Environ. Sci. Technol. 1992,26, 1666-1667
CORRESPONDENCE Comment on "Formic and Acetic Acids in Coastal North Carolina Rainwater"
(3) Cragin, J. H. Anal. Chin. Acta 1979, 110, 313.
SIR: In a recent study of formic and acetic acids in North American rainwater, Avery et al. (1)implied that, during sample storage, the vessel material serves as a source of contamination. However, another potential source of these two acids, ambient air, was not addressed. An earlier study (2) performed in our laboratory identified vapor transfer of weak organic acids from ambient air as the major source of formic and acetic acid contamination in aqueous solutions when exposed intermittently. Our study involved storing samples of rain, melted snow, and deionized water in various open and sealed containers (polyethylene, polypropylene, Pyrex glass, and quartz) within a class 10 clean bench. Additionally, some samples were exposed to UV radiation to prevent biological activity. Results eliminated both container material and biological activity as major sources of these two acids and identified the contamination mechanism to be vapor absorption. The contamination rate in our laboratory for these two weak acids was 0.02-0.1 nM cm-2 h-' (2). Avery et al. (1)also observed significant concentration increases of formic and acetic acids in deionized water samples preserved with CHC1, and stored for 1month in a variety of plastic containers. Unfortunately, they give only a brief description of their storage and sampling procedures, so that comparison of their contamination rates with ours is not straightforward. We assume that all of their containers had the same shape (6-cm diameter) and volume (250 mL) and that the sample volume change between the initial (three-quarters full) and the final (half full after 1-month storage) analyses resulted from either the removal of several aliquots or evaporation caused by poorly fitting caps. Regardless of how air was exchanged, the rate of acetate and formate contamination in their laboratory was between 0.02 and 0.8 nM cm-2 h-l, which is of the same magnitude as observed in ours. Results of Avery et al. offer further support of the vapor transport mechanism by showing that refrigeration reduces the contamination rate significantly regardless of container material (1).This stands to reason because vapor pressures are lower at lower temperatures and the stagnant environment in an airtight refrigerator limits vapor flux. Avery et al. also observed a significant difference in contamination rates between old and new HDPE and LDPE bottles left out in the laboratory. This suggests that plastics may be pervious to acetic and formic acid vapors, as is the case with Hg (3), and the age of a bottle may influence the rate at which the gases diffuse through the container wall. Nonetheless, we feel that both our study and that of Avery et al. show vapor transport to be a major mechanism of sample contamination for formic and acetic acids. Registry No. HC(O)OH, 64-18-6; CH,C(O)OH, 64-19-7; HzO,
Cold Regions Research and Engineering Laboratory Hanover, New Hampshire 03755-1290
7732-18-5.
Literature Cited (1) Avery, G. B., Jr.; Willey, J. D.; Wilson, C. A. Enuiron. Sei.
Technol. 1991,25, 1875. (2) Hewitt, A. D.; Cragin, J. H. Atmos. Enuiron. 1991, %A, 453. 1666 Envlron. Sci. Technol., Vol. 26, No. 8, 1992
Alan D. Hewitt," James H. Cragin
SIR: The comments by Hewitt and Cragin and their recent paper (I) correctly point out the need for minimizing exposure of rain samples to laboratory air prior to analysis for formic and acetic acids. Contamination of our samples by laboratory air, however, cannot explain our observations and at most must be a minor contribution to the concentrations of formic and acetic acids in both our rain samples and storage experiments (2). The contamination rates from laboratory air reported in Hewitt and Cragin (1) are much greater than we observed, presumably because our containers were tightly sealed and theirs were open to the atmosphere. In our storage experiment [Table I in Avery et al. ( 2 ) ] , concentrations of formic and acetic acids were measured after 30 days in each container (Z), and at weekly intervals in some containers (3), which accounted for the volume change reported. The variables investigated were as follows: composition of bottle (HDPE, LDPE, Nylon), prior bottle use (new, rainwater storage only, old), storage location (laboratory, refrigerator, outside), and presence or absence of the preservative chloroform. The lowest average concentrations of formic (0.32 pM) and acetic (0.20 p M ) acids after 1 month of storage were obtained in HDPE bottles (new, used, or old; 60-, 125-, or 500-mL volume capacity) stored in the laboratory or refrigerator, without chloroform. These low concentrations may have resulted from exposure to laboratory air; any concentrations above this must come from other sources because all bottles were stored in the same laboratory. If it is assumed that these quantities of formic and acetic acids came from laboratory air permeating through plastic bottles that were half full of solution, and that the rate was constant over the 30-day time period [consistent with data presented in Avery (3)], then the rate of contamination in our experiments for the sum of formic plus acetic acids was between 0.000 23 and 0.00046 nmol cm-2h-l. This is obviously much lower than the comparable rate of up to 0.14 nmol cm-2 h-' reported by Hewitt and Cragin (1) in their open container experiments. The rates reported in the preceding comment are difficult to compare because the volume necessary to convert from nanomolar to nanomoles is not specified. Rainwater analyses were completed within 3 days (2) in contrast to the 30 days used in the storage experiment. Assuming linear contribution over time (3) and containers half full, our laboratory air could contribute less than 1% of the formic and acetic acids to rain samples with the average composition reported in ref 2. Deionized water stored for several days in the rainwater laboratory in HDPE carboys with loose lids was routinely analyzed along with samples, and no formic or acetic acids were detected, which further indicates limited addition from laboratory air. Rainwater samples collected from the same event and within a few meters of each other in different containers at times had very different formic and acetic acid concentrations, which strongly suggests container effects (3).
0013-936X/92/0926-1666$03.00/0
0 1992 American Chemical Soclety
The focus of the storage experiment (2) was not the small numbers obtained, which may reflect some contribution from air, but rather the very large concentrations sometimes obtained when rainwater samples were stored in bottles that were exposed to sunlight and/or chloroform. The argument presented by Hewitt and Cragin in their comment predicts that lower concentrations of formic and acetic acids should be found in samples stored outside compared with samples stored in a laboratory. In our experiment, samples stored outside had the highest concentrations, up to 550 kM formic acid and 120 pM acetic acid, which we attribute to contribution from plastic degraded by sunlight and/or chloroform. We hope this information will be useful to other investigators planning or conducting experiments outside in direct sunlight. The molar ratio of formic to acetic acid contributed to solutions from laboratory air was approximately 0.9 in the Hewitt and Cragin study (1). Three of the seven rainwater samples reported ( I ) also had molar ratios of formic to acetic acid less than 1. These ratios are lower than most other reported values (12) for rainwater (2-7). A rainwater study conducted in Hampton, Va, found average formic to acetic acid ratios of 1.4 in the growing season and 1.1 in the nongrowing season; these low ratios were attributed to a local, undefined source of acetic acid (8). Hewitt and Cragin should therefore consider the possibility of a local source of acetic acid in their geographical region, to assess whether their rates of contamination are representative. The recommended rainwater storage procedure, which is refrigerated storage in Pyrex containers (2) with chloroform (9) or tetrachloromercurate ( 4 ) as a preservative and analysis within days, is consistent with the experimental results of both Hewitt and Cragin (1) and Avery et al. (2). Note Added in Proof. We recently completed replication of some of Hewitt and Cragin's experiments (I)in our laboratory. We set out 100 or 200 mL of deionized water in open Pyrex containers (diameter 18.0 cm, height 9.5 cm) for 24 h and then analyzed the water for formic and acetic acids. Nine determinations were done in three adjoining laboratories, and three determinations were done outside the building. The indoor rates of contamination were variable from location to location and between the 2 days on which the experiments were run. Indoor rates of contamination for the sum of formic plus acetic acids varied from nondetectable (
