Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO(3)(-) in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying delta(18)O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO(3)) with low delta(18)O and USGS35 (NaNO(3)) with high delta(18)O and 'mass-independent' delta(17)O. The procedure used to produce USGS34 involved equilibration of HNO(3) with (18)O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO(3)(-) reference materials with a range of delta(18)O values and normal (mass-dependent) (18)O:(17)O:(16)O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (alpha) between [NO(3)(-)] and H(2)O decreases with increasing temperature from 1.0215 at 22 degrees C to 1.0131 at 100 degrees C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high (17)O:(18)O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO(3) (-) isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO(3)(-) samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of +/-0.2-0.3 per thousand, 1sigma): IAEA-N3 has delta(18)O = +25.6 per thousand and delta(17)O = +13.2 per thousand; USGS32 has delta(18)O = +25.7 per thousand; USGS34 has delta(18)O = -27.9 per thousand and delta(17)O = -14.8 per thousand; and USGS35 has delta(18)O = +57.5 per thousand and delta(17)O = +51.5 per thousand.

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high-temperature conversion techniques (HTC)a in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125 per thousand, an artificially enriched reference water (delta(18)O of +78.91 per thousand) and two barium sulfates (one depleted and one enriched in (18)O) were prepared and calibrated relative to VSMOW2b and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded: Reference material delta(18)O and estimated combined uncertainty IAEA-602 benzoic acid+71.28 +/- 0.36 per thousand USGS 35 sodium nitrate+56.81 +/- 0.31 per thousand IAEA-NO-3 potassium nitrate+25.32 +/- 0.29 per thousand IAEA-601 benzoic acid+23.14 +/- 0.19 per thousand IAEA-SO-5 barium sulfate+12.13 +/- 0.33 per thousand NBS 127 barium sulfate+8.59 +/- 0.26 per thousand VSMOW2 water 0 per thousand IAEA-600 caffeine-3.48 +/- 0.53 per thousand IAEA-SO-6 barium sulfate-11.35 +/- 0.31 per thousand USGS 34 potassium nitrate-27.78 +/- 0.37 per thousand SLAP water-55.5 per thousand The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for delta(18)O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios (delta(18)O) of nitrates, sulfates, or organic material should explicitly state in their reports the delta(18)O values of two or more internationally distributed nitrates (USGS 34, IAEA-NO-3, and USGS 35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the delta(18)O values at a later time should it become necessary.The high-temperature reduction technique for analyzing delta(18)O and delta(2)H is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

In their first paper on Buckminsterfullerene,1 Kroto and Smalley et al. proposed a hollow structure. Interest in theinclusion of atoms into the cavity to form "endohedral complexesn has continued. Early work concentrated on incorporating metals? but noble gas atoms were obvious possibilities. Several studies reported that collisions of fullerene cation radicals with helium and neon in a mass spectrometer led to addition of the mass of the noble gas atoms to the ions.S7 Many theoretical studiesB-15 have appeared with estimates of the energies of fullerenes containing noble gas atoms, the dynamics of motion inside the cavity, and spectroscopic properties, even though none of these substances has been prepared in quantity.We have recently shown16 that fullerene prepared in the standard manner (a graphite arc in a low pressure of helium17) contains helium in about one in a million molecules. Heating fullerenes in several atmospheres of 3He or neon at 600 "C led to inclusion of these atoms at similar levels. We proposed that (1) Kroto, H. W.; Heath, J. R.; OBrien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (2) Heath, J. R.; OBrien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.; Smalley, R. E. J. Am. Chem. Soc. 1985,107,7779-7780. (3) Weiske, T.; Bdhme, D. K.; HruUk, J.; KrHtchmer, K.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 884-886. (4) Schwarz, H.; Weiskc, T.; Bdhme, D. K.; HruUk, J. In Euckminrterfullerenes; Billups, W. E., Ciufolini, M. A,, Eds.; VCH Publishers: New (5) Ross, M. M.; Callahan, J. H. (7) Caldwcll, K. A.; Giblin, D. E.; Hsu, C. S.; Cox, D.; Gross, M. L. J. Am. (8) Breton, J.; Gonzalez-Platas, J.; Girardet, G. (IS) Cardini, 0.; Procacci, P.; Salvi, P. R.; Schettino, V. Chem. Phys. (16) Saunders, M.; JimCnez-Vhzqucz. H. A,; Cross, R. J.; Poreda, R. J. (17) KrHtchmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. 3734. 8362. 8741. 1864-7869.this process takes place by the reversible breaking of one or more bonds, opening a "window" large enough for entry of atoms. The noble gases were detected by heating samples in the modified mass spectrometer operated by one of us (R.J.P.). 16 We have now incorporated krypton using -15 atm at 600 OC for 2 h. An 18.5-mg sample of the resulting fullerene yielded 1.6 X 1 V cm3 of 84Kr on heating (1 atom in 410 OOO molecules), with a release pattern similar to that reported for helium and neon. Since several theoretical papers predicted that krypton and xenon would not fit into CM, we wanted to try to incorporate xenon.To investigate the spectroscopic and other properties of these interesting noble gas compounds, it is necessary to increase the fraction of fullerene molecules occupied. We have done this by carrying out the incorporation step at high pressures of the gas. One of us (S.M.) operates a facility with high-pressure steel vessels that can be heated behind appropriate shielding. Usually, a pump is employed to compress the gas into the vessel. Using expensive gases (e.g., 3He), this is a disadvantage since large amounts o...

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