study was performed last year (2). Standard EM1 air sampling bags made from ‘I’edlar PVF Film (Du Pont Type no. 200SG40TR) were used. California urban ambient, air samples were kept for 500 h arid the loss in CO concentration was 9%) or less. (The same instrumental methods were used as in the Texas study.) These results are in sharp contrast to the Texas study, which shows a 43% loss of CO in only 100 h for Tedlar bags (type not specified). T h e 5X greater loss in CO in one-fifth the time suggests that there may be effects caused by differences in bag material, manufacturing techniques, preconditioning of the bags, ambient air constituents, methods of hag storage, or other unknowns. Additional work using Tedlar bags can be cited. For example, Lonneman et al. recently reported ( 3 )that Tedlar bags can be used successfully for hydrocarbon sampling when care is taken to prevent sunlight from photooxidizing the sample. (Other references are available on request.) We submit, therefore, that ‘l’edlar air sampling bags can be-and are being----usedas air sample containers when proper precautions are followed in the choice of PVF’ film, heat sealing of the bags, and storage of air samples. Regarding the validity of bag sampling, it must be pointed out t h a t the sequential sampler that was evaluated by the authors, which showed poor correlation with continuous analyzers because it had rubber diaphragm pumps and gun1 rubber tubing, is no loriger on the markrt. T h e EM1 AQS-11, which uses piston pumps and ‘rygon tubing, was not part of this evaluation and does not exhibit the poor performance characteristics of the obsolete unit (see ref 2; other references available on request). AQS-I1 systems have been produced for a wide variety of uses in North America and Europe. Many units are in daily operation collecting carbon monoxide samples; others are used in special studies of more exotic pollutants such as benzene and radon. We submit, therefore, that the validity of the bag sequential sampling technique has been---and is being- estahlished by continuing practical application. EM1 would welcome further evaluations of the AQS-I1 by qualified researchers.
Literature Cited (1) l’olasek, .J. C., Bullin, J . A,, Enuiron. Sci. 7‘whrioi.. 12, 7(?8 12 (1978). ( 2 ) Garharz, tJ., Sperling, R., t’eache, M., “l’inle-Iiite#ratt.d IJrhan
Carbon Monoxide Measurements”, 1977. ( 3 ) I,onneman, W. A , , Seiln, R. I,,, 13ufnlini, .J. ,J,, E t t ~ > i r o Snc i . ?‘t~clrrioi.,12, 45s)- ($3 (1978).
Roger B Sperling Environmental Measurements, Inc. 215 Leidesdorff St. San Francisco, Calif. 941 11
SIH: This is i n reference to the paper on the bag sequential sampling technique for ambient air analysis by cJ. C. Polasek and J. A. Rullin (I.:S&T,12, ’i08-.12(1978)l. T h e authors indicated that when the intakes for the hag samplers and the continuous analyzer were placed s i d by ~ side, CO concentrations from the two methods did not correlate well. T h e authors attributed this to the asymmetry of the sample ports on the sarnpler. However, they preseiitrd no evidence to support this conclusion. When t h e samples were drawn through a (mnnion header, the concentrations agreed t o within f 1 ppni for at least 90% of the data points for CO concentrations of 1 8 ppm. ‘This indicates that the hag sampler and continuous analyxer do indeed indicate the same (:Oconcentrations for the same air 610
Environmental Science & Technology
sample. Therefore, the lack of correlation in the earlier experiment may be due to actual differences in the CO concentrations of the different air samples that were analyzed. T h e experiment in which two continuous analyzers were attached to the cominon header showed that CO concentrations along roadways vary widely and a t frequencies u p to 2 cycles/min. If the CO concentrations show large temporal variations, then. under these conditions, it seems plausible that they would also show large spatial variations. which would give rise t o the effect mentioned above. This could be tested by repeating the first experiment, in which t h e intakes for the bag samplers and continuous analyzer were placed side by side, with the exception that the continuous analyzer is replaced with another bag sampler. If there is an actual difference of the CO concentrations in the samples measured, there would still be no correlation, even though the same measurement method was used for each sample.
Allen Hoffman 273 Henry St. Brooklyn, N.Y. 11201
SIR: T h e letters by Kuntz, Sperling, and Hoffman center on two distinct points, which must be addressed separately. T h e first and most common comment concerned the conclusion that ‘I’edlar hags were unsuitable for use in an air monitoring study. T w o of the letters state that, in separate tests conducted in several different laboratories, results were obtained which contradict those of the Texas study. There are a numher of possihle reasons for this. First and most important is the difference in concentration of the pollutants heing measured. As Ih. Kuntz points out, a material may behave quite differently at different concentrations. Moe (2) also noted this effect. Mr. Sperling mentions a number of other possible factors which could account for the differences. Only two of the unknowns mentioned, hag material and manufacturing techniques, art’ completely under the manufacturer’s control. The material factor was eliminated by a check with the Texas State Department of Highways and I’uhlic Transportation, which showed that the bags used in the Texas study were also type 200 S(X0‘I’K. We have talked with I>uT’ont personnel in their films group at Buffalo, N.Y. (telephone numher 71G-87B4420). They advised us that they recommend only their 200 SCXU‘rH and 200 SG-IO‘I’H films for air sampling work. These two films are their 4- and 1-mil versions o f 99% or greatrr purity PVE’.There are 70 to 80 other types of I’VF wit,h various dyes, stiffeners, thinners. etc. Manufac,turing techniques are a distinct possibility to consider, meaning that tests should be run on any bags obtained from ;in unfamiliar nianufacturer. All reasonable precautions were taken in the Texas study in regard to conditioning and storage of the hags, as detailed in the article. Ambient air constituents are not under anyone’s control. Usually, if the constitwrits can he characterized, there is no need t o collect them. T h e contradictor>-results obtained by other groups highlight the fact that pi,llutant behwvior at ambient levels is a complex subject, Othcr groups (,?, 4 ) have contacted us indicating that they had confirmed our results. It should he pointed out that haviiig hags which give good results for part of‘the range o t intereht are useless f o r monitoring studies unless the reason for failure and range of failure in he precisely pinpoiiittxl. To date this has not been accomplished by
0013-936X/79/09 13-0610$01.OO/O @ 1979 American Chemical Society
