Environ. Sci. Technol, 1992, 26, 697-702
A Passive Sampler for Measurement of Carbon Monoxide Using a Solid Adsorbent Klyoung Lee,+Yuklo Yanaglsawa, *,+ Masakazu Hlshinuma,t John D. Spengler,+ and Irwln H. Billlck~ Harvard School of Public Health, 665 Huntington Avenue, Boston, Massachusetts 021 15,and Gas Research Institute, 8600 West Bryn Mawr Avenue, Chlcago, Illinois 60631
w A new passive sampler has been developed for measuring time-weighted average carbon monoxide concentrations. An adsorbent for the new sampler is Y-type zeolite of which sodium ion was partially exchanged with Zn ion. Carbon monoxide is transferred by molecular diffusion through a small diffusion tube inserted into the adsorbent layer. The collected CO is thermally desorbed from the adsorbent and transformed to methane before the analysis by gas chromatography with a flame ionization detector. Low levels of CO concentration can be reliably measured by the newly designed sampler. A linear relationship between CO exposures and the amounts of collected CO was maintained from 30 to 1600 ppmh, regardless of CO concentrations, sampling time within 1day of exposure, and any environmental factors. The sampler is compact, inexpensive, nonhazardous, and easy to use. The properties of the passive sampler are applicable to large-scale studies of personal CO exposure and indoor/outdoor air pollution studies. Introduction Carbon monoxide is thought to be one of the most common pollutants in our environment, The main source of CO is incomplete combustion of carbonaceous fuels and the principal health effects are due to a reduction of oxygen transport. Since CO is an inevitable product of any combustion process, personal and stationary CO monitorings are necessary. To conduct large-scale studies of the CO monitoring, a sampler should be compact, inexpensive, simple to use, and nonhazardous. Therefore, passive samplers are promising for large-scale studies of CO monitoring. A passive sampler is generally simple in structure and easily used by people of all ages with simple instruction. Passive samplers can be used to measure personal exposures and stationary levels. Thus, temporal and spatial variabilities of air pollution can be determined by passive samplers. Such passive samplers have become an important tool in air pollution studies. Passive samplers consist of a diffusion barrier, a collection medium, and a sampler case. The feature of a passive sampler depends upon the collection medium. So far, various collection media have been used for the sampling of several toxic air pollutants. These media can be classified into two types: liquid and solid reagents. The liquid media utilize chemical reaction for the absorption of the pollutants. The liquid reagents are typically coated onto a mechanical support. A n example of a liquid reagent is triethanolamine, which is widely used to collect NO2 (e.g., Palmes tube (I), Yanagisawa badge ( 2 ) ) . The solid reagent or adsorbent utilizes chemisorption and/or physisorption. Activated carbon, a typical solid adsorbent, is used to collect volatile organic compounds. Adsorbates are recovered by either thermal desorption or extractions using solvents. The analysis using thermal Harvard School of Public Health. *Gas Research Institute. 0013-936X/92/0926-0697$03.00/0
desorption is usually quick and easy. The sensitivity of the thermal desorption is expected to be higher than that using solvent extraction. Therefore, thermal desorption is an attractive procedure for the analysis of a large number of passive samplers. However, artifact formation, due to elevated temperatures needed for the thermal desorption, should be validated carefully. Several passive samplers for carbon monoxide were developed using the chemical reagents that reacted with CO (3, 4 ) . The reactions were seen as a change in color of the reagents. For instance, Vapor Gard, which is one of the few commercialized passive samplers for CO, packs reagents impregnated on silica gel in a glass tube that has one open end, Exposure dosages are directly readable from the length of color change. However, the accuracy of the stain tube is at best f20% (4) because the border of color change is not clearly demarcated and because of nonuniformity of dispersions of reagents on supports. There have not been passive samplers designed for CO that use adsorbents such as zeolite, active carbon, polymer, and activated alumina. Difficulties in using such adsorbents have been as follows: (1)the decay of the sampling rate with increased CO concentrations adjacent to the adsorbent, which is due to low adsorption capacities for CO; (2) low recovery efficiency with the thermal desorption resulting from strong interactions, called chemisorption, between the adsorbent and CO molecules; and (3) high blank values due to decompositionsof organic compounds on the adsorbent or the adsorbent itself. Difficulties such as those mentioned above can be overcome by (1)adopting an appropriate adsorbent which has a high adsorption capacity and can adsorb CO reversibly, (2) designing the sampler t~ have a low sampling rate and high adsorption efficiency, and (3) using an adsorbent made out of inorganic compounds. The new sampler utilizes Zn-Y-zeolite to overcome these difficulties. The high adsorption capacity was obtained by using Y-type zeolite in which Na+ ion was partially exchanged with Zn2+ ion. A low sampling rate and high adsorption efficiency was attained by using a fused-silica capillary column (i.d. 0.32 mm, length 50 mm) as a diffusion path with one end inserted into the adsorbent layer. This Zn-Y-zeolite adsorbs CO reversibly and is made out of inorganic compounds. The use of an appropriate adsorbent and new design allows the following: (1)to decrease the concentration of CO adjacent to the adsorbent, (2) to thermally desorb with high recovery efficiency and (3) to expand the dynamic range of the sampler. The amount of CO collected by the sampler was quantitatively measured by gas chromatography with a flame ionization detector (GC/FID) after thermal desorption and transformation of CO to methane by a methanator.
Experimental Section Structure of Passive Sampler. Figure 1A shows the structure of the passive sampler. A fused-silica capillary column (Hewlett-Packard, i.d. 0.32 mm) was used for the diffusion tube (length 50 mm). Both sides of the glass tube
0 1992 American Chemical Society
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Nylon filament
Zn-Y z e o l i t e
D i f f u s i o n tube I.D. I 0.32m m
(150 mg)
Glass t u b e O.D. = 0.63 crn I.D. = 0.4 crn
Rubber s e p t u m
,
I Stainless steel screen
(A) O i l cjay
cap
Storage tube
(5)
Flgure 1. (A) Structure of passive sampler and (B) passive sampler inside storage tube.
(0.d. 6 mm, i.d. 4 mm, length 65 mm) were sealed by rubber septa (Aldrich). Adsorbent (150 mg of Zn-Y-zeolite) was fixed in the middle of the glass tube with glass wool and screen. One opening of the diffusion tube was located in the center of the adsorbent layer. The efficiency of CO adsorption was increased by the configuration, since CO could diffuse in all directions of the adsorbent layer. A nylon filament (diameter 0.254 mm) was inserted into the diffusion tube to keep the diffusion path open and prevented accidental blocking by the adsorbent during shipping and storage. The filament was removed for sampling. The passive sampler was stored in a storage tube, as shown in Figure 1B. Preparation of Adsorbent and Assembly. A particle size of Na-Y-zeolite (LZY-52, the Linde Division of the
Union Carbide Corp., Old Ridgebury Rd., Danbury, CT 06817) was adjusted by using sieves in sizes ranging from 20 to 40 mesh. Partial ion exchange for Zn2+ ion was conducted according to Egerton and Stone (5) as follows: The sieved Na-Y-zeolite (20 g) was washed with a buffer solution of sodium acetate and acetic acid (pH 5). The zeolite and 3 mL of the buffer solution were mixed with ZnC1, (Aldrich) aqueous solution (0.2 M, 500 mL) and heated at 100 OC for 2 h. The adsorbent was filtered out, washed with deionized water, dried overnight at 70 OC, and stored over a saturated ammonium chloride solution for 3 weeks. After the Zn-Y-zeolite was calcined at 500 OC for 5 h, 150 mg of the adsorbent was poured into the glass tube previously assembled with the diffusion tube. The assembly of the sampler was completed in a CO-free glovebox. Analytical Procedures. A thermal desorption unit and gas chromatography with a flame ionization detector (GC/FID) were designed for a semiautomated analysis of the sampler and made by Sibata Scientific Technology (Tokyo, Japan). A schematic diagram of the integrated system is shown in Figure 2. The analytical procedures were designed to thermally desorb CO from the sampler, remove interfering gases by a preseparation column, separate CO from other air components by a main separation column, convert CO to methane by a methanator, and determine the amount of the converted CO by FID. These analytical steps were carried out by switching two valves installed in the GC/FID system. The preseparation column (Porapaq-Q, 40-60 mesh, 28 cm of a 6-mm-0.d. stainless steel column) and main separation column (Molecular Sieve 13X, 40-60 mesh, 97 cm of 6-mm-0.d. stainless steel column) were placed in an oven at 50 "C. The methanator consisted of a nickel catalyst packed in a glass tube (0.d. 5.1 mm, i.d. 3.18 mm, length 9 cm), kept at 400 "C with hydrogen supplied at a flow rate of 10 mL/min. In a desorption mode, the sampler was heated at 80 "C for 2 min in a sampler oven. Valves were set to back-flush position (B) and desorption position (D) for the first 1min
METHAN ATOR
MAIN SEPARATION COLUMN
(A) A n a l y t i c a l s y s t e m (GC/FID)
Back-Flush
Normal
Desorption
(B) V a l v e p o s i t i o n s Flgure 2. Schematic diagram of the analytical system for the passive sampler.
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Injection
80%, were used in the exposure experiment. The mixed gas passed deionized water to establish 80% relative humidity in the exposure chamber. The relative humidity in the exposure chamber was measured by a humidity sensor (Vaisala, Model HMP 113A). Temperature was controlled by placing the exposure chamber in an incubator (Precision Model 815). The wind effects of the samplers were determined by using a wind tunnel, made from acrylic tubes, as shown in Figure 4. Mixed CO gas was directed into the wind tunnel and was circulated between the inner and outer cylinders with a fan. The wind velocity at the inner cylinder was measured by an anemometer (Kurz, Model 415-M-D) at the center of the wind tunnel. Two wind velocities, 0.5 and 1.0 m/s, were introduced into the sampler. The samplers, placed parallel and perpendicularly to the wind direction, were exposed to 40 ppm CO for 4 h at 80% relative humidity. The samplers were placed in the storage tube before and after the exposure. Exposure was stopped by sealing the cap of the storage tube with oil clay. The storage effects of exposed and blank samplers were tested for up to 1 month. The blank samplers in the storage tubes were stored in an enclosed chamber with 100 ppm CO for 1 month. The samplers exposed to known concentrations of CO were stored in an enclosed chamber with room air for 1 month. The samplers were taken out and analyzed at 0, 1, 7, 14, 21, and 28 days of storage.
Motor
.................... Speed oontrolkr
system
0
Humidity sensor
n
CO cylinder
Chart recorder Exposure chamber
Flgure 3. Diagram of exposure chamber system.
(B-D position). Then the back-flush valve was switched to normal position (N) to connect the preseparation column with the main separation column (N-D position). An injection mode began after 2 min of the desorption mode. When the desorption valve was switched to the injection position (I),nitrogen carrier gas carried the desorbed gases from the sampler into the main separation column through the preseparation column (N-I position). To remove interferant gases such as hydrocarbons from the desorbed gases, the back-flush valve and then the desorption valve were switched to B and D after 1min in the injection mode (B-D position). The interval of the injection mode was experimentally determined to ensure that all the CO but less hydrocarbons were introduced to the main separation column. Since FID is not sensitive to CO, the methanator was installed after the main separation column to reduce CO to methane before the detection by FID. The analysis of one sampler was completed in 5 min. Exposure Experiments. The samplers were exposed to known concentrations of CO in a glass chamber. Figure 3 shows the diagram of the exposure chamber. The volume of the exposure chamber was 4 L, and the flow rate of CO gas, balanced with nitrogen, was 0.5 L/min. The concentration in the exposure chamber was continuously monitored with an Ecolyzer (Interscan, Series 4000) and simultaneously validated by GC/FID. For the cross-check of CO concentrations, 1mL of a sample gas was taken from the chamber by a gas-tight syringe and injected into the GC/FID through a direct injection port placed at the sampler oven. Two extreme relative humidities, 0 and /
7
Anemometer
Definitions of Sampling Constant and Sampling Rate In this paper, a sampling constant and a sampling rate are defined as follows. The amount of CO collected by the sampler can be derived from Fick's law, by the assumption of a steady state. The amount of CO is proportional to a driving force and exposure time. The driving force, AC, is the difference in the CO level at the inlet of the diffusion path (C) and at the outlet opening in the adsorbent layer (Ccl).
W = SACt (1) where W is the amount of CO collected by the adsorbent (pmol), S is the sampling constant (mol/h), AC is the driving force (ppm) (C - CJ, and t is the exposure time (h)* If CO adsorption at the outlet opening in the adsorbent layer is very quick, C, is assumed to be zero. The sampling constant can be calculated using an ambient CO concentration as the driving force. Nonzero concentrations at the outlet opening in the adsorbent layer caused by saturation decrease the driving force.
~~~l~~~~ ChartrRcordRr
-
.......
CO cylinder
3 t t
188 cm
g* cm
Table I. Constancy of Sampling Rate in Different Exposure Conditions
co
expt
total CO exposure: PPm.h
sampling time, h
average CO concn, PPm
relative humidity, %
collected,* nmol
sampling rate,* mL/day
cv, %
1 2 3 4 5 6 7 8 9 10
32 42 90 206 1072 1080 1080 1320 1581 1870
3.15 4.15 2 2.3 11.9 24 24 14.5 17.6 21
10 10 45 90 90 45 45 91 90 89
80 80 0 0 80 0 80 80 80 80
0.16 0.23 0.48 1.05 5.52 5.96 5.60 6.82 8.04 8.53
2.73 2.91 2.87 2.74 2.77 2.97 2.79 2.78 2.73 2.45
7.0 3.5 4.2 3.4 8.0 3.5 3.9 7.0 6.3 16.3
aTotal CO exposure is calculated by continuous measurement with ecolyzer. *Averages of four or five samplers exposed simultaneously.
The sampling rate (F)at 293 K can be calculated from the sampling constant. The sampling rate is expressed as the volume of air sampled in 1 day.
F = S (mol/h)
X
22400
X
293/273 (cm3/mol) X 24 (h/day) (2)
where F is the sampling rate (mL/day). The sampling rate is a function of the diffusion coefficient of CO in the diffusion tube, the cross-sectional area, and the length of the diffusion tube. The sampling rate could be described as follows: F = Dco A / L X 3600 X 24 (3) where Dco is the diffusion coefficient of CO (cm2/s),A is the cross-sectional area of the diffusion tube (cm2),and L is the length of the diffusion tube (cm). An apparent diffusion coefficient of CO (Dco) can be calculated from eq 3 using the sampling rate obtained by eq 1 and eq 2.
Results The optimal configuration of the sampler was sought by manipulating the inner diameter of the diffusion tube. Three sizes of the fused-silica capillary column, 0.25-,0.32-, and 0.53-mm i.d., were compared by exposing the samplers to 45 ppm CO for 24 h at 0 and 80% relative humidities. The sampling rates of the sampler using 0.25-,0.32-, and 0.53-mm diffusion tube were 1.85,2.97, and 6.53 mL/day at 0% RH and 1.68, 2.79, and 6.05 mL/day at 80% RH. The sampling rates were obtained from five or six samplers and the Coefficients of variation of simultaneouslyexposed samplers were less than 6%. Sampling rates are predicted to be a linear function of the cross-sectional area of the diffusion path from eq 3. The sampling rates of the 0.53-mm diffusion tube sampler were not linear to the cross-section area and were smaller than expected, unlike those of a 0.25-mm and a 0.32-mm diffusion tube sampler, as shown in Figure 5. This suggested that the 0.53-mm diffusion tube sampler was saturated with CO and/or water molecules in an exposure of 1080 ppmh. The sampling rates for the 0.25- and 0.32-mm diffusion tube samplers were similar at 0 and 80% relative humidity, indicating these samplers were not saturated with CO and/or water molecules. However, the sampling rate of the sampler with a 0.25-mm diffusion tube, which was very fragile, was so small that its minimum detection limit was higher than that of a 0.32-mm diffusion tube sampler. Since the 0.32-mm diffusion tube sampler had the optimum sampling rate and showed less effects from humidity, it was chosen for the sampler. Passive samplers having the 0.32-mm-i.d. diffusion tube were tested using three different CO concentrations, 10, 700
Envlron. Scl. Technol., Voi. 26, No. 4, 1992
I
0
0.05 0.1 0.15 0.2 Cross-sectionalarea of diffusion path
I
0.25
Figure 5. Relationship between cross-sectional area (mm2)of diffusion tube and sampling rate (mLlday) in CO exposure of 1080 ppmsh.
45, and 90 ppm, under various exposure periods and two humidities. CO exposures, defined as the product of CO concentration in the chamber and exposure period, were calculated from the continuous measurements by the Ecolyzer. Four or five samplers were exposed simultaneously. Table I shows detailed experimental conditions and results, such as CO concentrations, exposure periods, relative humidities, sampling rates, and coefficients of variation. The coefficients of variation among four or five samplers simultaneouslyexposed were less than 8% except for experiment 10. The relationship between the amount of CO recovered and the exposures up to 1580 ppmh was linear. The sampling constant and the sampling rate of the sampler with the 0.32-mm diffusion tube calculated from eqs 1and 2 using the average of 64 data points were 4.78 X 10” mol/h and 2.76 mL/day, respectively. By comparing experiments 5-7, in which total exposures were almost identical, we could conclude that the sampling rates were not affected by different CO concentrations, exposure periods, and relative humidities. When the samplers were exposed to similar CO exposures composed of different CO concentrations and different sampling periods, as shown in the experiments 5 and 7, their sampling rates were almost equal, indicating the sampling rate was linear in terms of the CO concentration and exposure period. Relative humidity was the only variable with differences in experiments 6 and 7. The amounts of CO collected in these experiments were not statistically different from each other, suggesting no relative humidity effects under these experimental conditions. Consistency in the sensitivity of CO adsorption is necessary to measure precise average CO concentrations. The consistency of the sampling rate was tested by two sets of experiments: experiments 3, 6, and 7, and experiments 4, 5, and 8-10. The sampling rates were consistent over 24 h of exposure, as shown in Table I. When the samplers were exposed to 45 ppm CO for 2-24 h at either 0 or 80%
1
+
Stored in room air
+
1 0
-' 0
1
,
0
10 20 Temperature (C)
30
I I
40
::5 "
c 0
5
+
100 ppm of CO
J
10 15 20 Storage period (days)
Ee
+ c
25
30
Flgure 6. Temperature effects on sampllng rate of the sampler with CO exposure of 206 ppmh.
Flgure 7. Storage effect of blank samplers in storage tube for 1 month. Each point represents one sampler.
relative humidity, the sampling rates were slightly but not statistically different. For the second set of experiments, in which the samplers were exposed to 90 ppm CO gas at different exposure periods ranging from 2 through 21 h, the sampling rates were also consistent and similar to the experiments using 45 ppm except for experiment 10. In experiment 10, the samplers were exposed to 90 ppm CO for 21 h under 80% relative humidity. The maximum detection limit of the sampler was tested in a worst case scenario with relative humidity at 80%. The sampling rates obtained from the exposure experiments under 80% relative humidity (experiments 1, 2, 5, and 7-10) were very consistent except for experiment 10, in which the sampling rate was significantly lower than the others. This suggests that the exposure of 1870 ppmmh under 80% relative humidity exceeded the maximum detection limit of the sampler. Thus, the maximum detection limit at a relative humidity of 80% could be determined with CO exposures at 1581 ppm-h. The sampler has measured CO concentrations as low as 30 ppm-h. The minimum detection limit of the sampler probably fluctuates with the variation in the blank level. The amount of CO collected by the sampler in an exposure lower than 30 ppmh could be significantly affected by the variation of the amount of CO in a unexposed sampler. The unexposed sampler unavoidably contains a small amount of CO as a result of the assembly procedure. The standard deviation of the blank level on the sampler is usually f0.02 nmol. If a signal to noise ratio is assumed to be 2 for the minimum detection limit, the detection limit of 0.04 nmol of CO corresponds to exposures of 8 ppmmh. Under this signal to noise ratio, a coefficient of variation is expected to be 50% at an exposure of 8 ppmeh. At exposures of 30 ppmoh, the coefficient of variation is expected to be 13%. The increase of the sampling rate can reduce the effects of the variation of the blank level on the minimum detection limit. However, the effects of humidity can be prevented by the low sampling rate. Temperature effects on the sampling rate were not significant, as illustrated in Figure 6. The samplers were exposed to CO of 206 p p m h with a 0% relative humidity at various exposure temperatures, such as -5,5, 10,20,33, and 39 "C. The sampling rate was independent of temperatures in a range of -5 to 39 "C. The sampling rate did not change at wind velocities of 0.5 and 1 m/s when samplers were exposed to CO levels of 200 ppmeh in a wind tunnel. The cross-sectional area of the 0.32-mm diffusion tube is 0.08 mmz. The length of the diffusion tube is 50 mm. The configurations of the diffusion tube provide a low sampling rate of 2.8 mL/day. The low sampling rate of the sampler is not affected by different wind velocities and orientations.
Since the passive sampler must be sent to a laboratory for analysis, storage stability is very important. A blank sampler and an exposed sampler were placed in room air or in a container with 100 ppm CO. As shown in Figure 7, when the sampler was stored in room air, the CO amount in the blank sampler did not change for 1 month. Among the blank samplers stored in the container with 100 ppm CO, two out of eight samplers showed high amounts of CO. Except for those two data, significant changes were not observed for both of the storage conditions. For the exposed samplers, when they were stored in storage tubes, sealed by a cap containing oil clay, and placed in room air, the amounts of CO in the exposed samplers were not changed after up to 1 month of storage.
Discussion The sampling rate of a passive sampler for gaseous substances can be controlled by manipulating the length and inner diameter of the diffusion path. Lower sampling rates can be established by decreasing the inner diameter of the diffusion path or by increasing the length of the diffusion path, as expressed in eq 3. However, extension of the length of the diffusion path increases the time constant of the dynamic response. Therefore, reduction of the inner diameter is better and more viable. The longer diffusion path also causes difficulties in handling and wearing the sampler. Passive samplers using a diffusion tube of 0.32-mm i.d. measured the CO exposure between 30 and 1600 ppmh, regardless of the different CO concentrations, exposure times, and relative humidities. The samplers collected similar total amounts of CO, in which the total CO exposure was obtained by using different CO concentrations and sampling times. The observed sampling rate of a sampler with a 0.32-mm diffusion tube was independent of CO concentration and sampling time within a 1-day exposure. The amounts of CO collected, in which relative humidity was the only variable, were not significantly different from each other. Therefore, the sampling rate was not affected by relative humidity. The maximum detection is limited by the saturation of adsorbent with CO and/or water. As expressed by eq 1, the amount of CO collected is proportional to the driving force. A decrease in the sampling rate of the sampler with a CO exposure of 1840 ppm-h at 80% relative humidity was due to a decrease in the driving force. Under the assumption that all CO molecules at the outlet opening of the diffusion path are immediately adsorbed by the adsorbent, the driving force entirely depends on the concentration at the inlet opening of the diffusion path. If the rate of diffusion of the CO molecules along the diffusion path is greater than its rate of adsorption throughout Environ. Sci. Technol., Vol. 26, No. 4, 1992
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the adsorbent matrix, then the adsorbent sites immediately adjacent to the outlet opening will eventually become saturated with the CO molecules. The concentration of CO (C,)at the imbedded outlet opening will rise, and AC will fall correspondingly. The saturation of the adsorbent site is exacerbated by the presence of water molecules. The maximum detection limit of the sampler can be increased if the relative humidity is lower than 80% or if greater amounts of adsorbent are used. The calculation of a sampling constant is based on the assumption that the CO concentration a t the outlet opening of diffusion path in the adsorbent layer (C,)is zero. The diffusion coefficient of CO was calculated to be 0.201 cm2/s from the sampling constant and eq 3. The apparent diffusion coefficient of CO is in good agreement with the molecular diffusion coefficient of CO (Dco = 0.198 cm2/s) estimated from Hirschfelder's equation a t 20 O C (6). This agreement supports the assumption of zero CO concentration at the outlet of the diffusion path in the adsorbent layer (C,),which indicates no occurrence of saturation. Temperature can have an affect on sampling rates in two ways. The diffusion coefficient for an ideal gas is theoretically proportional to the 1 5 t h power of absolute temperature (7). Therefore, the sampling rate increases with an increase of temperature due to an increase of the diffusion coefficient. However, the increase of temperature also decreases the adsorption capacity of the adsorbent and increases the CO concentration adjacent to the outlet, thereby decreasing the driving force. Since the temperature effects are difficult to calculate, the effects of temperature were tested at various temperatures from -5 to 39 "C. The exposure experiment with these various temperatures showed no change in the sampling rate. The increase of the diffusion coefficient of CO may be counterbalanced with the temperature effect on the adsorption capacity. A simple plastic tube is used to store the sampler. The free volume of the storage tube with the sampler inside is 4 mL, which is about 1.5 times the sampling rate for a 1-day exposure of the sampler. Since all CO molecules in the inner space are likely adsorbed by the sampler during the storage, the blank level may increase by the amount of CO in the inner space, depending upon CO concentrations. To keep the CO concentration levels low, the sampler was assembled and placed in the storage tube inside a glovebox, where the CO concentration was usually less than 25 ppb. The maximum amount of CO in the inner space of the storage tube is 0.004 nmol, so the blank level is not significantly affected by CO concentrations inside the storage tube. The storage tube is also used to stop the
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exposure. However, when exposure is stopped, CO concentrations inside the storage tube cannot be controlled, unlike in assembly. Since CO concentrations in the storage tube may be high enough to affect the measurement significantly, oil clay was introduced to fill the space near the diffusion path. The oil clay can stop the exposure as well as prevent additional contamination from the air inside the storage tube. In some pilot studies, the use of oil clay seems to be inappropriate, because it requires two separate caps for one passive sampler. Therefore, we are searching for a modification. Conclusion A passive sampler for measurement of CO concentrations using Zn-Y-zeolite as a solid adsorbent was evaluated. The passive sampler for CO can measure CO concentration in a range of 30 and 1600 ppmsh with a precision of 8%. The sampler can measure CO exposure up to 1600 ppmmh regardless of CO concentration and exposure time. The performance of the passive sampler is not affected by several environmental factors, such as relative humidity, temperature, and wind velocity. A semiautomated analysis system thermally desorbs the collected CO from the adsorbent and transforms it to methane before analysis by gas chromatography with a flame ionization detector. Since the sampler is compact, inexpensive, and easy to use, it is appropriate for the measurements of personal CO exposure and CO concentrations in large-scale indoor/ outdoor studies. Registry No. CO, 630-08-0.
Literature Cited (1) Palmes, E. D.; Gunninson, A. F.; DiMattio, J.; Tomczyk, C. Am. Znd. Hyg. Assoc. J . 1976,27, 570-577. (2) Yanagisawa, Y.; Nishimura, H. Enuiron. Znt. 1982, 8 , 235-242. (3) MacConnaughey, P. W. U.S. Patent No. 3,507,623, April 21, 1970. (4) MacConnaughey, P. W.; McKee, E. S.; Pritts, I. M. Am. I n d . Hyg. Assoc. J . 1985, 46, 357-362. (5) Egerton, T. A.; Stone, F. S. J . Chem. SOC.,Faraday Trans. 1973, 69, 22-38. (6) Hirschfelder, J. 0.;Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; John Wiley & Sons: New York, 1954. (7) Shenvood, T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer, McGraw-Hill: New York, 1975; pp 8-53.
Received for review June 26, 1991. Revised manuscript received September 23,1991. Accepted November 20,1991. This research project was partially supported by the Gas Research Znstitute (Contract 5082-251 -0739) and Pacific Gas and Electric.
