184
Anal. Chem. 1991. 63. 184-186
(4) Lauer, H. H.; Colburn, J. C.; Grossman, P. D.; Nielsen, R . G.; Riggin, R. M.: Sittampalm, G. S.;Rickard, E. C. Anal. Cbem. 1989, 6 1 , 1186. ( 5 ) Tsuda, T.; Nomura, K.: Nakagawa, G. J . Cbromatogr. 1983, 264,
385. (6) Aguilar. M : Huang. X : Zare. R N J Cbromatogr. 1989. 480, 427 (7) Saitoh. T.; Hoshino, H.: Yotsuyanagi. T J . Cbromatogr 1989. 469, 175. ( 8 ) Gross, L.: Yeung. E. Anal. Cbem. 1990, 6 2 , 427. (9) Dasgupta, P ; Soroka. K.: Vithange. R. J Liq. Cbromatogr. 1987, 70. 3287. (10) Soroka. K , Vithanage, R , Phillips, D Walker, B Dasgupta, P Anal Cbem 1987. 59 629 ( 1 1) Balchunas, A.; Swaile, D.: Poweil, A . C.: Sepaniak, M. J. S e p . Sci. Tecbnol. 1988, 2 3 , 1691. (12) Hoyt, A.; Sepaniak, M. Anal. Lett. 1989, 2 2 , 861. (13) Kotrly, S.;Sucha. L. Handbook of Chemical Equilibria in Analytical Chemistry: John Wiley 8 Sons: New York, 1985. (14) Lange's Handbook of Chemistry, 13th ed.: Dean, J. A,, Ed : McGrawHill Book Co.: New York. 1985.
(15) Sepaniak, M. J.; Swaile, D. F.; Powell, A. C. J . Cbromatogr. 1989. 480, 185. (16) Murray, G.: Sepaniak, M. J . Liq. Cbromatogr. 1983, 6 , 931. (171 Jorgenson, J. W.; Rose, D. J., Jr. J . Cbromatogr. 1988, 447, 117. 118) Zare. R. N.: Burgi, D. S : Haung. X.: Pentoney, S. L., Jr. Anal. Cbem. 1988, 60. 2625. (19) Cunnane, S . Zinc: Clinicaland Biochemical Significance: CRC Press Inc.: Boca Raton. FL. 1988. (20) Atkins. P. W. Physical Cbemistry: W. H. Freeman and Co.: San Francisco, 1982.
RECEIVED for review ,June 20, 1990. Accepted October 10, 1990. This research was sponsored by the Division of Chemical Sciences, Office of Basic Sciences, U.S. Department of Energy, under Grant DE-FG05-86ER13613 with the University of Tennessee.
TECHNICAL NOTES Sample Tube for Medium-Pressure Nuclear Magnetic Resonance Spectroscopy in Liquid Xenon Solution Leslie D . Field,* Adrian V. George, Barbara A. Messerle, and Howard Ionn Department of Organic Chemistry, The Uniuersit3 of Sydne,, S j d n e J , N S W 2006, Australia
INTRODUCTION Nuclear magnetic resonance (NMR) spectra are recorded routinely on samples dissolved in solution at various temperatures. Containment of the sample can be difficult if components of the sample are extremely volatile or if the temperature is near the boiling point of the solvent. For high-pressure applications, specialized materials such as sapphire have been used for NMR spectroscopy on samples under pressures of up to several hundred atmospheres ( I ) . For low-pressure applications, NMR tubes with gas-tight concentric valves have been previously designed (2) and some are commercially available (5. Young (Scientific Glassware) Ltd . London, U.K., Catalog No. NMR/5, NMR/10). These are convenient tubes for handling liquids below their normal boiling points and may be employed a t pressures slightly above atmospheric pressure; however, they have the disadvantage that there is no safety mechanism to relieve pressure should it build up in the apparatus. In a completely sealed system, a rapid increase in internal pressure could result in a catastrophic failure of the tube with a consequent safety risk and/or possible damage to spectroscopic equipment. A sample tube for NMR spectroscopy incorporating a crude pressure release valve has been reported and used previously for applications involving condensed gases as NMR solvents (3). Although glass tubes have been employed to contain samples for NMR spectroscopy a t pressurs of a few atmospheres, the bursting pressure of such assemblies cannot be predicted reliably ( I ) . We report here the design of a sample tube for NMR spectroscopy a t medium pressure. The design incorporates a pressure relief mechanism that permits the safe handling of solutions a t moderate pressures (up to approximately 10 atm) and has been tested extensively for NMR measurements involving condensed gases as NMR solvents. EXPERIMENTAL SECTION Construction of the Medium-Pressure NMR Tube. A thick-walled Pvrex tube (outside diameter 8 mm, inside diameter 5 mm, length 1-10 mm) was fused to the body of a commercially available threaded gas-tight Pyrex valve (J. Young Ltd . London. 0003-2700/91/0363-0184$02.50/0
IJ.K., Catalog No. POR-8). The side arm of the valve body was sealed to provide an axially symmetric assembly. The valve plunger was disgarded, and an alternative stem was machined from free cutting brass rod, concentrically bored and perforated at the base to allow free flow of gases (Figure 1). The top of the brass body was fitted with a neoprene "0"-ring seal of a diameter appropriate to mate with a glass adaptor, allowing direct connection to a vacuum line. A plunger was fashioned from poly(tetrafluoroethylene) (PTFE) to fit loosely into the base of the brass valve body and shaped to sit on the glass of the valve casing. A retaining pin ensured that the PTFE plunger remained attached to the brass body during assembly and disassembly and allowed the plunger to be unseated when the body of the valve was loosened, for example, prior to the introduction of a gaseous sample. A spring was mounted between the head of the plunger and the brass stem. Spring compression (and hence the release pressure of the plunger) can be adjusted by the setting of the valve screw thread. If the pressure inside the tube were to build up beyond the preset level, the plunger would open and the excess gas pressure would vent through the central channel in the valve stem to the port in the top of the valve. Any vented gases can either be released directly to the atmosphere or be conducted away by tubing. The valve can be opened manually under controlled conditions, allowing recovery of volatile components from the sample. The requirement for the sample tube to be located in a highfield magnet (9.39 T) dictated that a spring of non-ferromagnetic material was essential. Stainless steel had unacceptably high ferromagnetism, and suitable springs were manufactured from phosphor bronze. Although phosphor bronze was acceptable for relatively low-pressure applications, it proved a difficult material to work into the dimensions required for high-strength springs. A shaped cylinder of die rubber (Blackwoods and Son, Smithfield, Australia, Catalog No. 05257706) was a satisfactory substitute for the spring for higher pressure applications. NMR Spectroscopy. The design of the pressure relief valve maintains the axial symmetry of the tube to minimize the perturbation of the magnetic field of the NMR spectrometer. The outer diameter of the thick-walled tube was chosen to suit the experiment (e.g., larger diameters permit easier sample loading with solids) and also the NMR instrumentation available. For NMR measurements requiring an external deuterium lock, the apparatus \vas constructed from an 8-mm thick-walled tube lo2 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 2, JANUARY 15, 1991
185
50
'IF I
m l
Figure 1. Expanded view of the construction of a tube for mediumpressure NMR spectroscopy. (a) Glass port to a vacuum line; (b) "0"-ring seals: (c) machined brass casing; (d) plunger retaining pin: (e) retaining nut: (f) pressure relief spring; (9) PTFE plunger: (h) valve casing; (i) concentric support; (i) thick-walled Pyrex tube (8-mm 0.d.): (k) outer tube (10-mm 0.d.); (I) deuterated lock substance; (m) sample.
cated inside a standard 10-mm NMR tube, using an annular support to ensure concentricity. The cavity between the tubes was filled with the lock material. The 8-mm/ 10-mm concentric tube arrangement was employed for 31Pand 'H NMR spectroscopy. NMR spectra were recorded by using a standard 10-mm probe tuned to 31Pat 162 MHz and 'H at 400 MHz in a vertical axis superconducting magnet. 31P spectra were acquired with composite pulse proton decoupling, and 'H spectra were acquired by using the normal decoupling coils of the probe. 31PNMR spectra were referenced to external, neat, trimethyl phosphite, taken as 140.85 ppm. 'H NMFt spectra were referenced to residual substrate resonances. The probe temperature was calibrated, prior to introduction of the sample, by using the chemical shift difference between the alkyl and hydroxyl signals in methanol (4). For experiments in which samples were irradiated with light at low temperature, the tube was supported within a vacuumjacketed Pyrex cylinder, positioned ca. 7 cm from a 125-W mercury vapor lamp. The sample temperature was maintained by a stream of nitrogen gas, precooled by passage through a heat-exchange coil immersed in a liquid nitrogen bath. Pressure Tests. The apparatus was tested thoroughly before use. For pressures up to 4 atm, the pressure relief behavior of the valve assembly alone was tested and calibrated independently of the glass tube. The reliable opening of the relief valve over a desired pressure regime was tested and calibrated by mounting the valve assembly on a manifold fitted with a pressure gauge and charging to a predetermined pressure with nitrogen gas from a cylinder. The whole apparatus was tested at higher pressures by condensing a substance of known vapor pressure (propane) into the tube, closing the pressure relief valve, encasing the assembly in a steel pipe, and allowing the tube to warm to a predetermined temperature to establish a known internal pressure.
RESULTS AND DISCUSSION NMR Spectroscopy in Liquid Xenon Solution. Liquified xenon has now found application in a number of experiments where it is necessary to use an inert solvent a t low temperatures (3). Although xenon is a liquid over a very limited temperature range at atmospheric pressure (mp -112 OC,bp -107 "C),the critical temperature is 17 OC, and the
I
I
,
,
15
,
.
,
,
70
,
,
,
,
65 PPH
,
,
,
,
60
,
,
,
,
,
,
,
,
55
Figure 2. (a) 31P NMR spectrum ('H decoupled) of a mixture of FeH,(DMPE), (5 mg) and toluene (50 mg) in liquid xenon solution (162 MHz, -95 "C); (b) after irradiation of 90 min at -100 O C .
liquid-phase range may be extended to about -60 "C by using pressures of no more than 10 atm (5). As an extension of our work on the photochemical activation of hydrocarbons using organometallic iron phosphate complexes (6) and to illustrate the use of the apparatus, we have examined the reaction of FeH2(DMPE)2[DMPE = 1,2-bis(dimethy1phosphino)ethanelwith arenes in liquid xenon solution. Studies of hydrocarbon activation by FeH2(DMPE)2 under photochemical conditions have been hindered by the lack of a solvent, which was itself not susceptible to attack by the reagent. Perfluoroalkanes (7) and bulky hydrocarbon solvents (8), e.g., 1,3,5-tri-tert-butylbenzene, have been employed previously as low reactivity solvents for such reactions, but low solubility of substrates and their relatively high melting points limits their use. The air-sensitive complex, FeH2(DMPE)2 (5 mg), was loaded into the thick-walled NMR tube in a drybox under an argon atmosphere. Degassed toluene (50 mg) was introduced and the pressure relief valve closed. The tube was attached to a vacuum line, cooled to liquid nitrogen temperature, and evacuated. Xenon (Xe (99.99%) was obtained from CIGHYTEC, Surry Hills, Australia, and was deoxygenated by three freeze-thaw cycles on a vacuum line before use) was condensed into the tube at liquid nitrogen temperature, and the pressure relief valve was closed with the spring compression set to allow the escape of gas should the internal pressure reach 5 atm. The sample tube was located concentrically in a 10-mm outer tube, containing diethyl-dlo ether as a low-temperature lock compound and the temperature was allowed to stabilize at -98 "C (methanol/liquid nitrogen slush). The sample tube was lowered into the precooled probe of a Bruker AMX400 NMR spectrometer and maintained a t -95
"C. Figure 2a shows a representative 31Pspectrum acquired on the sample, and Figure 2b shows the spectrum following irradiation at -100 "C for 90 min. Photochemical activation of the iron complex in the presence of toluene in liquid xenon resulted in ca. 40% conversion of the starting material to two major products. Complete disappearance of the starting complex occurred after several hours of irradiation. The major
l
l
l
'
l
186
Anal. Chem. 1991, 63, 186-189
products have been identified previously ( 9 ) as cis-Fe(H)(ptolyl)(DMPE), (26% after 90 min: 31PNMR 6 74.07 (1P m), 70.41 (1P m), 60.07 (1 P m), 53.52 (1P m); 'H NMR d -13.43 (Fe-H)) and cis-Fe(H)(m-tolyl)(DMPE):,(14% after 90 min: 31PNMR 6 74.01 (1P m), 70.41 (1 P m), 60.07 (1 P m). 52.49 (1 P m); 'H NMR 6 -13.49 (Fe-H)).
CONCLUSION We have designed, constructed, and tested a tube for NMR spectroscopy at medium pressures. The apparatus can be accommodated by commercial NMR spectrometers without the need for modification of the spectrometer or probe hardware. The design incorporates a pressure relief valve that permits the safe acquisition of NMR spectra of samples under a few atmospheres of pressure and enables the use of condensed gases as solvents a t temperatures above their normal boiling h i n t s . The apparatus has been used in the study of the photochemical activation of hydrocarbon C-H bonds in the presence of FeH,(DMPE), using liquid xenon as a solvent. Registry No. Xe, 7440-63-3.
LITERATURE CITED Roe, D. C. J. Magn. Reson. 1985, 63, 388-391. Vanni, H.; Earl, W. L.; Merbach, A. E. J. Magn. Reson. 1978, 29, 11-19. Gombler, W.; Willner, H. In?. Lab. 1984, 14 (5), 84-85. Sponsler, M. B.; Weiller. 8 . H.; Stoutland, P. 0.; Bergman. R. G. J , Am. Chem. SOC. 1989, 1 1 1 , 6841-6843. Van Geet, A. L. Anal. Chem. 1970, 4 2 , 679-680. Theeuwes. F.; Bearman, R . J. J . Chem. Thermodyn. 1970, 2 , 507-512. Baker, M. V.; Field, L. D. J. Am. Chem. SOC. 1987, 109, 2825-2826. Baker, M. V.; Fieid, L. D. J. Am. Chem. SOC. 1988, 108, 7436-7438. Baker, M. V.; Field, L. D. Organometallics 1988, 5 , 821-823. Hoyano, J. K.; McMaster, A. D.: Graham, W. A. G. J. Am. Chem. SOC.1983, 105, 7190-7191. Marx, D. E.; Lees, A. J. Inorg. Chem. 1988, 27, 1121-1122. Sakakura. T.; Ishida, K.; Tanaka, M. Chem. Lett. 1990, 585-588. Baker. M. V.: Field. L. D. J. A m . Chem. SOC. 1986, 108. 7433-7434 Tolman, C A , Ittei. S D , English, A D , Jesson. J P J Am Chem SOC 1979, 101 1742-1751
RECEIVED for review August 20,1990. Accepted September 2 7 , 1990. We gratefully acknowledge financial support from the Australian Research Council and The University of Sydney for a Gritton Postdoctoral Fellowship (B.A.M.).
Determination of Dimethyl Sulfoxide and Dimethyl Sulfone in Air Russell F. Lang*il Cooperative Institute for Marine and Atmospheric Studies, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149
Cheryl J. BrownZ NOAAIAOML Ocean Chemistry Diuision, 4301 Rickenbacker Causeway, Miami, Florida 33149
INTRODUCTION Dimethyl sulfoxide (DMSO) and dimethyl sulfone (DMSO,) are believed to be intermediates in the marine atmospheric oxidation pathway of dimethyl sulfide (DMS) to SO, and sulfate. Laboratory studies have shown that when DMS is photolyzed in air, DMSO and DMSOz are components of the reaction product mixture ( I ) . DMSO is also produced when DMS is photolyzed in the presence of IO ( 2 ) ,and DMSO:, is a minor reaction product when DMS and OH react ( 3 ) . The other major reaction products of DMS oxidation, methane sulfonic acid (MSA) and SO,, have been well documented in both laboratory studies ( 4 , 5 )and marine geochemical studies (6-9). DMSO and DMSO2,however, have not been thoroughly characterized in the marine environment. The original geochemical finding of DMSO and DMSO, in the marine environment reported total (gas phase + aerosol) concentrations in marine air and in rain (IO). These compounds have subsequently been reported in marine aerosols (11). An analytical method to determine DMSO and DMSOl in marine air needs to be (a) highly sensitive, since these compounds are present a t low nanogram per cubic meter concentrations, (b) highly selective, since a large number of other organic compounds would be present a t these low concentrations. In addition, the analytical system must be highly deactivated, since sulfur compounds are notoriously adsorptive. Existing methods developed for the determination of reduced organic sulfur gases typically utilize preconcentration
* To whom correspondence should be addressed. Current affiliation: Department of Chemistry, P.O. Box 249118, University of Miami, Coral Gables, FL 33124. Current affiliation: NOAA National Marine Fisheries Service, P.O. Box 271, La Jolla, CA 92038.
on solid adsorbents (I2-15), chemisorption onto gold (16,17), or cryogenic techniques (18-20). These methods all require subsequent thermal desorption into a gas chromatograph. They are not directly applicable to the determination of DMSO and DMSO, in air because of the low volatility of these two compounds, the boiling points being 189 and 238 "C, respectively (21). Preliminary experiments in our laboratory showed that DMSO and DMSO, could not be thermally desorbed quantitatively from Tenax, activated charcoal, or SE-30/silica support. Preconcentration on a solid sorbent, followed by solvent desorption with a polar organic solvent was considered the most promising analytical approach. Tenax was selected as the solid sorbent due to its low retention of water and high degree of stability and inertness (22-24). The analytical system chosen combines a high degree of sensitivity and selectivity by the use of packed-column gas chromatography coupled to a modified Hall electrolytic conductivity detector. This report presents for the first time, an analytical method for the determination of trace quantities of DMSO and DMSO, in air. It describes in detail the analytical method that was only briefly mentioned in the original geochemical report of DMSO and DMSO, in marine air (10).
EXPERIMENTAL SECTION Instrumentation and Reagents. Gas chromatography was performed on a Perkin-ElmerSigma 3B instrument equipped with a Hall 700A electrolytic conductivity detector and a HewlettPackard 3390A integrator. Detector sensitivity was increased by reducing the solvent flow rate to the conductivity cell from 0.80 to 0.20 mL/min. This was accomplished by coupling a 1/16 in. stainless steel Swagelok union with Teflon ferrules between the pump and the Teflon solvent delivery line. The flow rate was reduced to 0.20 mL/min by gradual tightening of the union nuts.
0003-2700/91/0363-0186$02.50/0P 199 1 American Chemical Society
