Anal. Chem. 1980, 52, 259-263
Detection limits were also obtained when the plasma gas flow contained 100% N2 at the same experimental conditions. Compared to the Ar-10% N2 ICP results presented in the table, all detection limits deteriorated by a factor of 2-5. It should be borne in mind that the present experimental parameters employed for the Ar plasma are generally similar to the optimum conditions used in other laboratories. However, the Ar-N2 ICP detection limits can be improved if the gas flow rates, the forward power, and the observation height are optimized. The Ar-ICP and the Ar-10% N2 ICP were also compared for the influence of 50 pmol/mL of A1 on the determination of 0.05 pmol/mL of Ca at the spectral line of 317.9 nm. The experimental parameters were similar to those for detection limit determinations, except for the observation height which was 15 mm for the two plasmas. The A1 suppressed the net Ca emission by 15 and 5% for the Ar plasma and the Ar-N2 plasma, respectively. Similar results also noted (11) by other investigators indicate that the Ar-N2 ICP is hotter than the corresponding Ar plasma.
LITERATURE CITED (1) R. M. Barnes, Crlt. Rev. Anal. Cbem., 7, 203 (1978). (2) V. A. Fassel and R. N. Kniseley, Anal. Chem., 46, I l l O A , 1155A (1974). (3) S.Greenfield, I. L. Jones, H. McGeachin, and P. B. Smith, Anal. Cbim. Acta, 74, 225 (1975).
(4)
(5) (6) (7) (8) (9)
(IO) (11)
(12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24)
259
P. W. J. M. Bournans and F. J. DeBoer, Spectrochim. Acta, Part 8,30,
309 (1975). R. H. Wendt and V. A. Fassel, Anal. Chem., 38, 337 (1966). Akbar Montaser and V. A. Fassel, Anal. Cbem., 48, 1490 (1976). S.Greenfield, Proc. Soc. Anal. Cbem,.,2, 111 (1965). S.Greenfield and P. B. Smith, Anal. Chirn. Acfa, 57, 209 (1971). S. Greenfield and P. 8. Smith, Anal. Cbim. Acta, 59, 341 (1972). S.Greenfield and H. McGeachin, Anal. Chim. Acta. 100, 101 (1976). M. Capitelli, F. Crarnarossa, L. Triolo, and M. Molinari, Combust. Flame, 15, 23 (1970). D. Truitt and J. W. Robinson, Anal. Cbim. Acta, 49, 401 (1970). D.Truitt and J. W. Robinson, Anal. Cbim. Acta, 50, 61 (1970). S. Greenfield, I. L. Jones, and C. T. Berry, Analyst(London), 89, 713 (1964). G. W. Dickinson and V . A. Fassei. Anal. Cbem., 41, 1021 (1969). P. W. J. M. Boumans and F. J. DeBoer, Specfrochim. Acta, Part 5 , 27 391 (1972). R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon, Anal. Chem., 46, 75 (1974). C. Veillon and M. Margoshes, Spectrochim. Acta, Part 8,23, 503 (1968). R. M. Barnes and S.Nikdel, J . Appl. Pbys., 47, 3929 (1976). R . M. Barnes and S.Nikdel, Appl. Spectrosc., 30. 310 (1976). M. Thompson, 8. Pahhvanpour, and S.J. Watton, Analysf(London), 103, 568 (1978). P. B. Zeernans, S. P. Terblanche, K. Visser, and F. H. Harnm. Appl. Specfrosc., 32, 572 (1978). B. Bogdain, Kontron ICP-Berichet, Ausgabe 2, February 1978. T. 8.Reed, Adv. High Temp. Cbern., 1, 284 (1967).
RECEIVED for review December 28, 1978. Resubmitted July 25, 1979. Accepted October 30, 1979. Presented in part a t the 1978 FACCS Meeting, Boston, Mass.
Determination of the Aqueous Chlorination Products of Humic Substances by Gas Chromatography with Microwave Emission Detection Bruce D. Ouimby,’ Michael F. Delaney,’ Peter C. Uden,’ and Ramon M. Barnes Department of Chemistry, GRC Tower
I, University of Massachusetts, Amherst, Massachusetts 0 1003
The aqueous chlorination products of humic and fulvlc acids have been examined by capillary column gas chromatography with an atmospheric pressure hellurn microwave emission detection system. The results indicate that in addition to trihalomethanes, significant numbers of chlorinated phenolic and/or other acidic compounds can be formed. I n addition, the presence of bromide ion in the chlorination mixture is shown to produce bromine-containing compounds. Since many of these compounds cannot be gas chromatographeddirectly, chemical derivatization with diazomethane is employed. The number of halogen-containing substances that appear in the element selective chromatograms is greatly Increased after methylatlon of the samples. Several halogenated phenols and aromatic carboxylic acids have been tentatively identified by retention times.
Halogenated organic compounds in finished drinking water have received much recent attention. In 1974 Rook ( 1 ) and Bellar, Lichtenberg, and Kroner (2) demonstrated that organic precursors in raw water are halogenated by chlorine, producing Present address: Hewlett-Packard Co., Route 41, Avondale, Pa.,
19311. * P r e s e n t address: D e p a r t m e n t of Chemistry, T u f t s University, M e d f o r d , Mass. 02154. 0003-2700/60/0352-0259$01.OO/O
trihalomethanes; a comprehensive review of the area has been published by Trussell and Umphres ( 3 ) . In general, studies indicate that the action of chlorine on humic substances in the raw water is the largest source of trihalomethanes. Very little data however regarding other possible chlorination products of humic substances are available and it is to these that the present study is addressed. Humic substances, amorphous, brown or black, hydrophilic, acidic, polydisperse substances with molecular weights ranging from several hundreds to tens of thousands ( 4 ,are classified as fulvic acid, humic acid, and hymatomelanic acid according to solubility in acid, alkali, and ethanol in the classical soil organic matter classification scheme. All three fractions are found in naturally colored waters, fulvic acid being at much higher concentration than hymatonielanic and humic acids (5, 6). It is accepted that the structures of humic materials vary with location and different conditions but consist mainly of aromatic polyhydroxy, polymethoxy, polycarboxylic acids with smaller amounts of sugars and nitrogen bases. It is not, however, clear to what extent the structures are held together by hydrogen bonds as opposed to being condensed with covalent bonds. Speculative structures were proposed by Dragunov (a,Christman and Ghassemi (8)as shown in Figure 1, and Schnitzer ( 4 ,based on various degradative or nondegradative techniques involving derivatization elemental analysis, functional group analysis, reducing properties, etc. (9). Q 1980 American Chemical Society
260
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 COOH
I
-N
Figure 1. Structure for the "color macromolecule" of humic material in natural water as proposed by Christman and Ghassemi (8)
It is expected that these materials will chlorinate by electrophilic aromatic ring substitution, due to strongly activating hydroxy and moderately activating methoxy groups. Phenolic compounds react with chlorine in aqueous media; the Manufacturing Chemists Association (10) studied the reaction (if any) of 14 compounds with chlorine under wastewater treatment conditions; phenol, for example, was found to react with chlorine to give a series of five chlorophenols plus nonaromatic oxidation products. Investigating the reaction of aromatic acids and aldehydes with chlorine, Hopkins and Chisholm (11) found that many reacted rapidly under basic conditions; included in this study were p-hydroxybenzoic acid, vanillic acid, anisic acid, mmethylbenzoic acid, vanillin, and 2,4-dihydroxybenzaldehyde. It is generally accepted that humic substances, at least in part, have lignin as a precursor substance, and that humic substances show some similarities to lignin (Le., in their degradation products (8)). In a study of aqueous chlorination reactions with lignin (12),it was found that aliphatic chlorocarboxylic acids, chloroguiacols, chlorocatechols, chloro-obenzoquinones, and chloromuconic acids were formed. We have used glass capillary column gas chromatography with atmospheric pressure helium microwave emission detection (MED) (13, 14) t o examine the halogenated chlorination products of humic substances. Because of the structural complexity of humic materials, chlorination may result in a large number of products, thus making capillary columns the method of choice. Since the primary objective was t o screen for halogenated compounds, the MED was chosen instead of interfaced gas chromatography/mass spectrometry (GC/MS) for the initial phase. In addition t o being much simpler and less expensive t o operate and maintain, the MED produces chromatograms with response only (within the selectivity limits of the system) to compounds containing chlorine or, by monitoring a different wavelength, only to compounds which contain bromine. The molar response factors for halogens contained in different chemical environments are relatively uniform (at least within ca. 15% ( 1 4 ) ) and thus the resulting chromatograms not only indicate which compounds contain a particular halogen, but also provide information on the relative amounts of that halogen contained in different analyte peaks. Once this information about halogen content is obtained, it can later be used to assign priorities as to which peaks must be examined mass spectrally. EXPERIMENTAL Gas Chromatograph-Microwave Emission Detector System. The complete GC/MED system employed and the
optimum instrumental parameters for detection of the halogens have been described in detail previously (13,14). A 100 m X 0.4 mm i.d. OV-225glass support coated open tubular (SCOT) column was prepared according to the method described by Cramers et al. (25) and was used throughout the investigation. The gas chromatograph (Varian 2440) was adapted for use with glass capillary columns by replacing the 1/4-in.0.d. stainless steel injection sleeve with a glass injection sleeve of 6-mm 0.d. and 1-mm i.d. having a stainless steel 1 / 4 inch X inch zero dead volume reducing union attached to the end. The column was supported in a metal screen cage and was connected to the injector sleeve by '/,6-inch 0.d. glass lined stainless steel tubing and a zero dead volume capillary union from a commercially available kit (Scientific Glass Engineering Pty., Melbourne, Australia). The exit of the column was connected to the effluent splitter (13) of the GC/MED system with '/,,-inch 0.d. stainless steel tubing, and the capillary tee fitting (SGE) which joined the column exit with this tubing allowed the introduction of a make-up gas. Preparation of Fulvic and Humic Acid Samples. Approximately 5 kg of an Al moderately well drained soil was obtained from a location about 50 feet from the shoreline of a local municipal reservoir. Humic and fulvic acids were extracted from the soil using a procedure similar to that of Snoeyink et al. (26). Approximately 2 kg of soil were placed in a glass column constructed to allow continuous upflow of 0.1 M sodium pyrophosphate in distilled deionized water (8 L) through the bed. A f k cycling for three days, the solution was centrifuged at 2000 rpm for 2 h to remove colloidal material and was then acidified to pH 1 with HC1 and stirred for 1 h. The precipitated humic acid was recovered by centrifugation as above, and dried under a stream of nitrogen. The fulvic acid which remained in solution was recovered by applying 1-L volumes of the remaining solution to a 30 cm X 3 cm i.d. column of XAD-8 macroreticular resin which had previously been washed with methanol and Soxhlet-extracted with ether, acetonitrile, and methanol each for 8 h. After the entire liter of solution was passed through the column, the bed was rinsed with pH 1 (HC1) distilled deionized water, and the fulvic acid was then desorbed by passing about 500 mL of 0.1 M NaOH through the column. The resulting fulvic acid solution was then adjusted to pH 7 (HCl), reduced in volume to about 7 5 mL by vacuum distillation, and taken to dryness under a stream of nitrogen. Laboratory Chlorinations. Humic and fulvic acid samples were chlorinated in the laboratory as follows. Approximately 200 mg of either humic or fulvic acid was dissolved in 100 mL of distilled deionized water and the pH adjusted to 6.6 (HCI). Three mL of an aqueous solution of sodium hypochlorite, determined by the iodometric method (17) to contain 37 mg/mL of available chlorine, were added to the sample solution which was then stirred at 25 "C for 30 min. To study the effect of bromide ion on the reactions, the same procedure was used except that 89 mg of sodium bromide was added to the sample solution prior to chlorination. After the 30-min contact time, the chlorination was stopped by adding 1g of hydroxylamine hydrochloride and stirring for 5 min followed by 1 g of sodium bisulfite. The chlorination mixture was then acidified to pH 1 (HC1) and extracted with two 25-mL portions of diethyl ether which had previously been distilled using a 3-ball Snyder distillation column. The resulting ether extract was placed in a tapered bottom flask fitted with a 3-ball Snyder distillation column and was reduced in volume to about 2 mL as described by Junk et al. (18). The resulting concentrate was split into two equal portions, one of which was treated with diazomethane in diethyl ether solution (prepared as described by Fales et al. (19)). The methylated and unmethylated extracts were further reduced in volume tc 7 5 pL and placed in glass injection vials with PTFE lined caps. The samples were then gas chromatographed immediately. Tap Water Sample. The reservoir next to the source of the soil sample investigated flows into a mixing reservoir where the water combines with that from another reservoir located a short distance away. The water from this mixing reservoir is then chlorinated with chlorine gas and flows into the distribution system. Four liters of this water were obtained from the tap in a building located downstream from the chlorinator. The sample was treated with 150 mg each of hydroxylamine hydrochloride and sodium bisulfite as described by Chriswell et al. (20).who recommend use of these reductants when phenols are to be an-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 CHLORINATED
FULVIC
CHLOR~NA-ED FUMIC
ACID
YON
I I1
NONMETHYLATED
METHYLATED
-
261
ACID I
l i
M E TH Y LATED
x ;CI(II) 4 8 1 Onm
21 0
170
130
90 -2°C
50 i min
Flgure 2. Chlorine selective capillary gas chromatogram of non-
methyhted chlorinated fuhic acid extract (above)and chlorinated fuhic acid extract methylated with diazomethane (below). Peak identities: (above) (1) chloroform, (2) trichloroacetic acid, (3) 1-chlorophenol; (below) (1) chloroform, (2) trichloroacetic acid methyl ester, (3) 2,4,6trichlorophenoI methyl ester, (4) Pchlorobenzdc acid methyl ester, (5) 3,5-dichlorobenzoic acid methyl ester, (6) 1-chlorophenol,(7) 2chlorophenol, (8) pentachlorphenol methyl ether. Column 100 m X 0.4 mm i.d. OV 225 glass suppor! coated open tubular (SCOT)column
alyzed t o ensure no structural changes on reaction. After acidification to pH 1 with HC1, the solution was passed at 4 mL/min through a column containing XAD-2 macroreticular resin, and the adsorbed organics were then recovered with distilled diethyl ether (see above). After reduction in volume of the ether extract to 2 mL, the sample was methylated with diazomethane,and then handled in the same manner as the samples discussed above. Materials. Diethyl ether, chloroform, and all of the inorganic reagents employed were purchased from Fisher Scientific Co. Bromodichloromethane and chlorodibromomethane were obtained from RFR Corp. (Hope, R.I.), bromoform from Matheson, Coleman and Bell (Norwood, Ohio), N-methyl-N-nitroso-Nnitroguanidine and the apparatus for diazomethane generation from Aldrich Chemical Co., (Milwaukee, Wis.), and the remaining chemicals from Eastman Kodak Co. The helium used in the GC/MED system was commercial grade, passed through a molecular sieve 3A trap. Gas Chromatographic Procedure. The injection port, transfer line, valve oven, and FID block were maintained at 240 "C throughout. The helium carrier gas flow was adjusted to 4 niL/min, and make-up gas added at the end of the column at 50 mL/min. The MED system was set to the optimal conditions for halogen detection (80 W microwave power to the cavity and a total flow rate of helium through the discharge tube equal to cn. 50 mL/min) and the spectrometer set to either the Cl(I1) 481.0-nm wavelength for chlorine selective detection or the Br(I1) 470.5-nm wavelength for bromine selective detection, according to procedures described previously (13,14). Using a l-gL micro syringe, 0.5 pL of the sample ether extract was injected with the high temperature divert valve placed in the "vent" position and the column temperature maintained at 50 "C. After 9 min (during which time the bulk of the solvent eluted), the valve was turned such that the column effluent (after passing through the splitter) was directed into the plasma, and the column temperature was programmed from 50 to 210 "C at 2 "C/min.
RESULTS AND DISCUSSION Laboratory Chlorinations. T o assess what type of organohalides are formed during the aqueous chlorination of humic substances, samples of fulvic and humic acids were chlorinated as described above. The ratio of chlorine to humic material was maintained a t a level representative of that commonly found in municipal water supplies, but the concentrations of chlorine and sample employed were about lo3 times greater, to simplify the concentration procedure.
210
770
130
-
90
53
50'c
2 ' ~ ~ r ~ i n
Figure 3. Chlorine selective capillary gas chromatogram of nonmethyhted chlorinated humic acid extract (above)and chlorinated humic acid extract methylated with diazomethane (below). Peak identities: (above) (1) chloroform, (2) trichloroacetic acid, (3) 1-chlorophenol; (below) (1) chloroform, (2) trichloroacetic acid methyl ester, (3) 2,4,6-trichlorophenoI methyl ether, (4) 1-chlorophenol, (5) pentachlorophenol methyl ether. Column as in Figure 2
The chlorine selective chromatogram from the ether extract (not methylated) of the chlorinated fulvic acid sample is shown in Figure 2a; the chromatogram is relatively uncomplicated. The compounds which have been tentatively identified by retention time correlation are chloroform, 1-chlorophenol,and trichloroacetic acid (the large, very broad peak). In a similar laboratory chlorination experiment using peat extracts as the source of humic material, Rook (21) also obtained relatively few chlorinated components in the ether extract, identifying chloral, chloroform, dichloromethane, dichloroacetic acid, trichloropropylene, chloroisopentanol, tetrachloroacetone, and pentachloroacetone by GC/MS. Since humic materials are known to contain substituted hydroxy and carboxylic acid functionalities on aromatic rings, it is reasonable to expect that chlorinated phenolic compounds and chlorinated aromatic acids would be formed upon chlorination. However, many phenolic compounds, particularly those containing more than one hydroxyl group, as well as aromatic acids, cannot be readily gas chromatographed directly. Thus, while the above experiments indicated relatively few chlorinated products from the interaction of aqueous chlorine and humic material, these results may be misleading, since two large classes of potential chlorinated products are excluded from the analysis under the conditions employed. One means of making phenolic and acidic compounds amenable to gas chromatographic analysis is t o convert the hydroxyl groups of phenols to methyl ethers and the carboxylic acid groups to methyl esters by means of a methylating reagent. Diazomethane was chosen a8 the methylating agent because it reacts under mild conditions (strong acids or bases are not required) yielding only volatile end products when used properly (22, 23). I t also has the ability to methylate both phenols and carboxylic acids and is one of the only methylating agents capable of completely esterifying all carboxyl groups on benzene polycarboxylic acids (e.g., benzenepentacarboxylic acid) (24). Diazomethane was used co methylate a portion of the ether extract discussed above, the resulting chlorine selective chromatogram being presented in Figure 2b. The number of chlorinated compounds in the chromatogram is greatly increased, indicating that a significant number of phenolic and/or acidic chlorinated compounds are formed. I t is noted that the very broad trichloroacetic acid peak is absent, being replaced by a large peak corresponding to methyl trichloroacetate. Other compounds tentatively identified
262
ANALYTICAL CHEMISTRY, VOL. 52, NO. 2, FEBRUARY 1980 CHLORINATED FULVIC ACID ( + N a B r )
NONMETHYLATED
210
170
130
90 +-
50 ~ " ~ t r n i n
50'C
Figure 4. Bromine selective capillary gas chromatogram of nonmethylated fulvic acid extract chlorinated in the presence of bromide (above) and extract methylated with diazomethane (below). Peak identiis: (above)(1) bromodichloromethane, (2) chlorodibromomethane, (3)bromoform, (4) 3-bromophenol; (below)(1) bromdichloromethane, (2) chlorodibromomethane, (3) bromoform, (4) 3-bromophenol methyl ether, (5)1-bromobenzoic acid methyl ester, (6) 34romophenol. Column as in Figure 2 include chloroform, 2,4,6-trichlorophenol methyl ether, methyl 2-chlorobenzoate, methyl 3,5-dichlorobenzoate,1-chlorophenol, 2-chlorophenol, and pentachlorophenol methyl ether. Identical chlorination experiments using humic acid were carried out, with the resulting chlorine selective chromatograms presented in Figure 3. Comparison of the chromatograms from the ether extracts before methylation for both fulvic and humic acid (Figures 2a and 3a) shows that while generally the same components are present in each case, significant differences exist between the relative quantities of a given component in the two samples. While in a few cases a particular compound is present in higher quantity in the fulvic acid extract, in most cases the amounts of chlorinated material are considerably higher in the humic acid extract. This observation is consistent with the findings of Babcock and Singer (25),in which humic acid was found to be more
5 14
reactive to chlorine than fulvic acid, producing significantly higher levels of chloroform when chlorinated under the same conditions. Comparison of the results from the methylated extracts of the fulvic and humic acid chlorination mixtures in Figures 2b and 3b again indicates that while in general the same components are present in each case, some compounds are present in significantly higher quantities in one extract compared to the other. For example, pentachlorophenol methyl ether is present a t the same level in both extracts, while the level of 2,4,6-trichlorophenol methyl ether in the humic acid extract is about 60 times that in the fulvic extract. The presence of bromide ion in water containing humic materials is known to result in the formation of bromine containing trihalomethanes (3)upon chlorination, presumably through the formation of hypobromite and related species from the oxidation of bromide by chlorine. It is thus expected that the presence of bromide should also result in the formation of phenols and carboxylic acids containing bromine and chlorine in a manner similar to that discussed above with respect to chlorine. T o confirm this prediction, the same chlorination procedure was carried out for fulvic acid with 89 mg of sodium bromide added to the mixture before chlorination. Bromine selective chromatograms for the nonmethylated and methylated extracts of this reaction mixture are presented in Figures 4a and 4b, respectively. As before, the chromatogram from the unmethylated sample is relatively uncomplicated, with the three bromine-containing trihalomethanes (bromoform, chlorodibromomethane, and bromodichloromethane) being the most significant peaks. Also tentatively indentified was 3-bromophenol. Methylation of the ether extract again resulted in a dramatic increase in the number of brominated compounds in the chromatogram, indicating the formation of many bromine-containing phenolic and/or acidic species. Tap Water Sample. In view of these findings, it is quite apparent that the chlorination of humic and fulvic acid in aqueous solution can result in the formation of halogenated phenolic and acidic compounds. As was noted earlier, however, while the ratios of chlorine to humic material in these experiments was kept in a range similar to that found in municipal water supplies, their concentrations were maintained higher by a factor of about lo3 to simplify concentration of
1
. 5 nm
Figure 5. Chlorine selective capillary gas chromatogram of tap water extract methylated with diazomethane (above) and parallel bromine selective chromatogram (below). Peak identities: (above)(1) chloroform, (2) bromodiihloromethane,(3)trichloroaceticacid methyl ester, (4) 2,4,&trichlorophenol methyl ether, (5)2-chlorobenzoic acid methyl ester, (6) pentachlorophenol methyl ether: (below) (1) bromodichloromethane, (2) 3-bromophenol methyl ether. Column as in Figure 2
Anal. Chem. 1980,
the reaction products. Since the presence of halogenated phenolic and acidic compounds formed during chlorination of drinking water is of potent.ial health significance,a tap water sample was analyzed in a similar manner. The particular sampling site (see Experimental) was chosen because one of the reservoirs from which the water is drawn for chlorination is located about 50 feet from the point where the soil for the humic and fulvic acid samples was obtained. Thus, the humic materials in the reservoir water would probably show a similarity to those extracted from the soil. Since the chlorine and humic materials are present in the municipal chlorination system a t concentrations approximately one thousand times lower than in the laboratory chlorinations, adsorption of the chlorination products from the tap water sample onto XAD-2 macroreticular resin was used to obtain a higher concentration factor. The resulting ether extract was then methylated, and the bromine and chlorine selective chromatograms are presented in Figure 5. As with the laboratory chlorination experiments, a significant number of chlorinated compounds are evident. Those that have been tentatively identified (by retention times) are chloroform, bromodichloromethane, methyl trichloroacetate, 2,4,6-trichlorophenol methyl ether, methyl 2-chlorobenzoate, and pentachlorophenol methyl ether (peaks 1-6). The bromine selective trace indicates fewer brominated compounds to be present, with bromodichloromethane and 3-bromophenol methyl ether being identified. T o obtain an estimate of the approximate level of these compounds in the original tap water sample, peak number 6 in Figure 5a, which corresponds to pentachlorophenol methyl ether, represents about 0.5 ng of chlorine entering the plasma. Accounting for the split ratio and assuming that the concentration and methylation steps proceed quantitatively, the concentration of pentachlorophenol in the original sample is estimated to be approximately 40 parts per trillion.
ACKNOWLEDGMENT T h e authors thank Richard K. Brehm for the use of the microwave cavity, and William G. Elliott of Elliott Labs,
263
52,263-267
Acton, Mass., for making available the echelle spectrometer and providing helpful suggestions with respect to its use.
LITERATURE CITED Rook. J. J. Water Treat. Exam. 1974, 23, 234. Bellar, T. A.; Lichtenberg, J. J.; Kroner, R. C. J . Am. Water Works Assoc. 1974, 66, 703. Trussell, R. R.; Umphres, M. D. J . Am. Water Works Assoc. 1978, 70, 604. Schnitzer, M.; Khan, S. U. "Humic Substances in the Environment"; Marcel Dekker: New York, 1972. Black, A.; Christman, R. J . Am. Water Works Assoc. 1963, 55, 897. Packham, R. Proc. SOC. Water Treat. Exam. 1964, 13, 316. Dragunov, S. I n "Soil Organic Matter", Kononova, M. M., Ed., Pergamon Press: New York, 1961. Christman. R. F.;Ghassemi, M. J . Am. Water Works Assoc. 1988, 58, 723. Kleinhampel, D. Albrecht-Thaer-Arch. 1970, 14, 3. ManufacturingChemists Assoc., EPA Project 12020 EXG, March 1972. Hopkins, C. Y.; Chisholm, M. J. Can. J . Res., Sect. 5 1948, 2 4 , 208. Das, B. S.; Reid, S. G.; Betts, J. L.; Patrick, K. J . fish. Res. 5d. Can. 1969, 2 6 , 3055. Quimby, B. D.; Uden, P. C.; Barnes, R. M. Anal. Chem. 1978, 50, 2112. Quimby. B. D.; Delaney, M. F.; Uden, P. C.; Barnes, R. M. Anal. Chern. 1979. 51. 875. Cramers, 'CI A.;Vermeer, E. A.; Franken, J. J. Chromatographia1977, IO, 413. Snoeyink, V. L.; McCreary, J. J.; Martin, C. J. "Activated Carbon Adsorption of Trace Organic Compounds", U.S. Environ. Prot. Agency Rep. EPA-60012-77-223 (December 1977); p 13. "Standard Methods fw the Examination of Water and Wastewater", 13th ed.; American Public Health Association: Washington, D.C., 1971. Junk, G. A.; Richard, J. J.; Grieser, M. D.; Witiak, D.; Witiak, J. L.; Arguello, M. D.; Vick, R.; Svec, H. J.: Fritz, J. S.; Calder, G. V. J . Chromatogr. 1974. 9 9 , 745. Fales, H. M.; Jaouni, T. M.; Babashak, J. F. Anal. Chem. 1973, 45, 2302. Chriswell, C. D.; Chang, R. C.; Fritz, J. S. Anal. Chem. 1975, 47, 1325. Rook, J. J . Environ. Sci. Techno/. 1977, 11, 478. Schlenk, H.; Gellerman, J. L. Anal. Chem. 1980, 32, 1412. Blau, K.; King, G., Eds. "Handbook of Derivatives for Chromatography"; Heyden and Sons: Philadelphia, Pa.. 1977; p 49. Schnitzer, M.; Desjardins, J. G. J . Gas Chromatogr. 1984, 2 . 270. Babcock, D.;Singer, P. Proc. 97th Ann. AWWA Conf. 1977, June: Paper 16-6.
RECEIVED for review August 8, 1979. Accepted November 9, 1979. This study was supported in part by Biomedical Research Support Grant RR 07048 to the University of Massachusetts.
Determination of Dissolved Nitrous Oxide in Aquatic Systems by Gas Chromatography Using Electron-Capture Detection and Multiple Phase Equilibration James W . Elklns' Center for Earth and Planetary Physics, Harvard University, Pierce Hall, Cambridge, Massachusetts 02 138
An accurate method for the determination of small concentrations of dissolved nttrous oxide (N20) in fresh and seawater systems using electron capture detector gas chromatography is discussed. Successive analyses are made of the head space after repeated equillbratlons of solutlon and an equal volume of ultra-pure gas. Precision better than 2 % can be achieved on sample sizes as small as 60 mL. Possible sources of error of this method are enumerated. Six samples can be analyzed per hour with this method. The variations of measured dissolved N20 concentration during long term storage of samples are small (2.3 % ) using the procedures discussed here. 'Present address: Gas and Particulate Science Division, N a t i o n a l B u r e a u of Standards, Washington, D.C. 20234 0003-2700/80/0352-0263$01,00/0
Nitrous oxide (NzO) is a trace constituent of the atmosphere, representing approximately 300 parts per billion of the total atmospheric mass. Possible changes in the level of atmospheric nitrous oxide have received considerable attention in recent years (1-4), although the geochemistry of the gas is poorly understood. The concern is that human activity could give rise to significant changes in the magnitude of source and sink strengths for nitrous oxide (5-7). It is therefore important to quantify sources and sinks for atmospheric NzO in both soil and water systems. The previous methods for determining dissolved N20, including that of Craig and Gordon @),have primarily used gas chromatography (GC). Early methods employed a technique developed by Swinnerton et al. (9) in which N 2 0 was purged from solution and trapped on a cooled molecular sieve, with subsequent analysis by a GC (10-1.3). The development of 0 1980 American Chemical Society
