Anal. Chem. 1997, 69, 4375-4380
Determination of δ18O and δ15N in Nitrate Kinga Re´ve´sz,*,† J. K. Bo 1 hlke,† and Tadashi Yoshinari‡
U.S. Geological Survey, 431 National Center, Reston, Virginia 20192, and Wadsworth Center, New York State Department of Health, and School of Public Health, State University of New York, Albany, New York 12201
The analyses of both O and N isotopic compositions of nitrate have many potential applications in studies of nitrate sources and reactions in hydrology, oceanography, and atmospheric chemistry, but simple and precise methods for these analyses have yet to be developed. Testing of a new method involving reaction of potassium nitrate with catalyzed graphite (C + Pd + Au) at 520 °C resulted in quantitative recovery of N and O from nitrate as free CO2, K2CO3, and N2. The δ18O values of nitrate reference materials were obtained by analyzing both the CO2 and K2CO3 from catalyzed graphite combustion. Provisional values of δ18OVSMOW for the internationally distributed KNO3 reference materials IAEA-N3 and USGS32 were both equal to +22.7 ( 0.5‰. Because the fraction of free CO2 and the isotopic fractionation factor between CO2 and K2CO3 were constant in the combustion products, the δ18O value of KNO3 could be calculated from measurements of the δ18O of free CO2. Thus, δ18OKNO3 ) aδ18Ofree CO2 - b, where a and b were equal to 0.9967 and 3.3, respectively, for the specific conditions of the experiments. The catalyzed graphite combustion method can be used to determine δ18O of KNO3 from measurements of δ18O of free CO2 with reproducibility on the order of (0.2‰ or better if local reference materials are prepared and analyzed with the samples. Reproducibility of δ15N was (0.1‰ after trace amounts of CO were removed. The nitrogen isotopic ratio (δ15N) has been used extensively as an indicator of the source of nitrate in the hydrosphere1-3 and as a measure of the degree of isotopic fractionation caused by chemical transformations such as denitrification.3,4 However, those applications sometimes can be limited by lack of discrimination or by local variations in the sources, sinks, and isotopic fractionation factors. The δ18O value of nitrate has the potential to resolve some of the ambiguities presented by δ15N data because some sources of oxygen in nitrate are isotopically distinctive and because the fractionation of the O isotopes is proportional to that of the N isotopes during common transformations such as denitrification.5-11 Simple, precise, and automated methods are, †
U.S. Geological Survey. E-mail:
[email protected];
[email protected]. Wadsworth Center and State University of New York. E-mail:
[email protected]. (1) Kreitler, C. W. Rep. Invest. Bur. of Econ. Geol. 1975, 83, 57 pp. (2) Heaton, T. H. E. Water S. Afr. 1986, 11, 199-208. (3) Hu ¨ bner, H. In Handbook of Environmental Chemistry, The Terrestrial Environment B, Fritz, P., Fontes, J. C., Eds.; Elsevier: New York, 1986, Vol. 2, pp 361-425. (4) Mariotti, A.; Landreau, A.; Simon, B. Geochim. Cosmochim. Acta 1988, 52, 1869-1878. ‡
S0003-2700(96)01052-9 CCC: $14.00
© 1997 American Chemical Society
therefore, needed for the analysis of both δ15N and δ18O of nitrate in environmental samples. Previous methods reported for δ18O analysis of N-bearing materials include those of (1) Amberger and Schmidt5 and Voerkelius,13 who prepared KNO3 for δ18O analysis by combustion with Hg(CN)2 at 550 °C, which was a modification of the method of Rittenberg and Ponticorvo12 for organic compounds; (2) Silva and others14 and Wassenaar,10 who prepared AgNO3 for δ18O analysis by combustion with graphite at 900 °C; and (3) Kornexl and others,15 who used an on-line procedure to prepare KNO3 for δ18O and δ15N analysis by combusting it with graphite at 1400 °C to produce N2 and CO2. Our experiments on O isotopic measurements in nitrate were motivated in part by the observations that (1) reported oxygen yields commonly were less than 100%, so the accuracies of the O isotopic values were not assured; (2) reported N isotopic analyses of nitrate samples combusted for O isotopic analysis have not been as precise as conventional N isotopic analysis,16-19 but the reasons were not fully documented; (3) reaction stoichiometries generally were not known or reported in detail; and (4) analyses were not reported for internationally distributed reference materials, so data from different laboratories could not be compared. In this paper, we describe a procedure for measuring δ15N and 18 δ O values of KNO3 by combustion with catalyzed graphite. We also report results from a series of experiments to identify the reaction products and to quantify the reaction stoichiometry. We compare our method with the Hg(CN)2 combustion method.5,8,12,13 Finally, we report provisional δ18O and δ15N values for some internationally distributed reference materials so that future interlaboratory comparisons can be made. (5) Amberger, A.; Schmidt, H. L. Geochim. Cosmochim. Acta 1987, 51, 26992705. (6) Bo ¨ttcher, J.; Strebel, O.; Voerkelius, S.; Schmidt, H. L. J. Hydrol. 1990, 114, 4413-4424. (7) Aravena, R.; Evans, M. L.; Cherry, J. A. Ground Water 1993, 31, 180-186. (8) Durka, W.; Schulze, E.-D.; Gebauer, G.; Voerkelius, S. Nature 1994, 372, (22/29), 765-767. (9) Kendall, C.; Silva, S. R.; Chang, C. C. Y.; Burns, D. A.; Cambell, D. H.; Shanley, J. B. Extended Synopses, International Symposium on Isotopes in Water Resources Management, Vienna, Austria, 20-24 March 1995; International Atomic Agency: Vienna, 1995; p 336. (10) Wassenaar, L. Appl. Geochem. 1995, 10, 391-405. (11) Bo¨hlke, J. K.; Ericksen, G. E.; Re´ve´sz, K. Chem. Geol. 1997, 136, p.153152. (12) Rittenberg, D.; Ponticorvo, L. Appl. Radiat. Isotopes 1956, 1, 208-214. (13) Voerkelius, S. Ph.D. Thesis, Tech. Univ. Munchen, 1990. (14) Silva, S. R.; Kendall, C.; Chang, C. C.; Radyc, J. C.; Wilkison, D. H. EOS, Trans. Am. Geophys. Union 1994, 280. (15) Kornexl, B.; Medina, R.; Schmidt, H. L. Isotopenpraxis Environ. Health Stud. 1994, 30, 215-218. (16) Fiedler, R.; Proksch, G. Anal. Chim. Acta 1972, 60, 277-285. (17) Minagawa, M.; Winter, D. A.; Kaplan, I. R. Anal. Chem. 1984, 56, 18591861. (18) Kendall, C.; Grim, E. Anal. Chem. 1990, 62, 526-529. (19) Boyd, S. R.; Rejou-Michel, A.; Javoy, M. Anal. Chem. 1994, 66, 1394-1402.
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METHOD The technique described herein is a sealed-tube combustion method based on pyrolysis of KNO3 with graphite, represented approximately by the following reaction:
4KNO3 + 5C f 2K2CO3 + 3CO2 + 2N2
(1)
followed by recombustion of the N2 fraction with Cu2O + CaO to remove trace quantities of CO. The isotopic ratios of oxygen and nitrogen in the nitrate were determined by analyzing all three major reaction products: CO2, K2CO3, and N2. Reagents and reaction conditions were chosen on the basis of thermodynamic and kinetic considerations. Theoretical Considerations. The reaction temperature (520 °C) was chosen on the bases of thermodynamic calculations20 and kinetic considerations to optimize decomposition of undesirable solid reaction products (KNO3, KCN, K2O2, K2O, KO2, KOCN), to minimize the production of undesirable gases (CO, N2O, NO, NO2), and to keep the equilibrium isotopic fractionation factors R(CO-CO2), R(NO-CO2), and R(NO-N2) as close to unity as possible.21 Pd and Au catalysts were tested for their effects on reaction kinetics. Gold has been shown to promote decomposition of N2O by Yosinari,22 Hinshelwood and Prichard,23 and Eley and Knights.24 Palladium has been shown to promote oxidation of CO by Ertl and Neumann.25 Stoichiometric conversion of KNO3 to K2CO3, CO2, and N2 would require a C/KNO3 molar ratio of 1.25 (reaction 1); however, experimental results indicate that the conversion was not stoichiometric and that excess graphite was advantageous for improving both oxygen and nitrogen yields. Graphite Combustion: Samples and Reagents. During development of the method, a laboratory KNO3 reference material designated RSIL-N11 (δ15N ) +3.5‰) was analyzed. For reporting purposes, the international isotopic reference materials IAEAN3 (δ15N ) +4.7‰) and USGS-32 (δ15N ) +180‰)26,27 were analyzed. The graphite combustion method was also tested with some natural nitrate samples having a large range of δ18O. The natural samples were from the Atacama Desert, Chile,11 and from shallow groundwaters in central Minnesota and Colorado.28,29 Natural or artificial mixed-salt samples were dissolved in deionized water, from which carbonate and sulfate were removed by acidification and addition of BaCl2 to precipitate BaSO4, which was separated by filtration. The nitrate-bearing filtrates were converted (20) JANAF Thermochemical Tables, 3rd ed.; American Chemical Society and the American Institute of Physics for the National Bureau of Standards: Washington, DC, 1985. (21) Richet, P.; Bottinga, Y.; Javoy, M. Annu. Rev. Earth Planet. Sci. 1977, 5, 65-110. (22) Yoshinari, T. In Denitrification in Soil and Sediment; Revsbech, N. P., Sorensen, J., Eds.; Plenum Press: New York, 1990; pp 129-150. (23) Hinshelwood, C. N.; Prichard, C. R. Proc. R. Soc. London 1925, A108, 211215. (24) Eley, D. D.; Knights, C. F. Proc. R. Soc. London 1966, A294, 1-19. (25) Ertl, G.; Neumann, M. Z. Phys. Chem. Neue Fogle 1974, 90, 127-134. (26) Bo¨hlke, J. K.; Gwinn, C. J.; Coplen, T. B. Geostand. Newsl. 1993, 17, 159164. (27) Bo¨hlke, J. K.; Coplen, T. B. Reference and intercomparison materials for stable isotopes of light elements. Proceedings of a consultants meeting held in Vienna, 1-3 December 1993; International Atomic Agency: Vienna, 1993; IAEA-TECDOC-825, pp 51-66. (28) Bo¨hlke, J. K.; Wanty, R.; Tuttle, M.; Delin, G.; Landon, M. EOS, Trans. Am. Geophys. Union 1994, 75, 154. (29) McMahon, P. B.; Bo ¨hlke, J. K. J. Hydrol. 1996, 186, 105-128.
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to KNO3-KCl solutions by ion exchange and were then freezedried to remove water. Graphite combustion experiments on KNO3 were conducted using C alone, C with Pd, C with Au, and C with Pd and Au catalysts together. Spectroscopic grade high-purity graphite was heated at 950 °C in a resistance-heated 9-mm-o.d. silica glass (quartz) tube for 48 h under vacuum to remove the surface oxide and increase the roughness of the surface.30,31 The graphite was cooled under vacuum and stored for up to several months in a glass bottle with a Teflon cap in a desiccator. Palladium-coated graphite was purchased from Johnson-Matthey Co. The Pd is estimated to constitute ∼1% by weight of the graphite. Gold catalyst was a 0.025-mm-thick gold foil (99.99% purity) cut into 1-cm × 0.25-cm pieces and inserted into the combustion tube. Procedure for Combustion of Nitrate with Graphite. Weighed amounts of KNO3 and C sufficient for several analyses were mixed and homogenized gently in an agate mortar. The amount of KNO3 varied from about 6 to 14 mg per analysis, and the C/KNO3 mole ratio was about 4. Aliquots of the mixture were transferred into quartz tubes that were closed at one end. A quartz-wool plug previously baked in air at 420 °C was inserted above each sample to prevent the mixed powders from migrating into the vacuum system. The loaded tubes were evacuated overnight while the temperature was held at 100 °C with heating tape wrapped around each tube to remove water from the surfaces. The tubes were sealed to a length of about 16 cm (about 6 cm3 internal volume). The sealed tubes were combusted at 520 °C for 24 h and then cooled by removing power from the oven, after which the temperature decreased by 60 °C in the first hour, 50 °C in the second hour, etc. to room temperature in approximately 24 h. The slow cooling promoted conversion of CO to CO2.6,13 Gases (mainly CO2 and N2) from combusted tubes were released in a glass ball-joint tube cracker attached to a vacuum line and separated cryogenically using a Toepler pump to collect the noncondensable fraction. The amount of noncondensable gas (mainly N2) was determined with the Toepler pump manometer, and aliquots of the gas were collected in two flame-sealed 9-mmo.d. quartz tubes. One of those tubes contained 0.1 g of CaO and 1.0 g of Cu + Cu2O and was recombusted for δ15N analysis according to the method of Kendall and Grim;18 the other tube was used for identification of interfering noncondensable gases. The CO2 gas was transferred cryogenically into a sample tube for yield measurement and for δ18O and δ13C analysis. The remaining solid in the combustion tube was put into a carbonate reaction vessel and reacted with phosphoric acid at 25 °C to release CO2 according to McCrea.36 Phosphoric acid with a specific gravity of 1.79 was prepared according to the method of Mook32 to assure 100% dissolution of solid carbonate. The CO2 yield was determined manometrically, and the sample was analyzed for δ18O and δ13C. For samples containing KCl, the acid reaction produced HCl, which could not be separated cryogeni(30) Kruprowski, A. Bull. Acad. Pol. Sci., Ser. Sci. Techn. 1964, XII, 737-742. (31) Gulbransen, E. A.; Andrew, K. F. Ind. Eng. Chem. 1952, 44, 1034-1051. (32) Mook, W. G. Ph.D. Thesis, Rijksuniversiteit Te Groningen, Groningen, The Netherlands, 1968. (33) Kendall, C.; Coplen, T. B. Anal. Chem. 1985, 22, 1437-1440. (34) Friedman, I.; O’Neil, J. R. In Data of Geochemistry; Fleischer, M., Ed.; U.S. Geological Survey Professional Paper 440-KK; U.S. Geological Survey, U.S. Government Printing Office: Washington, DC, 1977. (35) Coplen, T. B. Paleoceanogr. Curr. 1996, 11 (4), 369-370. (36) McCrea, J. M. J. Chem. Phys. 1950, 18, 849-857.
a All experiments were done with C + Pd + Au at 520 °C; uncertainties are given (1 standard deviation. b δ13C is expressed relative to VPDB.35 c δ18O is expressed relative to VSMOW.35 d R13C refers to R ) (δ13CCO2 + 1000)/(δ13CK2CO3 + 1000). e R 18O refers to R ) (δ18OCO2 + 1000)/(δ18OK2CO3 + 1000). f Calculated based on the measured N2 yield. g Sum of δ18O in KNO3 is calculated by mass balance using yields and δ18O values of CO2 and K2CO3. h Calcd δ18O in KNO3 is calculated by eq 3 using δ18O values of CO2. i The value of δ15N is expressed relative to air, normalized to +0.4 for IAEA-N1, and +180.0 for USGS-32; measured on noncondensable products of graphite combustion after recombustion with Cu2O. j Cl/NO3 ) 8, combusted together. k SO4/NO3 ) 2, SO4 removed before combustion (see text). l nd, no data.
3.6 ( 0.10 ndl 3.59 ( 0.11 4.58 ( 0.09 179.59( 0.10 -4.77 to 14.60 23.2 ( 0.5 23.0 ( 0.14 23.9 ( 0.74 22.68( 0.58 22.73( 0.19 3.7 to 51.4 23.49 ( 0.47 23.10 ( 0.05 23.77 ( 0.25 22.57 ( 0.38 22.89 ( 0.33 3.69 to 51.40 61 ( 3 60 ( 2 58 ( 3 62 ( 3 61 ( 3 59 ( 3 1.0081 ( 0.0007 98 ( 4 1.0079 ( 0.0003 102 ( 2 1.0082 ( 0.0005 105 ( 5 1.0089 ( 0.0006 99 ( 3 1.0082 ( 0.0007 102 ( 2 1.0082 ( 0.0006 99 ( 7 1.0042 ( 0.0046 1.0031 ( 0.0010 1.0052 ( 0.0004 1.0051 ( 0.0008 1.0042 ( 0.0016 1.0047 ( 0.0019 18.38 ( 0.75 18.26 ( 0.38 19.20 ( 0.84 16.93 ( 0.59 16.83 ( 1.34 0.23- 40.22 -18.08 ( 0.47 -15.73 ( 1.04 -17.68 ( 0.23 -17.78 ( 0.82 -16.65 ( 0.55 -17.27 ( 0.70 26.61 ( 0.51 26.33 ( 0.14 27.30 ( 0.74 25.94 ( 0.58 25.7 ( 0.19 6.45- 48.35 -12.95 ( 0.70 -12.67 ( 0.16 -12.61 ( 0.16 -12.73 ( 0.20 -12.50 ( 0.03 -12.63 ( 0.55 2.94 ( 0.12 3.04 ( 0.06 3.12 ( 0.18 2.97 ( 0.08 3.07 ( 0.09 3.04 ( 0.09 1.82 ( 0.11 1.79 ( 0.05 1.78 ( 0.21 1.86 ( 0.09 1.71 ( 0.03 1.86 ( 0.06 RSIL-N11 (n ) 22) RSIL-N11 KClj (n ) 6) RSIL-N11 SO4k (n ) 12) IAEA-N3 (n ) 14) USGS-32 (n ) 3) natural (n ) 17)
0.74 ( 0.04 0.78 ( 0.12 0.89 ( 0.17 0.74 ( 0.08 0.86 ( 0.08 0.87 ( 0.21
O CO2 sum of calcd yield (%)f yield (%)f δ18OKNO3 (‰)g δ18OKNO3 (‰)h R18Oe R13Cd δ18OK2CO3 (‰)c K2CO3
(‰)b
δ13C
δ18OCO2 (‰)c δ13CCO2 (‰)b O/N (M) K2CO3/N2 (M) CO2/N2 (M) KNO3 sample ID
(37) Revesz, K.; Bohlke, J. K.; Yoshinari, T. Proceedings of the IAEA International Symposium on Isotope Techniques in the Study of Past and Current Environmental Changes in the Hydrosphere and the Atmosphere, Vienna, Austria, 14-18 April 1997 (in press).
Table 1. Isotopic and Yield Data of Catalyzed Graphite Combustiona
RESULTS AND DISCUSSION Reaction Products of the Graphite Combustion. N2 and CO2 were the dominant products in the gas phase in all of the graphite combustion experiments. Yield of free CO2 was reproducible but consistently less than the total oxygen yield calculated on the basis of the N2 yields (Table 1). Analyses of the solid reaction products of the graphite combustion methods by X-ray diffraction indicated large amounts of K2CO3.37 The amounts of K2CO3 produced during combustion were determined from the amounts of CO2 released by phosphoric acid treatment of the solids. The ratios of the combined yields of O and N (O/N) accounted for by CO2, K2CO3, and N2 were close to 3.0 (Table 1), consistent with quantitative recovery of both elements from the KNO3. The amounts of N2 also were generally consistent with complete recovery but were not used directly to calculate yields because minor variations in the C/KNO3 ratios of sample mixtures caused uncertainties in the initial KNO3 weights. For most of the graphite experiments, approximately 60% of the O in the KNO3 could be accounted for by free CO2 and 40% by K2CO3. That product ratio (1.5) is somewhat different from the ratio expected from reaction 1 (1.0), implying that there may be another, unidentified sink for K. Nevertheless, it is evident that a large fraction of the O
δ15N (‰)i
cally from the CO2. The HCl prevented accurate yield measurements and δ determinations of the CO2; therefore, the HCl was removed by reaction with granular Zn cleaned according to the method of Kendall and Coplen.33 After a 20-min reaction time at room temperature, the remaining gas was passed through a liquid N2 trap to collect the CO2, which was then measured manometrically and transferred to the mass spectrometer. Values of δ18OK2CO3 were calculated from the measured values of δ18OCO2 by assuming that the O isotopic fractionation factor between the solid oxide and CO2 generated by phosphoric acid reaction of K2CO3 was the same as that for reaction of CaCO3 (R ) 1.010 25).34 The δ15N measurements of N2 gas were performed on a Finnigan MAT 251 mass spectrometer, and δ18O and δ13C measurements of CO2 gas were performed on a Dupont double-collecting mass spectrometer. The Finnigan mass spectrometer was used to determine the relative abundances of minor gases in the nonpurified, noncondensable gas aliquots that could indicate incomplete reaction or problems with sample handling (e.g., CO abundance from the 12/44 ratio, NO at m/z ) 30, NO2 at m/z ) 46, Ar at m/z ) 40, and CO2 and N2O at m/z ) 44). Solid reaction products were analyzed by X-ray diffraction for compound identification. Mercuric Cyanide Combustion Procedure. For comparison with the graphite combustion method, reference materials were also analyzed by the Hg(CN)2 combustion method used previously.5,8,12,13 KNO3 and Hg(CN)2 [KNO3/Hg(CN)2 ) 4/3 molar ratio] were weighed into 9-mm-od quartz tubes, evacuated overnight at 100 °C, sealed, heated in a programmable oven at 560 °C for 6 h, and cooled slowly. Gases from combusted tubes were separated cryogenically; CO2 gas was transferred cryogenically into a sample tube for yield measurement and for δ18O analysis. The solid combustion products were analyzed by X-ray diffraction, and δ13C and δ18O values were determined on the Dupont mass spectrometer.
Analytical Chemistry, Vol. 69, No. 21, November 1, 1997
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Table 2. Effect of Catalysts on the Graphite Combustion Techniquea type of oxygen type of reagent Catalyst yield (%)b graphite graphite graphite graphite Cu2O + Cu + CaOf
none Au Pd Pd + Au
92 ( 2 93 ( 4 101 ( 1 102 ( 2 ndg
12c (C)
30c (NO)
40c (Ar)
44c (CO2)
0.027 ( 0.008 0.031 ( 0.007 0.024 ( 0.002 0.022 ( 0.003 0.002f
1.349 ( 0.668 1.417 ( 1.369 0.023 ( 0.001 0.046 ( 0.022 0.033f
0.007 ( 0.003 0.007 ( 0.002 0.009 ( 0.004 0.009 ( 0.008 0.007f
0.227 ( 0.031 0.229 ( 0.040 0.153 ( 0.042 0.150 ( 0.044 0.054f
12/44c
δ15N (‰)d
0.119 ( 0.018 6.45 ( 0.40 0.135 ( 0.065 7.04 ( 3.11 0.157 ( 0.031 7.16 ( 0.41 0.146 ( 0.027 6.64 ( 0.91 0.037f +3.5f
δ18O (‰)e 23.44 ( 1.33 22.50 ( 1.00 23.02 ( 0.35 +23.42 ( 0.03 ndg
a All experiments were done with RSIL-N11 KNO ; uncertainties are given as (1 standard deviation. b Oxygen yield was calculated on the basis 3 of the N2 yield and the yield of free CO2 and CO2 generated from K2CO3. c Interfering masses are reported for representative samples prepared with different methods (voltage reading from the mass spectrometer detector relative to 100 for m/z ) 28); n ) 3. d The δ15N is expressed relative to air and normalized to +0.4 for IAEA-N1 and +180.0 for USGS32; measured on gas separated from CO2 but not recombusted to remove trace amounts of CO and NO; n ) 3. e δ18O is expressed relative to VSMOW;35 n ) 5. f Conventional δ15N analysis; no graphite combustion. g nd, no data.
liberated by the combustion of nitrate salts with graphite was converted to carbonate minerals, which have not been considered in previous reports. Effects of Catalysts in the Graphite Combustion. Catalysts had significant effects on the abundances of minor interfering gases, the apparent δ15N values of the noncondensable gas fraction, the total O yield, and the overall precision of the final δ18O values (Table 2). The δ15N values measured on noncondensable gases that were not recombusted with Cu2O apparently were affected by both CO and NO (Table 2). In experiments with C alone, high (m/z ) 30)/(m/z ) 28) and (m/z ) 12)/(m/z ) 44) ratios were observed with anomalous δ15N values and somewhat less than 100% yield in oxygen (Table 2). In experiments with C + Pd + Au, NO was not present, but apparent δ15N values generally were too high. Those high δ15N values are attributed to mass interference caused by small amounts of CO (indicated by elevated 12/44 ratios). While NO appears to have been eliminated in the presence of both catalysts, it is not clear that the catalysts had any significant effect on the abundance of CO. Neither Au nor Pd consistently eliminated NO when used alone, though Pd appears to have done so in the set of experiments summarized in Table 2. The average δ18O values were relatively insensitive to the presence of NO or CO, in part because they were only minor components, and also because the O isotopic fractionation factors R(CO2-NO) and R(CO2-CO) are not far from unity.21 However, the data in Table 2 and results of other experiments indicate that the standard deviations of δ18O were consistently small only when both catalysts were present. Subsequent discussion of the graphite combustion results focuses on those containing both Pd and Au, which produced the smallest amounts of NO and CO and gave the most reproducible results for δ18O (Tables 1 and 2). Nitrogen Isotope Results of the Catalyzed Graphite Combustion. The δ15N values could not be measured precisely in the N2 from untreated, noncondensable gases produced by graphite combustion, mainly because small amounts of isobaric CO interfered with the N2 peaks at masses m/z ) 28 and 29 (Table 2). Therefore, precise measurement of the δ15N required further treatment of the N2 gas by recombustion with Cu + Cu2O + CaO, after which the δ15N values of the noncondensable graphite reaction products compared well with those from conventional measurements in both accuracy and precision (Table 1). These results support the interpretation that all of the N originally present in the KNO3 was in the noncondensable gas products and that N2O was not a significant product of the graphite combustion. 4378 Analytical Chemistry, Vol. 69, No. 21, November 1, 1997
Figure 1. δ18O of K2CO3 versus the δ18O of CO2 for catalyzed graphite reaction at 520 °C. The filled symbols represent the measured δ18O values of both reaction products. The curve represents the δ18O of K2CO3 calculated from the measured δ18OCO2: δ18OK2CO3 ) [(1000+δ18OCO2) - 1008.3]/1.0083.
Oxygen Isotope Results of the Graphite Combustion. From the yields and the δ18O values of both the CO2 and carbonate combustion products, the isotopic composition of the total combined O released from KNO3 in each sample was calculated by isotopic mass balance (Table 1). The average δ18O values calculated in this way for the products of catalyzed graphite combustion were RSIL-N11 ) +23.46‰ (n ) 40); IAEA-N3 ) +22.68‰ (n ) 14); USGS-32 ) +22.73‰ (n ) 3). The 1σ standard deviations for the individual batches of tubes combusted together typically were 0.2‰ or better, whereas the standard deviations for all batches combined were approximately 0.5‰, indicating that small variations in the combustion conditions might have affected the data slightly. Therefore, the precision of the graphite combustion method can be on the order of (0.2‰ when local reference materials are included in each combustion run. The precipitation method for removal of SO42- from aqueous mixed salt samples did not alter significantly the δ18OKNO3 results (Table 1). In the graphite combustion experiments, the δ18O values of the free CO2 were consistently higher than the δ18O values of the carbonate fractions. The apparent O isotopic fractionation factor between the two phases (R ) [1000 + δ18OCO2]/[1000 + δ18OK2CO3]) was reproducible and had an average value of 1.0083 ( 0.0005. Analyses of various natural nitrate samples indicate that R was constant over a large range of nitrate δ18O values (Figure 1). The
Table 3. Isotopic and Yield Data of Hg(CN)2 Combustion
b
KNO3 sample ID
reagent type, temp
O yield (%)a
δ18O of CO2 (‰)b
calcd 18O KNO3c
RSIL-11 (n ) 5) RSIL-11 (n ) 5) USGS-32 (n ) 5) USGS-32 (n ) 5)
Hg(CN)2, 520 °C Hg(CN)2, 560 °C Hg(CN)2, 520 °C Hg(CN)2, 560 °C
66.28 ( 1.38 65.91 ( 0.44 66.89 ( 2.51 69.33 ( 1.21
27.79 ( 0.59 28.03 ( 0.44 27.19 ( 0.42 27.05 ( 0.15
23.45 23.69 22.85 22.71
a Oxygen yield calculated on the basis of the weight of the sample and the yield of free CO . Samples were approximately 10 mg of KNO . 2 3 δ18O is expressed relative to VSMOW.35 c δ18O of KNO3 was calculated according to eq 6.
measured value of R is similar to the equilibrium fractionation factor between CO2 and calcite at around 400-500 °C34 and, therefore, qualitatively consistent with equilibrium among the O-bearing combustion products of the graphite experiments (fractionation factors for K2CO3-CO2 are not known independently). Similarly, from the yields and the measured δ13C values of both the CO2 and K2CO3 combustion products, the total combined δ13C value and the C isotopic fractionation factor between the two phases were also calculated. The average sum of δ13C values (-14.1‰) was close to the original δ13C value of the graphite used in the combustion (-14.4‰), and the R value (1.003) was similar to the equilibrium fractionation factor between CO2 and calcite at around 400-500 °C.34 Because the O isotopic fractionation factors between the gas and solid reaction products were constant, it is possible to calculate the total δ18OKNO3 value from CO2 gas data alone:
δ18OKNO3 ) [(aδ18OCO2) + (b(1000 + δ18OCO2)/1.0083 1000)]/(a + b) (2) where a is the yield of free CO2 (in %) and b ) 100 - a. Results of those calculations yield standard deviations similar to those of the complete reconstructions using K2CO3 data. Furthermore, because the relative yields of the two O-bearing phases were relatively constant, it is possible to calculate the δ18OKNO3 values from the δ18O of free CO2 alone:
δ18OKNO3 ) 0.9967δ18OCO2 - 3.29
(3)
Results of those calculations have standard deviations of the δ18O of free CO2. The abbreviated procedures represented by eqs 2 and 3 eliminate several major steps in the analytical process and the possible experimental errors associated with them. Eliminating the analysis of K2CO3 avoids problems associated with the presence of Cl in the solid phase reaction products, which generates HCl when treated with phosphoric acid. Chloride, in mixed salt samples (KCl/KNO3 ) 8) had no measurable effect on the production or analysis of free CO2 and constructed δ18O of nitrate (Table 1). Subsequent experiments have yielded constant δ18O of free CO2 values for standards with KCl/KNO3 ratios from 0 to 5000. There was no evidence for significant variation in the results for sample sizes between 6 and 14 mg of KNO3; results for 2-6 mg samples were reproducible but required slightly different empirical correction coefficients in eqs 2 and 3. An error in the assumed isotopic fractionation factor associated with phosphoric acid treatment of K2CO3 (not known independently) could cause a minor systematic error in the final δ18O
values given for the KNO3 reagents, which are, therefore, designated as provisional values. Results of Hg(CN)2 Combustion. The major solid reaction product of the Hg(CN)2 combustion method identified by X-ray diffraction was KOCN.37 Though its abundance was not quantified, it is possible that this compound accounts for a significant fraction of the missing O in Hg(CN)2 experiments in which the yields of free CO2 are low;10 in our experiments with Hg(CN)2, free CO2 accounted for about 66% of the O in the reacted nitrate (Table 3). Although the Hg(CN)2 combustion method failed to convert nitrate oxygen to CO2 quantitatively, it yielded reasonably good precision in both the yield measurements (∼66%) and the δ18O measurement of CO2 ((0.5‰) (Table 3). The apparent O isotopic fractionation factor between free CO2 and the solid reaction product of the Hg(CN)2 combustion method (assumed to be largely KOCN) was estimated from analyses of RSIL-N11 combusted at 520 °C:
R ) (1000 + δ18OCO2)/(1000 + δ18OKOCN) ) 1.0126
(4)
where δ18OCO2 ) 27.79 (Table 3), and
δ18OKOCN ) (δ18OKNO3 - 0.66δ18OCO2)/0.34 ) 15.05‰
(5)
where δ18OKNO3 ) 23.46‰ (determined by graphite combustion). Because the relative yields of the two O-bearing phases were constant, it is also possible to calculate the δ18OKNO3 values from the δ18O of free CO2:
δ18OKNO3 ) 0.995δ18OCO2 - 4.2
(6)
where δ18OCO2 is the measured value after Hg(CN)2 combustion. There was no significant difference between the results of Hg(CN)2 combustion at 520 and 560 °C (Table 3). CONCLUSIONS Combustion of KNO3 with graphite plus catalysts at 520 °C yielded CO2, K2CO3, and N2 as the major products containing virtually all of the N and O. The δ15NKNO3 values determined from measurements on the N2 produced by the graphite combustion, separated from CO2 and recombusted with Cu + Cu2O, are both precise and accurate ((0.1‰) when compared with conventional N isotopic determinations. The δ18OKNO3 values determined from measurements of both the CO2 and K2CO3 also are precise ((0.2‰ or better for single combustion run; (0.5‰ for multiple combustion runs). The δ18O values of the international KNO3 reference Analytical Chemistry, Vol. 69, No. 21, November 1, 1997
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materials IAEA-N3 and USGS-32 determined by this method are both equal to +22.7 ( 0.5‰ (provisional values). The fraction of O (about 60% as CO2 and 40% as K2CO3) and the apparent oxygen isotopic fractionation factors (RCO2-K2CO3 ) 1.0083) produced by combustion and slow cooling were sufficiently constant that precise δ18OKNO3 values could be calculated from measurement of the δ18O value of free CO2 alone, with empirical corrections of the order of 3‰. The presence of chloride in the combustion tube did not have a measurable effect on the δ18OCO2 values. Combustion of KNO3 with Hg(CN)2 at 520 and 560 °C yielded similar results. Free CO2 accounted for about 66% of the O and had a δ18O value significantly different from that of the nitrate. Significant amounts of KOCN were identified in the solid products, and the δ18OCO2 values were consistent with RCO2-KOCN ) 1.0126. By analogy with the graphite combustion experiments, it is inferred that empirical correction factors could be applied to δ18OCO2 values to estimate δ18OKNO3 for the unknowns combusted with Hg(CN)2. Although provisional values of δ18OVSMOW are given for international reference materials IAEA-N3 and USGS-32, improved estimates and consensus values must await interlaboratory com-
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parisons with other materials or independent determination of the oxygen isotopic acid fractionation on treatment of K2CO3 with phosphoric acid. Also, it is emphasized that the empirical coefficients relating δ18O of CO2 and δ18O of KNO3 in eqs 2-6 may be expected to vary slightly with sample size, tube volume, combustion conditions, and other procedural variations; thus, local calibration and constant experimental conditions are necessary for both accuracy and precision. ACKNOWLEDGMENT We thank T. B. Coplen, R. R. Seal, and M. Vercouteren for valuable suggestions and discussion, J. Hannon, I. Hamblen, and J. Reel for assistance in the laboratory, D. Webster for X-ray diffraction analysis, and C. Kendall and L. Wassenaar for discussions about their methods. Received for review October 10, 1996. Accepted August 21, 1997.X AC9610523 X
Abstract published in Advance ACS Abstracts, October 1, 1997.
