Environ. Sci. Technol. WSS, 27, 2031-2034
Effects of the Diffusion Membrane on Passive Sampling P. P6rez Ballesta, E. Gonzhlez Ferradhs,' and A. Mliiana Aznar Department of Chemical Engineering, University of Murcia, 3007 1 Murcia, Spain Experiments were carried out to find the most effective membrane to use in a passive sampler which would minimize the influence of air velocity and direction on sampling rates. A stainless steel cloth witha 1-pmnominal filter rating was chosen. Sampling rates were correlated with various air velocities and the best-fitting equation was determined. The minimum recommended velocity was estimated to be 26 cmls.
Introduction The simplest way of describing passive sampling is using Fick's first law of diffusion, which defines the flow of mass, NA,as
N A = (DS/L)C (1) where C is the external (airbone) concentration of the substance concerned, L and S are the diffusion length and area of the sampler, respectively, and D is the diffusion coefficient in air of the gaseous substance. From equation 1, the airborne concentration of the substance can be obtained as follows: mL c=-=-
m tDS tSR where m is the mass collected in time t. The SR factor is known as sampling rate by analogy with active sampling systems. Air moving on the exposed surface of the sampler can affect the mass transfer in two ways: producing standstills that increase effectivediffusion length or generating eddies within the sampler that lead to reductions in diffusion length. In the former case, the effect is produced by the formation of a laminar boundary layer on the diffusion surface; this leads to increased transfer resistance and a decrease in sampling rate and, consequently, to a decrease in the estimated concentration (I). Conversely, if any turbulence reaches the inside of the sampler, the resulting convective movements increase sampling rates and lead to the overestimation of airborne concentration. The magnitude of these effects is determined, to a large extent, by sampler design. Thus, the reduction in sampling rate brought about by increased external resistance (boundary layer formation) can be diminished by increasing the diffusion length of the sampler,that is, by increasing its internal resistance. Greater diffusion lengthldiameter ratios in turn mitigate the effect of eddies within the sampler, and so a lengthrdiameter ratio greater than 3 is recommended for open-area samplers (2). Membrane samplers, however, are not affected by this design constraint and, in addition, minimize the effect of sampler position with respect to wind direction and prevent the deposition of particulate matter on the collection medium. The aims of this study were to choose a diffusion membrane for a prototype whose diffusion length and surface had previously been established to yield high sampling rates. For this purpose, the influence of various 0013-936X/93/0927-2031$04.00/0
0 1993 Amerlcan Chemical Soclety
diffusion membranes on the passive sampling of toluene was studied together with the influence of air velocity and sampler position on sampling rates. Theoretical Considerations. External resistance to mass transfer can be quantified as boundary layer thickness over the surface in question. In laminar condition, the boundary layer for a plate parallel to the flow of fluid is given by the expression (3) 6 = 4.65(~L'/V)~'~ (3) where v is the air kinematic viscosity, L' is a characteristic length of the sampler, and V is the air velocity. In turbulent condition, for the same geometry and orientation, the equation which determines the boundary layer thickness is (3) 6 = O.37(V/W1f5L'4'5
(4)
By analogy with both equations, the dependence of the boundary layer on air velocity can be expressed as 6 = k,IV (5) The values of 121 and n must be determined experimentally according to the fluidodynamic condition and orientation of the sampler. Thus, the external transfer coefficient, Kext,which is the inverse of the resistance offered by the boundary layer, Rext, can be expressed in the form: Kext= llRext= k,(1/6) = KV"
(6)
The total resistance to the mass transfer is given by the sum of the external and internal resistances: 'total = Rintr + Rext From eqs 6 and 7 the following can be obtained:
Rtod = Rintr + 1IKV"
(7)
(8)
Equation 8 shows that above certain air velocities external resistance does not significantly affect total resistance. However, as mentioned earlier, a higher air velocity can cause internal distortions (eddies), which in turn might lead to decreased internal resistance; this aspect-which depends on membrane "tightness"-needs to be tested experimentally. I t is convenient to express eq 8 in terms of sampling rate, as follows:
1ISR = l/SRi,
+ a/ V"
(9) where SR is the total sampling rate and a is a constant. From eq 9, Yangisaway et al. ( 4 ) deduced a simplified expression in the form:
SR = A + B In V (10) where A and B are constants. Equations 9 and 10 will be used later to fit the experimental values obtained with the sampler and membrane chosen. Environ. Scl. Technol., Vol. 27, No. IO, I993 2031
Table 1. Characteristics oP DiPfusiou Membranes type of mesh
square square
aperture size (m)
wire diameter (mm)
open area (%)
mesh
weight (Kg/m2,
25 10 1 (nominal) 6 7 (ahso1ute)b
0.025 0.01 0.035,0.025c
25 25
510 1270 375,23W
0.16 0.06 0.39
twilled dutch, weave (TDWG)' Manufactwedby Haver and Boeker, Germany. Values refer to filter rating. The fmt value is for the weft and the wmnd value for the warp.
\
\
ADSORBENT
Flgura 2. Passive sampler assembly.
Table 11. Characteristics of Activated C h o r 0 ~Uaed 1 as Adsorbent
origin
eoconut charcoal
PH specific surface, mYg pore volumep cmVg
5.9
0.30
particle size,mm
0.324.50
1130
For pores with diameter ranging from 35 to 1750 A. Flg~ml.Elementsofthepasslvesampler.A.Teflonrover;B.dHfusion membrane (wind screen). diffusion area, 8.04 an2:C and D, Temn retaining (C) and locking (0) rings that keep the diifuslon membrana Ught and at a specified distance (7 mm) from the adsorplbn layer: E and F. external aluminum (E)and internal stainless stwl (F) rings for holding wire meSh (G) tight; G. stain1888 steel wire mesh suppwt of O.lmmaperatues~efuconpecthgthelayerofadswbent;H,sempler Teflon base used as swbem deposn and back cover.
Experimental Section Passive Sampler. The experiments were carried out with a prototype designed in our laboratories, illustrated in Figure 1. The sampler incorporates a support so that it can be used for personal sampling. Figure 2shows the assembled sampler. High sampling rates (approximately 50 cm3/min for toluene) were obtained with our passive sampler. It is reusable and can be used with different adsorbents. We decided on stainless steel (manufactured by Haver & Boecker of Germany) as material for the diffusion membrane for ita advantages over polymeric (organic or inorganic) membranes: greater mesh uniformity that improves reproducibility and high resistance to chemicals and easy cleaning-in hot ethanol-which allows reuse. Also considered were the aperture size of the mesh and the arrangement of the threads in the membrane. Three types of membrane were selected for the assays. The charackistics are presented in Table I. Adsorbent. The adsorbent used for collection of the pollutant was obtained from preactivated coconut charcoal 2092
Envlmn. Sci. Technol.. Vol. 27. No. 10. 1993
subjected to a reactivation procedure (acid lixiviation, washed in deionized water, and activated in an inert atmosphereat 600OC). Thecharacteristies ofthecharcoal are presented in Table I1 (5). Analytical Method. The analytical method used irr based on that suggested by NOSH for the evaluation of organic vapors (6). Toluene was measured hy gas chromatographyafter desorption with carbon disulfide labeled with 1rLlmL benzene as internal standard. The adsorbent/desorhent ratio was established at 233 mg of active charcoaVmL of carbon disulfide. It should be noted that the desorption efficiencieswithin the concentrationrange6 studied must be calculated before carrying out the above procedure. Test Chamber. For the accurate evaluation of the sampler, a test chamber is necessary in which the most important variables that influence sampling can be controlled concentration, temperature, humidity, and air velocity. The test chamber is of a dynamic nature, and the contaminant can be introduced into a known flow of air in a controlled way. Figure 3 shows a simplified diagram of the setup. The airflow, which can he regulated between 0.2 and 7 L/min, is produced by a compressor and is purified in a multilayer fdter consisting of glass wool, silica gel, molecular sieve, and activated carbon. The air is humidified by means of a thermostated gas-liquid contactor with a surface areaof 5.3 an2. An Orion Sage 355injectingsyinge pump regulates the vaporized compound to be incorporated at arate rangingfrom 0.08to900cm/h Theexposure
Table 111. Overall Result of Membrane Selection Experimentse FILTER
HUMIDIFIER HYGROMETER
AR
EXPOSURE CHAMBER
SYRINGER PUMP
Flgure 3. Dynamic atmosphere generating system. AIR VELOCITY
A
PARALLEL
FACING
D
mesh size (pm)/position
no. of samplers
25ifacing 25/parallel 25/back 25/globd value lO/facing 1Oiparallel lO/back lO/global value l/facing l/parallel l/back liglobal value
6 6 6 18
6 6 6 18 6 6 6 18
SR (cm3/min) CV (%) 77.40 53.94 52.83 61.40 54.99 50.42 48.84 51.42 52.13 50.57 48.72 50.51
6.63 10.84 4.94 20.81 1.80 1.85 2.80 5.86 1.71 1.71 2.40 2.82
95 % confidence interval 70.42-84.37 46.03-61.85 49.29-56.36 54.45-68.34 51.92-56.30 49.19-51.65 47.03-50.65 49.82-53.02 51.16-53.10 49.66-51.48 47.49-49.96 49.92-51.50
Under the standard test conditions and air velocity of 90 cm/s.
C
BACK
Figure 4. Passive sampler positions to the airflow.
chamber with a capacity of 14.8 L has outlets on the sides for the adsorption tubes and analytical probes. A glass plate runs the length of the chamber supporting the passive samplers and permitting the generation of recirculation currents which can reach 0.02-2 m/s by means of an axial fan, regulated by a potentiometer. An internal electrical resistance controlled by a contact thermometer governs the temperature of the chamber. The system is provided with aspiration tubes regulated by critical orifices which permit the passage of 200 cm3/ min for active sampling. The chamber is connected online to a gas chromatograph so that the concentration inside may be followed during the experiments. Once a steady state has been reached, the accuracy and stability of the concentration in the chamber can be considered adequate, since deviations from the estimated concentration and from the average concentration do not exceed 2 % and 1% , respectively. Experimental Design. The aims of the experiments were 2-fold: (1) To choose the membrane that performed most satisfactorily when the sampler was positioned: (A) with its face placed parallel to the direction of the airflow, (B) facing the airflow, and (C)with its back to the airflow (Figure 4), under standard test conditions. The following are the standard test conditions: concentration, 1TLVTWA (ACGIH values) (7) [threshold limit values-time weighted average, over 8 h edited by the American Conference of Governmental Industrial Hygienists (e.g., for toluene, TLV-TWA = 100 ppm)]; humidity, -40%; temperature, 20 "C; air velocity, 90 cm/s; sampling time, 6 h. With this experiment, the most suitable windscreen was chosen. (2) To establish the equations that best reflect the relationship between sampling rate and air velocity for each of the three positions specified above, once a membrane was selected (in this case, DTW6, Table I),
Table IV. Sampling Rates According to Air Velocity and Sampler Position
velocity (cm/s)/position
no. of samplers
9/facing 9/parallel 9/back 9/global value 2Oifacing 2O/parallel 2O/back 2O/global value 36/facing 36/parallel 36/back 36/global value 60ifacing 6Oiparallel 6O/back 60/global value 90/facing 9O/parallel 90/back 90/global value 135/facing 135/parallel 135/back 135/global value 170/facing 170/parallel 170iback 17O//globalvalue
6 6 6 18 6 6 6 18
6 6 6 18 6
6 6 18
6 6 6 18 6 6 6 18 6 6 6 18
SR (cm3/min) CV(%) 43.77 38.47 32.39 38.93 47.87 43.72 39.22 41.47 51.12 46.93 45.99 48.64 52.18 49.41 47.47 49.69 52.13 50.57 48.73 50.47 54.21 52.69 51.93 52.95 55.03 53.97 52.73 53.84
1.41 7.55 0.32 12.50 2.46 2.20 1.13 5.72 2.04 2.65 1.20 4.75 1.73 1.76 2.09 4.31 1.76 1.71 2.40 3.29 1.82 1.68 2.51 2.64 2.22 2.29 0.69 2.49
95 % confidence interval 43.13-44.40 35.47-41.46 32.25-33.27 36.57-41.29 46.75-48.99 42.81-44.63 38.80-39.63 42.65-40.29 50.02-52.21 45.50-48.36 45.37-46.61 47.41-49.86 51.32-53.03 48.59-50.23 46.53-48.40 48.62-50.75 51.16-53.20 49.66-51.48 47.49-49.96 49.64-51.30 53.28-55.14 51.85-63.53 50.70-53.16 52.25-53.64 53.95-56.11 52.78-55.16 52.38-53.08 53.21-54.47
and finally to obtain the general equation applicable to the three positions. This equation would determine the minimum air velocity value above which sampling rates are not significantly affected. Mean air velocities of 9,20, 36, 60, 90, 135, and 170 cm/s were used; the rest of the experimental conditions were those outlined above.
Results and Discussion Table I11gives the results obtained for each membrane studied. Regarding the mean values of overall sampling rates and their corresponding variation coefficients, a significant reduction was observed as the aperture size of the mesh was reduced. As was to be expected, the airflow influenced the sampling rates more significantly when the exposed surface of the sampler directly faced the airflow and when the mesh size was greater. The results obtained with a l-pm nominal filter rating stainless steel membrane Envlron. Sci. Technol., Vol. 27. No. 10, 1993 2033
Table V. Fitting Equation of Sampling Rate vs Air Velocity
position
eq 9 1 = 1.73 X lo-' SR
facing R
eq 10
+ 1.946 X lo-'
0.992
vas,
SR = 36.84 + 3.59 In V 0.976 SR = 27.42 + 5.20 In V
parallel R
0.999
0.995
back
SR = 19.06
R
+ 6.75 In V
0.981 SR = 27.60 + 5.21 In V
mean R
0.974
corresponding to the mean of the three positions. Accepting a maximum deviation in 10% of the sampling rate at the standard velocity of 90 cm/s-50.6 cm3/min-the minimum recommended air velocity derived from the above equation is 26 cm/s, a value normally exceeded in personal monitoring (9). -
Fitted Ecuacian
Conclusions
I95 % C0"l Inferv.
*$
1
10
0' 0
20
40
60
80
100
120
140
160
1
180
Air velocity, cm/s Figure 5. Samplingrates at various air velocities and sampler positions.
(TDW6) were considered the most adequate for the subsequent experiments, and so this mesh was chosen for the design of the final sampler. Results obtained with the chosen membrane are presented in Table IV. In all cases an increase in sampling rate was observed with increased air velocity. This is due to a reduction in the boundary layer formed on the membrane surface and to the possible formation of eddies produced within the sampler. There was a greater difference between the sampling rates in the three positions as the air velocity decreased. This is due to the fact that the boundary layer effect is more strongly felt when the sampler has its back to the air. This position resulted in the lowest sampling rates of all, and the frontal position gave the highest rates. The sampling rates obtained at various air velocities and sampler positions were correlated using eqs 9 and 10, Table V; the parameters of eq 9 were optimized using the method of Hooke and Jeeves (8),with this equation providing the best fit. Figure 5 illustrates the mean sampling rates for each position and air velocity tested and also the fitting equation
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Environ. Scl. Technoi., Vol. 27, No. 10, 1993
The results obtained with the stainless steel mesh of 1-pmnominal filter rating are those best used for the design of the sampler; the influence on the sampling rate of air velocity and sampler position at air velocities above 26 cm/s is negligible.
Literature Cited (1) Tompkins, F. C.; Goldsmith, R. Am. Znd. Hyg. Assoc. J. 1977,38,371-377. (2) Lautenbeg, N. J.; Kring, E. V.; Morello, J. A. Am. Ind. Hyg. ASSOC.J. 1980, 41, 737-747. (3) Streeter, V. L.; Wylie, E. B. Mecanica de Fluidos, 3rd ed.; McGraw-Hill: New York, 1977; pp 250-257. (4) Yangisawa, Y.; Hemphill, C. P.; Spengler, J. D.; Ryan, P. B. Proc.-APCA Annu. Meet. 1986, 79th, 13 pp. (5) Walker, P. L., Jr.; Cariaso, 0. C.; Ismail, I. M. K. Carbon 1980, 18, 375-377. (6) U.S.National Institute for Occupational Safety and Health. Organic Solvent in Air-Analytical Methods. P & CAM 127, NIOSH Manual of Analytical Methods, 2nd. ed.; NIOSH: Washington, DC, 1977; Vol. 1, pp 127-1-127-7. (7) TLVs. Threshold Values and Biological Exposure Indices for 1991-92, American Conference of Governmental Industrial Hygienists, Cincinnati, OH. (8) Hooke, R.; Jeeves, T. A. J. Assoc. Comput. Mach. 1961,8, 212-229. (9) Goelzer, B.; O'Neill, I. K. Environmental Carcinogens Selected Methods of Analysis; Fishbein, L., O'Neill, I. K., Eds.; IARC Publications No. 68; IARC: Lyon, 1985; Vol7, Chapter 6, pp 124-125. Received for review July 31,1992. Revised manuscript received December 28, 1992. Accepted March 22, 1993.
