Environ. Sci. Technol. 1985, 19. 361-364
NOTES Tests for Fluorocarbon and Other Organic Vapor Release by Fluorocarbon Film Bags Nelson A. Kelly,+t Kelth L. Olson,* and Curtls A. Wongt
Environmental Science and Analytical Chemistry Departments, General Motors Research Laboratories, Warren, Michigan 48090
rn Tests were performed on several bags made from fluorinated ethene-propene copolymer film, commonly referred to as FEP-Teflon, to see if they release fluorocarbon vapors, as was recently reported by others. Special attention was given to determining if tetrafluoroethene and hexafluoropropene, the monomer units used to synthesize the film, were released. By use of an instrument that measured total gas-phase carbon, it was determined that at most 0.06 ppm of C of non-methane organic contamination was released into bags of clean air stored outdoors for up to 2 days. A more sophisticated gas chromatography/mass spectrometry technique was used to confirm that neither of the two precursor fluorocarbons was released into the bag at concentrations above the detection limit of -0.002 ppm for each compound. The latter conclusion applied to bags stored at -25 OC in a room as well as one stored in an irradiated chamber at 30-40 OC. These findings are contrary to those recently reported by another group who found large releases of fluorocarbon contaminants, especially hexafluoropropene, from similar bags. We conclude that not all FEP-Teflon film releases fluorocarbon vapors.
Introduction Plastic bags made from fluorocarbon polymers have been widely used in air pollution research (1-8). Over the years many virtues and pitfalls for a variety of different films have been investigated by air pollution researchers who employed such bags for the storage of air samples and calibration standards, as well as the irradiation of smog producing mixtures. For the latter purpose, the film most widely used is made of fluorinated ethene-propene copolymer (FEP-Teflon, E. I. du Pont de Nemours & co., Inc., Wilmington DE), since it is inert, transparent, and photochemically stable (7). Even this material, however, has shortcomings that must be considered by scientists using it in smog chamber studies. We have observed that both hydrocarbons and nitrogen oxides contaminated air samples stored in FEP-Teflon bags (9). Although we believe that the source of such contaminants was permeation into the bags from outside and that permeability is a characteristic general to all FEP-Teflon film, such problems can be surmounted by simple conditioning procedures (7). Recently, a potentially very serious drawback to the use of FEP-Tefon film for either storage containers or smog chambers was reported by Lonneman and co-workers (10). They observed that large amounts of several low molecular weight fluorocarbons were released by FEP-Teflon. The main contaminant was hexafluoropropene, one of the Environmental Science Department.
* Analytical Chemistry Department. 0013-936X/85/0919-0361$01.50/0
monomer units in FEP-Teflon synthesis. The emitted fluorocarbon vapors also caused ozone formation when irradiated in the presence of NO,. Although Lonneman et al. speculated that the fluorocarbon emissions from bags might be relatively inefficient as ozone producers, i.e., they state that halogenation of ethene and propene reduces the reaction rate with OH and 03, there is some evidence to the contrary. For example, C2F3C1,the compound most similar to C2F4 for which the reaction rate with OH has been measured (the C2F4-OHand C3F6-OHreaction rates have not been reported yet), reacts with OH faster than does C2H4(11). Furthermore, the peroxy radicals produced in air following OH attack on olefinic fluorocarbons may be even more efficient in converting NO to NO2than their hydrocarbon counterparts. By use of the one fluorinated peroxy radical that has been studied, as an example, CF302,its reaction with NO is twice as fast as CH3O2 (12). These simple analogies lead us to speculate that if fluorocarbons are released by FEP-Teflon, this is a potentially serious problem for researchers who wish to use Teflon reactors to study free-radical chemistry. Previous work in our laboratory concerning the contamination of FEP-Teflon bags showed that the bags were contaminated by a myriad of hydrocarbons (9). From our previous data, however, we could not be certain that some of the contaminants were not fluorocarbons for two reasons. First, contaminants were not identified beyond noting that they were organic compounds that appeared as peaks on a GC column in the range where C5-Cl0 hydrocarbons eluted. Second, fluorocarbons could have been present but not detected due to their poor response in a flame ionization detector. Therefore, it was necessary to test our bags for fluorocarbon release to see if the contamination reported by Lonneman et al. was present in our experiments. The tests were designed to determine the total gas-phase carbon released by the film, as well as the concentration of the two fluorocarbons, tetrafluoroethene and hexafluoropropene, i.e., the two monomers used to make FEP-Teflon film.
Experimental Section Eight 450-L bags, one 50-L bag, and one 2ooO-L bag were used for the experiments. All were made of FEP-Teflon, type A. The 450-L bags and the 2000-L bag were purchased from Livingstone Coatings Corporation, Charlotte, NC; the 50-L bag was purchased from Alltech Associates, Deerfield, IL. The heat seals on the Livingstone bags were made by raising the temperature of the film to its melting point (260-280 "C) for as short a time as possible while still yielding an adequate seal. Seven of the 450-Lbags were made from 0.05-mm thick film while one 450-L bag, the 2ooO-L bag, and the 50-L bag were made from 0.12-mm film. All the 450-L bags as well as the 2000-L bag were
0 1985 American Chemical Society
Environ. Sci. Technol., Vol. 19, No. 4, 1985 361
fitted with FEP-Teflon valves for adding and removing gas samples, as described previously (7). For purposes of identification, bags 1,2,4, and 6-9 had volumes of 450 L and were made of 0.05-mm film, bag 3 was a 2000-L bag made of 0.12 mm film, bag 5 was a 450-L bag made of 0.12-mm film, and bag 10 was a 50-L bag made of 0.12-mm film. Two types of experiments were conducted. In the first type, bags 1-6 were flushed once (with -30 L) and then filled with hydrocarbon-free air (zero air) from Scott Speciality Gases, Troy, MI; they were stored outdoors during Aug 1981at a site in downtown Detroit, MI. A field study designed to measure the ozone formation properties of Detroit's air was in progress at this site. The bags were suspended -3 f t above the ground in a field behind the Detroit Science Center which is -3 km north of the central business district. Measurements of total hydrocarbons (THC), methane (CH,), and non-methane organic compounds (NMOC) were made in the bags after the zero air was stored for various periods of time. A Byron Model 401 total organic analyzer was used. This analyzer uses a flame ionization detector (FID) to measure THC and CH4. Unfortunately, the THC measurement is prone to error because different organic compounds have different response factors in a FID. For example, hexafluoropropene and dichlorodifluoromethane gave responses of 0.65 and 0.10 ppm of C relative to methane (1ppm of C). However, NMOC were measured on the Byron analyzer by chromatographically separating them from CHI, oxidizing them to C02with a cupric oxide catalyst heated to 700 "C, and then catalytically converting the COzto CHI to give a ppm of carbon (ppm of C) response. The detection limits for this instrument were -0.05 ppm of C for all three components, THC, CHI, and NMOC. A second type of experiment, in which specific tests for hexafluoropropene and tetrafluoroethene were conducted, used bags 7-10. Bag 7 was used to prepare a standard containing 0.2 ppm of both hexafluoropropene and tetrafluoroethene; i.e., this bag was not checked for fluorocarbon contamination but rather was used to develop the experimental technique as we will discuss later. The hexafluoropropene and tetrafluoroethene were obtained from PCR Research Chemicals (Gainsville, FL) and were injected into bag 7 with gas-tight syringes as it was being filled with zero air. Bags 8-10 were tested for the presence of fluorocarbon vapors, but slightly different filling and/or handling procedures were used on each one prior to the analysis. Bag 8 was the only one of the three to be preflushed with -30 L of zero air before it was filled and stored. Bag 9 was irradiated in an indoor smog chamber for 8 h after it was filled and before it was tested for contaminants. The first-order photolysis coefficient for NOz in the smog chamber (this is a measure of the light intensity) was -0.2 min-l. Bag 10 was simply filled with zero air and stored in the laboratory. However, after it was checked for gaseous fluorocarbon contaminants, bag 10 was used to make up 0.01- and 0.2-ppm fluorocarbon standards in order to check the sampling protocol, which we will describe next. Two different sampling protocols were used, and since both yielded useful data, they will be described. In the initial method, used on bags 7 and 8, gases were sampled by drawing air from each bag into a 30 cm X 3 mm 0.d. loop of stainless steel tubing immersed in liquid oxygen. Two liters of air was pumped through the stainless steel trap at 1 L/min with a Bendix Model 44 air sampling pump, An improved sampling technique was used on bags 9 and 10. For this technique, gases were sampled by 362 Environ. Sci. Technol., Vol. 19, No. 4, 1985
Table I. Increases in THC, CH4, and NMOC (As Measured with the Byron Analyzer) in Bags of Zero Air Stored Outdoors for Various Times storage time before bag ATHC, ACH,, ANMOC, bag" analysis,bh conditionc ppm of C ppm ppm of C 1 2 3 4 5 6
4 6 6 7 7 47
new used used new new used
0.07
