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Mass Transfer Effects in a Nitric Oxide Dosimeter J. S. Nadeau, M. E. Treen, and D. G. B. Boocock“ Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A4
Mass transfer studies have resulted in the selection of a membrane for a nitric oxide passive dosimeter. The mass transfer rate of nitric oxide to a free radical reagent on silica gel increases nonlinearly when wind velocity is raised from stagnant conditions to 7 m/s. When a reinforced poly(methylvinylsiloxane) membrane with a thickness of 0.05 mm is attached to the front of the plate, a constant mass transfer rate occurs at all wind velocities in the range studied. Reagent surface loading on the plate is best controlled by changes in reagent solution concentration rather then by use of multispotting procedures. Blank readings may be decreased significantty by pretreatment of the plate preparation room with small ozone concentrations.
We recently reported a reagent which when supported on thin-layer silica gel plates formed the basis of a nitric oxide passive dosimeter ( I ) . This reagent, a nitronylnitroxide free radical 1 reacts with nitric oxide to produce an iminonitroxide free radical 2 as shown in Equation 1.
The reagent is applied to the plate in chloroform solution and after exposure, the product is separated by thin-layer chromatography, extracted with a measured volume of p dioxane and quantified by electron spin resonance. We report here on the selection of a membrane to buffer mass transfer effects caused by wind. This involved the study of the mass transfer of nitric oxide in membrane-covered and membrane-free plates. We also report on techniques which may be used to reduce blank readings. T h e reaction of nitric oxide with the reagent 1 on the plate is mass transfer controlled ( I ) . It is therefore essential that the transport rate of nitric oxide to the reagent be independent of wind velocity. Therefore the boundary layer over the plate must not be part of the limiting resistance. I t is usual to deliberately introduce a limiting resistance into a passive dosimeter. This resistance may be a membrane as in West‘s vinyl chloride ( 2 )and sulfur dioxide (3) dosimeters, or it may be some kind of air gap as in the general dosimeter described by Tompkins and Goldsmith ( 4 ) and the tubular type of nitrogen dioxide dosimeter described by Palmes ( 5 ) . T h e geometry of our collection/reaction surface indicated that a membrane would be the simplest and most convenient resistance layer. Analogy between Dosimeters and Electrical Metering. T h e term “limiting resistance” is used when discussing both mass transfer and electrical circuits. T h e analogy of a dosimeter to an electrical measuring circuit is very useful, particularly to those who may not be familiar with mass transfer concepts. Figure 1 shows an electrical circuit which 0003-2700/78/0350-1871$01,00/0
is analogous to t h e nitric oxide dosimeter containing a membrane. Rb, R,, R,, and R, are resistances equivalent to the mass transfer resistances of the boundary layer, membrane, silica gel, and reagent-nitric oxide reaction, respectively. Rb in the dosimeter would generally decrease as wind velocity increases. R, in the dosimeter would be made up of resistances due to molecular diffusion, Knudsen diffusion, and surface diffusion in the silica gel. R, may become limiting if the reagent surface loading falls belou a certain value for a particular nitric oxide concentration. T h e reaction of nitric oxide with the reagent would then be chemically controlled. This would be equivalent to changing the response of the meter in the electrical circuit and would obviously be undesirable. Finally, t h e E M F of the electrical circuit is analogous to the nitric oxide concentration difference across the dosimeter. This may be taken as the ambient nitric oxide concentration in most cases. T h e boundary layer, in the absence of a membrane or air gap, is lusually the limiting and variable resistance. T h e membrane or air gap added to the dosimeter should therefore have a significantly larger resistance and this will result in a decrease in sensitivity of the device. The Membrane. Reiszner and West (3)have studied the permeation of sulfur dioxide in various membranes and concluded t h a t permeation rates are high in silicone membranes and are not greatly affected by changes in temperature. We had access to membranes which have been specially made for evaluation as artificial lung material (Sci-Med Inc. 13070 County Rd. 6, Minneapolis, Minn., 55441). These membranes are not listed in the manufacturer’s catalogue but are available on request. After some preliminary studies, we chose a type 2A membrane which is silica-filled poly(methylvinylsi1oxane) reinforced with a polyester knit. T h e membrane is available in two forms and we used t h e one in which the knots of the reinforcing material are placed upwards in the pouring process and thus appear on the rough side of the membrane. T h e overall material thickness is 0.17 mm (0.007 in.) but the actual membrane thickness is 0.050 m m (0.0020 in.). We used the membrane with the rough side facing the plate. We have studied the mass transfer of nitric oxide to membrane-covered plates and membrane-free plates in a simple wind tunnel using ambient air. Reagent Application Procedure. We have also studied the flexibility in the method of applying reagent to the plates. In our initial studies, plates were spotted with single 10-wL portions of a 6.28 X lo-’ M solution of the reagent in chloroform ( I ) . We first investigated the effect of changing the concentration of the reagent in the spotting solution. Six different concentrations ranging from 0.81 X lo-* M t o 6.28 X lo-’ M were used. Plates were exposed to 0.38 ppm nitric oxide in nitrogen for 1 h in a 1000-L chamber ( I ) . T h e plate response was directly proportional to the reagent solution concentration. This relationship is due to changes in spot area and not t o a change in surface loading of reagent. Earlier studies showed that the reaction of nitric oxide and reagent is mass transfer controlled. Figure 2, which displays the results of multispotting procedures (see below), shows that, as expected, there is a limit to the spot size which can be obtained using 1O-fiL portions of spotting solution. I t appears, from ‘C 1978 American
Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Figure 1.
Electrical circuit analogous to nitric oxide dosimeter
Figure 2. Plate response as a function of the number of 10-pL superimposed spots of rea ent solution. Upper points 6.28 X lo-* M, lower points 3.14 X 10-9 M
a consideration of the two data points representing single spots, that for a reagent concentration of 10.84 X M, the spot would be close to this limiting size. In a second investigation, the effect of using multiple superimposed spots was studied. The chloroform from a IO-pL spot was allowed to evaporate before applying a second spot at the identical position on the plate. Successive spots could be applied in the same way. Two reagent concentrations, 6.28 X lo-' and 3.14 X 10~-'M were used. Plates, carrying up t o six superimposed spots, were exposed to 0.38 ppm nitric oxide in nitrogen for 1 h. T h e plate responses are shown in Figure 2 . Curves have heen drawn only to show the trend in results. T h e same maximum response is reached using both spotting solutions and this occurs with six successive spottings of the weaker solution and three successive spottings of the stronger solution. This again demonstrates that the reagent eventually reaches the same outer limit that the chloroform reaches before it evaporates. The increase in spot area with successive spottings was observed visually. It was also noticed that the annulus of reagent a t the periphery of the spot became wider and darker as successive spots were applied, thus indicating that most of the reagent was located at the edge of the spot. T h e results of these two studies indicate that when the reagent is applied in spot form, then the spot size and reagent surface loading are best controlled by changes in the reagent solution concentration and the volume of reagent applied, and not by use of superimposed spots. B l a n k Readings. T h e lower liniits of detection for the method are strongly determined by the magnitude of blank readings. T h e iminonitroxide blank is produced by two processes. First! product 2 can be formed from the reagent by nitric oxide or strong light prior to its application to the plate and, secondly, product can be formed during plate handling and preparation. This latter effect is important when measuring typical ambient concentrations of nitric oxide over very short periods such as 1 h. In our wind tunnel studies (see below), blanks were initially as high as 80% of the sample plate readings. One obvious method for reducing blanks caused by product in the reagent, would involve a thin-layer separation prior to plate exposure. We studied this possibility using a single spot of reagent, 6.28 X M, on a plate which
was slightly longer than usual. T h e reagent spot spreads during elution and increases in area by 3 6 7 ~after 30 min. The area then remains constant for at least the next Xl-min period of elution. The increase in area was measured from the response of the eluted plates. For the second separation, made after plate exposure, elution was carried out in the opposite direction: that part of the plate carrying the product blank having been cut off at an appropriate distance from the reagent spot. Lye did not employ the above procedure because. in our wind tunnel studies, the major source o f t h e blank was plate preparation. We therefore removed nitric oxide from our plate preparation room with ozone by turning on a 6-in. pencil-type UV lamp for 10 min. T h e lamp was switched off and 10 min later the plates were prepared. In this way, blanks were reduced to 8%'and less in the wind tunnel studies. Ozone dosage should be carefully designed for the plate preparation area to avoid any hazard to personnel.
EXPERIMENTAL Plate Preparation. The silica gel plates used were Eastman Kodak 6061 tgpe containing a poiy(viny1 aiicohol) binder. Mhough these have now been replaced by the manufacturer with type 13179 which contain a poly(acry1ic acid) binder, we continued to use them so that we could relate our results to our previous work (1). The plate size has now been reduced to 5 X 'L cm thus allowiq: 40 of them to be cut from a 20 X 20 cm sheet. The plates were preconditioned by drying them for at least 0.5 h at a temperature of 80 "C. The temperature is limited by the thermal stability of the polyester backing. Twenty minutes prior to plate preparation, a six-inch pencil-type ultraviolet lamp (Pen-Ray SCT 1) was switched on in the preparation room (dimensions,3 m X 4 m X 3 m high) for 10 min. Sample plates and plates for blanks were spotted alternately as described previously ( I ) and covered with the type 2.4 membrane so that the rough side of the membrane faced the plate. Commercially available invisible tape (0.75 inch wide) was immediately fastened on the membrane over the spot area. This is a convenient method of reducing the plate response to zero. Plate storage has been described previously (I). Wind Tunnel Studies. The Kind tunnel consisted of metal ductwork with a square cross section and 30-cm sides. Ambient air was drawn from outside the building: around a 99" turn, through vertical baffles, and along a 6-m linear section. A rigid polyester door was installed at the midpoint of the linear section. At the exhaust. a Dehli single inlet exhauster fan was driven by a constant speed Delco AC motor (1726 rpni!. The air velocity was varied by adjusting a flap at the tunnel entrance. Immediately prior to measurement, prepared plates were introduced through the door and taped face up on the lower surface of the tunnel. The adhesive tape was removed from the face of the membrane-covered plates. The membrane was removed completely in the case of membrane-free plates. The temperature of the air was measured 15 cm downstream. Air velocity was measured 'L cm above the tunnel floor using a pitot tube and a Wilk- Lambrecht slant manometer. Nitric oxide and nitrogen dioxide time-weighted average concentrations were measured as described previously ( I ) . The tunnel inlet was located on a second floor facing an EastGIvest street in Toronto. On a weekday this street tgpically carried 1000 vehicles per hour in a dagTime nonrush-hour period (1O:OO a.m.-ll:UO a.m.). Plate exposures were made for either 1 or 2 h. For 1-h exposures. parallel nitric oxide sampling was performed over the first and last 20 min. For 2-h exposures. sampling was carried out in the initial and final 50 min. The ambient nitric oxide concentrations varied between 0.02 and 0.12 ppm. For air velocities of zero. plates were exposed in a 1000-L chamber under natural convective conditions. After exposure, the plates were developed in a 2 : l by volume mixture of n-pentane and diethyl ether. This elution mixture differed from that described previously ( 1 ) in that the carbon tetrachloride component was replaced by more n-pentane. It was found that the presence of carbon tetrachloride can cause the radicals to undergo reversible salt formation on the plates. This is presumably caused by the trace amounts of hydrochloric acid which can form in carbon tetrachloride coxtaining small amounts of water. After development, the product radical 2 was extracted
ANALYTICAL CHEMISTRY, VOL. 50, NO. 13, NOVEMBER 1978
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Figure 3. Response for membrane-free plate as a function of air velocity
air velocity ( m 1 s ) Figure 4. Response (cmlppm NO) for membrane-covered plate as a function of air velocity
from the silica gel with dioxane and spectra were obtained using a Varian-E4 spectrometer as described previously ( I ) . All results were expressed as a plate response in the form plate response =
A(2 2 ) -
where c is the concentration of nitric oxide in ppm, t is the time in hours, H,and Hb are the ESR peak heights in cm of the product extracted from the sample and blank plates, respectively, and G , and G b are the respective receiver gains of the instrument.
RESULTS AND DISCUSSION Plate responses for membrane-free and membrane-covered plates, plotted as a function of air velocity, are shown in Figures 3 and 4, respectively. I t should be noted that the response scale in Figure 4 has been expanded compared t o that in Figure 3. Simple boundary layer theory predicts that when the boundary layer has the limiting resistance, then for laminar flow the plate response should be proportional to the square root of the wind velocity, c, and for turbulent flow the response should vary as ( c ) ~ These ~. relationships should hold until t h e plate resistance becomes part of t h e limiting resistance. The plates may be considered as being infinitely long when taped to the bottom of the wind tunnel and thus the flow should be turbulent in most of the velocity range studied. T h e results shown in Figure 3 are not inconsistent with t h e above theory, especially if there is a small laminar-flow region between the first two data points. However, there is one serious inconsistency in the data shown in Figures 3 and 4. It can be seen that introduction of the membrane reduced the plate response only 75% under natural convective conditions.
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Using the previous analogy this w o d d be equivalent to R , being equal to 3Rb. This observation. together with steepness of the response curve in Figure 3. indicated that the membrane should not be capable of completely buffering wind effects, whereas Figure 4 demonstrates that the membrane is very effective. One reasonable explanation for this anomaly is that a t least part of the limiting resistance in the membrane-free plate is within the silica gel layer. even under natural convective conditions. In fact, earlier studies showed a mass transfer rate in natural convective conditions which was concentration dependent above about 0.5 ppm nitric oxide, and this was attributed to diffusional processes on the silica gel ( I ) . It is suggested that the airflow over the membrane-free plates penetrates t h e silica gel layer and increases t h e mass transfer coefficient in it. In the absence of such an effect, the curve in Figure 3 would be much less, steep. T h e membrane would prevent the penetration and thi!;, together with the extra resistance provided by the membrane, would account for its effectiveness. The introduction of the membrane eliminates the nonlinear portion of the calibration curve in the range 0.5-1.00 ppm nitric oxide. This is expected because, at 1.0 ppm nitric oxide, the theoretical concentration above the silica gel is 0.25 ppm and a t this concentration the diffusion coefficient of nitric oxide on the silica gel is still in the linear region. Even in a nitric oxide concentration of 4.0 ppm, for which the theoretical concentration above the silica gel is 1.0 ppm, the plate response should exceed the linear value by only 7.5%. More resistive membranes would be used for measurement of higher nitric oxide concentrations because in the present system a membrane-free plate can tolerate only 0.7 ppm nitric oxide for eight hours before the plate response changes. In this situation, diffusional effects on the isilica gel would become negligible. However, the concentration dependent diffusional effects could be a problem if the reagent were employed as a colorimetric reagent, supported on silica gel in a flow-through tube-type dosimeter. T h e above results may also have relevance to dosimeters using activated charcoal. In particular, the results emphasize that the evaluation of membranes and air gaps as limiting resistances for dosimeters which contain solid adsorbents, should include air flow conditions, especially if the maximum sensitivity is required for the dosimeter. With the membrane in place, the dosimeter is 450 wm thick but in some applications a light shield may be required. Present Studies. The device is currently being tested with t h e new plates. A carbon-14 labeled reagent is also being studied so that the product radical may be measured by liquid scintillation counting, a procedure which we believe will be more convenient than electron spin resonance.
LITERATURE CITED (1) J. S. Nadeau and D.G. B. Boocock, Anal. Chem., 49, 1672 (1977). (2) L. H. Nelms, K. D. Reiszner, and P. W West, Anal. Chem., 49, 994 (1977). ( 3 ) K. D. Reiszner and P. W. West, fnviron Sci. Techno/.,7. 526 (1973). (4) F. C. Tompkins and R. L. Goldsmith. " A new personnel dosimeter for the monitoring of industrial pollutants", paper presented at the AiH conference, Atlanta. Ga.. May 1976. (5) E. D. Palmes, A. F. Gunnison, J. DiMattio, and C. Tomczyk, Am. I n d . Hyg. Assoc. J . . 37,570 (1976).
RECEIVED for review May 3, 1978. Accepted August 7, 1978. Work supported by Grants from the National Research Council of Canada and a scholarship from the McAllister Foundation of the University of Toronto.
