Environ. Sci. Technol. 1989, 23, 1246-1249
Interlaboratory Methods Comparison for the Total Organic Carbon Analysis of Aquifer Materials Robert M. Powell'
Northrop Services, Inc., R. S . Kerr Environmental Research Laboratory, P.O. Box 1198, Ada, Oklahoma 74820 Bert E. Bledsoe U S . Environmental Protection Agency, R. S . Kerr Environmental Research Laboratory, Ada, Oklahoma Gary P. Curtls
Stanford University, Stanford, California Richard L. Johnson
Oregon Graduate Center, Beaverton, Oregon The total organic carbon (TOC) content of aquifer materials has been found to have significant effects on the movement of pollutants in the subsurface environment. Accurate quantification of TOC is therefore of great importance to research in groundwater contamination. However, large discrepancies have been observed when laboratories employing different methods determine TOC on the same aquifer material. This study was undertaken to evaluate the extent of these differences and examine their causes. Five subsurface samples were distributed and analyzed for both total carbon and total organic carbon by several instrumental techniques at four laboratories. Results indicate that when the material is adequately prepared with regard to homogeneity and removal of carbonate minerals, reasonably comparable values are obtained for all the instruments used, with the exception of a wet oxidation technique that is insufficiently oxidizing. Trends in subsurface research indicate a need for methods with improved accuracy and precision to lower detection limits and increase reliability. These methods will require standardization of analytical techniques, particularly sample preparation.
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Introduction Numerous methods, both classical and instrumental, exist for the determination of total organic carbon (TOC) in soils (1-3). These methods were, however, developed primarily for use in topsoil analysis, where agricultural suitability of the soil is the primary concern and the value of TOC is relatively high. Since the movement of hydrophobic pollutants such as neutral pesticides through natural sediment is dependent on sorption by the organic carbon of the sediment (4), and especially since there is concern about whether TOC levels of 99% for every sample. Duncan's multiple range test (8)was then applied to the data at the 95% significance level to determine which results might he rejected on a statistical basis. The results from laboratory 2 were found to he significantly different from all other results in every case except sample CS1, where no signifcant difference was found between laboratory 2 and laboratory 1. Table 111 provides the mean TOC and percent RSD values for the samples when the 5% HCI and laboratory 2 analyses are discarded as outliers. Total carbon analyses (Figure 3) resulted in interlaboratory percent RSDs from 24 to 5270, the most similar methods (Leco WR-12 and Leco (3-344) agreeing clasely and the most dissimilar methods (methanator and headspace analysis) at opposite extremes. Due to the
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Figure 4. TOC % RSD correlation to TCITOC.
smaller number of analyses and the offsetting values from laboratories 1and 2, it was impossible to designate any of these analyses as statistical outliers. Although many factors undoubtedly contribute to the analytical variability of TOC analysis, an attempt was made to determine whether any of the obvious possibilities might make a major contribution. Correlations to TOC variability as % RSD, excluding the laboratory 2 and 5% HC1 analyses, were investigated. The factors evaluated were as follows: (1) TOC mean value. Were the variations between laboratories due to instrumental limitations, particularly regarding the low TOC samples where detection limits might be a consideration? (2) TC mean value. Were the TOC variations related to the total carbon quantity? (3) TC/TOC ratio. Since the TC of some samples was almost entirely due to TOC, could a correlation be found with the ratio of the two, indicating an increase in TOC variation with higher inorganic carbon content? Correlations for the three factors were modeled by linear regression (9),and although none of the tested factors can be ruled out, the TC/TOC ratio had by far the most significant correlation of the three tested, with a coefficient of determination of 0.81 and a significance of 96% (Figure 4). This indicates that one can generally expect greater variability of TOC results for samples with high inorganic (carbonate) carbon than for those with low amounts, even though results from samples known to be incompletely digested are discarded. It suggests that the TC/TOC ratio is likely to be more significant than any other factor, including instrumental detection limits, in obtaining accurate quantification for low TOC level materials. It is apparent that improvements in carbonate carbon removal methods could alleviate some of the discrepancies that have been observed between laboratories.
Conclusion With the study sample CS5 as an example, a 1270% relative difference in TOC value could be obtained by two researchers if one performed wet oxidation on his sample while the other used a 5% HC1 digestion followed by high-temperature combustion. Had these been routine samples, the results might have been accepted as correct. Although this study determined the results from laboratory 2 and the 5% HC1 digestions on CS1 and CS5 to be outliers, this was only possible due to the additional data available from the other analyses. The low wet-oxidation results from laboratory 2 indicate that TOC analysis of solid subsurface materials should contain a high-temper-
ature oxidation step until an adequately oxidizing wet technique can be demonstrated. Additionally, the TOC outliers resulting from the 5% HC1 digestions illustrate the need for an acid concentration sufficient to reproducibly and quantitatively remove carbonate minerals. Even with careful preparation of sample splits and the removal of known outliers resulting from incomplete carbonate digestion and inadequate oxidation, sample CS1 shows that TOC values of aquifer materials at the 0.03% level can still be expected to vary by as much as 39% between laboratories using different methods. This variation is strongly correlated to the TC/TOC ratio, with higher ratios resulting in increasing interlaboratory variation. It is necessary to develop and agree on standard methods of sample pretreatment to increase the reliability of TOC analyses. These methods will need to address adequate digestion of the sample, in terms of acid type and purity, concentration,digestion time and temperature, and sample grain size. Aquifer material reference standards with low TOC values should also be developed and analyzed with routine samples for quality control.
Acknowledgments We thank William Ball, Roger Cosby, Marvin Piwoni, Joseph Michalowski, Steve Schmelling, and the staffs of both the US. Environmental Protection Agency and Northrop Services, Inc., at the Robert S. Kerr Environmental Research Laboratory in Ada, OK. Registry No. C, 7440-44-0.
Literature Cited (1) Hesse, P.R. A Textbook of Soil Chemical Analysis, 1st ed.; Chemical Publishing Co., Inc.; New York, 1972; pp 204-210. (2) Allison, L.E. Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties;Black, C. A., Ed; American Society of Agronomy, Inc.; Madison, WI, 1979; pp .. i3wi378. (3) Metson, A. J.; Blakemore, L. C.; Rhoades, D. A. N . 2.J. Sci. 1979, 22, 205-228. (4) Karickhoff, S.W.; Brown, D. S.; Scott, T. A. Water Res. 1979,13,241-24a. (5) Banerjee, P.; Piwoni, M. D.; Ebeid, K. Chemosphere 1985, 14,1057-1067. (6) Schwarzenbach, R. P.;Westall, J. Enuiron. Sci. Technol. 1981,15, 1360-1367. (7) Curtis, G. P.;Reinhard, M.; Roberts, P. V. In Geochemical Processes at Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.; ACS Symposium Series 323; American Chemical Society: Washington, DC, 1986; pp 191-216. (8) Steel, R. G.D.; Torrie, J. H. Principles and Procedures of Statistics; McGraw-Hill Book Co., Inc.: New York, 1960. (9) Rafferty, J.; Norling, R.; Tamaru, R.; McMath, C.; Morganstein, D. Statworks. Statistics with Graphics for the Macintosh; DataMetrics, Inc., Cricket Software: Philadelphia, PA, 1985.
Received for review November 19, 1987. Revised manuscript received November 4,1988. Accepted May 1,1989. Although the research described in this article has been funded wholly or in part by the United States Environmental Protection Agency under Contract 68-03-3315 to Northrop Services, Inc., Environmental Sciences, and under in-house research programs, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency and no official endorsement should be inferred.
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