Environ. Sci. Technol. 1988, 22, 845-848
A General Gravimetric Dilution Technique for Preparing Trace Calibration Gases: N,O Calibration Gas Preparation Walter D. Komhyr,” Ellsworth G. Dutton, and Thayne M. Thompson Air Resources Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80303 -
~~~~
w A method is described for calibrating trace reference gases in concentrations of parts per million to parts per trillion. As applied to N20, of atmospheric concentration of about 300 parts per billion, the method consists of gravimetric preparation of a mixture of pure C 0 2 and N 2 0 in amounts roughly equivalent to the proportions of these gases in the atmosphere and dynamic dilution of this gas mixture to approximate atmospheric concentrations of COP and NzO with COP- and N20-free air. Analysis of the dilution gas mixture for COP content, using standard nondispersive infrared analyzer techniques, combined with accurate gravimetric information about the relative masses of C02 and N20 in the dilution gas mixture, yields the gas mixture NzO concentration. Calibration gases such as CC1,F and CC12F2,in parts per trillion, can be prepared in a similar manner but with N 2 0serving as the calibration gas.
Introduction Global pollution of the atmosphere by anthropogenic trace gases that are photochemically and radiatively active and that have a potential for ozone destruction and climate modification has become a subject of considerable scientific concern in recent years. Apart from the “greenhouse” effect of COz, the list of important radiatively active trace gas species has grown to include CCl,F, CC12F2,03,CHI, NzO, CHC1F2, CH3CCl,, CCl,, CBrF,, CzF& CF,Cl, and CHF, (I). Trace atmospheric constituents exist also (e.g., CO and NO) that, although not themselves radiatively active, alter the chemistry of the troposphere, thereby perturbing the radiatively important gases. The need for accurate global monitoring of these trace gases to assess their growth rates, sources, and sinks is clear. For such measurements, highly accurate and stable calibration gas standards are required in the concentration range of parts per million (ppm) to parts per trillion (ppt). We describe a general gravimetric dilution technique for establishing such calibration gas standards and provide details concerning preparation of N 2 0 calibration gas whereby pure C 0 2 and N 2 0 are combined gravimetrically in an accurately known proportion and then diluted with COz- and NzO-free air to concentrations approximating those in the atmosphere. The N 2 0 concentration in the resulting gas mixture is then derived from an accurate measurement, by standard infrared analyzer techniques, of the gas mixture’s C o n content. The methodology resembles that of Craig and Gordon (2) and Weiss et al. (3), who used a manometrially calibrated nondispersive infrared C 0 2 analyzer for determinations of COP in N20/ COP-in-air calibration gas samples that, however, were prepared by a volumetric technique. Preparation of N 2 0 Calibration Gas N2O and C 0 2 were combined gravimetrically in a proportion roughly equivalent to that present in the atmosphere, in a size H (0.0433 m3) chrome-molybdenum steel cylinder (tank 3078) fitted with a packless valve. (The gas mixture was subsequently diluted with N20- and COP-free
air to serve as an NzO calibration gas standard.) The molar ratio of NzO to C 0 2 to be put into tank 3078 was derived as follows: Let no = number of moles of NnO-free, C02free, dry air in a sample of the calibration gas; nl = number of moles of CO,; and n2= number of moles of N20. Taking the atmospheric C 0 2 and N 2 0 mole fractions to be 330 ppm and 300 ppb, respectively, we have
300 -
io9
(2)
(0.9091 x 10-~)n,
(3)
n2
no + nl
+ n2
N
n2 -=
no
and
n2
N
In preparing the gas mixture, care was taken to ensure that fractionation of the gases did not occur in the tank due to partial liquefaction of COP. The vapor pressure of COz is 5.824 X lo6 N m-2 at 21.1 “C. At this temperature, partial C02 liquefaction will occur in a size H tank if more than 4564 g of C02 are put into it. To avoid fractionation, tank 3078 was filled with 3178.77 f 0.10 g of C 0 2 and, in accordance with eq 3,2.9279 f 0.0001 g of N20. The work was accomplished by Liquid Carbonic Corp., Los Angeles, CA, using a manual, gravimetric technique and employing class S weights traceable to the National Bureau of Standards. Research-grade C 0 2 and N 2 0 were used of purity 99.995% and 99.99%, respectively. Uncertainties associated with the masses of C02and N 2 0 indicated above represent limits of mass resolution of the balances used in weighing the gases. Uncertainties in the masses of the balance weights used were considerably smaller (13). Synthetic zero air was used for dilution of the N20-C02 tank gas mixture. The air was prepared gravimetrically according to the following specifications: (1) oxygen concentration to be 20.9 f 0.2%, with a certified absolute accuracy of f0.02%; oxygen to be 99.995% pure and derived from electrolysis of water (2) argon concentration to be 0.9 f 0.1%, with a certified absolute accuracy of fO.Ol%; argon to be 99.998% pure (3) remainder to be 99.998% pure nitrogen (4) synthetic air to be hydrogen-free, with moisture content less than 5 ppm Use of electrically derived oxygen was specified, since it is difficult to separate argon completely from oxygen prepared from liquefied air. Hydrogen mixed with electrically prepared oxygen, on the other hand, is readily burned off. Results of oxygen and argon concentration analyses ( 4 ) of several tanks of synthetic air prepared by Liquid Carbonic Corp. indicated that the above specifications for the zero air were generally well met by the vendor. Figure 1is a schematic diagram of the dilution apparatus used for preparing the N 2 0 calibration gas. Apart from the NZ0-CO2 gas mixture, the zero air supply, and the C02 (Ascarite) scrubber, the apparatus consists of flowmeters (F) and 0.098 and 0.049 cm diameter stainless steel tubing cleaned with acetone, a 10% nitric acid and 5% hydro-
Not subject to U.S. Copyright. Published 1988 by the American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 7, 1988
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320 I
I
1 BOULDER,COLORACC JULY-OCTOBER, 1982
3101
11
0
t a a
0, 300
Ascarite .0 r
m x Z
2 Zero Air
Figure 1. Schematic diagram of apparatus used for preparing N20 calibration gas. Flowmeters are indicated by F, mixing valves by V.
fluoric acid aqueous solution, distilled water, and dry nitrogen. The flowmeters are fabricated from glass and stainless steel. Mixing valves (V) shown in Figure 1were not used in the original apparatus. Because chromatographic tests on the zero air indicated that it contained no detectable N20, an NzO scrubber was not used in line with the zero air supply. In using the apparatus of Figure 1,flow rates of the gases are first adjusted approximately to values indicated on the figure. The NzO calibration gas emanating from the apparatus is passed through an NDIR COz analyzer, and minor adjustments are made to the gas flow rates to obtain an “on scale” analyzer COz reading in the range of 300-360 ppm. After the COz signal trace stabilizes, N20 calibration samples are collected in stainless steel cylinders or glass flasks that have been flushed with the calibration gas. Because the mole fraction ratio n2/nl is conserved when the N20-CO2gas mixture in tank 3078 is diluted with zero air, the exact NzO mole fraction in a sample of calibration gas can be determined from an accurate measurement of the sample’s CO, mole fraction (nl/no) and the relation n2 = n1 nz = -nl mzm1’ no
no nl
no %mi
(4)
where m, = mass of COz in tank 3078, ml’= molecular weight of COz (44.0098), m 2= mass of N 2 0 in tank 3078, and m2/ = molecular weight of NzO (44.0128). The CO, mole fractions (nl/no) of the prepared NzO calibration gas samples were determined with a semiautomatic NDIR COPanalyzer apparatus (5) and NOAA/ Geophysical Monitoring for Climatic Change (GMCC) secondary standard CO, calibration gases ( 4 ) traceable to the Scripps Institution of Oceanography 1982 manometric COz scale (6). The absolute uncertainty associated with determination of the COPmole fractions is believed to be f0.2 ppm but may be as large as h0.6 ppm (7). A sample, quantitative evaluation of eq 4 reveals the accuracy with which the N 2 0 mole fraction can be determined in a batch of prepared N 2 0 calibration gas: nz -- 330.56 f 0.6 _ no
(2.9279 f 0.001)(44.0098 f 0.001) (3.178.77 f 0.10)(44.0128 f 0.002) = 304.45 f 0.49 ppb 106
Thus, barring significant systematic errors, the gravimetric dilution technique decribed herein is potentially capable of yielding mole fractions of NzO calibration gases accurate to within several tenths of 1%. A crudely assembled calibration apparatus similar to that shown in Figure 1was first used to prepare 10 N 2 0 calibration gas samples by the gravimetric dilution tech846
Environ. Sci. Technol., Voi. r2, No. 7, 1988
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TANK 3072 C02 = 329 6 PPMV AT C02 = 329 6 PPMV, N2O = 299 7 i 2 1PPBV SLOPE -0 1952 i 0 1507 PPBV N20/PPMV CO2 I
I
1
I
I
a value of 304.1 f 3.4 ppb. (By this time tank 3072 air had been depleted.) The result obtained was 303.7 f 1.2 ppb, in good agreement with the value obtained by the gravimetric dilution technique. COP interference via reactions 5-8 given above was confirmed (15) by alternately passing air containing 226 ppb of NzO and 297 ppm of C02, and the same air from which C 0 2 was removed with an Ascarite filter, into an HP-5890 electron capture gas chromatograph equipped with a Porasil A column. The Ascarite trap reduced the yielding ~ response of the chromatograph to NzO by ~ 2 6 % and N20change of 0.20 ppb/ppm COP The fact that both experiments gave virtually identical results with gas chromatographs that were different but equipped with the same type of Porasil column operated under similar conditions in 1982 and 1986 suggests that the C02interference effect is reproducible and that the Porasil column is unable to separate C 0 2 from N20 completely. Preparation of CC1,F and CC1Z2 Calibration Gases To illustrate the method of preparing trace calibration gases at concentrations of parts per trillion, we consider the trace pollutants CC1,F and CC12Fzpresent in the atmosphere at concentrations of about 230 and 400 ppt, respectively. Following the procedures described under Preparation of NzO Calibration Gas, highly purified N20, CC13F,and CC12F2gases are gravimetrically mixed into a tank, and the gas mixture is then diluted with zero air so that the resulting calibration gas contains approximately ambient atmospheric concentrations of the three trace gases. CC13F and CClzFz concentrations are then determined from an accurate determination of the NzO concentration in the calibration gas, with NzO standard gases (established as described under Preparation of NzO Calibration Gases). If atmospheric concentrations of N20,CCl,F, and CC12F2 are taken to be 310 ppb, 230 ppt, and 400 ppt, respectively, then the molar proportions of the various gases to be put into the tank are n3
-
(0.7419 X 10-3)n2
(9)
n4
-
(1.2903 X 10-3)n2
(10)
and
where n2, n3, and n4 are the required number of moles of N20, CClSF, and CC12F2,respectively. Taking into account the molecular weights of NzO, CC13F, and CCl2F2,namely, 44.0128, 137.368, and 120.914, respectively, and also the gas vapor pressures at 21.1 "C, namely, 5.238 X lo6, 0.092 X lo6, and 0.585 X lo6,respectively, calculations indicate that if 3000 g of NzO are put into a size H cylinder, all components of the gas mixture will remain gaseous at normal room temperature. Suitable amounts of CC1,F and CClzFz to be put into the tank according to eq 9 and 10 are approximately 6.92 g of CC1,F and 10.62 g of CC12F2. Uncertainties in weighing the gases are fO.1 g for N20 and *0.0001 g for the other gases. Purities of commercialy available N20, CC13F, and CClZF2are 99.99%, 99.9%, and 99.0%, respectively (16). A three-stage dilution apparatus for preparing CC1,F and CClzFz calibration gas mixtures is illustrated in Figure 3. Filters are employed in the zero airstream to eliminate from it traces of the calibration gas constituents. Provision is made to add research-grade COz to the calibration gas, if needed. When the apparatus is used, gas flow rates are set approximately at values indicated in Figure 3. Samples of the calibration gas are then analyzed for N20 content with
Figure 3. Schematic diagram of apparatus constructed for preparing CCI,F and CCI,F, calibration gases. Flowmeters are indicated by F, mixing valves by V.
a gas chromatograph and for COz content with an infrared COz analyzer. The CC1,F mole fraction (n3/no) of the sample gas mixture is then determined from n3 --
n2 n3 n2 m3m2' - - = --
(11) no no nz no % m i where mz = mass of N20 in tank 5127618, m2/ = molecular weight of N 2 0 (44.0128), m3 = mass of CC1,F in tank 5127618, and m i = molecular weight of CC1,F (137.368). The mole fraction of the CC12F2component of the Calibration gas may be determined similarly. Barring systematic errors, the quantity contributing the main uncertainty, estimated to be about 0.5%, to Calibration gas CC1,F values determined from eq 11 is the measured N 2 0 mole fraction, nz/no. For CC12F2,an additional error of 1%stems from 1%impurity in this gas procured from commercial sources.
Summary and Conclusions The gravimetric dilution method described for preparing NzO and other trace calibration gases involves no highly accurate measurements of pressure, temperature, and gas flow rates. Determinations of gas masses in gram and kilogram quantities are made accurately with ease, as are highly accurate COz mole fraction measurements using standard infrared analyzer techniques. Allowing equilibrium conditions to be established when diluting the pure gas mixtures minimizes systematic errors. Uncertainties at the 95% confidence level in the mole fractions of N 2 0 calibration gases, prepared relative to highly accurately known Concentrations of C02, are estimated not to exceed *0.5%. For CC1,F calibration gas standards, prepared relative to known N20 concentrations, achievable mole fraction uncertainties are likely to be less than 4~1.5%. Potential maximum uncertainties associated with the mole fractions of CC12Fzcalibration gases, which are commercially available at only 99.0% purity and which are also prepared relative to N 2 0 calibration gases, are estimated to be *3.0%. Acknowledgments Appreciation is expressed to R. A. Rasmussen, Oregon Graduate Center, who performed N 2 0 calibrations of NOAA/GMCC tank 3072 air during 1977-1983; to R. F. Weiss, Scripps Institution of Oceanography, for his 1980 N2O calibration of tank 3072 air; and to R. K. Leonard, CIRES, University of Colorado, for assistance with computer processing of the data. The COz calibration scale of C. D. Keeling, Scripps Institution of Oceanography, was Environ. Sci. Technol., Vol. 22, No. 7, 1988
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used in preparing the N20 calibration gas standards. Registry No. N20, 10024-97-2.
Literature Cited (1) Ramanathan, V.; Cicerone, R. J.; Singh, H. B.; Kiehl, J. T. J . Geophys. Res. 03: Atmos. 1985,90, 5547-5566. (2) Craig, H.; Gordon, L. I. Geochim. Cosmochim. Acta 1963, 27, 949-955. (3) Weiss, R. F.; Keeling, C. D.; Craig, H. J. Geophys. Res. C8: Oceans Atmos. 1981,86, 7197-7202. (4) Komhyr, W. D.; Harris, T. B.; Waterman, L. S. J . Atmos. Oceanic Technol. 1985,2, 82-88. ( 5 ) Komhyr, W. D.; Waterman, L. S.; Taylor, W. R. J. Geophys. Res. C 2 Oceans Atmos. 1983,88, 1315-1322. (6) Keeling, C. D.; Bacastow, R. B.; Guenther, P. R.; Moss, D. J. Scripps reference Gas Calibrating System for Carbon Dioxide in Air Standards; revision of 1982 report prepared for the Environmental Monitoring Program of the World Meteorological Organization; Scripps Institute of Oceanography: La Jolla, CA, 1983; 31 pp. (7) Komhyr, W. D.; Harris, T. B. WMO Special Environmental Report No. 14; World Meterological Organization: Geneva, Switzerland, 1980; pp 73-78.
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(8) Thompson, T. M.; Komhyr, W. D.; Dutton, E. G. NOAA Technical Report ERL 428-ARL 8; NOAA Air Resources Laboratory: Boulder, CO, 1985; 124 pp.
(9) Rasmussen, R. A.; Khalil, M. A. K. Science (Washington, D.C.) 1986,232, 1623-1624. (10) Komhyr, W. D.; Gammon, R. H.; Harris, T. B.; Waterman, L. S.; Conway, T. J.; Taylor, W. R.; Thoning, K. W. J . Geophys. Res. 0 3 : Atmos. 1985, 90, 5567-5596' (11) Peterson, J. T.; Komhyr, W. D.; Waterman, L. S.; Gammon, R. H.; Thoning, K. W.; Conway, T. J. J. Atmos. Chem. 1986, 4, 491-510. (12) Phillips, M. P.; Sievers, R. E.; Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C. Anal. Chem. 1979, 51, 1819-1825. (13) Payne, J., Liquid Carbonic Corp., private communication, 1980. (14) Weiss, R. F., Scripps Institution of Oceanography, private communication, 1980. (15) Elkins, J. NOAA Air Resources Laboratory, private communication, 1986. (16) Young, Q., Liquid Carbonic Corp., private communication, 1985. Received for review A u w t 11,1987. Revised manuscript received January 28, 1988. Accepted February 10, 1988.
