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Anal. Chem. 1981, 53, 458-461
and 656 pg/L OC1- was analyzed for OC1- via the two procedures. Results appear in Table 111. It is apparent from these data that the luminol CL technique is suitable for the routine determination of OC1- in drinking water and that it gives results comparable to the standard DPD colorimetric method in terms of accuracy and freedom from major interferences but is more precise and has a detection limit over 2 orders of magnitude lower. Equal NHzCl and OC1- levels lead to only a ca. 1 % error in both methods. The CL technique requires only one reagent solution and is rapid in that about 90 samples/h can be run once solutions are prepared. The detection limit, lower than achieved with any-other standard method for OCl-, allows for studies of the fate of residual chlorine after reaction with water constituents. LITERATURE CITED (1) ”Standard Methods for the Examination of Water and Wastewater”, 14th 4.;American Public Health Association: Washington, DC, 1975; pp 304-349.
Isaccson, U.;Wettermark, G. Anal. Chlm. Acta 1976, 83, 227-239. Isaccson, U.; Wettermark, G. Anal. Lett. 1978, 7 7 (I), 13-25. Seitz, W. R. J . Phys. Chem. 1975, 79, 101-115. Seliger, H. H. “Llght and Life”; McElron, W. D., Gloss, B., Eds.; John Hopkins Press: Baltimore, MD, 1961; pp,,200-205. Seitz, W. R.; Hercules, D. M. Chemiluminescence and Bioluminescence”; Cormier, M. J., Hercules, D. M., Lee, J., Eds.; Pienum: New York, 1973; pp 427-449. Flsher Chemical Index 71C; Fisher Scientific Co.: Pittsburgh, PA. Cotton, F. A.; Wiikinson, G. “Advanced Inorganic Chemlstry”, 3rd ed.; Intersclence: New York, 1972; pp 477-479, 878-879. Hoyt, S.; Ingle, J. D. Jr. Anal. Chlm. Acta 1976, 87, 163. Montano, L. A; Ingle, J. D.,Jr. Anal. Chem. 1979, 57, 926-930. Marino, D. F.; Ingle, J. D., Jr. Anal. Chern. 1981, 53, 294. Chapin, R. M. J. Am. Chem. SOC.1934, 58, 2211. Seitz, W. R.; Hercules, D. M. Anal. Chem. 1972, 44, 2143.
RECEIWDfor review July 7,1980. Accepted December 9,1980. Acknowledgment is made to the National Science Foundation (Grant No: CHE 7616711 and CHE 7921292) for partial support of this research. Presented in part a t the 62nd Canadian Chemical Conference, 1979, Vancouver, British Columbia.
Rubber Disk Passive Monitor for Benzene Dosimeter Michael V. Sefton,” Ennlo L. Mastracci,‘ and John L. Mann’ Depaltment of Chemical Engineering and Applied Chemistty, University of Toronto, Toronto, Ontario, M5S lA4, Canada
A disk (3.75 mm X 12 mm dlameter) cut from a sheet of gum rubber has been designed and callbrated to act as a passive dosimeter for benzene in the workplace. The disk absorbs the amblent benzene and Is both ilmltlng resistance to mass transfer and collection element. There was a dlrect relationship between the benzene concentration in the CSz extract and the product of ambient concentration and the square root of exposure tlme over the range of 3-25 ppm (8 h exposure). While the 95% confidence interval Is less than f2 ppm at exposures corresponding to the TLV (10 ppm, 8 h) or hlgher, the error increases at lower concentratlons (f2.5 ppm at 5 ppm, 8 h) due to the apparent trace presence of benzene In blank disks and the llmlted sensitivity of the flame ionizatlon detector at these concentrations. Although field testing Is still required, the rubber disk may be a useful alternative to charcoal badge dosimeters for organic-vapor monitoring.
Passive monitors are becoming an increasingly desirable means of conducting industrial hygiene surveys in the workplace. For organic vapor dosimetry, these monitors typically consist of an activated charcol collecting element separated from the workplace air by a membrane or “draft shield” (1-5). The membrane acts as a limiting resistance to control the rate of mass transfer to the collection element and free the monitor from the effects of worker movement and air currents in the workplace. After exposure, the organic vapor is desorbed thermally or with carbon disulfide or other appropriate solvent and the amount collected quantified by gas chromatography. Unlike conventional active dosimeters which employ a pump to draw a sample of air through a bed of activated ‘Current address: Syncrude Canada Ltd., Ft. McMurray, Alberta, Canada. Current address: Imperial Oil Ltd., Toronto, Ontario, Canada. 0003-2700/81/0353-0458$01.00/0
charcoal, passive dosimeters are less cumbersome and therefore more readily tolerated by the worker. Since there is no pump to be calibrated nor battery pack to be recharged, they are more reliable in the field and easier to use than active dosimeters. However, they have yet to be approved by OSHA for routine compliance monitoring. A simpler, less expensive monitor, which does not suffer from the disadvantages associated with activated charcoal, has been designed and subjected to laboratory evaluation. For benzene dosimetry, this monitor consists of a small natural rubber disk, 3.75 mm thick and 12 mm in diameter, which acts as both collection element and rate controlling membrane. The rubber disk absorbs vapor at a rate which is proportional to the ambient concentration, governed by the diffusivity and solubility of the vapor in the rubber. The diffusion process within the rubber provides the limiting resistance to mass transfer, conventionally associated with the separate membrane. The dissolution of the vapor in the rubber (Le., absorption), rather than adsorption at specific sites as on activated charcoal, is the means of vapor collection or trapping. DOSIMETER DESIGN The size and shape of the dosimeter must be adjusted according to the nature of the vapor of interest and the material chosen for the disk. Natural rubber was a good choice for benzene because of the high diffusivity and solubility in the rubber and the ready availability of the material. HOWever, it would have been desirable to choose a material which contained no contaminating material extractable in carbon disulfide. By use of the properties of the material, the intended extraction and analytical procedures, and the desired sensitivty (as related to the recognized maximum exposure levels), the size and shape of the dosimeter and the constraints governing its use can be defined. The absorption of an organic vapor in a rubber disk can be described in the initial stages (i.e., with less than 60% of 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981 baffle
the equilibrium uptake), by (6)
450
paired discs on mylar film septum
where Mt = mass gained at time t , M, = equilibrium uptake, S = the ratio of exposed area to volume of the disk (specific surface area), D = diffusivity of organic vapor in the solid, and t = time. It is assumed that D is constant because of the low concentrations involved in this work. The complications of diffusion in glassy or semicrystalline materials are avoided by using wholly amorphous materials above the glass transition temperature. The use of eq 1and the limitation of M t / M , < 0.6 imposes a constraint on the area/volume ratio for an 8-h dosimeter, that
s I(3.13
x 10-3)/~1/2
sampllng chamber
(2)
for a diffusivity with units of cm2/s. Equation 2 dictates the shape of the dosimeter. Since the amount of vapor in the extracting solvent is simply Mt corrected for the extraction efficiency and M, is related to the solubility or partition coefficient, K , of the vapor in the rubber disk, the concentration, C, of vapor in the extracting solvent is given by
where C = concentration in extracting solvent (ppm, w/w), Y = ambient concentration (ppm, v/v), E = extraction efficiency (the fraction of absorbed vapor which becomes dissolved in the extracting solvent), K = distribution constant [ (g/cm3, in dosimeter)/(ppm, in air)], VD = volume of dosimeter (cm3), Vs, ps = solvent volume, density, K' = a constant for any dosimeter system. Equation 3 defines a linear relationship between ambient concentration Y and the measured concentration C after a constant exposure time provided all the parameters (in particular, E , K , and D)are constant. The size of the dosimeter, VD, is defined by the detection limit, C-, of the analytical procedure (gas chromatography) and the minimum required detection level, Ymi,,. Thus (4) The size of the dosimeter however is further limited by the fact that the disk will absorb some of the extracting solvent. Thus Vs must be greater than the volume of solvent which is absorbed by a disk of volume VD; i.e.
-1> -VD 4
hw w
D to vent
vs
where q = volume of solvent absorbed by 1 cm3 of disk, a swelling ratio. EXPERIMENTAL SECTION Dosimeter Preparation. Disks, 12 mm in diameter, were cut from a 3.75 mm thick sheet of gum rubber (Ontario Rubber Co., Toronto, Ontario), with a specific gravity of 0.98. To be used as dosimeters, they were glued to a small piece of Mylar film so that only one face and the edge of the disk were exposed to the benzene atmosphere, giving a disk with an area/volume ratio of 6.0 cm2/cm3. Laboratory Evaluation. Pairs of disks were exposed for 1-17 h at benzene concentrationsof 9.2 to 26 ppm in a Lucite chamber which was connected to a dynamic dilution system (Figure 1). Dilute benzene-in-air mixtures were prepared by diluting a calibrated benzene/air mixture (26 ppm, Matheson of Canada,
air benzene/air from gas cylinders Flgure 1. The dynamic dilution system and sampling chamber.
.
Toronto, Ontario). More concentrated mixtures were prepared from an air stream saturated with benzene at 8 "C and produced by using standard techniques (7). Dosimeter Analysis. After benzene exposure, the disks were removed from the backing and placed in screwcap vials with a Teflon-rubber septum (Chromatographic Specialties, Brockville, Ontario) to which was added 2.0 mL of carbon disulfide (>99% CS2,spectroscopic grade, "Photrex" Reagent, J. T. Baker Chemical, Toronto, Ontario) for 2 h to extract the absorbed benzene. The extraction of some higher molecular weight, less volatile material necessitated modification of the standard benzene/CS2 gas chromatographic analysis procedure (8). After the benzene peak had appeared, the column was back-flushed to the detector by automatic activation of a four-way valve. This reduced the total analysis time from over 80 min without back-flush to less than 16 min without any need for temperature programming. The gas chromatograph was a Model 5830A (Hewlett-Packard, Toronto, Ontario) with HP 18850A terminal and a flame ionization detector. The column was 20 ft long, in. diameter, containing 10% FFAP on Chromosorb W, A/W, DMCS, 80/100 mesh (ChromatographicSpecialties). The column temperature was 135 "C, injection temperature 200 "C, and detector temperature 300 "C. Helium flow rate was 13 cm3/min. One-microliter injections were used for the CS2extracts and 1.0-mL injections were used for the benzene/air mixtures; no back-flushing was needed for the air samples. The detection limit of this analysis was 0.25 ppm (w/w) benzene in CS2. The instrument response was linear to more than 70 ppm (w/w). Determination of Fundamental Parameters. The diffusivity of benzene in the rubber at 30 "C was determined by measuring the rate of weight gain of a sample of rubber exposed to 0.05% (v/v) benzene in air using the Cahn electrobalance (Cahn Instruments, Cerritos, CA) according to standard techniques (9). The equilibrium uptake was measured under the same conditions, and at lower concentrations, as part of this determination. The swelling ratio, q, was determined by weighing samples of rubber before and after swelling in CS2and correcting to a volume ratio by using the pure component densities. The extraction efficiency was estimated by placing 2 mL of CS2 containing varying amounts of benzene in contact with 207 mg of rubber. At equilibrium the concentration of benzene remaining in the solution was determined and plotted against initial concentration. The slope of this line is the extraction efficiency, E. RESULTS The amount of benzene in the CS2 extracting solution is plotted against the product of the ambient benzene concentration and the square root of the exposure time in Figure 2. Contrary to eq 3, this calibration curve was not straight but indicated the presence of a concentration dependence in E
460
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981 Y @ 8 HOURS (pprn)
.O'
1 /,/.///'
60-
50-
I'
40
30
-
/p'
/" I
Ell
/'
20 -
I/TIME x
,
Y ( x io. ppm sec M)
Figure 2. Calibration curve of rubber disk dosimeter for benzene exposure. Concentration of benzene in CS2 after extraction, C (ppm, w/w), plotted against the product of ambient benzene concentration, Y (pprn, v/v), and the square root of exposure time, t'" (s'"). The
additional abscissa scale gives
Y for
an exposure time of 8 h.
or K' or both. A blank dosimeter, not exposed to benzene, contained material which on GC analysis appeared to be benzene (same retention time). As a result this curve did not pass through the origin, making it more difficult to use in the low exposure range. From the electrobalance measurements the benzene diffusivity in the rubber at 30 "C was 2.2 X cm2/s agreeing with the literature value of 2.0 x cm*/s for benzene at near-zero concentrations in a different sample of natural rubber a t 30 "C (10). The distribution constant at 30 "C was 1.14 X lo4 g/(cm3 rubberappm) (v/v) from the slope of a (linear) plot of equilibrium uptake against benzene concentration from 600 ppm (v/v) to 0.05% (v/v). The swelling ratio was 4.76 g of CS2/g of rubber or 3.7 cm3/cm3. The extraction efficiency was estimated from Figure 3. At low equilibrium concentrations the curve has a slope of 1, indicating a 100% extraction efficiency. Unlike the recovery of benzene from activated charcoal in a badge dosimeter, addition of CS2 to a rubber disk containing benzene results in two phases a t equilibrium: CS2 swollen rubber and CS2 liquid. The extraction efficiency
is then determined by the partition coefficient of benzene between these two phases. For an E of 1, the concentration of benzene in the CS2 available for sampling is the same as the concentration of benzene in the CS2 that has been absorbed by the rubber; this is thermodynamically reasonable at low concentrations. Thus the product of the measured concentration and the total mass of C S 2 (corresponding to 2 mL) would be equal to the total amount of benzene initially present in the rubber and, therefore, the extraction eftkiency is 1. The slope (and extraction efficiency) greater than one a t higher concentrations merely reflects the preferential partitioning of the benzene into the CS2 liquid; E > 1 does not imply that more mass is present after extraction than before. It should be noted, however, that equilibrium has been reached from the opposite direction to what would normally be the situation in dosimeter use. Nevertheless, the extraction efficiency at equilibrium should be identical, within the rather limited accuracy of this method.
DISCUSSION To design the dosimeter, we corrected the measured diffusivity and distribution constant to 25 "C by using the temperature coefficients reported in the literature (10, 11) for other samples of natural rubber. The corrected values were
0
10
,
,
,
20
30
40
3
INITIAL CONCENTRATION OF BENZENE IN CS,, pprn (wW
Figure 3. Extraction efficiency as defined by eq 6, determined from the opposite direction to normal dosimeter use. The concentration of benzene in CS2 at equilibrium with the rubber plotted against the initial concentration prior to mixing with rubber.
1.3 X lo-' cm2/s and 1.24 X lo4 g/(cm3.ppm), respectively. Assuming E = 1 and q = 3.7 cm3/cm3,the design equations 2 , 4 , and 5 became S I8.68
VD
0.271 > - L
vs
4.403 -
YmhS
for 8 h of exposure ( t )and a minimum detectable GC concentration of 0.3 ppm (C-) with a 1 pL injection. The disk with only one face exposed had an area/volume ratio of 6.0 and a VD/Vs of 0.21 with 2 mL of CS2 used to extract the absorbed benzene. The calculated detection limit according to these design criteria was 3.5 ppm, in reasonable agreement with the experimental results. According to these equations exposing more of the surface of the disk (up to the 8.68 limit) should result in a lower minimum detectable concentration (at S = 8.68, Ymh = 2.4 ppm). The simplest way to use the disk dosimeter would be to use Figure 2 as a calibration curve given the exposure time, t, and the measured concentration in the CS2ext.ract to calculate the ambient benzene concentration, Y. For 8-h exposures, this calibration curve covers a measurement range of 3-25 ppm (v/v) as indicated on the additional abscissa scale. At the low exposure end (5 ppm, 8 h), however, the 95% confidence interval (estimated graphically from the confidence interval on C) gives an error of f 2 . 5 ppm which is beyond NIOSH (National Institute for Occupational Safety and Health) acceptability (*35% for
