ACKNOWLEDGMENT
more, the solutions here have somewhat higher ionic strengths than those employed in a direct conductometric method, so that a greater uncertainty in activity coefficients results. For this reason, along with the assumptions required, the present method seems limited to a precision of approximately sr0.05 pK, unit for acids with pK, = 0.0. However, the lower sensitivity of calculated pK, values to sample purity seems to recommend the mixed acid method for certain practical applications.
The author gratefully acknowledges the assistance of ProfessorL. M.Schwartz in this work, RECEIVED for review December 7, 1970. Accepted March 19, 1971. Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Measurement of Atmospheric Nitrous Oxide Using a Molecular Sieve 5A Trap and Gas Chromatography Miles D. LaHue, Herman D. Axelrod, and James P. Lodge, Jr. National Center for Atmospheric Research, Boulder, Colo. 80302
The presence of nitrous oxide, NzO, in the atmosphere has not .received much publicity, because the gas is not a direct product of technological pollution. Nevertheless, the concentration of NzO is about 250-300parts per billion (ppb or Two possible sources of N 2 0 are soil bacteria and photochemical reactions. One would believe that the bacterial source would vary seasonally, while a photochemical source might produce a constant NzO level. In order to discriminate between these two possible conditions, a very reproducible method for NzOanalysis is needed. Junge (I) reviewed some of the earlier methods of NzO measurement. Most used infrared spectrometry, but the measurements suffered from large COZand water interferences. Gas chromatographs with a thermal conductivity detector typically do not have the sensitivity to measure ambient NzO (300ppb) with a 5-ml air sample. Recognizing this fact, Bock and Schutz (2) concentrated NzOon molecular sieve SA. After the sample was taken, the molecular sieve was placed under vacuum and heated to 250-300 OC. The released NzO was pulled off by a Topler pump and stored in a gas buret prior to analysis by gas chromatography. This procedure can possibly cause erroneous values; Schiitz (3) reported thermal decomposition of NzO above 300 OC, and it is conceivable that reducing the pressure prior to transfer could remove NzOadsorbed on the molecular sieve surface. LaHue et al. ( 4 ) also used a molecular sieve and a heating technique, but found the reproducibility to be poor. Leithe and Hofer ( 5 ) recommended trapping atmospheric NzO on silica gel submerged in a dry ice-acetone bath and then heating the sample to drive the NzO onto a gas chromatographic column. However, the need to use a cold bath renders this method unsuitable for field work. The use of molecular sieve offers the advantage of allowing sample collection at any location. Furthermore, a transfer technique not involving heat or evacuation would bypass the ~
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(1) C. E. Junge, “Air Chemistry and Radioactivity,” Academic Press, New York, N. Y . , 1963, pp 81-84. (2) R. Bock and K . Schiitz, J . Anal. Chem., 237, 321 (1968).
( 3 ) K . Schiitz, Ph.D. thesis, University of Mainz, Germany, 1966. (4) M. D. LaHue, J. B. Pate, and J. P. Lodge, Jr., J . Geophys. Res., 75, 2922 (1970). ( 5 ) V. W. Leithe and A. Hofer, Allg. Prukt. Chem., 19, 78 (1968).
problems mentioned above and would be simple and inexpensive (no Topler pump or vacuum system would be required). The method described below uses a molecular sieve trap for sampling, but the transfer technique has been significantly altered. A stream of water-saturated He passed over the sieve displaces the NzO. The water-displaced NzO is trapped on a silica gel column submerged in a dry ice-isopropyl alcohol bath. The silica gel column is then heated to transfer the NzOinto a gas chromatograph. EXPERIMENTAL
Sampling System. Activated molecular sieve 5A (l/16-in. pellets) was obtained directly from Linde Division, Union Carbide Corp., and used without further preparation. (The NzO content of the unexposed molecular sieve should be determined, because this commercially activated molecular sieve was found to contain about 1 pg NzO/ll grams. Fur,thermore, each batch of molecular sieve must be thoroughly mixed to ensure the same NzO background for all sample tubes.) A sampling train similar to that described by LaHue et al. (4) was used with critical orifices controlling the sampling rate. The air was first passed over CaS04 (8 mesh) and Ascarite (8-20 mesh) to remove most of the atmospheric water and COa prior to reaching the molecular sieve. The molecular sieve tube was made from stainless steel */*-in. 0.d. X 8-in. L with Swagelok fittings at each end. The tube contained 11 grams of molecular sieve. Such a tube can be sealed and mailed without fear of leakage and breakage. Transfer of NnO to Gas Chromatographic Sample Loop. Prior to introducing the NzO into the gas chromatograph, the experimenter must remove the NzO from the molecular sieve. (Molecular sieve 5A has a great affinity for water and will easily displace other trapped gases.) A He flow saturated with water vapor was passed over the molecular sieve to displace the NzO and to carry the liberated NzO past CaS04 and Ascarite into a U-tube filled with activated 8-20 mesh silica gel in a dry ice-isopropyl alcohol bath (-80 “C). (The substitution of Na for He did not give satisfactory results.) Figure 1 shows the experimental system. The U-tube T was fitted with Whitey toggle valves for isolation. Initially, dry He (bypassing the sample tube) flowed through the system with T at room temperature. This allowed all the air in the system to be purged with the He still flowing. T was then submerged in the bath and allowed to reach bath temperature. The sample tube was then placed into the system, and V ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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Table I. Measurement of Atmospheric NzO a t Boulder, Colo. Date Time N20 found, ppb
Figure 1. Sample transfer system. NzO is removed from the molecular sieve and is adsorbed by the silica gel A . "4 in. i.d. X 3 in. L. tube filled with 8-20 mesh Ascarite B. Dry ice-isopropyl alcohol bath, -80 "C C. 8/4-in.i.d. X 12-in. L. tube filled with 8 mesh CaSO, S. Stainless steel sample tube. T. 3/a-in.0.d. X 16-in. 1. thin walled stainless steel tube packed with activated 8-20 mesh silica gel V. 4-way stopcock W . Water scrubbing towers for saturation of He
26 October 1970
0900930 12w1230 15 w 1530
30 1 307 30 1
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0900-0930 12w1230 15W1530
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0900-0930 1 200- 1230 1500- 1 530
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matograph as follows: The U-tube was placed in a 140 "C oil bath and connected to the gas sampling valve. After 10 min, the gas sampling valve was switched, the toggle valves were opened, and the N 2 0 was transferred onto the gas chromatographic column. The flush time was 2.5 min. Figure 2 shows the instrumental parameters used. The peak area of the NzO was measured and the amount of NzO was determined from a standardization curve.
was turned to allow the He flush to be saturated with water prior to reaching the sample (S). (The transferring can be carried out with the same stainless steel sample tube as described in the previous section.) The He flow rate was 2 1./mi n ./ After a 40-min transfer time, the molecular sieve was toially water saturated, and the CaS04(C) before the U-tube T, became warm from water breakthrough. A 5-min dry flush (accomplished by switching V ) was used after the wet flush to ensure total removal of the N20 from the transfer system. After the flush was completed, the toggle valves on Twere closed and T was removed from the bath. Analysis of Sample. The amount of NzO in the sample was measured by a Perkin-Elmer 900 gas chromatograph with a thermal conductivity detector and a Carle Model 2014 gas sampling valve. The gas chromatograph was calibrated with known r l volumes of a J. T. Baker mixture of 5 % NzO in Nt injected into the instrument with a gas syringe (Precision Sampling, Inc., Baton Rouge, La.). (The detector response was linear over the calibration range.) The N20 sample in T was introduced into the gas chro-
RESULTS
The foregoing method was used to measure atmospheric NzO at the National Center for Atmospheric Research three times each day for three consecutive days. Table I shows the results of these measurements. During this brief period the N 2 0 concentration does not appear to vary; the average value was 305 ppb, with a 1.2% standard deviation. These values are in the range of those reported by Junge ( 1 ) . DISCUSSION
Sampling Procedure. The molecular sieve sample tube was made from stainless steel with Swagelok fittings to prevent the molecular sieve from being exposed to the atmosphere while the sample was stored. Experiments indicated that a polyethylene tube would continue to leak after being "sealed" and that long term storage or transport would not be possible 20'C
t
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Air Peaks from Sample Injeclion
r-----PERKIN-ELMER 900 Delector Thermal Conductivily Column Flow 5 0 c c / m i n He Column 12' x 11'. Stainless Steel Porapok Q 80/100mesh
512X
Figure 2. Typical gas chromatogram for atmospheric N 2 0 sample
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
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