a04
Anal. Chem. Wag, 55, 604-608
Spectrophotometric Determination of Atmospheric Nitrogen Monoxide and Nitrogen Dioxide after Collection with [ N ,"-Ethylene bis(salicy1ideniminat0)] cobalt Kolchlro Ishll* and Kazuyukl Aokl Tokyo Metropolitan Research Institute for Environmental Protectlon, Yurakucho 2-7- 1, Chiyoda-ku, Tokyo 100, Japan
A new method for sampling and determining NO and NO, in the atmosphere Is investigated. Atmospheric NO and NO, are collected in a solution of [N,N'-ethylenebls(salicyiideniminato)lcobak (Co(saien)) in odichlorobenzene. NO reacts with Co(salen) reversibly yielding a nitrosyl complex (Co(saien)NO), which is converted to a nitro complex by air oxidation in the copresence of aminoethanethioi in an 83% yield. NO, Is absorbed by Co(saien) solution irreversibly. The product compound can easily be decomposed by stripping with aqueous alkaline soiutlon, yielding the NOz- ion quantitatively. The nitro compound and the NO,- were quantified spectrophotometricallywith a Griess-type reagent. I n usual sampling conditions, 0,, NH,, SO,, CO, and H,O dld not Interfere with the present method. The detection limits were 2 ppb-h (NO) and 1 ppboh (NO,).
Nitrogen monoxide (NO) is emitted as exhaust gas from automobiles and industrial plants into the atmosphere, and is photooxidized to the more toxic nitrogen dioxide (NO,) (1-4). Moreover, these nitrogen oxides as well as reactive hydrocarbons are known to cause photochemical smog formation (1-3). Chemiluminescence methods (CLD) for the two oxides (5) in ambient air have been widely used because of their ability to detect NO and NO, (NO + NOz) even a t low concentrations. However, no appropriate wet chemical techniques for direct measurement of NO have been developed. Only a modified Griess-Saltzman method for continuous NO2 measurement is being used for continuous NO measurement (6, 7). In this conventional method, NO is indirectly determined by oxidizing it to NOz, measuring the NOz, and then converting the results to NO concentration. However, this oxidation step is not yet clarified, and various oxidizers such as CrO, have been tested (8). Recently, Neadeau et al. (9) have reported a method for measuring NO by electron spin resonance (ESR) using a free radical reagent. A simultaneous measuring technique of NO and NOz has been tried by polarography under a nitrogen atmosphere using Fe(edta) (10). Much research has been done on the absorbing method of NO with use of metal complexes. Among many such metal complexes, the authors have been interested in [N,N'-ethylenebis(salicylideniminato)]cobalt (Co(salen)) which was reported to take up NO irreversibly (11) and to take up Oz reversibly (12). In an inert solvent such as chloroform, Co(salen) absorbs NO to form a five-coordinate nitrosyl compound (Co(sa1en)NO) (11). NO, coordinated to Co(salen) and its ring-substituted derivatives, is known to form nitro compounds by reaction with oxygen (13). This reaction needs bases Co(sa1en)NO
+ '/,02+ B + Co(salen)NOz
(1)
where B represents a Lewis base such as pyridine or tributylphosphine. This reaction offers two suggestions for 0003-2700/83/0355-0604$01.50/0
collection and determination of atmospheric NO, i.e., (1)Co (sa1en)NO formed in the absorbing solution disappears by reacting with oxygen catalyzed by basic substances in ambient air (generally NHJ, and (2) NO can be spectrophotometrically determined by a Griess-type reagent, by converting Co(sa1en)NO to a nitro compound. The present authors have investigated the fundamental characteristics of Co(salen) and found that Co(salen) solution in o-dichlorobenzene (o-DCB) absorbs directly NO2 as well as NO at approximately 100% efficiency without the influence of oxygen in air, and that NO and NOz can be simultaneously determined spectrophotometrically by using one absorber. The details of examination on the conditions and procedures for simultaneous determination of NO and NO2 are presented.
EXPERIMENTAL SECTION Reagents. Co(salen) was synthesized according to the method reported by Nishikawa and Yamada (14). The absorbing solution was prepared by dissolving 32.5 mg of Co(salen) in 0.1 L of o-DCB. The Griess-type reagent was made by dissolving 20 g of sulfanilamide, 50 mg of N-(1-naphthy1)ethylenediamine dihydrochloride, and 50 mL of phosphoric acid in 1L of distilled water. The aminoethanethiol (AET) solution was prepared by dissolving 1 g of AET hydrochloride in 0.1 L of ethanol and standardized by iodometry. All reagents were of analytical reagent grade and used without further purification. NO gas, NO2gas, and NO standard gas for calibration of a CLD NO, analyzer (Bendix 8108-B) were supplied from gas cylinders (Takachiho). Ozone was generated from Oz by a silent arc discharge instrument, diluted by air, and monitored with a Bendix O3analyzer. Apparatus. In studying O3interference, NO in Co(sa1en)NO was determined by the system similar to that used by Cox (15). Nitrogen was used as a carrier gas. In the reaction vessel was placed 5 mL of p-toluenesulfonic acid solution in 1-butanol (2% (w/v)). A 0.1 mL sample was injected into the solution by means of a syringe. NO dissociated from Co(salen)NOand was measured by the NO, analyzer (15). Quantitative evaluation of the samples was performed by comparison with the standard NO gas on the basis of peak areas. The concentration of Co(I1) was measured by a atomic absorption spectrometer (Seiko SAS727). Absorption Experiment. Figure 1shows a schematic diagram of the absorption apparatus. NO and NOz gases diluted by Nz or air (0.5-10 ppm) were bubbled into the absorbing solutions (5 mL) in the absorbing vessel. The absorbing vessel is a test tube (25 mL) into which Teflon tubing (1mm i.d.) is inserted. The concentrations of NO and NOz at the outlet of the absorbing vessel were monitored with the CLD NO, analyzer. The gas flow rate could be conditioned by drawing Nz or air at ca. 130 mL/min. Air Sampling. A sampling train was assembled by connecting in series the absorbing vessel, a solvent trap, an orifice, an air pump, and a gas meter. Air was bubbled into the absorbing solution (5 mL) for 2 h at the rate of ca. 130 mL/min. Equilibrium Measurement. In the absorption experiment system (Figure l),Nz was bubbled into the absorbing solution of known ratios of [Co(salen)]/[Co(salen)NO] (from 1/5 to 5/1). The output NO concentration was measured. Analytical Procedure for Determination of NOz. When sampling was completed, 0.1 mL of methanol was added to the absorbing solution. After 20 min, the solution was shaken strongly 0 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
2
0 0.01
,
j
0.1
1
[CO-Sa
605
len] /mM
Flgure 3: Relation between NO absorption efficiency (at t =O, In Figure 2) and Co(sa1en)concentration: inlet concentration of NO, 9.8 ppm; volume of absorblng solution, 5 mL.
(atm)) and absorbed NO concentration, assuming the system is in equilibrium state pNO
i / min
Figure 2. Concentration change of NO and NOp at the outlet of the absorbing vessel: Inlet concentration, 9.8 ppm (NO and NO,); concentration of Co(salen),0.14 mM.
with 4 mL of NaHC03 solution (0.01 M)for 30 s. The aqueous layer was pipetted into a test tube (the organic layer was to be used for NO determination) and once washed with 2 mL of zinc diethyldithiocarbamate (Zn(ddtc)z)solution in CC14 (0.5% (w/v)). To 3 mL of the washed aqueous layer, 2 mL of the Griess-type reagent was added. The absorbance was measured at 540 nm. Analytical Procedure for Determination of NO. The organic layer from which NOz had been removed was shaken strongly with 5 mL of Micheal's buffer solution (pH 4, CH3COOH-CH3COONa)for 10 s. A small amount of anhydrous NaZSO4was added to the washed organic layer for dehydration. Three milliliters of the dehydrated organic layer was pipetted into a centrifuge tube, and then 0.06 mL of N-ethylmorphorinesolution in o-DCB (4% (v/v)) and 0.1 mL of AET solution were added. Twenty minutes were allowed for Co(salen)NO to react with AET, and then 4 mL of the Griess-type reagent was added. The mixture was shaken strongly for 30 s, was allowed to stand for more than 10 min, and was centrifuged (up 500g). The aqueous layer was carefully pipetted into an absorbing cell to measure the absorbance (at 540 nm). The absorbed amounts of NO and NOz in the absorbing solution were calculated from the absorbance-concentration curves described below. Calibration Curves. Known volumes of NO and NOz gyes, adjusted to 0.5 ppm in nitrogen, were drawn through the absorbing solutions and the solutions were analyzed according to the analytical procedures. From the absorbances and the absorbed amounts of NO and NOz, two absorbance-concentration curves were obtained. RESULTS AND DISCUSSION Absorption of NO and NOz by Co(sa1en) Solution. When 9.8 ppm of NO and NO2 gases (in Nz) were separately passed into the system (Figure l),the concentrations of NO and NO2 at the outlet (Cmt (pprn)) changed as shown in Figure 2. The same results were obtained in the case of air-diluted samples, meaning no influence of Oz.The efficiency of NO absorption decreased gradually according to the increasing amount of absorbed NO. The NO absorption efficiency ( 4 - Cout/Cin,Gin: NO concentration (pprn) at the inlet of the absorbing vessel) can be related to NO vapor pressure ( P N O
= coutpO
[Co(salen)NO] 1 = [Co(salen)li - [Co(salen)NO] ITPNo (2)
where Po denotes atmospheric pressure (atm), subscript i means initial state, and KPNo is defined in eq 4. The relation between the efficiency of NO absorption (at t = 0, in Figure 2) and the concentration of Co(sa1en) solution is shown in Figure 3, at the rate of ca. 130 mL/min. The efficiency of NO absorption increases, when the concentration of Co(sa1en) increases. Because the solubility limit of Co(salen) in o-DCB is ca. 1mM, the concentration of absorbing solution was set to be 1 mM. The NO absorption efficiency of the solution was at least 99% for sample gases containing 9.84.8 ppm NO. On the other hand, the efficiency of NOz absorption was approximately loo%, independent of the amount of absorbed NOz, until the Co(sa1en) solution was nearly saturated with NOz, where the efficiency began to decrease rapidly (Figure 2). NO- a n d NO,-Co(salen) Complexes. The NO uptake by Co(salen) was found to be reversible by the following experimental results: (1)NO was regenerated by bubbling N2 into or heating the Co(salen) solution saturated with NO, and (2) the Co(salen) solution, which had been once saturated with NO and heated to purge absorbed NO, absorbed again as the same amount of NO as in the former saturation. On the other hand, the experiment with NO2 was made in a similar way. As a result, it was found that 1mol of NOz is irreversibly bound with 1 mol of Co(sa1en). Stoichiometry and Equilibrium Constant of t h e Absorbing Reaction of NO. Since Hill's coefficient is 1,one molecule of NO binds with one molecule of Co(sa1en). Co(sa1en) NO + Co(sa1en)NO (3) The equilibrium constant KpNO, defined by eq 4,was determined in the temperature range of 35-65 "C. KpNo= [Co(salen)NO]/([Co(salen)]PNO) (4)
+
Since the obtained KpNovalues were independent of N2 flow rates, it could be concluded that this system was in equilibrium. KpNoand enthalpy change ( A H o )of Co(sa1en)-NO, Co(salen)-Oz, and Fe(edta)-NO systems are summarized in Table I. Table I shows the following: (i) at room temperature, Co(salen) forms a more stable nitrosyl complex than Fe(edta) does, and (ii) theoretically, Co(sa1en) should absorb NO quantitatively even at the parts-per-billionlevel when the ratio of [Co(salen)NO]/ [Co(salen)] is 0.01 or less. Limit of Sampling Volume. Losses of NO from the absorbing vessel can be estimated by the following calculation: the concentration of Co(sa1en)NOin the absorbing solution is assumed to be represented by eq 5, where V,, Vo,and Vl [Co(salen)NO] = VgCin/(VoVl)
(5)
606
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
Table I. Thermodynamic Data of NO and 0, Coordination t o Metal Complexes system (metal complex-gas) solvent temp, "C Co(sa1en)-NO
o-dichlorobenzene
Co(sa1en)-0, ( I 6)
pyridine dimethylformamide 0.1 M tetraethylammonium perchlorate
20 20
water water
20 60-70 38.5 55.0
35 50 65
Co(sa1en)-0, ( I 7) Fe(edta)-NO ( 1 8 ) Fe(edta)-NO (19)
0
70.0 a
K (L6.mol-') in eq 7.
In torr-'.
In kcal/mol.
K p , atm-'
4.5 x 10' 1.1 x 107
2.4 X lo6 >105a
640 0.128
AH", kJ/mol -.88.6
-59 -85
0.089 0.18 x 1 0 - 3 d
0.850 X 2.38 x 10-d
-14.1' -15.8'
H/K,.
denote volume of sampled air (L), molar volume (L), and volume of absorbing solution (L), respectively. Equation 6 is obtained from eq 4 and eq 5. The limiting volume of
sampled gas, estimated with eq 6, is 125 L at 30 "C to maintain 99% absorption efficiency, theoretically. Solvent. In polar solvents (20) such as pyridine, Me2S0, and N,N-dimethylformamide, Co(sa1en) absorbs O2 and consequently forms a dimeric p-peroxo complex (16) which is less soluble and is likely to precipitate.
B + Co(sa1en) + O2 + B-Co(salen)02 B-Co(salen)02
(7)
+ B-Co(sa1en) B-Co(sa1en)-0-0-Co(sa1en)-B
(8)
Furthermore, this p-peroxo species does not absorb NO. On the contrary, in inert solvents (20) such as toluene, acetone, THF, 2,2'-dimethoxydiethyl ether, and benzene, Co(sa1en) neither absorbs O2 nor forms a p-peroxo complex (16). Among the most inert solvents, we selected o-DCB as the solvent, because its vapor pressure is low enough to minimize losses of solvent in air sampling. It is recommended that fresh absorbing solution should be prepared at least every 2 weeks and that the stored absorbing solution should be heated to purge NO prior to use. Determination of NO2. Once NOz is collected in the absorbing solution, it remains stable at least for 24 h. Methanol was added so as to dissolve the precipitate formed by NO2 absorption. To strip NOz into aqueous solutions, either NaHC03 solution (0.01 M) or NaOH solution (0.1 M) can be used. Absorbed NO2 is stripped into the aqueous layer as NO2- in one shaking, and the absorbed NO remains in the organic layer. Next, for removal of any trace amount of Co(II), the aqueous layer was washed with Zn(ddtc)z solution. A linear relationship was obtained between the absorbance of colored solution with the Griess-type reagent and the amount of absorbed NOz. From this relation, and the volume change in the stripping operation together with molar absorptivity of NOz-, absorbed NO2was considered to be converted to NO2-, quantitatively. Determination of NO. Once NO is collected in the absorbing solution, it disappears at the rate of 4% /day at 25 O C and of 2%/day a t 0 OC. Remaining Co(salen), which interferes the development of color for determination of NO, has to be removed. When the organic layer containing Co(salen) and Co(sa1en)NO is washed with aqueous buffer solution (pH 4), 98% of the Co(sa1en) is removed from the organic layer; however Co(sa1en)NO remains in the organic layer. Though Co(sa1en)NO decomposes in a strong acid solution instantaneously, it does so very slowly (ca. 7%/h) a t pH 4.
[bEd/rnM
Figure 4. Relation between absorbance (at 540 nm) and AET concentration in o-DCB.
Since free AET is likely to suffer air oxidation, its hydrochloride was used and N-ethylmorphine was added as a neutralizer. Oxidation of NO Group. It was not known until now how the NO group is oxidized with O2in the copresence of AET, but it seems to be similar to the mechanism reported by Clarkson and Bas010 (13). In the case of using pyridine, which was used by Clarkson and Bas010 (13)as a oxidation catalyst, in place of AET, oxidized Co(salen)NOsolution did not make the Griess-type reagent color. Pyridine seems to prevent transference of the NO2 group into the aqueous layer by coordinating the complex. Appropriate Concentration of AET. The relation between AET concentration in the organic layer and the absorbance colored by the Griess-type reagent is shown in Figure 4. When the concentration of AET was higher than 1mM, the absorbance was constant. In the lower concentration range of AET, the absorbance was lower because of incomplete oxidation of the NO group. From this result, the volume of AET solution (0.04 M) to be added to the organic layer (3 mL) was determined to be 0.1 mL. Conversion Ratio of NO to NOz. A linear relationship was obtained between the amounts of absorbed NO and the absorbances of color developed with the Griess-type reagent. The calibration curve varies with the volume of Griess-type reagent used (V). Moreover, the products of V and the absorbance (A) are not constant. Most of AET and Co(I1) are extracted into the aqueous layer in this color development process. It was observed that the color development decreased according to the increasing concentrations of AET and COW) in the aqueous layer, i.e., the decreasing volume of the Griess-type reagent, V. Values of l/(AV), (mL-l) obtained against V, (mL, in parentheses) were the following: 0.376 (4), 0.364 (5), 0.357 (6), 0.355 (7),0.356 (8),0.359 (9),0.349 (lo), 0.350 (13),0.352 (17), and 0.345 (20). The relation between 1/(AV) and 1 / V can be expressed as in eq 9, where (Ano is the (AV) value
ANALYTICAL CHEMISTRY, VOL. 55, NO. 4, APRIL 1983
607
Table 11. Comparative Studies of CLD with the Present Method
t
/ hr
Flgure 5. Decrease of NO in Co(saien)NO in the presence of NH,: (0) under a N, atmosphere: (0)under an air atmosphere.
amt of NO, amt of NO,, PPb.h PPb.h sampling presprestime, ent ent Japan Mean methmethTime CLD od CLD od
date Sept 1 7 , 1 9 8 1 Sept 1 8 , 1 9 8 1
Sept 22, 1981 Sept 24, 1981 Sept 25, 1981 Sept 28, 1981 -
00
5
10
10-12 12-14 14-16 7-9 9-11 11-13 13-15 9-11 11-13 9-11 11-13 13-14 14-15 15-16 10-13
16 9 19 36 21 7 10 19 11 20 44 49 79 56 13
18 12 21 37 25 12 13 22 13 14 31 49 77 56 22
48 41 47 40 51 42 44 51 50 71 45 42 45 50 50
47 41 47 39 46 38 40 49 47 69 44 45 57 60 55
15
01passed throogh/pL
Plgure 8. Decrease of NO in Co(saien)NOwith increasing amount of O3passed: (0) [Co(saien)] = 0, (0)[Co(salen)]/[Co(saien)NO] = 1.
in the absence of AET and Co(I1) and k is a constant. In this experiment, (AV), was obtained as 0.340, which corresponded to 2.78 pg of NOz-. Since the absorbed amount of NO was 3.34 pg (as NOz), it was concluded that 83% of NO was converted to NO2-. Interferences. For the determination of ambient NO, the interfrences by possibly coexisting trace substances were studied. (NH,). One liter of N2 containing 1250 ppm of NH3 was bubbled into the absorbing solution containing Co(salen)NO (1.25 pmol/9 mL). Since the amount of NO did not decrease in a N2 atmosphere, and decreased in an air atmosphere (Figure 5), NH3 was considered to catalyze the oxidation by oxygen like pyridine. Clarkson and Basolo (13)analyzed the mechanism of reaction in eq 1. They showed that the rate of the reaction is first order in Co(salen)NO and that the rate constant can be expressed by [B] and [O,]. When the concentration of B is low, the rate constant can be simplified as k[NH3], where k is a constant. In this experiment, NH3 concentration was low and koW/ [C~(salen)NO]~ was obtained at 12%/h, where kobsd is the rate constant and subscript i denotes initial state. In ambient air, the concentration of NH3 is usually 10 ppb or less, the ratio becomes ca. 10T4%/h. Therefore, the interference of NH3 was negligible. (03). One ppm of O3 was bubbled into the absorbing solution containing a known amount of Co(salen)NO. Changes of the amount of NO against the amount of O3 passed are shown in Figure 6. This shows that the reaction of O3with Co(salen)NO is equimolar, and that O3reacts with Co(salen) in preference to Co(sa1en)NO. In air sampling, the concentration of Co(salen) is much higher than that of Co(salen)NO, therefore no Co(sa1en)NO is lost. (CO, SOZ, and Water). When CO (94 ppm) in air, SOz (16 ppm) in air, and humidified air were bubbled into the absorbing solutions containing a known amount of Co(sa1en)NO a t the rate of 130 mL/min for 1 h, no significant interferences were found. Mutual Interference of NO and NOz. Absorption rate of NO2 by the absorbing solution is higher than that of NO. NO (3.8 ppm), NO2 (3.8 ppm), and the mixture of NO (3.8 ppm) and NOz (3.8 ppm) in air (1 L) were bubbled into three
absorbing solutions, respectively. The absorbed amounts of NO and NO2 in simultaneous absorption were equal to those in the separate experiments. Therefore, the mutual interference was not observed up to 3.8 ppm of NO and 3.8 ppm of NOz. Comparative Studies of CLD with the Present Method. Ambient air was introduced into the CLD NO, analyzer and into the present absorbing solution, and NO and NOz were determined, respectively. Table I1 shows the comparisons of the present method with CLD measurement. These values are in good agreement. The mean squares of the differences between CLD and the present methods are 5 ppb-h both for NO and NOz. The standard deviations of the blank measurements (n = 10) were 0.005 absorbance unit (NO) and 0.003 absorbance unit (NOz). Therefore, the detection limits of NO and NOz are thought to be 2ppb.h and 1 ppb-h, respectively.
ACKNOWLEDGMENT The authors are sincerely grateful to H. Sato of Yokohama National University for reading the manuscript, and to T. Aranoo and K. Yoshizumi of the Research Institute for assistance in preparing the manuscript. Registry No. NO, 10102-43-9; NO2, 10102-44-0; Co(salen), 14167-18-1.
LITERATURE CITED Lelghton, P. A. “Photochemistry of Air Pollution”; Academic Press: New York, 1981; p 152. Tuesday, C. S.“Chemical Reactions In Urban Atmospheres”; Eisevier: New York, 1971; p 45. Altshuller, A. P.; Bufallnl, J. J. Envlron. Scl. Technol. 1971, 5 , 39-64. ”Air Quality Criteria for Nitrogen Oxides”; Environmental Protection Agency: Washington, DC, 1971. Fontllln, A.; Sabadell, A. L.; Ronco, R. J. Anal. Chem. 1970, 42, 575-579. Saitzman, 6. E.; Mendenhall, A. L., Jr. Anal. Chem. 1984, 3 6 , 1300-1304. Stern, A. C. “Air Pollution”, 3rd ed.; Academic Press: New York, 1978; Vol. 3, p 267. Levaggl, D.; Kothny, E. L.; Beisky, T.; de Vera, E.; Mueller, P. K. Envlron. Scl. Technol. 1974, 8 , 348-350. Nedeau, J. S.; Treen, M. E.; Boocock, 0.G. 8. Anal. Chem. 1978, 50, 1871-1873. Hamamoto, 0.; Uchlyama, S.; Nozaki, K.; Muto, G. Bunseki Kagaku 1979, 28, 118-121. Earnshaw, A.; Hewlett, P. C. Larkworthy, L. F. J . Chem. SOC. 1985, 47 10-4723. Tsumakl, T. Bull. Chem. SOC.Jpn. 1938, 13, 252-280. Clarkson, S. G.; Basolo, F. Inorg. Chem. 1973, 72, 1528-1534. Nlshikawa, H.; Yamada. S. Bull. Chem. Soc. Jpn. 1984, 3 7 , 8-12. Cox, R. D. Anal. Chem. 1980, 52, 332-335.
608
Anal. Chem. 1983, 55, 608-612
(16) Cesarottl, E.; Gullottl, M.; Paslnl, A,; Ugo, R. J . Chem. SOC.,Dakon Trans. 1977,757-763. (17) Hammerschmidt, R. F.; Broman, R. F. J . Nectroanal. Chem. 1979, 99. 103-110. (18) Hasui, H.; Osuo, H.; Ohmlchl, H.; Fukuzyu, Y.; Tarul, H. Nlppon Kagaku Kaishi 1970,447-455. (19) Hishinuma, Y.; KaJi, R.; Aklmoto, H.; Nakajima, F.; Morl, T.; Kamo, T.;
Akikawa, Y.; Nozawa, S. Bull. Chem. SOC. Jpn. 1979, 52, 2863-2865. (20) Florlani, C.; Caiderazzo, F. J . Chem. SOC.A lQW, 946-953.
RECEIVED for review August 23, 1982. Accepted December 8, 1982.
Determination of Trace Quantities of Dimethyl Sulfide in Aqueous Solutions M. 0. Andreae* and W. R. Barnard Department of Oceanography, Florida Stare University, Tallahassee, Florida 32306
A method Is described for the determination of dimethyl sulfide (DMS) at the nanogram level In aqueous solutions. DMS Is removed from aqueous samples by sparging with a He carrier gas stream. The volatile DMS Is trapped cryogenically with liquid nitrogen on a chromatography column that serves as both the trapping and the separatlon mechanism. After controlled heating to separate DMS from interfering compounds, DMS Is detected by a flame photometrlc detector. The detection iimlt is 0.03 ng of S (DMS), corresponding to a concentration of 0.3 ng L-' for a 100-mL sampie. Precision is 6.2%. Accuracy, sample storage, and strlpping eff iciency are also discussed. This procedure has been used to measure DMS in a variety of natural waters.
Models for the global mass balance of sulfur all indicate the need for a volatile, biogenically derived compound to transfer sulfur from the sea to the atmosphere in order to account for the total atmospheric sulfur content. Efforts to identify this compound have resulted in the measurement of several reduced-sulfur compounds including dimethyl sulfide, carbon disulfide, and carbonyl sulfide, in both the atmosphere and the oceans (1-5). The discovery that living organisms produce volatile, methylated sulfur compounds (6) has led to the investigation of the importance of dimethyl sulfide (DMS) in the transfer of sulfur from the sea to atmosphere (1-3, 7-9). Because of the need for a rapid, precise, and accurate method for the determination of trace amounts of DMS in seawater to help establish its role in the transfer of sulfur from sea to air, we have developed the method described below and tested it on over 200 seawater samples from various areas of the Atlantic Ocean and Gulf of Mexico. DMS has been measured previously. Rasmussen (5) has measured DMS in pond waters by gas-liquid equilibration and GC-FPD determination. Another method involving extraction of DMS via CCll and HgClz from large volumes of seawater (15 L) has also been reported (1). DMS in the atmosphere has been preconcentrated on gold beads and analyzed using a similar detection system (4). Dimethyl sulfoxide (DMSO) has been determined as DMS after borohydride reduction by one of us (IO),but improvements in the detection system have increased the sensitivity of the instrument by 2 orders of magnitude. The procedure detailed below represents the first report of a method for the determination of DMS in water samples that includes the analytical parameters accuracy, precision, detection limit, and a con-
sideration of sample storage problems and interferences. Our method has several advantages over previous methods: it is rapid, accurate, and requires only small sample volumes. The instrument described below has also been used successfully aboard an oceanographic research vessel, eliminating the need for sample treatment to assure stability until samples can be analyzed onshore.
EXPERIMENTAL SECTION Apparatus. The instrument configuration is depicted schematically in Figure 1. Known-volume sample loops of Teflon tubing are connected to an Altex series 202 six-way rotary sampling
valve (Rainin Instrument Co., Woburn, MA), The He carrier gas stream passes through a 19-mm o.d., 36 cm long stainless steel pipe packed with a combination of activated charcoal (50-200 mesh, Fisher Scientific, Pittsburgh, PA) and molecular sieve (Union Carbide Type 4A, Fluka Chemical Co., Hauppauge, NY) to remove trace sulfur gases from the He carrier stream. From this scrubber the carrier is routed through the sample loop and the six-way valve and is used to inject the sample into the bubbling chamber through a glass frit at the base of the chamber. From the bubbling chamber, the carrier gas stream passes through a short length of Tygon tubing and into a 15 cm long, 12 mm 0.d. Pyrex drying tube fiied approximately two-thirds full with K2C03 (Mallinckrodt, Inc., Analytical Reagent Grade) to remove moisture from the carrier gas stream. The gas stream then passes into a 6 mm o.d., 30 cm long glass U-tube filled with 15% OV3 on Chromosorb W AW-DMCS 6C-80 mesh. This U-tube serves both as a trap and a separation column. About 2 m of 5 Q m-l Chrome1 wire is wound around the trap and connected to a variable transformer to allow controlled heating of the trap. The trap is connected to a Pyrex burner similar to that described by Braman et al. (4). Gas flow rates for this burner are 125 mL m i d for air and 110 mL min-l for H2 and the He carrier gas flow is 100 mL min-'. This burner is enclosed in an aluminum housing which is flanged to a photomultiplier system (98246 PMT and QL30F PMT housing, EM1 GenCom Inc., Plainview, NY). Power for the PMT (700 V) is suiplied by a Model 7101 high-voltage supply and the PMT current is measured by a Model 3A27 electrometer (both manufactured by Pacific Precision Instruments, Concord, CA). A Hewlett-Packard HP 3390A integrator is used to record and integrate the chromatographic peaks. Standards. A gaseous diffusion tube with a known permeation rate was initially used to standardize the instrument. We used a wafer device with a nominal permeation rate of 2 ng of DMS/min (Metronics ASSOC., Inc., Santa Clara, CA). This device was standardized against a gravimetrically calibrated permeation device by J. M. Ammons (1980, personal communication) and has an actual permeation rate of 2.84 ng of DMS/min at 30 OC. The wafer device was later replaced by liquid DMS standards which were prepared by dissolving liquid DMS (ICN Pharmaceuticals, Inc., Plainview, NY) in degassed ethylene glycol. Standards
0003-2700/83/0355-0608$01,50/00 1983 American Chemical Society
