Correspondence Comment on “Evaluation of the Diffusive Gradient in a Thin Film Technique for Monitoring Trace Metal Concentrations in Estaurine Waters” Dunn et al. (1) have shown the application of a passive technique to measure low ambient concentrations of trace metals. They use the terminologysDiffusive Gradient Technique (DGT)sand state that it has generated considerable interest over the past decade. However, I would submit that DGT is the same as “passive dosimetry”, a general method for measuring ambient concentrations that has been used for decades to measure exposure of workers to chemicals in the work place. My colleagues and I applied the passive dosimetry concept to analyze trace organic contaminants in water in the late 1980s and published those results in this Journal (2). Our initial design used a 1-cm diffusion barrier (holes drilled in Plexiglas barrier) behind which we placed GAC as the adsorbent. A second generation of dosimeter was constructed in which we used a Nucleopore filter to shorten the diffusion barrier to 10 µm, thus greatly increasing the sensitivity of the dosimeter (e.g., 10 ppm of hydrocarbons could be measured with an exposure time of 14 h as compared to 120 h for the thicker diffusion barrier) (3). The application of passive dosimetry to measurements of contaminants in water has also found its way into a textbook in our field (4). As to the application of DGT (or passive dosimetry) for environmental monitoring, the authors note correctly that * Corresponding author e-mail:
[email protected].
5268
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 22, 2003
the dynamic nature of estuaries gives DGT an advantage over grab sampling because a “continual time integrated response to the analyte in solution” can be obtained. However, the effect of varying concentration over exposure time on the diffusional flux influences the reporting of the “average” concentration. In effect, diffusional flux shifts from that of simple steady state to the more complex, unsteady state. The passive dosimetry literature in the field of measuring air contaminants addresses this issue (e.g., for the assumption of a sinusoidal variation in concentration), although it should still be considered an open research question.
Literature Cited (1) Dunn, R. J. K.; Teasdale, P. R.; Warnken J.; Schleich, R. R. Environ. Sci. Technol. 2003, 37, 2794-2800. (2) DiGiano, F. A.; Elliot, D.; Leith, D. Environ. Sci. Technol. 1988, 22, 1365-1367. (3) DiGiano, F. A.; Leith, D.; Elliott, D.; Pate, B.; Napfel C. M.; Anthony, T. R. Application of Passive Dosimetry for Detection of Leaking Underground Storage Tanks; Water Resources Research Institute Report 268; The University of North Carolina: Chapel Hill, 1992. (4) Weber, W. J., Jr.; DiGiano, F. A. Process Dynamics in Environmental Systems; Wiley-Interscience: New York, 1996.
F. A. DiGiano* Department of Environmental Sciences and Engineering University of North Carolina Chapel Hill, North Carolina 27599-7431 ES030532Q
10.1021/es030532q CCC: $25.00
2003 American Chemical Society Published on Web 10/22/2003
