(9) Jarman, R. T., ibid., 82,352 (1956). (10) Magarvey, R. H., J . Meteorol., 14,182 (1957). (11) Martin, A., Barber, F. R., Atmos. Enuiron., 8,325 (1974). (12) Ford, R. E., Furmidge, C. G. L., “Impact and Spreading of Spray Drops on Foliar Surfaces”, Sci. Chem. Ind. Monograph 25, Symp. on Wetting, pp 417-432,1967. (13) Bikerman, J. J., I n d . Eng. Chem., 13,443 (1941). (14) Cheng, L., I n d . Eng. Chem., Process Des. Deu., in press
(1977). (15) Giffin, E., Muraszew, A., “The Atomization of Liquid Fuels”, p p 2-6, Wiley, New York, N.Y., 1953. ~ (16) Magono, C., J . Meteorol., 1 1 , (1954).
Received for reuiew March 15, 1976. Accepted September 21, 1976.
PAN Measurement in Dry and Humid Atmospheres William A. Lonneman U.S. Environmental Protection Agency, Environmental Sciences Research Laboratory, Research Triangle Park, N.C. 277 1 The effects of water on the gas chromatographic (GC) analysis of peroxyacetyl nitrate (PAN) are not observed in our laboratory studies. A series of PAN calibrations demonstrates the independent nature of water and PAN on our GC systems. On the basis of these results, the water anomaly observed by others is probably unique to their system. A recent publication by Holdren and Rasmussen ( I ) dealt with the effect of moisture on peroxyacetyl nitrate (PAN) analyses. In brief, the authors reported on a series of experiments that were performed with a variety of GC-electron capture systems which demonstrated a reduced response for PAN when samples were a t relative humidities of 30% or lower. They observed as much as a tenfold decrease in PAN response when the relative humidity of the sample air was near 0%. The authors suggest that a possible cause for this water anomaly was a column sample interaction. In the past 10 years, we have performed many GC-electron capture analyses for PAN and PAN-type compounds in both ambient atmospheres and smog chamber irradiation studies of hydrocarbon-nitrogen oxides mixtures. In those studies air samples of PAN were analyzed a t relative humidities ranging from 30 to 100%with no apparent water vapor effect upon the PAN response. The lack of such an effect, however, was more clearly demonstrated during instrument standardization where PAN standards in both humid and dry atmospheres were analyzed with no apparent effect on PAN response. Presented here are the results of three such calibrations.
Experimental and Discussion The GC system for PAN used in our smog chamber studies was a 9 ft X in. o.d. glass column packed with 10% Carbowax 600 on Gas Chrom Z. This length of GC column was used to permit resolution of alkyl nitrate products. Retention times for PAN and water vapor with this system were 8.8 and 23 min, respectively. The GC system was operated a t 30-min intervals to analyze the chamber contents. A typical procedure for standardization was to replace one of the 30-min chamber samples with a series of PAN calibrations. These standardizations were performed by the dynamic dilution of calibrated cylinders of PAN with dry nitrogen. An example of such a calibration is shown in Figure 1. In this case, a 10.2 ppm PAN standard was dynamically diluted by a factor of 170 with dry nitrogen to produce a PAN sample of 60 ppb which was injected onto the GC column. Dry nitrogen or air was the preferred diluent since either eliminated the necessity of waiting for the resolution of a water peak. In this way, PAN standards could be repeated several times during this 30-min time period. Generally, the first PAN standard sample was injected onto the GC immediately after the resolution of the PAN peak from 194
Environmental Science & Technology
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METHYL N I T R A T E
Figure 1. Dynamic calibration for PAN using 10.2 ppm standard cylinder of PAN in dry nitrogen and cylinder of prepurified grade nitrogen, February 2, 1971 Column used was 270 cm X 3.2 mm 0.d. glass column packed with 10% Carbowax 600 on Gas Chrom 2
the previous chamber sample. This permitted a 14-min period for the elution of the first PAN standard peak before the elution of the water peak from the previous chamber sample. A second PAN standard sample was injected onto the GC column immediately after the elution of the first PAN standard peak. A graphic illustration of these standards is shown in Figure 1. In effect, Figure 1shows two identical 60 ppb PAN samples on the GC column, one with water a t approximately 50% relative humidity from the previous chamber sample with the possibility of a water-PAN interaction and one without water and no possibility of a interaction occurring. If PAN and water interactions occurred in the GC system, one would expect to see a definite difference in response between the two PAN standard samples; however, no difference was observed. Similar calibrations were performed many times during a two-year chamber program with PAN concentrations ranging from 25 to 300 ppb with no water anomalies observed. In field study calibrations, the usual procedure for PAN standardization is the syringe injection of an aliquot from a concentrated PAN standard bag into a second bag containing a metered volume of dry air or nitrogen. The concentrated PAN bags are calibrated by IR absorptivities ( 2 ) .For example, in one specific case, a 10-cc aliquot of a 108 ppm PAN sample was injected into a Tedlar (PVF) bag containing 100 1. of dry nitrogen. Since Tedlar is permeable to water from the surrounding atmosphere, the water content inside the bag increased with storage time. Results of two such calibrations are given in Figures 2 and 3. Since the permeation rate of water through Tedlar is 2.8 g/(lOO in.*) (24 h) (mil) a t 39.5 “C ( 3 ) , it takes very little time for the relative humidity in the bag to
L
-PAN
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METHYLNITRATE
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PAN
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Figure 2. PAN standard of 10.6 ppb concentration in dry nitrogen prepared in a 2 mil Tedlar PVF bag, August 21, 1.975 Column used was 90 cm X 3.2 mm 0.d. glass column packed with 10% Carbowax 600 on Gas Chrom 2
METHYL NITRATE
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24 houri
c
c
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Figure 3. PAN standard of 31.0 ppb in dry nitrogen prepared in 2 mil Tedlar PVF bag, August 21, 1975 Column used was 90 cm X 3.2 mm 0.d. glass column packed with 10% Carbowax 600 on gas Chrom Z
approach the relative humidity of the surrounding atmosphere. In these experiments the relative humidity of the surrounding atmosphere was approximately 50%. If the water anomaly occurs at low relative humidities, one would expect a difference in PAN response between the first sample a t less than 10% relative humidity and the third sample at approximately 50??relative humidity. Since this was not observed, we conclude that the water anomaly does not occur in our GC system. In fact, in one instance (Figure 3), the PAN peak decreased approximately 15% after 24 h storage; however, this decrease was due to PAN deterioration on the
surfaces of Tedlar as indicated by a corresponding increase in methyl nitrate-a usual degradation product ( 4 ) . Based on these results and our experiences, we suggest that the anomaly reported by Holdren and Rasmussen ( I ) is probably related to the preparation of the GC-electron capture system. When using columns prepared from either Carbowax 400 or 600, we experienced difficulties whenever the GC substrate was heated to 100 "C for conditioning purposes. At these temperatures the substrate appears to decompose as evidenced by a pungent odor. When this occurred, the PAN response deteriorated, and quantitative elution of PAN was impossible. When the GC substrate was prepared and conditioned at considerably lower temperatures (approximately 40 "C), we detected no odor, and we observed quantitative elution of PAN at good sensitivities (less than 1ppb). On occasions stored substrates at room temperature developed similar pungent odors suggestive of autooxidation; such substrates would not permit quantitative elution of PAN a t good sensitivity. At times, we have been suspicious of column deterioration due to impurities in some of the cylinders of carrier gas used. Our experience also suggests that column tubing material is an important factor. We have used only glass tubing in an effort to assure completely inert surfaces. We have attempted to replace glass with either stainless steel or Teflon for better rigidity and durability, but have not been completely satisfied with these alternatives. Brass and copper surfaces as well as heated inert surfaces seem to cause loss of PAN. For this reason, only glass, Teflon, and stainless steel materials have been used for the sample valve and connecting tubing in the construction of the GC system. Detector temperatures for our systems have been maintained anywhere from 25 to a maximum of 50 "C. At these low temperatures, however, deterioration of detector sensitivity due to deposit buildup occurs more readily; therefore, overnight heating of the detector at 150 "C was frequently performed.
Conclusion The problems observed by Holdren and Rasmussen ( I ) reflect a water vapor effect that is unique to the system used by these investigators. I t is possible that their system has active sites that cause PAN loss. If so, water vapor may deactivate such sites resulting in a greater PAN response. Literature Cited (1) Holdren, M. W., Rasmussen, R. A,, Enuiron. Sci. Technol., 10, 185 (1976). (2) Stephens, E. R., Anal. Chem., 36,928 (1964). (3) Du Pont Technical Bulletin TD-3 on Gas and Vapor Permeabilities of Tedlar film, Wilmington, Del. (4) Stephens, E. R., "Advances in Environmental Science and Technology", Vol 1,p 119, J. N Pitts, Jr., and R. L. Metcalf, Eds., Wiley-Interscience, New York, N. Y., 1969.
Received for review April 19,1976. Accepted September 29, 1976.
Volume 11, Number 2, February 1977
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