carbon could be determined if samples were dried inside the system. Thus liquid samples in platinum boats would be inserted into a cold tube, purged with argon, and heated to, for example, 85 “C to volatilize water, C02, and highly volatile organics such as CHC13. The water vapor would be removed to prevent destabilizing the plasma, and total volatile carbon might be determined. However, this procedure would be slower and would result in broader emission peaks, which are most difficult to measure. Also, preliminary work using MnOz (rather than V205)as oxidant has yielded promising results, since it yields oxygen only as required. Thus boats need to be refilled less frequently. In principle, nitrogen and phosphorus can also be determined by plasma emission spectrometry. Although the major emission lines of nitrogen lie in the vacuum UV region (near 112 nm) and are not easily accessible, there are strong CN bands at 388,387, and 386 nm. If nitrogen can be reproducibly converted to CN, total organic nitrogen could be determined. Phosphorus has emission lines at 253, 254, and 255 nm which are readily accessible, but there are considerable problems in volatilizing inorganic phosphates. An instrument which could simultaneously determine organic nitrogen and organic carbon would be exceedingly useful, particularly since most presently used procedures are slow and yield incomplete recoveries.
ACKNOWLEDGMENT The authors thank W. N. Mills and R. Donnelly for assistance with the experimental work.
LITERATURE CITED (1) C. E. Van Hall, J. S. A. Franko. and V. A. Stenaer. Anal. Chem.. 35, 315 (1963). (2) G. W. Fuhs, J . fhycol., 5 , 312 (1969). (3) D. W. Memzel and R. F. Vaccaro, Llmnol. Oceanogr., 9, 136 (1964). (4) Oceanography International Corp. “Operating procedures for the total carbon system model 05248 (preliminary)”, 1974, Oceanography International Coro.. Coileae Station. Texas. (5) Perkin-Elmer, Instructionkd procadures manual for Model 240 Elemental Analyzer, 1969, Perkin-Elmer, Norwalk, Conn. (6) Envirotech CorD., Organic carbon analyzer, Dohrmann Division, Santa Clara, Calif. (7) G. Telek and N. Marshall, Mar. Biol., 24, 219 (1974). (8) A. K. McCormack, S. C. Targ, and W. D. Cooke, Anal. Chem., 37, 1470 11965).
(9) (10) (11) (12)
(13) (14) (15) (16)
C.-A:’Bache and D. J. Lisk, Anal. Chem., 37, 1470 (1965). C. A. Bache and D. J. Lisk, Anal. Chem., 38, 783 (1966). C. A. Bache and D. J. Lisk, Anal. Chem., 39, 786 (1967). H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, Anal. Chem., 42, 876 (1970). E. Canelli and D. G. Mitchell, Water Res., 9, 1093 (1975). D. A. House, Chem. Rev., 62, 165 (1962). J. M. Baldwin and R. E. McAtee, Microchemical J., 19, 179 (1974). P. J. Wangersky, “Analytical Methods in Oceanography”, T. R. P. Gibb, Jr., Ed., ACS, 1975, Washington, D.C.
RECEIVED for review January 10,1977. Accepted May 5,1977.
Shipboard Measurement of Dissolved Nitrous Oxide in Seawater by Electron Capture Gas Chromatography Yuval Cohen School of Oceanography, Oregon State University, Corvallis, Oregon 9733 1
A method has been developed for shlpboard measurement of dissolved nitrous oxide (N20)In seawater. N20Is stripped from solutlon, adsorbed on molecular sieve, and subsequently analyzed by electron capture gas chromatography. I t takes 17 min to analyze a 125-cm3 seawater sample. The precision of measurement is about 2 % and overall accuracy is estlmated at 3%. The method was used to study the N20 distribution In the Eastern Tropical North Pacific.
The geochemistry of nitrous oxide (N20)has recently drawn considerable attention because of its involvement in the chemistry of atmospheric ozone ( I , 2). For evaluation of all sources and sinks for atmospheric N20, its marine geochemistry is of special interest. Data on the oceanic distribution of N 2 0 is also desirable for a better quantitiative understanding of the marine nitrogen budget. Dissolved N20 in seawater has been measured by various gas chromatographic techniques (3-5). Hahn’s ( 4 ) method is suitable for shipboard measurements. However, sample size (5 L), analysis time (more than 1 h per sample), and quite complicated transfer procedures pose severe limitation on the number of samples which can be analyzed a t sea and on the amount of synoptic data which can be obtained from conventional water samplers. T o overcome these difficulties, a precise method for shipboard analysis of dissolved N20 in seawater was developed whereby N 2 0 is stripped from solution 1238
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
by the technique of Swinnerton et al. (6),adsorbed on molecular sieve, and subsequently analyzed by electron capture gas chromatography. The high sensitivity of the high temperature electron capture detector (ECD) for N 2 0 (7) enables a significant reduction in sample size and analysis time. The method can be also used for analysis of N 2 0 in air; however, existing methods are more suitable for air analysis (8, 9).
EXPERIMENTAL Seawater Sampling and Preconcentration of NzO. Seawater samples are taken in glass 125-cm3gas-analysisbottles with Teflon stopcocks. The bottles are filled from below and allowed to overflow about two thirds of their volume. Samples are stored in the dark at 5 “C until analyzed-typically within less than 14 h from sampling. The analysis system, shown schematicallyin Figure 1,utilizes nitrogen for both the stripping and carrier gas streams. Ultra-pure nitrogen is further purified by passing it through an Oxisorb trap in order to increase the detector standing current and to reduce the N 2 0 blank to below detection limit. Flow of both streams is controlled with Brooks model 8744 flow controllers. Sample bottles are connected to the system as shown in Figure 1,and the connecting lines are evacuated with both 4-way valves (A and B) in the solid line position. Then valve B is turned to the dashed line position, the sample bottle stopcocks are opened, and the stripping gas, flowing at 120 cm3/min, forces the sample into the stripper (34 mm 0.d. x 200 mm, coarse frit). When all the water has been pushed through the frit, stripping of the dissolved gases begins. The gases swept from solution pass through Drierite and molecular sieve type 3A for removal of water vapor, through
cI
loot
5001
600
Figure 1.
I
I
Switchng,heatmg,
0.2
0.4
0.6
0.8
,I 1.0
CONCENTRATION (pprnV)
NO ,
Gas flow diagram of apparatus used for measurement of
N20
0
J,
Figure 3. Vertical N 2 0 distribution at three hydrographic stations in the Eastern Tropical North Pacific. Station locations: (0)12' 31.7'
N, 111' 52.3' W; (0) 13' 45.1' N, 111' 52.3' W; (A) 08" 42.4' N, 105' 56.2' W. Dashed line is the N 2 0 equilibrium solubility (average for the three stations)with respect to air of 287 ppbv N 2 0 mixing ratio
cedure used for analysis of unknown samples. The calibration mixture is injected via a gas sampling valve (Carle model 5521) with a 0.2-cm3sample loop at ambient temperature (measured to k0.05 "C with a calibrated thermometer Pressure in the sample loop is measured to h1 mm Hg with a solid state pressure sensor (National Semiconductor model LX1703A) with a digital readout. The concentration of the unknown liquid sample is given by
c =A- ., - .V,- . -T,oc P, A,
V , T, Po
TIME (min.)
Figure 2.
Gas chromatogram of a seawater sample taken from the
N 2 0 maximum in the Eastern Tropical North Pacific. Output attenuation
is X512
Ascarite for removal of COz, and, finally, through a U-shaped stainless steel trap (6.35 mm 0.d. X 300 mm) packed with 30/40 mesh molecular sieve type 13X for collection of NzO. During collection,the trap is held in an ice-water bath and switching valve C (Carle model 5521) is in the solid line position. Using the equation given by Weiss and Craig (IO),the calculated time required to reduce the concentration of N20 in the stripper by a factor of e is less than 1min. To ensure complete extraction of NzO, samples are stripped for 10 min. To introduce the next sample, the shutoff valve at the end of the line is closed and the stripper is drained to slightly above the level of the drain valve. Thus, no air contamination is introduced into the system between consecutive samples. Gas Chromatography. When stripping is complete, the NzO adsorbed on the molecular sieve is transferred to the gas chromatograph by switching valve C to the dashed line position and immediately heating the molecular sieve trap to 250 "C with an electrically heated aluminum block. The trap is heated for 2 min a t which time valve C is turned back to its original position. Analysis is performed with an Analog Technology model 140-A pulsed electron capture detector with a tritiated scandium source. The chromatographic column is stainless steel (3.2 mm 0.d. X 3 m) packed with 60/80 mesh molecular sieve type 5A. Operating conditions are as follows: column oven temperature, 250 "C; detector temperature, 320 "C; carrier flow rate, 30 cm3/min; detector standing current, typically 1.7 X lo-' A. Output signal is displayed on a Hewlett-Packard model 7128A recorder and the peaks are integrated with a Hewlett-Packard model 3373B integrator. The N 2 0 retention time is about 4.5 min (Figure 2). Total analysis time is approximately 17 min. Calibration. Calibration is by injection of a commercial N20/N2 mixture into the stripper when it is filled with seawater that has been previously stripped of its dissolved gases. The calibration mixture is subsequently analyzed by the same pro-
where A, and A, are the peak area for NzO in the unknown sample and the calibration mixture, respectively; C, is the NzO concentration in the calibration mixture; V, is the sample loop volume; VI is the liquid volume; P, and T, are the pressure and absolute ' and are the pressure temperature in the sample loop; and P and absolute temperature at standard conditions. C, is in units of C,. A nonideality correction (8) is negligible in this case. Calibration is performed every 2-3 h of operation. Air Analysis. Air samples are taken in 50-cm3glass bottles similar to those used for seawater sampling. Analysis is carried out through the stripper by the same procedure used for seawater analysis so that no separate calibration procedure is required.
RESULTS Nitrous oxide concentrations in seawater and air samples from the Eastern Tropical North Pacific (ETNP) were measured aboard R/V Wecoma during January 1977 (Weloc 77 Cruise, Leg 1). More than 200 seawater samples from various depths between the sea surface and 3000 m were analyzed. The precision of analysis was evaluated from 15 sets of duplicate shipboard measurements covering the range of concentrations from 140 to 870 ppbv. The standard deviation for these measurement,s was f 6 ppbv, or 2.2% of the average of all the duplicate measurements. Overall accuracy of the analysis is estimated a t 3.070,the main source of uncertainty being the calibration mixtures used. Comparison of two commercial standards (Matheson Gas Products and Airco Industrial Gases), analyzed by the manufacturers t o f2.0% showed agreement with the manufacturer's specifications. Sixteen air samples were analyzed during the cruise. The average NzO mixing ratio found was 287 f 9 ppbv (3.170, one relative standard deviation). As an illustration of the performance of the method, the vertical NzO distribution, down t o 600 m, a t three stations in the E T N P is shown in Figure 3. Also plotted is the NzO ANALYTICAL CHEMISTRY, VOL.
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equilibrium solubility with respect to air of 287 ppbv NzO mixing ratio. The observed vertical distributions of N 2 0 at the three stations are essentially the same, the water being supersaturated a t all depths. Such distributions are typical of stations located outside the core of the extensive oxygen minimum zone off the Central American coast. A detailed discussion of the cruise results will appear elsewhere (11).
DISCUSSION The method described here proved to be convenient for shipboard operation and performed as well at sea as in the laboratory. The high sensitivity of the ECD for NzO makes it possible to carry out the analysis at relatively high signal attenuations which minimize the effects of the ship's motion on detector response. The ECDs selectivity eliminates interference from COzand traces of Nzretained on the molecular sieve trap and allows a direct transfer procedure from the trap to the chromatographic column. Experimental conditions such as column temperature, flow rates, and the temperature to which the trap is heated (as long as it is between 200 OC and 300 "C) are not critical. The values listed in the Experimental section gave optimal results for the N20 peak shape and the separation between the switching peaks and the N 2 0peak. Proper drying of the gas stream that exits the stripper is critical. When Drierite alone is used, traces of water vapor from one sample interfere with the next sample. The combination of the high capacity of Drierite and the high efficiency of molecular sieve type 3A proved satisfactory. Within the precision of the method, storage of seawater samples for up to 15 h under the conditions described in the Experimental section did not have any apparent effect on their N20 concentrations. If one wishes to store samples for longer periods of time, the possibility of alteration of their NzO concentrations due to microbial activity should be investigated and, if necessary, the samples should be poisoned. Yoshinari (5) found marked changes of the NzO concentration in samples from the oxygen minimum in the Atlanic Ocean after they were stored for about one month a t room temperature. T o accurately calibrate the system, the ideal approach would be to analyze standard solutions of known NzO content. Unfortunately this cannot be done because the solubility of N 2 0 in seawater is not yet accurately known. The calibration method used here, whereby a standard mixture is injected into the stripper filled with prepurged seawater, simulates the over-all sample treatment and, therefore, is a better approach to the ideal than direct injection of standards onto the chromatographic column. The variability of N20 mixing ratio in the air samples analyzed is greater than that found by other workers in the same general geographic region (8,12). This is possibly due to the limited number of samples analyzed. However, the average absolute atmospheric N20 mixing ratio in the ETNP reported here (287 ppbv) is, within experimental error, the same as that found by Weiss (12) in the Eastern Pacific (296 ppbv) and by Singh et al. (9) near the California Coast (296
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ppbv). The good agreement among the findings obtained by three different laboratories using different analytical techniques lends confidence to the results reported here. The accuracy of the seawater analysis should be comparable to that of the air analysis as the same analytical procedure is used. The average atmospheric NzO mixing ratio in the Eastern Pacific reported by Rasmussen et al. (8) (332 ppbv) is about 13% higher than the values listed above. Intercalibration of the various methods currently used for NzO analysis is highly desirable in order to resolve the systematic differences among them. While this work was in progress, Rasmussen et al. (13) reported the use of a multiple phase equilibration technique for shipboard measurements of dissolved N2O in seawater. Their method is comparable to the method described here in terms of sample size and total analysis time, but their precision is lower than that reported here by about a factor of two. Improved precision, small sample size, and relatively short analysis time are the major advantages of the method presented. During the Weloc 77 cruise, sampling plans for hydrographic stations could be based on results obtained from previous stations and, in addition to NzO, a large amount of data including salinity, dissolved oxygen, pH, and nutrients could be obtained from 1-L water samples.
ACKNOWLEDGMENT I thank Louis I. Gordon for his helpful suggestions and constructive criticism throughout the course of this work. Creative discussions with Marvin D. Lilley were invaluable. Mark J. Borgerson kindly made available for me the pressure sensor circuitry. I very much appreciate the assistance of the captain, crew and scientific party aboard R/V Wecoma during the Weloc 77 cruise.
LITERATURE CITED M. B. McEiroy, J. E. Elkins, S. C. Wofsky, and Y. L. Yung, Rev. Geopbys. Space Pbys., 14, 143 (1976). P. J. Crutzen, Geopbys. Res. Lett., 3 , 169 (1976). H. Craig and L. I. Gordon, Geochim. Cosrnocbim. Acta, 27, 949 (1963). J. Hahn, Anal. Cbem., 44, 1889 (1972). T. Yoshinari, Mar. Cbern., 4, 189 (1976). J. W. Swinnerton. V . J. Linnenbaum. and C. H. Cheek, Anal. Cbem.. 3 4 , 483 (1962). W. E. Wentworth and R. R. Freeman, J . Cbromatogr., 79, 322 (1973). R. A. Rasmussen, J. Krasnec, and D. Pierotti, Geophys. Res. Lett., 3 , 615 11976) \
-I
H:B. Singh, L. J. Saias and L. A. Cavanagh, paper presented at the 69th Annual Meeting of the Air Pollution Control Association, June 27-July 1, 1976, Portland, Ore. R. F. Weiss and H. Craig, Deep Sea Res., 20, 291 (1973). Y . Cohen and L. I. Gordon, unwblished work, Oreaon State University, Corvaiiis, Ore., 1977. R. F. Weiss, unpubilshed work, Scripps Institution of Oceanography, La Jolla. Calif.. 1977. R . A. Rasmussen, D. Pierotti, J. Krasnec, and B. Halter, Report on the cruise of the Alpha HelixResearch Vessel, March 5 to 20, 1976, N.S.F. Grant No. OCE-75 04688 A03.
RECEN~D for review March 10,1977. Accepted April 28,1977. This research was supported by the Office of Naval Research through contact N00014-76-C-0067 under project NR083-102.
