Anal. Chem. 1996, 68, 1493-1498
Sequential Method for the Field Determination of Nanomolar Concentrations of Dimethyl Sulfoxide in Natural Waters Rafel Simo´,* Joan O. Grimalt, and Joan Albaige´s
Department of Environmental Chemistry, CID-CSIC, Jordi Girona 18-26, 08034 Barcelona, Catalonia, Spain
A procedure for the determination of dimethyl sulfoxide (DMSO), along with dimethyl sulfide (DMS) and dimethyl sulfoniopropionate (DMSP), at nanomolar levels in natural waters has been developed. After removal of DMS by purge and cryotrapping, DMSP is removed by the same method after alkaline hydrolysis, and DMSO is reduced to DMS using a combination of sodium borohydride and hydrochloric acid. The DMS produced is stripped, cryotrapped, and analyzed by gas chromatography. Detection of 3 pmol of DMSO was achieved, resulting in a detection limit of 0.05 nM for a 50 mL sample. Mean yield for standards in the range 0.7-130 nM (n ) 31) was 95%, and mean precision (as coefficient of variation) was 14%. Precision for replicates of natural seawater samples was always e10%. Mean yields of the sequential analysis of DMS + DMSP + DMSO (1.5-25 nM) standard mixtures in seawater were >90% for the three species. Filtered seawater samples stored frozen (-20 °C) showed no significant changes in DMSO concentration. Since DMSP is the only compound, other than DMSO, that gives rise to DMS upon reduction with NaBH4, tests were performed to ensure that DMSP is quantitatively removed before the DMSO analysis. Boranes were identified as the substances that produced two major peaks in the chromatogram of the reduction products. Adequate chromatographic conditions were established to avoid their coelution with DMS. This entire protocol allows the sequential determination of DMS, DMSP, and DMSO in natural waters and is suitable for field work, so it should be very useful in studies of dimethyl sulfur biogeochemistry. Some examples of DMSO (along with DMS and DMSP) measurements in the Mediterranean Sea are presented. Dimethyl sulfoxide (DMSO) is a well-known chemical used as a solvent, a lubricant, and an additive in industrial applications, but its occurrence in aquatic environments arises mainly from natural processes. In fact, DMSO is a major pool of dissolved sulfur in seawater, where it occurs at nanomolar levels,1-8 and a (1) Andreae, M. O. Limnol. Oceanogr. 1980, 25, 1054-1063. (2) Lee, C.; Wakeham, S. G. Chem. Oceanogr. 1989, 9, 1-51. (3) Gibson, J. A. E.; Garrick, R. C.; Burton, H. R.; McTaggart, A. R. Mar. Biol. 1990, 104, 339-346. (4) Ridgeway, R.; Thornton, D.; Bandy, A. J. Atmos. Chem. 1992, 14, 53-60. (5) Bates, T. S.; Kiene, R. P.; Wolfe, G. V.; Matrai, P. A.; Chavez, F. P.; Buck, K. R.; Blomquist, B. W.; Cuhel, R. L. J. Geophys. Res. 1994, 99, 7835-7843. (6) Kiene, R. P.; Gerard, G. Mar. Chem. 1994, 47, 1-12. (7) Hatton, A. D.; Malin, G.; McEwan, A. G.; Liss, P. S. Anal. Chem. 1994, 66, 4093-4096. 0003-2700/96/0368-1493$12.00/0
© 1996 American Chemical Society
significant constituent in other natural waters1,6,9,10 and in the atmosphere.9,11,12 The implication of DMSO in the biogeochemical cycle of dimethyl sulfide (DMS) has been anticipated.5,8,13-15 DMS is the most abundant volatile sulfur compound in surface seawater,16 and a key species of the global sulfur cycle.17 Seawater DMS is produced by plankton, mostly from algal dimethyl sulfoniopropionate (DMSP). Upon release to the air, DMS undergoes oxidation to form acidic aerosols and condensation nuclei, thereby affecting tropospheric chemistry and cloud reflectiveness over the oceans.18 Current evidence suggests that the dynamics of DMS in water is influenced by photochemical and microbial processes.14,19 Laboratory studies have shown that DMSO can be a potential source and sink for DMS20-23 through these two types of processes. However, the significance of the role of DMSO in the DMS cycling has still to be assessed. This task is hampered by the scarcity of reliable analytical methods for DMSO determination in the environment. Owing to the difficulties in the measurement of nanomolar DMSO levels in water matrices with direct chromatographic methods,11,24,25 reduction and subsequent gas-phase analysis of the DMS produced appears to be the technique of choice. Sodium borohydride26 and titanium trichloride6 have been used as reducing agents. However, in both procedures, DMSP acts as an (8) Simo´, R.; Grimalt, J. O.; Albaige´s, J. Deep-Sea Res., in press. (9) Harvey, G. R.; Lang, R. F. Geophys. Res. Lett. 1986, 13, 49-51. (10) Wakeham, S. G.; Howes, B. L.; Dacey, J. W. H.; Schwarzembach, R. P.; Zeyer, J. Geochim. Cosmochim. Acta 1987, 51, 1675-1684. (11) Watts, S. F.; Watson, A. J.; Brimblecombe, P. Atmos. Environ. 1987, 21, 2731-2736. (12) Berresheim, H.; Tanner, D. J.; Eisele, F. L. Anal. Chem. 1993, 65, 84-86. (13) Malin, G.; Turner, S. M.; Liss, P. S. J. Phycol. 1992, 28, 590-597. (14) Kiene, R. P. In Microbial growth on C1 compounds; Murrell, J. C., Kelly, D. P., Eds.; Intercept: Andover, 1993; pp 15-33. (15) Simo´, R.; Grimalt, J. O.; Pedro´s-Alio´, C.; Albaige´s, J. Mar. Ecol. Prog. Ser. 1995, 127, 291-299. (16) Andreae, M. O. Mar. Chem. 1990, 30, 1-29. (17) Bates, T. S.; Lamb, B. K.; Guenther, A.; Dignon, J.; Stoiber, R. E. J. Atmos. Chem. 1992, 14, 315-337. (18) Charlson, R. J.; Lovelock, J. E.; Andreae, M. O.; Warren, S. G. Nature 1987, 326, 655-661. (19) Kiene, R. P. Mar. Chem. 1992, 37, 29-52. (20) Brimblecombe, P.; Shooter, D. Mar. Chem. 1986, 19, 343-353. (21) Taylor, B. F.; Kiene, R. P. In Biogenic Sulfur in the Environment; Saltzmann, E., Cooper, W. J., Eds.; ACS Symposium Series 393; American Chemical Society: Washington, DC, 1989; pp 202-221. (22) Zhang, L.; Kuniyoshi, I.; Hirai, M.; Shoda, M. Biotechnol. Lett. 1991, 13, 223-228. (23) Juliette, L. Y.; Hyman, M. R.; Arp, D. J. Appl. Environ. Microbiol. 1993, 59, 3718-3727. (24) Paulin, H. J.; Murphi, J. B.; Larson, R. E. Anal. Chem. 1966, 38, 651-652. (25) Lang, R. F.; Brown, C. J. Anal. Chem. 1991, 63, 186-189. (26) Andreae, M. O. Anal. Chem. 1980, 52, 150-153.
Analytical Chemistry, Vol. 68, No. 9, May 1, 1996 1493
interference for DMSO, so that correction after separate DMSP analysis has to be applied. The formerly reported borohydride reduction proved to be hardly reproducible because of its strong dependence on the operational conditions. In fact, very few applications have been reported since its publication.1,10 Recently, this lack of reproducibility has been compensated by the use of isotope dilution mass spectrometry,4 but the need of a mass spectrometer makes this new method hard to use in field work. Gibson et al.3 used stannous chloride as reducing agent in a seawater application of a method formerly developed for foodstuffs.27 However, the usefulness of this work is difficult to evaluate since details of the experimental procedure were not described. Recently, an enzyme-linked method for DMSO reduction has been reported.7 Besides giving good accuracy, the method avoids interferences, including DMSP. However, its application is limited by the availability of the enzyme isolated for that study. The need for reliable DMSO measurements to go along with DMS and DMSP data in studies of sulfur biogeochemistry has prompted us to investigate an analytical method suitable for field work which did not require additional instrumentation beyond that used for DMS and DMSP analyses. For this purpose, we have reexamined and worked out the early borohydride reduction method26 to make it sound and reproducible. We present here a procedure for DMSO measurement which is integrated as an additional step in the analyses of DMS and DMSP, thus constituting a sequential protocol for the determination of dimethyl sulfur species in natural waters. EXPERIMENTAL SECTION Standards, Reagents, and Materials. DMSO (HPLC grade, 99.5%) was obtained from Carlo Erba (Milano, Italy). For preparation of standards, DMSO was weighted into a volumetric flask and diluted with fresh MilliQ water to make a 0.13 mM stock solution. Addition of 1 mL of HCl (25%) per 100 mL of solution, and storage at 4 °C in the dark, prevented DMSO decomposition over several weeks. This stock was diluted to a working stock of 26 µM, which was used for further preparation of standards. DMSP‚HCl was obtained from Research Plus (Bayonne, NJ), and standards were prepared following the same procedures as for DMSO. A certified permeation tube (208 ng/min, VICI Metronics, Santa Clara, CA) was the source of DMS. Sodium borohydride (98%, Aldrich Chemical Co., Milwaukee, WI) was used in the form of 0.045 g pellets. Hydrochloric acid (25% w/w) and 0.1 g pellets of sodium hydroxide were used for acidification and basification, respectively. DMS losses by adsorption onto glass surfaces were prevented by silanization of all glassware (sampling bottles, reaction vials, purge flask, cryogenic trap loops) with dimethyldichlorosilane (5% in toluene, Fluka, Buchs, Switzerland) and rinsing with HCl diluted in distilled water before use. Instrumental Setting. Purge and Cryotrap Device. A glass bubbling flask was used for the stripping of volatiles (including DMS) from water. The flask was provided with a Teflon-capped side-port through which the sample was introduced and with a fritted glass diffuser through which the sparging gas (nitrogen, 99.999% quality) was supplied. The traps for cryogenic preconcentration were U-shaped borosilicate glass tubes (20 cm × 10 mm o.d. × 6 mm i.d.). The (27) Anness, B. J. J. Sci. Food Agric. 1981, 32, 353-358.
1494
Analytical Chemistry, Vol. 68, No. 9, May 1, 1996
lower portion of the tubes was packed with quartz wool to increase the condensation surface. The traps were conditioned before use by nitrogen flushing at 80 °C. Series of five U-tubes were connected to two six-way PTFE rotating valves (Rheodyne, Cotati, CA) by means of 1/16 in. PTFE tubing. One position was shortcircuited, allowing the loops to be closed without interrupting the gas flow. During preconcentration, the traps were immersed in liquid argon. A Nafion dryer (Perma-Pure Inc., Toms River, NJ), with a countercurrent of dry air, was located between the purge flask and the cryogenic trap to avoid ice blocking in the trap loop. Gas Chromatography. A gas chromatograph especially designed for the analysis of volatile sulfur compounds28,29 was used for both the lab and the field determinations of DMS. It is equipped with a flame photometric detector (Perkin-Elmer, Norwalk, CT) and a 1.4 m × 1/8 in. Teflon (FEP) column filled with Carbopack BHT-100 (Supelco, Bellefonte, PA) and heated/cooled with Peltier elements. Detector gas flow rates were 95 and 170 mL min-1 for hydrogen (99.999% quality) and synthetic air, respectively. The carrier gas was 99.999% quality nitrogen, additionally purified by passage through molecular sieves and cryogenic traps. Base line separation of DMS from other sulfur volatiles (CS2, DMDS, CH3SH, COS, H2S) was achieved using a temperature program from 50 °C to 100 °C and a carrier gas flow rate of 20 mL min-1. Peak areas were recorded with a HewlettPackard 3393A integrator. The small dimensions and the robustness of the apparatus make it suitable for field work, since it can be safely transported to the studied environment. The chromatograph can even be set up in a mobile lab (e.g., a van, boat) without major disturbance of the signal activity. Procedures. The entire protocol for the analysis of DMSO after sequential determination of DMS and DMSP is shown in Figure 1. Removal (and Determination) of DMS and DMSP. DMS measurements were performed according to a modified method based on a cryotrapping gas chromatographic technique described elsewhere.28 For standards prepared in MilliQ water, volumes of 25 or 50 mL were taken with a Teflon tube attached to a glass syringe and injected into the purge device through the Teflonfaced septum. For natural samples, the Teflon tube was replaced with a filter unit holding a 2.4 cm GF/F filter (Whatman, Maidstone, UK) connected to a needle. The collected volumes were injected with this filtration unit by application of a very gentle pressure. Volatiles were stripped with nitrogen at a flow rate of 150 mL min-1 during 20 min and cryotrapped at the temperature of liquid argon (-186 °C). With these conditions, quantitative DMS removal from water is achieved.28 Once sparging was completed, the cryotrap was connected to the gas chromatograph by means of the six-port valves. Desorption was performed by quickly placing the loop in hot water (70-80 °C). The desorbed volatiles were cryofocused in a second, smaller cryogenic trap (W-shaped, 25 cm × 1/8 in.), also in liquid argon, located at the inlet of the chromatographic column. After a desorption/cryofocusing time of 90 s, the valves of the collection cryotrap were then short-circuited so that the carrier gas flowed directly through the cryofocusing loop to the column. Injection proceeded by quickly placing the cryofocusing loop in hot water. This cryofocusing step prevented peak widening caused by the difference in the diameters between the cryotrap and the column and separated (28) Simo´, R.; Grimalt, J. O.; Albaige´s, J. J. Chromatogr. 1993, 655A, 301-307. (29) Haunold, W.; Georgii, H. W.; Ockelmann, G. LC-GC Int. 1992, 5, 28-35.
reaction time, the purge device was connected to the cryogenic trap for preconcentration of the stripped volatiles. Application of a gentle flow (maximum 40 mL min-1) of the sparging gas during the first 3 min after NaBH4 addition allowed the slow dissolution of borohydride and its effective reaction with DMSO. When the reaction time was completed, the sparging flow rate was lowered again (
