Correspondence Comment on “Problems Associated with Using Filtration to Define Dissolved Trace Element Concentrations in Natural Water Samples” SIR: The conventional operational distinction between the dissolved and particulate phase in natural waters is based on filtration using 0.4- or 0.45-µm filters. Because the suspended particle distribution can include a substantial amount of material less than 0.4 µm in diameter (1, 2) and because these fine colloidal particles can have substantial concentrations of certain trace elements (3), it is clear that the conventionally defined operational dissolved phase does not properly represent the truly dissolved concentrations of some elements. Accurate determination of the truly dissolved concentrations of trace elements is important in part because it is in the dissolved phase that these elements are most available to the biota, either as trace nutrients or toxic contaminants. A number of workers have explored artifacts in the determination of dissolved trace elements associated with the method of filtration (4-6). Most recently, Horowitz et al. (7) have nicely demonstrated that different filtered trace element concentrations can be obtained from the same water sample even when using different types of filters having the same nominal pore size (0.4/0.45 µm). In particular, their results emphasize that progressive clogging of a filter can reduce its effective pore size, thereby resulting in progressively changing concentrations in the filtrate. Horowitz et al. (7) suggest that there are three viable options for dealing with filtration artifacts: (1) the use of very high surface area filters so that the filtrate represents a well-defined physical separation based on the nominal pore size of the filter, (2) the use of a sample pretreatment such as centrifugation to limit filtration artifacts, and (3) the exclusion of the colloidal phase from the filtrate. They conclude that “it is probably far simpler and more effective to specify the use of large surface area membrane filters than to either pretreat samples or to try to remove the majority of the colloidal material prior to filtration”. Indeed, they note that this strategy forms the basis of new USGS parts-per-billion protocols for dissolved trace elements (8). We do not disagree that the large filter area strategy advocated by Horowitz et al. (7) and tentatively adopted by the USGS Office of Water Quality is simple. However, this strategy is short-sighted for the simple reason that its result has little meaning from either physical-chemical, toxicological, or chemical transport viewpoints. Additionally, the authors fail to note that, even if all workers used this filtration method, there could still be differences in the analytical result due to differences in the ways that various storage and analysis techniques will solubilize and determine the colloidal fraction. * Corresponding author e-mail address:
[email protected].
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A further problem with the high surface area strategy advocated by Horowitz et al. (7) and tentatively adopted by the USGS (8) is revealed by an examination of the capsule filtration data presented in their Tables 2 and 3. These data show that the filtrate concentration changes substantially with volume filtered. This is an understandable consequence of clogging that occurs even with high surface area cartridge filters. Thus, in order for all workers to obtain similar results, there will be a need for an agreement on a standardized filtration volume. Presumably, such a standardized volume would need to be large enough so that researchers requiring large volumes would not need to switch filters too frequently but also small enough so that researchers requiring small volumes need not waste time filtering large unneeded volumes. Such a volume standard would further emphasize the arbitrary nature of the cartridge filter result. It should also be recognized that most trace element researchers rinse their filters and filtration apparatus with sample prior to collecting filtrate. This rinsing procedure helps assure that any residual fluids from cleaning the filter and apparatus do not contaminate the sample and that the filter and apparatus are at least briefly equilibrated with the sample. With high surface area cartridge filters, a substantial rinsing with the sample may be necessary. Indeed, the extremely high zinc values Horowitz et al. (7) report for their first aliquots from Gelman capsule filters could well be indicative of the rinsing out of residual contamination. Part of the reason for Horowitz et al. (7) advocating the simple high surface area filtration method appears to stem from their concern for the complexity and expense of excluding the colloidal fraction from the dissolved phase. However, there may be simple methods available that can yield reliable determinations of dissolved phase trace element concentrations. Specifically, Taylor and Shiller (9) discuss the method of exhaustive filtration whereby a low surface area polycarbonate screen filter is first clogged by the sample before filtrate is collected. They demonstrated that, for iron in the Mississippi River and some of its tributaries, exhaustive filtration yields results comparable to tangential flow ultrafiltration. It is true, as pointed out by Horowitz et al. (7), that exhaustive filtration is a somewhat subjective procedure because it does not provide a welldefined filter pore size. However, if enough of the colloidal trace elements are excluded by exhaustive filtration so that colloids no longer substantially contribute to measured filtrate concentrations, then the procedure will provide a reasonable representation of the dissolved phase. We note that many trace element researchers do use low surface area polycarbonate screen membrane filters and do rinse their apparatus with sample prior to collection of filtrate. That these workers have obtained reproducible and geochemically interpretable results for a variety of dissolved trace elements in world rivers (10-14) lends credence to
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the exhaustive filtration approach. Clearly though, more work should be done to compare the results of exhaustive filtration with those of ultrafiltration so as to better establish the circumstances under which this methodology does or does not provide accurate dissolved phase measurements.
Literature Cited (1) Buffle, J.; Leppard, G. G. Environ. Sci. Technol. 1995, 29, 2169. (2) Buffle, J.; Leppard, G. G. Environ. Sci. Technol. 1995, 29, 2176. (3) Benoit, G.; Oktay-Marshall, S. D.; Cantu, A., II; Hood, E. M.; Coleman, C. H.; Corapcioglu, M. O.; Santschi, P. Mar. Chem. 1994, 45, 307. (4) Kennedy, V. C.; Zellweger, G. W.; Jones, B. F. Water Resour. Res. 1974, 10, 785. (5) Laxen, D. P. H.; Chandler, I. M. Anal. Chem. 1982, 54, 1350. (6) Horowitz, A. J.; Elrick, K. A.; Colberg, M. R. Water Res. 1992, 26, 753. (7) Horowitz, A. J.; Lum, K. R.; Garbarino, J. R.; Hall, G. E. M.; Lemieux, C.; Demas, C. R. Environ. Sci. Technol. 1996, 30, 954. (8) Horowitz, A. J.; Demas, C. R.; Fitzgerald, K. K.; Miller, T. L.; Rickert, D. A. Open-File Rep.sU.S. Geol. Surv. 1994, No. 94539. (9) Taylor, H. R.; Shiller, A. M. Environ. Sci. Technol. 1995, 29, 1313. (10) Boyle, E. A. In Copper in the Environment, Part I; Nriagu, J. O., Ed.; John Wiley: New York, 1979. (11) Measures, C. I.; Edmond, J. M. Earth Planet. Sci. Lett. 1983, 66, 101. (12) Shiller, A. M.; Boyle, E. A. Nature 1985, 317, 49. (13) Yee, H. S.; Measures, C. I.; Edmond, J. M. Nature 1987, 326, 686. (14) Palmer, M. R.; Edmond, J. M. Geochim. Cosmochim. Acta 1993, 57, 4947.
Alan M. Shiller* Institute of Marine Sciences University of Southern Mississippi Stennis Space Center, Mississippi 39529
Howard E. Taylor U.S. Geological Survey 3215 Marine Street Boulder, Colorado 80303 ES960387Z
Response to Comments on “Problems Associated with Using Filtration To Define Dissolved Trace Element Concentrations in Natural Water Samples” SIR: Shiller and Taylor do not appear to take issue so much with the scientific content of Horowitz et al. (1) as with one conclusion and particularly the new U.S. Geological Survey (USGS) protocol for inorganic constituents in filtered water (2). This is really a policy rather than a scientific issue. The USGS engages in two types of studies: (1) routine monitoring and specific projects carried out in cooperation/ collaboration with local, State, and Federal agencies or (2) basic/applied research. The current regulatory definition * Corresponding author e-mail address:
[email protected]. † Present address: I.U.C.N.-World Conservation Union, 380 St. Antoine, W., Suite 3200, Montreal, Quebec H2Y 3X7, Canada. ‡ Present address: Multisources, 2875 Rue Holt, Montreal, Quebec H1Y 1P7, Canada.
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for dissolved constituents also is the almost universally accepted operational onesmaterials passing a 0.45-µm membrane filter. This is a major issue when USGS ‘clients’ and other government agencies, including Environment Canada, must produce data that meet that “regulatory definition”. The new USGS protocol was designed to (1) meet the regulatory definition of dissolved constituents, (2) generate consistent and comparable data while minimizing filtration artifacts (1), and (3) minimize potential contamination. This protocol is now standard operating procedure for all USGS studies addressing regulatory issues. In non-regulatory-related studies, USGS personnel may select any procedure deemed appropriate, such as that used by Taylor and Shiller (3). The current operational/regulatory definition of ‘dissolved’ probably has little physicochemical/thermodynamic meaning. However, as long as the environmental/regulatory community opts to define dissolved constituents on the basis of a physical separation (e.g., filtration), this problem will remain (see ref 1). Evaluations of operationally defined dissolved concentrations against toxicity do not appear to exist; however, current regulatory limits are surrogates for inferred toxicity because they are supposed to be based on the statistical treatment of toxicological data. As such, why is one operational definition of dissolved more appropriate for inferring or predicting toxicity than another? Finally, fluxes in fluvial systems for artifactaffected elements tend to be dominated by suspended particulate matter-associated constituents; whereas most dissolved contributions for the same elements are minimal (4, 5). Hence, one operational definition of dissolved should be as appropriate as another. As noted, even capsule filters are subject to clogging, with concomitant decreases in filtrate chemical concentrations for certain constituents (1). The new USGS protocol deals with this issue; it lists specific volumes (the minimum necessary to meet analytical requirements) and a specific order for the removal of various filtrate aliquots for subsequent chemical analyses so that consistent and comparable data can be produced (2). A 250-mL filtrate aliquot is collected first (the initial 25 mL is used to rinse the system and sample bottle and is discarded) and is used for the determination of artifact-affected constituents. Subsequent aliquots are used for the determination of nonartifact-affected constituents. Shiller and Taylor question the cleanliness of capsule filters and, presumably, the procedures used to clean/ condition them and cite the ‘high’ Zn values reported in Horowitz et al. (1) as proof. However, capsule filters initially were used by Windom et al. to determine sub-microgram per liter trace element concentrations in several U.S. rivers (5). Furthermore, the USGS extensively evaluated capsule filters prior to selecting them. Zn and other blank concentrations were low (Table 1). Similar results were found during other studies (1; Table 1). Although the new USGS protocol originally was to be used at the g1 µg/L level, field blanks indicate it is effective well below that range (Table 1). Horowitz et al. warned that using high surface area filters likely would raise some trace element concentrations, not as a result of contamination but due to the inclusion of greater amounts of trace element-rich colloidal material in the filtrates (1). ‘Elevated’ Zn concentrations, among others, are the result. The new USGS protocol incorporates extensive quality control procedures, including field blanks, to monitor potential contamination
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a Unpublished data, A. J. Horowitz, USGS, Atlanta, GA; analyses of 500 mL DIW conditioning solutions by the USGS Branch of Analytical Services, Arvada, CO. b Unpublished data, A. J. Horowitz, USGS, Atlanta, GA; analyses by the USGS Branch of Analytical Services, Arvada, CO. c Unpublished data for the lab studies in Horowitz et al., 1996; analyses by the USGS Branch of Analytical Services, Arvada, CO, and Geological Survey of Canada, Ottawa, Canada. d Unpublished data, Scott Phillips, USGS, Towson, MD; analyses by Howard Taylor, USGS, Arvada, CO. e Samples processed by USGS, Massachusetts District Office, Marlborough, MA; analyses by Martin Shafer, Water Chemistry Program, University of Wisconsin, Madison, WI. f Samples processed by USGS, Virginia District Office, Charlottesville, VA; analyses by the USGS Branch of Analytical Services, Arvada, CO. g Samples processed by USGS, Virginia District Office, Charlottesville, VA; analyses by Texas A&M University, College Station, TX.
