studies reported here because of its capability for thermal desorption as well as for comparability with pump-based Tenax GC sampling systems. The monitor was developed through a contractual effort ( 1 ) with the former Monsanto Research Corp. of Dayton, OH, and was evaluated for short-term, low-level measurement of several volatile organic chemicals.
EXPERIMENTAL SECTION Equipment. The PSD used in these studies was a dual-faced cylinder constructed entirely of stainless steel. The device, shown exploded in Figure 1,measured 1 in. (3.8 cm) 0.d. X in. (1.2 cm) and weighed 36 g. The internal diameter was 1 /8 in. (3.5 cm), except for the central portion, which was reduced to 1 3/ls in. (3.0 cm) to provide for a precisely defined compartment to contain the sorbent. Pairs of 200-mesh (33.6% open area) wire screens and perforated plates (1.0-mm-diameter holes, 35 holes/cm2, 28% open area), placed on each side of the sorbent bed, served as diffusion barriers. The sorbent used in these studies was Tenax GC (Enka NV, The Netherlands); about 0.4 g was required to fill the cavity. Prototypes were constructed at a cost of less than 25 dollars each. The device may be easily assembled and disassembled with snap-ring pliers and reused with the same sorbent. After use, the PSD was thermally desorbed in a specially constructed oven coupled with a cryogenic trap (77 K). Desorbed chemicals were subsequently released into a gas chromatograph for analysis. Through an arrangement of six-port high-temperature switching valves, both the PSDs and Tenax tubes from active sampling devices could be compared sequentially. Initial tests (I) were conducted in an exposure chamber constructed from a 2-L borosilicate glass reaction kettle fitted with an “0”-ring-sealed Teflon lid. A standard clamp ring sealed the lid, which was provided with three ports to admit test gas mixtures, allow withdrawal of chamber contents for analysis (by sorption on actively pumped Tenax GC tubes), and permit insertion of probes to measure temperature and humidity. The chamber could accommodate three PSDs at a time. Subsequent studies (2) were carried out in a 200-L glass chamber constructed by joining two opposing bell jars to a central anodized aluminum ring. This ring was provided with numerous ports for loading, sampling, monitoring devices, etc. Stirring within the chamber was achieved by using a completely sealed internal fan that was magnetically coupled to an external variable-speed drive unit, For these investigations, a stainless steel shroud and swirl dampener were placed over the fan to achieve well-directed flow within the chamber and a uniform flow pattern near the devices. Prior to each chamber trial, the chamber was thoroughly flushed with zero-grade nitrogen, and direct gas analyses were made by 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57. NO. 1, JANUARY 1985 * 215 Table I. Comparison of Passive Single-Face Sampling for 1 h with Actively Pumped Tenax Tubes'
STAINLESSSTEEL PERFORATED PLATI
compound
SORBENT CONTAINMENT &RE*
chloroform 1,Z-dichloroethane carbon tetrachloride trichloroethylene 1,1,2-trichloroethane tetrachloroethylene chlorobenzene
-
(I
Figure 1.
Thermally desorbable passive sampling device
gas chromatography to confirm the initial condition of the chamber. The test chemicals were added by directly injecting an appropriate liquid or gas mixture of the pure chemicals. Dilution then was carried out in the chamber as needed to ohtain the desired concentrations. Once loaded, the chamber was operated in a dynamic mode with periodic samples withdrawn at about 50 cm3/minfor GC analysis. For this purpose, the chamber was directly coupled to a capillary column GC through a heated transfer line. Simultaneous samples were also taken with Tenax GC tubes, Appropriate makeup gas was added to maintain the chamber a t about 0.25-cm H20 above ambient pressure. Analytical results were used to calculate the time-weighted average concentrations within the chamber required to determine PSD sampling rates. Velocity measurements were made using an Alnor anemometer. Most of the tests were performed at 50 cm/s (1M) ft/min). Relative humidity (RH) in the chamber was varied from 7% to 92%. Exposures were normally made at 7-10% RH. Three PSDs at a time were loaded into the chamber using the sample holder shown in Figure 2. The holder waa a stainless steel cylinder with a central rod for attachment of the devices. For a given experiment, the devices were attached t~ the holder under a protective dean atmosphere (glovebox) and the cylinder was closed, flushed with zero-grade nitrogen by means of the gas porta, and inserted into the chamber. One end of the cylinder was locked in place while the other end protruded through a seal in the aluminum ring. The concentrations of test chemicals could then be adjusted in the chamher without exposure of the badges. A t the start of a chamber exposure, the outer shell of the cylinder was partially withdrawn through the seal, leaving the devices suspended from the central rod. At the completion of the trial, this procedure was reversed, and the entire assembly was removed to a glovebox ,TEFLON
air concentration, ppbv ~SD/ chamber Tenax tube PSD tube ratio 11.6 13.7 8.8 10.4 10.2 8.2 12.1
11.1 13.7 10.6 10.2
12.9 13.8
11.1
12.5
8.6 11.0
13.7
9.7 12.1
12.4
1.17 1.00 0.91 1.19 1.13 1.59 1.13
Flow rate for Tenax tuhes, 28 cm3/min.
for disassembly and preparation of the collector for analysis. A Hewlett-Packard 571 I gas chromatograph, equipped with a Hall microelectrolytic conductivity detector (Tracor 700) and an HNu photoionization detertor and a Varian 3700 GC equipped with tandem electron capture and photoionization detectnrs were used in these investigations. Both packed (2 mm i.d. X 242 cm glass. SP-1000 on 60-to 80-mesh Carhopak B)and fused silira capillary (25 and 50 m, SI?-30coated, columns were employed. The PSDs were generally desorbed into the analytical system at 200 O C in stainless steel chambers which could he loaded and unloaded under nitrogen. Cleanup ~ n ~ i s t of e dovernight bakeout at 225 "C with zero-grade nitrogen purge in a specially designed oven accommodating 12 PSDs.
RESULTS A N D DISCUSSION Initial evaluation of the PSD centered on direct comparisons between the device and actively pumped Tenax sampling tubes. Comparisons were made by exposing PSDs in triplicate in the 2-L exposure chamber to various gas mixtures and sampling simultaneously through a tube packed with Tenax CC. The volume of gas passing through the pumped tube was measured and the equivalent sampling rate fur the PSD determined hy dividing the mass of material collected on the PSD per unit time by the mass.to-volume ratio determined by simultaneous active sampling. The results from I-h, sin. gle-face exposure teats are shown in Tahle 1. Similar results were ohtained for dual-face sampling. Additional small chamber studies were undertaken to demonstrate the effects of fluctuating concentrations of air pollutants on the PSD response. I n these experiments, the syringe drive rate on the sample generation system was varied over the exposure time, thus varying the chamber gas concentrations. An active sampling tube was used to sample the exposure chamber during the test to verify concentrations. Concentrations were varied from 1 to 10 to 100 ppbv over a 1-h period. Results of these experiments were essentially identical with those shown in Tahle I. Linearity was determined by triplicate 1-,2.. 4-, 8,and 16-h exposures at concentrations of 1, 10, and 100 ppbv. Results are presented graphically in Figures 3 and 4 for two levels of loading. Response wa9 ohserved M be linear for all teut
O-RING SEAL
CYLINDER F b r e 2. Loading and support device for exposure chamber testing Of passke sampling devices. Gas ports are fw the purpose Of purging the cylinder with clean nitrogen prior to insertion into the exposure chamber.
216
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 550
J1
J2
500
CX
-
/-
450
E
400
F
350
tc
Q S
300
Flgure 5. Boundary layer over PSD surface.
4 A
0 0
250
F
z
;
200
150
100
50
0 CONCENTRATION x TIME, ppbv-hr
Flgure 3. Response of the PSD vs. sorbent loading.
5,000
I
4,500
t
4,000
P
3.500
3,000
0 0
cF
2
t
2,500
2.000c
0 TETRACHLOROETHYLENE(l4mml
l'oool
0 CHLOROBENZENE (9 mml
1.1.2 -TRICHLOROETHANE (60 mm CARBON TETRACHLORIDE (90mm] A TRICHLOROETHYLENE ( 6 0 mm) A 1 . 2 . DICHLOROETHANE (61 mm) 0 CHLOROFORM (160 mm)
YO
500
0 0
0
, dg F I
I
I
I
I
I
100
200
300
400
500
600
CONCENTRATION x TIME, ppbv-hr
Figure 4. Response to long exposure times or high loadings. Decreased response approximately parallels compound vapor pressure (given as mmHg in parentheses).
chemicals for accumulations of 1hg or less of the compound by the sorbent. At high sorbent loadings, nonlinear behavior
was observed for the more volatile compounds (Figure 4).The degree of divergence from linearity was generally proportional to the vapor pressure of the compound. The cause of this effect is believed to be reversibility of the sorption process. At higher loadings, significant vapor pressures may exist at the face of the sorbent (particularly for weakly sorbed compounds) to appreciably reduce sampling rates according to Fick's first law; Le., Co is no longer negligible. A study of the effects of reversible sorption on the performance of the PSD is the subject of a subsequent publication (3). The effects of air velocity on the PSD performance were studied under laboratory and field conditions. In laboratory tests, air flow rates of 0.3 and 3.5 L/min through the 2-L exposure chamber gave equivalent sampling rates. However, studies conducted under carefully controlled conditions in the 200-L chamber showed that the sampling rates for the device under a linear face velocity of 5 cm/s (0.1 mph) were on the average 64% of those obtained at 50 cm/s (1mph). These empirical results agreed almost exactly with predictions based on boundary layer theory ( 4 , 5 ) . Obviously, when a passive sampler removes molecules from the air space surrounding the device, the concentrations of those chemicals must be replenished by bulk movement of the air in order to achieve a steady-state condition. Whenever there is bulk flow over a surface, there is a drag between the gas stream and the stationary surface, with the result that a relatively stagnant boundary layer develops near the surface. The thickness of this boundary layer depends on the velocity and the distance from the leading edge of the surface. Surface shape and roughness also affect the boundary layer development but were not considered here. The significance of this boundary layer is that it presents resistance that is additional to the intrinsic resistance that is determined by the physical structure of the device. A simplified representation of the boundary layer effect is given in Figure 5, where the collector is separated from the external atmosphere by a series of channels having length 1 and a total area of A,. The boundary layer thickness is represented by 6, and x represents the position of the surface of the PSD. Under steady-state conditions, the mass fluxes J1 and J2must be equal. If the total face area of the device is Ab, the mass-transfer rates can be written as
and Ab
m 2
= D$C,
-
C,)
where C, represents the concentration of the chemical a t the face of the device. Because A,/Ab = t, the effective porosity
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
1
1
1
1
1
1
1
1
1
I
1
I
10-
-
09-
-
-
MEASURED RFi/RFin
CHEMICAL
0.82 0.52 0.58 0.57 0.55 0.74 0.71
CHLOROFORM 1.1.1-TRICHCORO ETHANE CARBON TETRACHLORIDE TRICHLOROETHYLENE TETRACHLOROETHYLENE BENZENE CH LO R O BENZENE
Mean = 0.64 = 0 . 11
0 1 -
217
0.2
c
1
0
1
1
1
1
1
1
1
I
FACE VELOCITY, c m / s
Flgure 6. Calculated effect of face veloclty on passive monitor sampling rate. Data given in inset derived empirically from chamber measurements at 5 and 50 cm/s.
and Aoe/l = 9.70 cm for single-face sampling or 19.40 cm for dual-face sampling. It should be noted that the open recess a t the face of the device was included in these calculations. For the circular PSD, d = 2 r / n = 0.96 cm and 6 = 1.55/u1/' (u in centimeters per second). Therefore, the velocity correction factor is
Table 11. Calculated Sampling Rates compound
D, cm2/minn
RW,cm3/minb
chloroform l,l,l-trichloroethane carbon tetrachloride trichloroethylene tetrachloroethylene benzene chlorobenzene
5.33 & 2.1% 4.76 f 1.0% 4.97 f 0.8% 5.25 f 0.9% 4.97 0.3% 5.59 f 1.6% 4.48 f 2.5%
79.5 71.0 74.2 78.3 74.3 83.4 66.8
*
or a t 50 cm/s
nTaken from ref 6. b A t an air velocity of 50 cm/s. of the sampling surface, and because in a steady-state sampling condition ml = m2,then
1
- =CX c, - cx
(3)
68
or
c, c,
Z/€6
-=-
1
+
(4)
1/66
Now 8 is a function of air velocity and the PSD dimensions and is defined ( 4 ) as 4.64 (pd/vp)'/' 6=
1.026S,1/3
for air at 25
O C
(5)
where p is the viscosity, d is the mean distance from the leading edge of the surface, p is the density of the air, and S, is the Schmidt number ( p / p D ) . Now the PSD shown in Figure 1 is a multilayer structure, and in determining the effective e/1, the several layers must be treated as a series of diffusional resistors. There are three values of 1, (0.127,0.0107, and 0.0655) and ti (1.0, 0.379, and 0.395) derived from the open areas and diffusion path lengths made up by the coarse plates, fine screens, and retainer rings. Since these structures are located on each side of the sorbent, their sums must be doubled. Thus,
(e/l)-' = z ( c i / l i ) - ' = 0.729 i
or
c / l = 1.37
(64
and the effective sampling rate for dual-face sampling is cDA
RbO= -j-
= 14.920 (in cm3/min)
(8)
The rates calculated for some volatile organic compounds by using eq 8 are shown in Table 11. It should be noted that the rates given in Table I1 are minimum rates for the device; Le., they are based on the assumption that the open recess at the face of the badge is part of the diffusion barrier. An upper limit estimate of these rates would yield values about 20% higher. An appreciable portion of the observed variance in the sampling rate data can be assigned to variation in the device construction. The devices used in this study were prototypes; therefore, this problem could easily be remedied by improved precision in the fabrication procedures. For the test procedures, precision averaged about 22%, with about half of this value attributable to the devices. Relative velocity effects, expressed as RU/RBO, are shown in Figure 6. Empirical measurements of R5/Rb0are given in the inset. Figure 6 implies that the sampling rate approaches zero as the air velocity goes to zero. This is somewhat misleading because the calculated rate represents only the steady-state rate. Under perfectly quiescent sampling conditions, the diffusion layer would continuously expand to infinity, taking an infinite time to do so. During this time, the device would continue to sample in a transient mode at an ever decreasing rate. However, from a practical standpoint, it appears that the PSDs sample at reasonably constant rates
218
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table 111. Field Exposurea of PSD
compound
air concn X time ( C t ) , ppbv-h PSD sample interval, h 5-h 1 2 3 4 5 exposure
Table IV. Comparison of PSDs with Pumped Activated Charcoal Tubes as Personal Monitors for 1,8-Dichloroethane subjectn
Passive Sampler 1,2-dichloroethane trichloroethylene 1,1,2-trichloroethane tetrachloroethylene chlorobenzene
0.27
0.29
0.24 0.20 -0.10
-1.1
1 2
25.9
3 4
21.0 144.0
0.16
1.48
1.47
0.48
1.50
0.90
0.48
0.38
0.25
0.38
0.15
0.12
8.6
6.7
5.1 27.8 18.2 1027.0
a Exposure time, 5.6 h. *Single face sampling. Equivalent sampling rate assumed to be 30 cm3/min. CFlow rates, 46.2-49.0
cm3/min.
8.4
18.2
10.4
0.90
51.0
0.66 51.0
Pumped Tube (28 cm3/min) 1,2-dichloroethane trichloroethylene 1,1,2-trichloroethane tetrachloroethylene chlorobenzene
6.3
0.67
0.58 0.38 0.25 0.11 0.16
measured air concn, mg/m3 PSDb charcoal tubeC
0.34
0.14
0.13
0.43
0.22
0.15
0.49
0.59
0.11
0.17
7.5
9.6
OTernperature = -12 "C; wind velocity
5.4 220 cm/s.
at face velocities above about 15 cm/s (0.3 mph). Normal air velocities may range from a low of 5-10 cm/s (0.1-0.2 mph) to as high as 225 cm/s (5 mph) in indoor situations. Outof-doors, air velocities are likely to always exceed 15 cm/s. Thus, the expected variation in rates'would be 0.65Rm-1.15Rm. Although velocities of 25-50 cm/s are routinely generated by movement of an active person, such velocities may not be attained by an office worker or a sedentary subject located in a building with poor air circulation. This problem may be further compounded by the shielding effect of the body and clothing when the device is worn close to the body (as opposed to being attached to an extremity). However, under usual indoor working conditions the expected rate would be R50f 5%. For stationary indoor air monitoring, the PSD may be placed near a small electric fan as an inexpensive, unobtrusive means of assuring sufficient air flow around the device. Since Tenax GC is a hydrophobic sorbent, air humidity was not expected to affect performance. This was confirmed through controlled chamber evaluations at 87% and 92% RH. The effects of temperature and pressure on the PSDs were not measured explicitly; however, diffusion-limited devices are known to be relatively insensitive to ambient changes in these parameters, While the diffusion coefficient of gases is theoretically proportional to T1.5/P,in actual practice the concomitant air volume changes result in apparent sampling rate changes of ca. 0.2% 1°C and virtually no pressure effects. Outdoor field evaluations under a range of conditions (-15 to +35 "C and 20-80% RH) showed no significant effects of temperature and pressure on sampling rate. Results from outdoor tests conducted during winter months are presented in Table 111. Five PSDs were exposed for 1 h each a t sequential 1-h intervals, while a sixth device was exposed continuously for 5 h. Samples were also collected with active Tenax GC tubes for 1h each during the first, third, and fifth hours. Two additional PSDs were fortified with known quantities of gaseous chlorobenzene in the 2-L chamber to determine recoveries. One of the fortified devices was exposed for 1 h during the fifth interval. Generally good agreement
Table V. PSD Storage Stability Studies found after 12 compound
initial loading, ng
days, ng
loss, %
chloroform 1,2-dichloroethane carbon tetrachloride trichloroethylene 1,1,2-trichloroethane tetrachloroethylene chlorobenzene
25.0
22.4 21.0
10 10 50 18 16 10 15
23.3 21.0
10.5
24.3 27.5
20.0 23.2 26.6 27.0
29.7 32.1
can be seen in comparing 1-h exposures of the PSD and pumped tubes. Comparison of the total quantity of compound collected ( C t ) in 5 h with the sum of the amounts collected in five consecutive 1-h exposures shows excellent agreement for two of five chemicals. In a personal monitoring test, the PSD and pumped activated charcoal tubes (NIOSH) were worn simultaneously by workers exposed to 1,2-dichloroethane in a chemical manufacturing plant. Results are given in Table IV. Even at these very high air concentrations and long exposure time (5 h), good agreement was achieved between the PSDs and charcoal tubes with one exception. This may have been due to the position of the PSD with respect to the charcoal sampling tube; the former was shielded by protective clothing. However the possibility of reduced response due to overloading (reversible sorption) of the PSD cannot be dimissed. Tests to determine the effectiveness of packaging to prevent contamination of the PSDs were also conducted. Precleaned devices were placed into sealed friction-lid metal cans (paint can type). These cans were pretreated by baking in an oven at 250 "C for 2-4 h to remove volatile organics from the metal surfaces. Activated charcoal was also placed in the can to preferentially sorb organic contaminants during storage. Background levels were not found to change significantly after up to 4-week exposures of the package to laboratory environments. A cursory study was conducted to determine the extent of compound loss during storage of the PSDs. Initial storage tests involved exposing a series of devices to 1ppbv concentrations of the test chemicals and observing compound recovery with respect to time. Replicate devices were analyzed initially to establish the concentrations a t the beginning of storage. The remainder of the PSDs were capped with friction-tight caps, placed in screw-cap glass jars, and maintained at ambient laboratory conditions for 12 days. Results are given in Table V. Significant losses were observed only in the case of carbon tetrachloride, which showed a decrease of 50% over the 12-day period. Losses of carbon tetrachloride may be due to chemical breakdown during storage or on subsequent thermal desorption. Recoveries of other compounds were at acceptable levels but may be improved by better packaging and environmental controls.
219
Anal. Chem. 1985, 57,219-223
CONCLUSIONS The thermally desorbable passive air monitor discussed here has several advantages over commercial PSDs designed for volatile organic chemicals. Sampling rates are equivalent to those of the most commonly used commercial devices, which employ activated charcoal as the sorbent (7). However, a major advantage of this PSD over the charcoal-based commercial devices is the greatly enhanced sensitivity achievable by thermal desorption. When solvent desorption is required, less than 0.5% of the collected sample can be analyzed (e.g., 5 pL of 1mL of extraction solvent). Thermal desorption and analysis of the entire sample affords a 200-fold (or greater) increase in sensitivity, permitting the measurement of ppb concentrationswith less than 1-h exposure times. On the other hand, while 24-h exposures are possible for most volatile organic compounds for charcoal-based PSDs, the thermally desorbable PSD may be restricted to short sampling times for the more volatile chemicals. When long sampling periods are desired, it may be possible to add additional diffusion barriers to reduce the effective sampling rates. In fact, since far more sample than needed is usually collected even at ppb levels, sampling rates could be reduced without significantly affecting analytical sensitivity. The use of other thermally desorbable sorbents such as Spherocarb (Analabs, North Haven, CT) may also permit extended exposures. The anticipated effectof air velocity across the face of the PSD on the sampling rate was found to be essentially correct, and the devices should not be used under conditions where air flow is consistently below about 15 cm/s. Indeed, all passive devices are subject to boundary layer effects and should not be used under quiescent conditions. No apparent effects of high wind velocities (up to 900 cm/s) have been observed in actual field exposures (1).However, the correction factor (eq 6b) approaches unity as the air velocity increases. Therefore, at 900 cm/s, f(ve/l) = 0.93 and the concentration C, of a gas at the face of the PSD should be 93% of its true concentration C,. When Tenax GC or other hydrophobic sorbents are used, the PSD is not affected by high atmospheric humidities. Charcoal-based devices have been found to suffer from greatly
reduced sampling rates at 80% RH and above (2). Since PSDs cannot be "turned off", care must be exercised to assure hermetic sealing of the devices before and after exposure and to minimize exposure in the laboratory. For field studies conducted by these investigators, the PSDs have been sealed in small jars or cans, which are stored and transported in a larger can or jar containing activated charcoal to prevent contamination. The devices should be packaged and unpackaged in a glovebox under a clean atmosphere. Exposure to the laboratory air prior to placement in the desorption oven should be limited to a few seconds.
ACKNOWLEDGMENT We thank J. E. Strobe1 and J. V. Pustinger, formerly of Monsanto Research Corp., for their contributions to this work.
LITERATURE CITED (1) Wooten, 0. W.; Strobel, J. E.; Pustinger, J. V.; McMillin, C. R. "Passive Sampling Devlce for Ambient Air and Short-Term Personal Monitoring", Proceedings of the 3rd National-Symposium on Recent Advances in Pollutant Monitoring of Ambient Air and Stationary Sources, Raleigh, NC, May 1983; U S . Environmental Protection Agency: Research Triangle Park, NC, 1984; Report 600/4-84-050. (2) Lewis, R. G.; Coutant, R. W.; Wooten. G. W.; McMillln, C. R.; Mullk, J. D., paper presented at the 1983 Spring National Meeting of the American Institute of Chemlcal Englneers, Houston, TX, March 1983. (3) Coutant, R. W.; Lewis, R. G.; Muiik, J. D. Anal. Chem., following paper in this Issue. (4) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. "Transport Phenomena", Wlley: New York, 1960; Chapters 4 and 19. (5) Schllchting, H. "Boundary Layer Theory"; McGraw-Hill: New York, 1955. (6) Lugg, G. A. Anal. Chem. 1966, 4 0 , 1072-1077. (7) Coutant, R. W.; Scott, D. R. Environ. Sci. Techno/. 1982, 16, 4 10-4 13.
RECEIVED for review June 20,1984. Accepted September 26, 1984. Although the research described in this article was funded wholly or in part by the U.S.Environmental Protection Agency through Contracts 68-02-3469 and -3487, it has not been subjected to Agency review. Therefore, it does not necessarily reflect the views of the Agency, and no official endorsement should be inferred. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Passive Sampling Devices with Reversible Adsorption Robert W. Coutant* Battelle Columbus Laboratories, Columbus, Ohio 43201 Robert G.Lewis and James Mulik Environmental Monitoring Systems Laboratory, U S . Environmental Protection Agency, Research Triangle Park, North Carolina 27711 The mechanlcs of passlve sampllng utilizing thermally reverslble adsorptlon are discussed. A general model relating sampllng rates to device deslgn and spectflc sorbate/sorbent properties Is developed, and a slmpllfled thin-bed model Is presented. Results of a laboratory evaluation of a thln-bed passlve dosimeter employlng Tenax GC as the sorbent for sampllng of 17 volatile organlc compounds are used to Iiiustrate the appilcabliity of the model. The results provlde guldelines for the design and use of passlve monitors employing reverslble adsorptlon.
Most commercially available passive sampling devices 0003-2700/85/0357-0219$01.50/0
(PSD) for volatile organic compounds employ activated carbon. With this sorbent, the sorption process is thermally irreversible (in a practical sense) and solvent desorption is normally used to recover the sorbates. This process is satisfactory for a variety of industrial hygiene applications for which these devices were originally intended. However, the use of these devices for sampling of ambient air concentrations (0.1-10 ppvb) can impose severe limitations on the analytical techniques (I).Passive devices which employ thermally reversible adsorption, on the other hand, offer several advantages particularly well suited to low level sampling and analysis of volatile organics. These include (1)independent from solvent contamination, (2) increased sensitivity because of the availability of the whole sample for analysis, and (3) more 0 1984 American Chemical Society
