Environ. Sci. Technol. 1984, 18, 978-981
On the other hand, acetone has a number of drawbacks as a solvent. The most serious is its light sensitivity; the free radicals formed photolytically can react with PAH in the sample and lead to the loss of PAH and the formation of new compounds not initially present in the water sample. In addition, evaporation is slow even at room temperature, and it is not as good a solvent for PAH as methylene chloride. The highest boiling point solvent tested was acetonitrile. Although recovery factors are generally good with this solvent, it is difficult to volatilize and has a tendency to mask lower boiling PAH during gas chromatographic analysis. As can be seen from the data for the Snyder column reduction, the solvent peak substantially obscured the naphthalene peak so that its peak height could not be compared with the peak height of the standard solution. In conclusion, we believe that good recoveries are possible with most of the apparatus we tested. The most important factors influencing good recoveries are the analyst’s experience with a particular method and a particular solvent and the amount of time that the analyst can afford to spend on each solvent reduction. Registry No. Naphthalene, 91-20-3; fluorene, 86-73-7; pyrene, 129-00-0;chrysene, 218-01-9;7,12-dimethylbenz[a]anthracene, 57-97-6;benzo[a]pyrene, 50-32-8;perylene, 198-55-0;water, 7732-18-5.
Literature C i t e d (1) Junk, G. A.; Richard, J. J.; Grieser, M. D.; Wittiak, D.; Wihak, J. L.; Arguello, M. D.; Vick, R.; Svec, H. J.; Fritz, J. S.; Calder, G. V. J . Chromatogr. 1974,99, 745-762. (2) Longbottom, J. E.; Lichtenburg, J. J., Eds. “Methods for Organic Chemical Analysis of Municipal and Industrial Wastewater”; U.S. Environmental Protection Agency, Environmental Monitoring and Support Laboratory: Cincinnati, OH, 1982. (3) Shinohara, R.;Koga, M.; Shinohara, J.; Hori, T. Bunseki Kagaku 1977,26(12),856-864. (4) Murray, D. J. J . Chromatogr. 1979,177, 135-140. (5) Grob, K.; Grob, K., Jr.; Grob, G. J . Chromatogr. 1975,106,
299-315. (6) Smith, S.R.;Cordes, H. F.; Johnson, J. H. Naval Weapons Center, China Lake, CA, 1978,NWC T M 3279. (7) Higgins, C. E.; Guerin, M. R. Anal. Chem. 1980, 52, 1984-1987. (8) Mieure, J. P.; Dietrich, M. W. J . Chromatogr. Sci. 1973, 11, 559-570. Received for review January 13, 1983. Revised manuscript received June 18,1984. Accepted June 28,1984. T h e work upon which this publication is based was supported in part by funds provided by the office of Water Research and Technology (Project A-084-CT), U.S. Department of the interior, Washington, DC, as authorized by the Water Research and Development Act of 1978 (P. L. 95-467).
Evaluation of Ultrafiltration for Determining Molecular Weight of Fulvic Acid George R. Alken US. Geological Survey, Box 25046, MS 407, Federal Center, Denver. Colorado 80225
Two commonly used ultrafiltration membranes are evaluated for the determination of molecular weights of humic substances. Polyacrylic acids of M , 2000 and 5000 and two well-characterized fulvic acids are used as standards. Molecular size characteristics of standards, as determined by small-angle X-ray scattering, are presented. Great care in evaluating molecular weight data obtained by ultrafiltration is needed because of broad nominal cutoffs and membrane-solute interactions. Introduction
Although molecular weight fractionation data of humic substances are very valuable, obtaining this information is usually difficult. Each of the many methods available for determining molecular weights has its own set of problems to consider when the data are analyzed. One such method, which has received attention in the last 15 years (1-1 1) ,especially for determining molecular weights of aquatic humic substances, is ultrafiltration. Ultrafiltration actually provides information about molecular size, and results are dependent on molecular configuration and charge. In theory, ultrafiltration is a simple process (12, 13): under hydrostatic pressure, solute molecules smaller than the size cutoff of the membrane are passed, along with solvent, through the micropores of the membranes; larger solutes are retained and concentrated. In practice, however, a number of problems, inherent in ultrafiltration, should be noted: (1)Because the micropores are not uniform in size, the molecular weight cutoffs given by the manufacturer are not as sharp as would be expected. The nominal molecular weight cutoff usually represents the particle size that will be 90% retained (14). 978
Environ. Sci. Technol., Vol. 18, No. 12, 1984
(2) The separation process in ultrafiltration is dependent on both pressure and concentration gradient ( I ) . As the volume of solvent in the cell decreases, the concentration of high molecular weight solutes increases, resulting in a breakthrough of higher molecular weight solutes. Buffle et al. (4) recommend that the filtrate volume never exceed 90% of the initial total volume. (3) The reactivity of humic and fulvic acids with other dissolved species, with colloidal particles, and with each other to form aggregates can seriously affect the separation of these species. Buffle et al. (4) report that the interactions of these materials in solution are serious factors leading to irreproducible results with ultrafiltration. As the filtration of solution proceeds, the composition of species in the filtration cell changes, with an increase in concentration of the retained, high molecular weight material. As the humic substances become more concentrated, the possibilities for interaction also increase. (4)Humic and fulvic acids can also interact with the membrane surface (4). Adsorption of humic substances to the membrane can alter the surface chemistry and result in charge repulsion of organic anions by the membrane. It is apparent that this seemingly simple process is quite complicated. Membrane characteristics, the mechanisms of filtration, and the chemical complexity of humic substances all contribute to the problem. In the past, compounds such as vitamin B-12 and cytochrome c have been used as standards to compare with separations carried out for humic substances (9). These compounds may not be chemically similar enough to humic substances to adequately define membrane performance for humic substances. In this report, the Amicon UM-10 and PM-10 ultrafiltration membranes have been evaluated with a soil fulvic acid, an aquatic fulvic acid, and polyacrylic acids
Not subject to U.S. Copyright. Published 1984 by the American Chemical Society
with molecular weights of 2000 and 5000.
Experimental Section Reagents. The soil fulvic acid used in this study was extracted from the spodic horizon of the Lakewood soil series collected near Wilmington, NC, according to the procedure of Malcolm (15).This material is 51.7% carbon, 3.5% hydrogen, 41.2% oxygen, 1.1% nitrogen, and 1.9% ash. The aquatic fulvic acid was isolated from the Suwannee River near Fargo, GA, by adsorption chromatography onto the methylmethacrylate resin XAD-8 and processed according to the procedure of Thurman and Malcolm (16). This material is 51.3% carbon 4.3% hydrogen, 42.9% oxygen, 0.56% nitrogen, and lo4 70 50 30
19 15 75 85 26/74 60 36/88 3
Literature Cited Manka, J.; Rebhun, M. Water Res. 1982, 16, 399-403. Tuschall, R. L.; Brezonik, P. L. Limnol. Oceanogr. 1980, 25,495-504. Giesey, J. P.; Briese, L. A. Chem. Geol. 1977,20,109-120. Buffle, J.;Deladoey, P.; Haerdi, W. Anal. Chim. Acta 1978, 101, 339-357. Maurer, L. G. Deep-sea Res. 1976, 23, 1059-1064. Schindler, J. E.; Alberts, J. J.; Honick, K. R. Limnol. Oceanogr. 1972, 17, 952-957. Gjessing, E. T. Environ. Sci. Technol. 1970, 4, 437-438. Moore, R. M.; Burton, S. D.; Williams, P. J.; Young, M. L. Geochim. Cosmochim. Acta 1979, 43, 919-926. Ogura, N. Mar. Biol. 1974,24, 305-312. Wilander, A. Schweiz. 2. Hydrol. 1972, 34, 190-200. Brown, M. Mar. Chem. 1975,3, 253-258. Cross, R. A.; Strathmann, H. In “An Introduction to Separation Science”; Karger, B. L.; Snyder, L. R.; Horvath, C., Eds.; Wiley-Interscience: New York, 1973; pp 469-496. Michaels, A. S. In “Progress in Separation and Purification”; Perry, E. S., Ed.; Interscience: New York, 1968; pp 297-333. Swift, R. S. In “Humic Substances: Geochemistry, Isolation, Characterization”; Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P., Eds.; Wiley-Interscience: New York, in press. Malcolm, R. L. J . Res. U.S. Geol. Surv. 1976, 4, 37-40. Thurman, E. M.; Malcolm, R. L. Environ. Sci. Technol. 1981,15,463-466. Wershaw, R. L.; Pinckney, D. J. J . Res. U.S. Geol. Surv. 1973, I , 701-707. Thurman, E. M.; Wershaw, R. L.; Malcolm, R. L.; Pinckney, D. J. Org. Geochem. 1982,4, 27-35.
membrane used UM-10 UM-10 PM-10 PM-10 PM-10 UM-10 UM-10 PM-lO/UM-lO UM-10 PM- 10/UM-10 PM-10
data. Broad, nominal molecular weight cutoffs and solute interactions with membrane surfaces make the analysis of ultrdilitration data for these solutes very difficult. On the basis of the data presented in this paper, the PM-10 membrane appears to be free of interactions with the fulvic acids used in this study and is more suitable than the UM-10 membrane for studies of fulvic acid; however, great care needs to be exercised to avoid drawing erroneous conclusions. A number of other ultrafiltration membranes of various compositions and size cutoffs are available. Before any of these membranes are used for the molecular size fractionation of humic substances, they should be carefully and thoroughly evaluated. Registry No. PM-10, 9061-70-5; UM-10, 92396-46-8; polyacrylic acid (homopolymer), 9003-01-4.
Received for review March 23, 1984. Accepted August 7, 1984.
Yields of Glyoxal and Methylglyoxal from the NO,-Air m- and p-Xylene
Photooxidations of Toluene and
Ernest0 C. Tuazon, Roger Atkinson,” HdlQne Mac Leod, Helnr W. Blermann, Arthur M. Winer, William P. L. Carter, and James N. Pltts, Jr. Statewide Air Pollution Research Center, University of California, Riverside, California 9252 1
rn The yields of the ring cleavage products glyoxal and methylglyoxal from the reactions of OH radicals with toluene and m-and p-xylene in the presence of parts per million concentrations of NO have been determined in 1 atm of air at 298 f 2 K by using in situ long path-length Fourier transform infrared absorption spectroscopy and differential optical absorption spectroscopy. The yields of glyoxal and methylglyoxal derived after correction for their photolysis and reaction with OH radicals were the following respectively: from toluene, 0.111 f 0.013 and 0.146 f 0.014; from rn-xylene, 0.104 f 0.020 and 0.265 f 0.035; from p-xylene, 0.120 f 0.020 and 0.111 f 0.015. These data are important inputs to chemical models of the NO,-air photooxidations of these aromatic hydrocarbons.
pollution of their emissions into the atmosphere. However, despite numerous kinetic, product, mechanistic, and computer modeling studies, the reaction pathways involved in the NO,-air photooxidations of the aromatic hydrocarbons are still incompletely understood (3-6). Kinetic and environmental chamber studies have shown that under atmospheric conditions the sole loss process of the aromatic hydrocarbons is due to reaction with the hydroxyl radical (3,6,7). These OH radical reactions have been shown to proceed via two pathways, namely, H atom abstraction from the substituent alkyl groups and OH radical addition to the aromatic ring (6-12) (taking toluene as an example) CHz
I
Introduction Aromatic hydrocarbons are important constituents of gasoline ( I , 2) and other commerical fuels (2),with gasoline having an aromatic content of -2545% ( I ) . Black et al. ( 1 ) have shown that the aromatic content of the total (tailpipe plus evaporative) hydrocarbon emissions is in the range 10-30%. Thus, a complete knowledge of the atmospheric chemistry of the aromatic hydrocarbons is necessary to assess the impacts on photochemical air 0013-936X/84/0918-0981$01.50/0
0 1984 American Chemical Society
(1)
Environ. Sci. Technol., Vol. 18, No. 12, 1984
981
