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Effect of face velocity on performance of diffusive samplers
Ann. occtq~ Hy ., Vol. 40, No. 4, pp. 461-476,
Cqyrirht
Pergamon
Publiokd
1996
0 1996 i dish tJcct&mtimal Hy&ne society
by Ekvin
Sciace
Ltd. Printed in Great Britain ooo3-4878/% s15.00+0.00
0003-4878(%)00082-8
EFFECT
OF FACE VELOCITY ON PERFORMANCE DIFFUSIVE SAMPLERS
OF
Hajime Hori and Isamu Tanaka Department of Environmental Health Engineering, Institute of Industrial Ecological Sciences, University of Occupational and Environmental Health Japan, l-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807, Japan (Received in final form 21 July 1995) Abstract-Two types of diffusive samplers, the DuPont Pro-Tek G-AA Gasbadge (sampler A) and the 3M #3500 Organic Vapor Monitor (sampler B), were exposed to toluene vapour under various face velocities (O-2.0 m s-l). The vapour was prepared in an exposure chamber made of stainless steel and equipped with a small wind channel. The vapour concentration measured by the sampler B was almost the same as that in the chamber except when the face velocity was zero. However, the vapour concentration measured by the sampler A increased with the increasing face velocities. This tendency was increased when the airflow direction was perpendicular to the surface of the sampler. Copyright 0 1996 British Occupational Hygiene Society.
INTRODUCTION
Diffusive samplers are widely used for the measurement of organic vapours in work environments and personal exposure levels. Several kinds of diffusive samplers have been developed for organic vapours, such as the #3500 Organic Vapor Monitor (3M), the Pro-Tek G-AA/G-BB Gasbadge (DuPont), the Passive Gastube (Shibata Scientific) and so on. Although various shapes of samplers have been developed, the basic principle of the diffusive sampler is the same. Typically, it consists of a diffuser (diffusing zone) and a collector. The vapour molecules that enter the diffusing zone move to the inside of the sampler by molecular diffusion and are then absorbed in the collector. The diffusive samplers do not need a pump so that they are small and light compared with conventional charcoal tubes. This feature is especially useful for personal monitoring because the monitor can be easily attached to the collar of a worker’s clothes. However, we have to clarify several points to confirm the reliability of the diffusive samplers. For example, the response characteristics of the sampler under fluctuating vapour concentrations, and the effect of airflow surrounding the sampler. We have been studying the collection characteristics of organic vapours by the diffusive samplers experimentally and theoretically, and we have found that the diffusive samplers were proved to be useful with fluctuating vapour concentrations (Hori and Tanaka, 1993). This result gives very useful information because the vapour concentration in the work environment is always fluctuating. It has been reported that the effect of the face velocity on the collection characteristics is small (Samimi and Falbo, 1985; Lautenberger et al., 1980; Hallberg and Rudling, 1989; Moore et al., 1984; Van Der Wal and Morekerke, 1984; Pengelly et al., 1994; Naus et al., 1987; E&wood et al., 1990; Kring et al., 1984; Cassinelli et al., 1987; Kennedy et al., 1987; Levin et al., 1987). However, 467
468
H. Hori and I. Tanaka
when the vapour concentration was measured by the diffusive samplers and charcoal tubes simultaneously in the workplaces, the values measured by the diffusive samplers were sometimes higher than those by the charcoal tubes, especially when the air stream in the workplace was strong (unpublished data). This suggests that the air velocity may affect the vapour concentration measured by the diffusive samplers. If this is the case, the measured value should be corrected by the face velocity. In the case of personal sampling, the air stream may always exist at the face of the sampler because the worker moves around in the workplace even when the air is stationary, so that it is important to know the effects of the face velocity when diffusive samplers are used. We investigated the effect of face velocity on the performance of the diffusive samplers using two types of samplers under various face velocities (O-2.0 m s-l).
EXPERIMENTAL
Samplers Two different types of samplers, the DuPont Pro-Tek G-AA Gasbadge (sampler A) and the 3M #3500 Organic Vapor Monitor (sampler B), were tested. Both samplers use similar charcoal sheetsas the collector, but the structure of the diffuser is different. Sampler B has a membrane on its face in order to prevent the invasion of the wind. However, sampler A has no membranes but instead has many small holes (1 mm in dia.) in the diffuser. The response time of sampler A is about 6 times shorter than that of sampler B becausethe entry resistance is small (Kodama et al., 1983). Experimental apparatus Figure 1 shows a schematic diagram of the experimental apparatus. A vapour generator (3 in Fig. 1) has been developed by the authors (Hori and Yanagisawa, 1994). The exposure chamber (volume = 0.1 m3) (5 in Fig. 1) is made of stainless steel with a stainless steel T-shaped pipe (4 cm dia. and 20 cm long) connected to the top of the chamber. The organic vapour obtained from the generator is continuously introduced into the chamber from the side branch of the pipe. By evacuating the air
5 6
Fig. 1. Schematic diagram of the experimental apparatus. 1, needle valve; 2, rotameter; 3, vapour generator; 4, thermostatic bath; 5, exposure chamber; 6, wind chameb 7, diffiive sampler; 8, pump; 9, auto gas sampler; 10, FID-gas chromatograph.
Effect of face velocity on samples
469
in the chamber using an exhaust pump (8 in Fig. l), dilution air is introduced from the top of the pipe. The air in the chamber is automatically sampled by an auto gas sampler (GS-5OOOA,Gasukuro Kogyo, Japan) (9 in Fig. 1) at intervals of 5 min, and the vapour concentration is determined by a gas chromatograph (CC) (Model 370, Gasukuro Kogyo, Japan) (10 in Fig. 1) equipped with a flame ionization detector (FID). In the chamber, a small wind channel (6 in Fig. 1) that consists of a fan and a cylinder has been installed. The air velocity in the wind channel is controlled by a volt slider and the velocity at the end of the channel is measured by a hot wire anemometer (Anemomaster Model 6061, Kanomax, Japan). The organic vapour used in this experiment was toluene. Experimental procedure
When the air velocity and the vapour concentration in the chamber reached the desired value, a diffusive sampler was inserted into the chamber from the side and exposed to the vapour for 30 min. After sampling, the charcoal pellet was transferred to a test tube and 5 ml of carbon disulphide added for extraction. After being left for more than 1 h with occasional shaking, 2 ~1 of the sample was injected into the GC using a microsyringe and the collected amount of vapour was determined. The vapour concentration (C), was calculated using Equation (1): cmm3) = 25
SRt,
24 460 x x lo6 (pw>,
M
(1)
where m is the adsorbed amount (g), fsis the sampling time (min), M is the molecular weight of the vapour (toluene, M= 92.12) and SR is the sampling rate (cm3 min- ‘).
RESULTS
(1) Pro-Tek G-AA Gasbadge The relationship between the measured vapour concentration by the G-AA Gasbadge and the concentration in the chamber at various face velocities is shown in Figs 2-4. The flow direction was perpendicular to the face of the sampler in these figures. The vapour concentration measured by the sampler was lower than that in the chamber when the face velocity was zero (Fig. 2), which is the same tendency as shown in previous reports (Samimi and Falbo, 1985; Lautenberger et al., 1980; Hallberg and Rudling, 1989; Moore et al., 1984;Van Der Wal and Morekerke, 1984; Hahne, 1990; Mulik et al., 1989; Harper and Pumell, 1987; Levin et al., 1987). The regression line that passesthrough the origin is also shown in the figures. The slope of the regression line increased with increasing face velocity, indicating that the vapour concentration becomes higher as the face velocity increases. Figure 5 shows the effect of the face velocity based on the regression line in Figs 2-4. In this figure, the dotted line (relative concentration = 1) indicates that the vapour concentration measured by the sampler is the same as in the chamber. This figure also shows that the vapour concentration measured by the sampler increased almost linearly as the face velocity increased. When the face velocity was 2.0 m s-l, the measured concentration was almost twice the value in the chamber.
470
H. Hori and I. Tanaka
concentration
in chamber
(ppm)
Fig. 2. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity= 0 m s-l).
0
100 200 300 400 500 600 7 IO
concentration
in chamber
(ppm)
Fig. 3. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Rro-Tek G-AA) (face velocity = 1.0 m s-l).
E-
9Q 800 ii! 2
r i
. 1 .
600
2 2 400 .-0’ g 200
.
/
\ c=
1923c,
r=0901 .
1’
9s
0
L 0
200
concentration
400
600
in chamber
800
IC
(ppm)
Fig. 4. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity= 2.0 m s-l).
Effect of face velocity on samples
471
.g 2.5 C, /c~= 0\\828V E 8 s 0 P 'E '3 B
1.5 1.0 -o.5 2.0 0.0 ,_ 0.0
.
0.5 1.0 1.5 2.0 Face velocity (m/s)
2.5
Fig. 5. Effect of face velocity on measured vapour concentration. Relative concentration means ratio of concentrations measured by the sampler and those in the chamber (Pro-Tek G-AA).
0
100
200
Concentration
300
8 “‘8
400
500
600
i
in chamber (ppm)
Fig. 6. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity = 2.0 m s-I, the flow direction is parallel to the face of the sampler).
We also studied the effect of flow direction on the performance of the sampler. Figure 6 shows the result when the flow direction was parallel to the sampler. The vapour concentration given by the sampler also tended to be higher than the actual one, but when compared with Fig. 4 the effect of the face velocity was smaller than in the case when the flow direction was perpendicular to the sampler face. (2) 3M #3500 Organic Vapor Monitor
Figures 7-9 show the comparison of vapour concentrations between measured values determined by the sampler and actual values in the chamber when the face velocity was 0, 1.0 and 2.0 m s-r, respectively. Figure 10 shows the effect of the face velocity on measured vapour concentration. The measured vapour concentration was almost the same when the face velocity was between 0.5 and 2.0 m s-‘. Only when the face velocity was zero were the values given by the sampler lower than the actual values.
H. Hori and I. Tanaka
412
g 400.
a, & n
c = 0.683C,
E 300. 300 z
r = 0 958 \
l ’0
g 200 200.-E ‘“m 2 IOO-
‘“m 2 100 iz iz .ss 0:I::00
100
200
Concentration
300
400
500
in chamber (ppm)
Fig. 7. Relationship between the vapour concentration in the chamber and the concentration measured by the difliuive sampler (3M #3500) (face velocity = 0 m s-l). 500, F2 400. t 2 3oo
= Cr=0997 0 936Cm
/ /’
;-.r
,’
s i? 200
/I//
Concentration
in chamber (ppm)
Fig. 8. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (3M #3500) (face velocity = 1.0 m s-l).
DISCUSSION
Diffusive samplers are useful for the measurement of organic vapours in work environments and personal exposure levels but it is suspected that air velocity may affect vapour concentrations measured using them. In this study, we used two typical samplers to investigate the effect of face velocity. In both cases, the measured vapour concentrations were lower than the actual ones when the face velocity was zero. The reason for this is that when there is no movement of air in the workplace, the boundary layer or gas film is well formed near the surface of the sampler (Lewis et al., 1985; Harper and Purnell, 1987). When air moves around the sampler, the thickness of the boundary layer reduces, so that the length of the diffusion path close to that of the diffuser. Figures 7 and 10 show that the measured concentration by the sampler is almost the same as that in the chamber
Effect of face velocity on samples 500 /
/
z
c=1017c,
/
r = 0 994
g 400 J
\\___
a, g 300 z O” 5 200. .5 ‘E IOO4
473
l
/
0
100
200
Concentration
300 in chamber
400
5 CIO
(ppm)
Fig. 9. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (3M #3500) (face velocity=2.0 m s-l).
O.OO~
25
Face velocity (m/s)
Fig. 10. Effect of face velocity on measured vapour coaceatratioa. Relative concentration means ratio of concentrations measured by the sampler and those in the chamber (3M #3500).
when the air velocity was 0.25 m s-’ for both samplers. This suggests that the boundary layer effects could be ignored when the face velocity is greater than 0.25 m s-i. Collection of the vapour in the diffusive samplers is controlled by the sampling rate (SR), which is shown (theoretically) as follows: SR=$
(2)
where A is the surface area and D is the diffusion coefficient. Equation (2) indicates that the sampling rate is inversely proportional to the length of the diffusion path, L. In the boundary layer, because air is stationary and convection does not occur, the vapour molecules outside of the boundary layer have to move by diffusion to reach the sampler. In this case, becausethe total diffusion path of molecules is longer than that of the diffuser, the actual sampling rate decreases. However, Equation (1) indicates that the sampling rate is directly proportional to the collected amount of
H. Hori and I. Tanaka
414
vapour when the vapour concentration is the same.When considering Equations (1) and (2), the vapour concentration at a low face velocity for measured concentration is lower than the actual if the sampling rate is used without any corrections. When an air stream exists, the thickness of the boundary layer becomes so small that the length of this additional diffusion path can be ignored. In these experiments, the measured vapour concentration using sampler A increased with increasing face velocity, while sampler B was not affected by the face velocity. This difference might be caused by the difference in the structure of the diffuser. Sampler B has a membrane at the face of the sampler to prevent any invasion of wind. Becausethe wind cannot pass through the membrane, the vapour concentration given by sampler B was almost the same as the concentration in the chamber unless the face velocity was too small (Fig. 10). However, sampler A does not have a membrane but instead has many small holes (1 mm in dia. and 3.5 mm in length) in the diffuser. The wind cannot pass through these holes because one end of the hole is closed off by the collector even when the air stream is high. However, a meniscus may be formed in the holes as shown in Fig. 11. In this case, the air can enter the holes up to point A in the figure, resulting in the effective diffusion zone decreasing from L to L,. This result will have a greater affect on the sampling rate. Therefore, when the face velocity increases,the adsorbed amount given in Equations (1) and (2) also increases. As a result, the measured vapour concentration was higher than actual. In order to reduce the effect of the face velocity for this type of sampler, because the meniscus radius is so small, a sampler with holes of smaller diameter may be more effective. Another reason that the measured vapour concentration was larger than actual is that part of the air passesthrough the holes. If there is a small gap existing between the diffuser and the collector, some of the air will pass through the diffuser. If this is the case, not only the diffusion but also the effect of the air flow will be included in the sampling rate, which will also cause a higher measured vapour concentration. Lautenberger et al. (1980) also tested the effect of face velocity on the sampling rate for the velocity range of O-l .5 m s-l using sampler A. They concluded that the sampling rate was not affected by the face velocity except when the face velocity was L A
La
Air
> hole
~~// Face of the sampler
collector
Fig. 11. schematic drawing of Pro-Tek G-AA Gasbadge: L is the length of the diffusion path of the sampler, and I&. is the effective length of the diffusion path when air flows.
Effect of face velocity on samples
475
zero. However, their measurements were obtained when the air flow direction was parallel to the sampler, except for the face velocity of 0.5 m s-l. In our experiments, the effect of the face velocity when the air flow direction was parallel to the sampler was also smaller than when the flow direction was perpendicular (Fig. 8). The reason for this is that the air stream from the side of the sampler is effective in reducing the thickness of the boundary layer but it may have little effect on the reduction of the length of the diffusion path becausethe airflow and the hole direction are different. From Fig. 7, the relationship between the actual vapour concentration in air, C, and the value measured by the sampler, C,,,, is shown as follows for sampler A: C = G/(0.828
V),
where V is the face velocity (m s-r). Equation (3) is useful for estimating the exact vapour concentration for sampler A when air velocity in the workplace is between 0 and 2.0 m s-r.
REFERENCES Cassinelli, M. E., Hull, R. D., &able, J. V. and Teass,A. W. (1987) Protocol for the evaluation of passive monitors. In An Alternative Approach to Workplace Air Monitoring. Proceedings of an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders,K. J.), pp. 19&202. Royal Society of Chemistry, London. Elliwood, P. A., Groves, J. A. and Pengelly, M. I. (1990) Evaluation of a diffusive sampler for formaldehyde. Ann. occup. Hyg. 34, 305-313. Hahne, R. M. A. (1990) Evaluation of the GMD Systems, Inc., Thermallydesorbable diffusional dosimeter for monitoring methyl chloride. Am. ind. Hyg. Ass. J. 51,96-101. Hallberg, B.-O. and Rudling, J. (1989) A diffusive sampler of gaseous pollutants by collection in liquid media. Ann. occup. Hyg. 33,61-68. Harper, M. and Pumell, C. J. (1987) Diffusive sampling-A review. Am. ind. Hyg. Ass. J. 48,214218. Hori, H. and Tanaka, I. (1993) Response characteristics of the diffusive sampler at fluctuating vapor concentrations. Am. ind. Hyg. Ass. J. S&95-101. Hori, H. and Yanagisawa, Y. (1994) A new vapor generator of multicomponent organic solvents using capillary effect. Environ. Sci. Technol. 27,2023-2030. Kennedy, E. R., Cassinelli, M. E. and Hull, R. D. (1987) Verification of passive monitorperformance: Applications. In An Alternative Approach to Workplace Air Monitoring, Proceedingsof an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders, K. J.), pp. 203-208. Royal Society of Chemistry, London. Kodama, Y., Matsuno, K., Tanaka, I. and Akiyama, T. (1983) Response time of a new passive monitor for organic vapors. Jpn J. ind. Hlth 25, 181-185. Kring, E. V., Damrell, D. J., Henry, T. J., DeMoor, H. M., Basilio, A. N. and Simon, C. E. (1984) Laboratory validation of a new passive calorimetric air monitoring badge for sampling hydrogen sulfide in air. Am. ind. Hyg. Ass. J. 45, l-9. Lautenberger, W. J., Kring, E. B. and Morello, J. A. (1980) A new personal badge monitor for organic vapors. Am. ind. Hyg. Ass. J. 41, 737-747. Levin, J.-O., Lindahl, R. and Andersson, K. (1987) A diffusive sampler for sub-parts-per-million levels of formaldehyde in air using chemosorption on 2,4dinitrophenylhydrazine-coated glass fiber filters. In An Alternative Approach to Workplace Air Monitoring. Proceedings of an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders, K. J.), pp. 345-350. Royal Society of Chemistry, London. Lewis, R. G., Coutant, R. W., Wooten, G. W. and McMillin, C. R. (1985) Thermally desorbable passive sampling device for volatile organic chemicals in ambient air. Analyt. Chem. 57, 214-219. Moore, G., Steinle, S. and Lefebre, H. (1984) Theory and practice in the development of a multisorbent passive dosimeter system. Am. ind. Hyg. Ass. J. 45, 145-153. Mulik, J. D., Lewis, R. G., McClenny, W. A. and Williams, D. D. (1989) Modification of high-efficiency passive sampler to determine nitrogen dioxide or formaldehyde in air. Analyt. Chem. 61, 187-189. Naus, C. J., McAvoy, D., Broder, I. and Smith, J. W. (1987) A non toxic diffusional dosimeter for sulfur dioxide. Am. ind. Hyg. Ass. J. 48, 1001-1003.
416
H. Hori and I. Tanaka
Pengelly, M. I., Groves, J. A. and Turnbull, G. B. (1994) The application of a validation protocol to calorimetric diffusive sampler. Ann. occup. Hyg. 38, 161-170. Samimi, B. and Falbo, L. (1985) Comparison of standard charcoal tubes with Abcor (NMS) gasbadges within controlled atmosphere. Am. ind. Hyg. Ass. J. 46, 49-52. Van Der Wal., J. F. and Morekerken, A. (1984)The performance of passive diffusion monitors for organic vapours for personal sampling of painters. Ann. occup. Hyg. 28, 39-47.
Cqyrirht
Pergamon
Publiokd
1996
0 1996 i dish tJcct&mtimal Hy&ne society
by Ekvin
Sciace
Ltd. Printed in Great Britain ooo3-4878/% s15.00+0.00
0003-4878(%)00082-8
EFFECT
OF FACE VELOCITY ON PERFORMANCE DIFFUSIVE SAMPLERS
OF
Hajime Hori and Isamu Tanaka Department of Environmental Health Engineering, Institute of Industrial Ecological Sciences, University of Occupational and Environmental Health Japan, l-1 Iseigaoka, Yahatanishi-ku, Kitakyushu 807, Japan (Received in final form 21 July 1995) Abstract-Two types of diffusive samplers, the DuPont Pro-Tek G-AA Gasbadge (sampler A) and the 3M #3500 Organic Vapor Monitor (sampler B), were exposed to toluene vapour under various face velocities (O-2.0 m s-l). The vapour was prepared in an exposure chamber made of stainless steel and equipped with a small wind channel. The vapour concentration measured by the sampler B was almost the same as that in the chamber except when the face velocity was zero. However, the vapour concentration measured by the sampler A increased with the increasing face velocities. This tendency was increased when the airflow direction was perpendicular to the surface of the sampler. Copyright 0 1996 British Occupational Hygiene Society.
INTRODUCTION
Diffusive samplers are widely used for the measurement of organic vapours in work environments and personal exposure levels. Several kinds of diffusive samplers have been developed for organic vapours, such as the #3500 Organic Vapor Monitor (3M), the Pro-Tek G-AA/G-BB Gasbadge (DuPont), the Passive Gastube (Shibata Scientific) and so on. Although various shapes of samplers have been developed, the basic principle of the diffusive sampler is the same. Typically, it consists of a diffuser (diffusing zone) and a collector. The vapour molecules that enter the diffusing zone move to the inside of the sampler by molecular diffusion and are then absorbed in the collector. The diffusive samplers do not need a pump so that they are small and light compared with conventional charcoal tubes. This feature is especially useful for personal monitoring because the monitor can be easily attached to the collar of a worker’s clothes. However, we have to clarify several points to confirm the reliability of the diffusive samplers. For example, the response characteristics of the sampler under fluctuating vapour concentrations, and the effect of airflow surrounding the sampler. We have been studying the collection characteristics of organic vapours by the diffusive samplers experimentally and theoretically, and we have found that the diffusive samplers were proved to be useful with fluctuating vapour concentrations (Hori and Tanaka, 1993). This result gives very useful information because the vapour concentration in the work environment is always fluctuating. It has been reported that the effect of the face velocity on the collection characteristics is small (Samimi and Falbo, 1985; Lautenberger et al., 1980; Hallberg and Rudling, 1989; Moore et al., 1984; Van Der Wal and Morekerke, 1984; Pengelly et al., 1994; Naus et al., 1987; E&wood et al., 1990; Kring et al., 1984; Cassinelli et al., 1987; Kennedy et al., 1987; Levin et al., 1987). However, 467
468
H. Hori and I. Tanaka
when the vapour concentration was measured by the diffusive samplers and charcoal tubes simultaneously in the workplaces, the values measured by the diffusive samplers were sometimes higher than those by the charcoal tubes, especially when the air stream in the workplace was strong (unpublished data). This suggests that the air velocity may affect the vapour concentration measured by the diffusive samplers. If this is the case, the measured value should be corrected by the face velocity. In the case of personal sampling, the air stream may always exist at the face of the sampler because the worker moves around in the workplace even when the air is stationary, so that it is important to know the effects of the face velocity when diffusive samplers are used. We investigated the effect of face velocity on the performance of the diffusive samplers using two types of samplers under various face velocities (O-2.0 m s-l).
EXPERIMENTAL
Samplers Two different types of samplers, the DuPont Pro-Tek G-AA Gasbadge (sampler A) and the 3M #3500 Organic Vapor Monitor (sampler B), were tested. Both samplers use similar charcoal sheetsas the collector, but the structure of the diffuser is different. Sampler B has a membrane on its face in order to prevent the invasion of the wind. However, sampler A has no membranes but instead has many small holes (1 mm in dia.) in the diffuser. The response time of sampler A is about 6 times shorter than that of sampler B becausethe entry resistance is small (Kodama et al., 1983). Experimental apparatus Figure 1 shows a schematic diagram of the experimental apparatus. A vapour generator (3 in Fig. 1) has been developed by the authors (Hori and Yanagisawa, 1994). The exposure chamber (volume = 0.1 m3) (5 in Fig. 1) is made of stainless steel with a stainless steel T-shaped pipe (4 cm dia. and 20 cm long) connected to the top of the chamber. The organic vapour obtained from the generator is continuously introduced into the chamber from the side branch of the pipe. By evacuating the air
5 6
Fig. 1. Schematic diagram of the experimental apparatus. 1, needle valve; 2, rotameter; 3, vapour generator; 4, thermostatic bath; 5, exposure chamber; 6, wind chameb 7, diffiive sampler; 8, pump; 9, auto gas sampler; 10, FID-gas chromatograph.
Effect of face velocity on samples
469
in the chamber using an exhaust pump (8 in Fig. l), dilution air is introduced from the top of the pipe. The air in the chamber is automatically sampled by an auto gas sampler (GS-5OOOA,Gasukuro Kogyo, Japan) (9 in Fig. 1) at intervals of 5 min, and the vapour concentration is determined by a gas chromatograph (CC) (Model 370, Gasukuro Kogyo, Japan) (10 in Fig. 1) equipped with a flame ionization detector (FID). In the chamber, a small wind channel (6 in Fig. 1) that consists of a fan and a cylinder has been installed. The air velocity in the wind channel is controlled by a volt slider and the velocity at the end of the channel is measured by a hot wire anemometer (Anemomaster Model 6061, Kanomax, Japan). The organic vapour used in this experiment was toluene. Experimental procedure
When the air velocity and the vapour concentration in the chamber reached the desired value, a diffusive sampler was inserted into the chamber from the side and exposed to the vapour for 30 min. After sampling, the charcoal pellet was transferred to a test tube and 5 ml of carbon disulphide added for extraction. After being left for more than 1 h with occasional shaking, 2 ~1 of the sample was injected into the GC using a microsyringe and the collected amount of vapour was determined. The vapour concentration (C), was calculated using Equation (1): cmm3) = 25
SRt,
24 460 x x lo6 (pw>,
M
(1)
where m is the adsorbed amount (g), fsis the sampling time (min), M is the molecular weight of the vapour (toluene, M= 92.12) and SR is the sampling rate (cm3 min- ‘).
RESULTS
(1) Pro-Tek G-AA Gasbadge The relationship between the measured vapour concentration by the G-AA Gasbadge and the concentration in the chamber at various face velocities is shown in Figs 2-4. The flow direction was perpendicular to the face of the sampler in these figures. The vapour concentration measured by the sampler was lower than that in the chamber when the face velocity was zero (Fig. 2), which is the same tendency as shown in previous reports (Samimi and Falbo, 1985; Lautenberger et al., 1980; Hallberg and Rudling, 1989; Moore et al., 1984;Van Der Wal and Morekerke, 1984; Hahne, 1990; Mulik et al., 1989; Harper and Pumell, 1987; Levin et al., 1987). The regression line that passesthrough the origin is also shown in the figures. The slope of the regression line increased with increasing face velocity, indicating that the vapour concentration becomes higher as the face velocity increases. Figure 5 shows the effect of the face velocity based on the regression line in Figs 2-4. In this figure, the dotted line (relative concentration = 1) indicates that the vapour concentration measured by the sampler is the same as in the chamber. This figure also shows that the vapour concentration measured by the sampler increased almost linearly as the face velocity increased. When the face velocity was 2.0 m s-l, the measured concentration was almost twice the value in the chamber.
470
H. Hori and I. Tanaka
concentration
in chamber
(ppm)
Fig. 2. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity= 0 m s-l).
0
100 200 300 400 500 600 7 IO
concentration
in chamber
(ppm)
Fig. 3. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Rro-Tek G-AA) (face velocity = 1.0 m s-l).
E-
9Q 800 ii! 2
r i
. 1 .
600
2 2 400 .-0’ g 200
.
/
\ c=
1923c,
r=0901 .
1’
9s
0
L 0
200
concentration
400
600
in chamber
800
IC
(ppm)
Fig. 4. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity= 2.0 m s-l).
Effect of face velocity on samples
471
.g 2.5 C, /c~= 0\\828V E 8 s 0 P 'E '3 B
1.5 1.0 -o.5 2.0 0.0 ,_ 0.0
.
0.5 1.0 1.5 2.0 Face velocity (m/s)
2.5
Fig. 5. Effect of face velocity on measured vapour concentration. Relative concentration means ratio of concentrations measured by the sampler and those in the chamber (Pro-Tek G-AA).
0
100
200
Concentration
300
8 “‘8
400
500
600
i
in chamber (ppm)
Fig. 6. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (Pro-Tek G-AA) (face velocity = 2.0 m s-I, the flow direction is parallel to the face of the sampler).
We also studied the effect of flow direction on the performance of the sampler. Figure 6 shows the result when the flow direction was parallel to the sampler. The vapour concentration given by the sampler also tended to be higher than the actual one, but when compared with Fig. 4 the effect of the face velocity was smaller than in the case when the flow direction was perpendicular to the sampler face. (2) 3M #3500 Organic Vapor Monitor
Figures 7-9 show the comparison of vapour concentrations between measured values determined by the sampler and actual values in the chamber when the face velocity was 0, 1.0 and 2.0 m s-r, respectively. Figure 10 shows the effect of the face velocity on measured vapour concentration. The measured vapour concentration was almost the same when the face velocity was between 0.5 and 2.0 m s-‘. Only when the face velocity was zero were the values given by the sampler lower than the actual values.
H. Hori and I. Tanaka
412
g 400.
a, & n
c = 0.683C,
E 300. 300 z
r = 0 958 \
l ’0
g 200 200.-E ‘“m 2 IOO-
‘“m 2 100 iz iz .ss 0:I::00
100
200
Concentration
300
400
500
in chamber (ppm)
Fig. 7. Relationship between the vapour concentration in the chamber and the concentration measured by the difliuive sampler (3M #3500) (face velocity = 0 m s-l). 500, F2 400. t 2 3oo
= Cr=0997 0 936Cm
/ /’
;-.r
,’
s i? 200
/I//
Concentration
in chamber (ppm)
Fig. 8. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (3M #3500) (face velocity = 1.0 m s-l).
DISCUSSION
Diffusive samplers are useful for the measurement of organic vapours in work environments and personal exposure levels but it is suspected that air velocity may affect vapour concentrations measured using them. In this study, we used two typical samplers to investigate the effect of face velocity. In both cases, the measured vapour concentrations were lower than the actual ones when the face velocity was zero. The reason for this is that when there is no movement of air in the workplace, the boundary layer or gas film is well formed near the surface of the sampler (Lewis et al., 1985; Harper and Purnell, 1987). When air moves around the sampler, the thickness of the boundary layer reduces, so that the length of the diffusion path close to that of the diffuser. Figures 7 and 10 show that the measured concentration by the sampler is almost the same as that in the chamber
Effect of face velocity on samples 500 /
/
z
c=1017c,
/
r = 0 994
g 400 J
\\___
a, g 300 z O” 5 200. .5 ‘E IOO4
473
l
/
0
100
200
Concentration
300 in chamber
400
5 CIO
(ppm)
Fig. 9. Relationship between the vapour concentration in the chamber and the concentration measured by the diffusive sampler (3M #3500) (face velocity=2.0 m s-l).
O.OO~
25
Face velocity (m/s)
Fig. 10. Effect of face velocity on measured vapour coaceatratioa. Relative concentration means ratio of concentrations measured by the sampler and those in the chamber (3M #3500).
when the air velocity was 0.25 m s-’ for both samplers. This suggests that the boundary layer effects could be ignored when the face velocity is greater than 0.25 m s-i. Collection of the vapour in the diffusive samplers is controlled by the sampling rate (SR), which is shown (theoretically) as follows: SR=$
(2)
where A is the surface area and D is the diffusion coefficient. Equation (2) indicates that the sampling rate is inversely proportional to the length of the diffusion path, L. In the boundary layer, because air is stationary and convection does not occur, the vapour molecules outside of the boundary layer have to move by diffusion to reach the sampler. In this case, becausethe total diffusion path of molecules is longer than that of the diffuser, the actual sampling rate decreases. However, Equation (1) indicates that the sampling rate is directly proportional to the collected amount of
H. Hori and I. Tanaka
414
vapour when the vapour concentration is the same.When considering Equations (1) and (2), the vapour concentration at a low face velocity for measured concentration is lower than the actual if the sampling rate is used without any corrections. When an air stream exists, the thickness of the boundary layer becomes so small that the length of this additional diffusion path can be ignored. In these experiments, the measured vapour concentration using sampler A increased with increasing face velocity, while sampler B was not affected by the face velocity. This difference might be caused by the difference in the structure of the diffuser. Sampler B has a membrane at the face of the sampler to prevent any invasion of wind. Becausethe wind cannot pass through the membrane, the vapour concentration given by sampler B was almost the same as the concentration in the chamber unless the face velocity was too small (Fig. 10). However, sampler A does not have a membrane but instead has many small holes (1 mm in dia. and 3.5 mm in length) in the diffuser. The wind cannot pass through these holes because one end of the hole is closed off by the collector even when the air stream is high. However, a meniscus may be formed in the holes as shown in Fig. 11. In this case, the air can enter the holes up to point A in the figure, resulting in the effective diffusion zone decreasing from L to L,. This result will have a greater affect on the sampling rate. Therefore, when the face velocity increases,the adsorbed amount given in Equations (1) and (2) also increases. As a result, the measured vapour concentration was higher than actual. In order to reduce the effect of the face velocity for this type of sampler, because the meniscus radius is so small, a sampler with holes of smaller diameter may be more effective. Another reason that the measured vapour concentration was larger than actual is that part of the air passesthrough the holes. If there is a small gap existing between the diffuser and the collector, some of the air will pass through the diffuser. If this is the case, not only the diffusion but also the effect of the air flow will be included in the sampling rate, which will also cause a higher measured vapour concentration. Lautenberger et al. (1980) also tested the effect of face velocity on the sampling rate for the velocity range of O-l .5 m s-l using sampler A. They concluded that the sampling rate was not affected by the face velocity except when the face velocity was L A
La
Air
> hole
~~// Face of the sampler
collector
Fig. 11. schematic drawing of Pro-Tek G-AA Gasbadge: L is the length of the diffusion path of the sampler, and I&. is the effective length of the diffusion path when air flows.
Effect of face velocity on samples
475
zero. However, their measurements were obtained when the air flow direction was parallel to the sampler, except for the face velocity of 0.5 m s-l. In our experiments, the effect of the face velocity when the air flow direction was parallel to the sampler was also smaller than when the flow direction was perpendicular (Fig. 8). The reason for this is that the air stream from the side of the sampler is effective in reducing the thickness of the boundary layer but it may have little effect on the reduction of the length of the diffusion path becausethe airflow and the hole direction are different. From Fig. 7, the relationship between the actual vapour concentration in air, C, and the value measured by the sampler, C,,,, is shown as follows for sampler A: C = G/(0.828
V),
where V is the face velocity (m s-r). Equation (3) is useful for estimating the exact vapour concentration for sampler A when air velocity in the workplace is between 0 and 2.0 m s-r.
REFERENCES Cassinelli, M. E., Hull, R. D., &able, J. V. and Teass,A. W. (1987) Protocol for the evaluation of passive monitors. In An Alternative Approach to Workplace Air Monitoring. Proceedings of an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders,K. J.), pp. 19&202. Royal Society of Chemistry, London. Elliwood, P. A., Groves, J. A. and Pengelly, M. I. (1990) Evaluation of a diffusive sampler for formaldehyde. Ann. occup. Hyg. 34, 305-313. Hahne, R. M. A. (1990) Evaluation of the GMD Systems, Inc., Thermallydesorbable diffusional dosimeter for monitoring methyl chloride. Am. ind. Hyg. Ass. J. 51,96-101. Hallberg, B.-O. and Rudling, J. (1989) A diffusive sampler of gaseous pollutants by collection in liquid media. Ann. occup. Hyg. 33,61-68. Harper, M. and Pumell, C. J. (1987) Diffusive sampling-A review. Am. ind. Hyg. Ass. J. 48,214218. Hori, H. and Tanaka, I. (1993) Response characteristics of the diffusive sampler at fluctuating vapor concentrations. Am. ind. Hyg. Ass. J. S&95-101. Hori, H. and Yanagisawa, Y. (1994) A new vapor generator of multicomponent organic solvents using capillary effect. Environ. Sci. Technol. 27,2023-2030. Kennedy, E. R., Cassinelli, M. E. and Hull, R. D. (1987) Verification of passive monitorperformance: Applications. In An Alternative Approach to Workplace Air Monitoring, Proceedingsof an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders, K. J.), pp. 203-208. Royal Society of Chemistry, London. Kodama, Y., Matsuno, K., Tanaka, I. and Akiyama, T. (1983) Response time of a new passive monitor for organic vapors. Jpn J. ind. Hlth 25, 181-185. Kring, E. V., Damrell, D. J., Henry, T. J., DeMoor, H. M., Basilio, A. N. and Simon, C. E. (1984) Laboratory validation of a new passive calorimetric air monitoring badge for sampling hydrogen sulfide in air. Am. ind. Hyg. Ass. J. 45, l-9. Lautenberger, W. J., Kring, E. B. and Morello, J. A. (1980) A new personal badge monitor for organic vapors. Am. ind. Hyg. Ass. J. 41, 737-747. Levin, J.-O., Lindahl, R. and Andersson, K. (1987) A diffusive sampler for sub-parts-per-million levels of formaldehyde in air using chemosorption on 2,4dinitrophenylhydrazine-coated glass fiber filters. In An Alternative Approach to Workplace Air Monitoring. Proceedings of an International Symposium, Luxembourg (Edited by Berlin, A., Brown, R. H. and Saunders, K. J.), pp. 345-350. Royal Society of Chemistry, London. Lewis, R. G., Coutant, R. W., Wooten, G. W. and McMillin, C. R. (1985) Thermally desorbable passive sampling device for volatile organic chemicals in ambient air. Analyt. Chem. 57, 214-219. Moore, G., Steinle, S. and Lefebre, H. (1984) Theory and practice in the development of a multisorbent passive dosimeter system. Am. ind. Hyg. Ass. J. 45, 145-153. Mulik, J. D., Lewis, R. G., McClenny, W. A. and Williams, D. D. (1989) Modification of high-efficiency passive sampler to determine nitrogen dioxide or formaldehyde in air. Analyt. Chem. 61, 187-189. Naus, C. J., McAvoy, D., Broder, I. and Smith, J. W. (1987) A non toxic diffusional dosimeter for sulfur dioxide. Am. ind. Hyg. Ass. J. 48, 1001-1003.
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Pengelly, M. I., Groves, J. A. and Turnbull, G. B. (1994) The application of a validation protocol to calorimetric diffusive sampler. Ann. occup. Hyg. 38, 161-170. Samimi, B. and Falbo, L. (1985) Comparison of standard charcoal tubes with Abcor (NMS) gasbadges within controlled atmosphere. Am. ind. Hyg. Ass. J. 46, 49-52. Van Der Wal., J. F. and Morekerken, A. (1984)The performance of passive diffusion monitors for organic vapours for personal sampling of painters. Ann. occup. Hyg. 28, 39-47.