2035
Anal. Chem. 1908, 6 0 , 2035-2040
sensitivity is to use a bypass valve to direct the hydrogen gas outside the outer sleeve of the G P T while the acidic sample solution is pumped through the inner tube by using the continuous mode until a relatively large sample volume has passed. Accumulation of AsC13will take place in the gas phase, and the entrapped gas can be passed through the atomizer to yield a transient signal. This technique will be analogous to the pressure and cold trapping techniques sometimes used in the hydride generation (18). Further, if a heated gas generation cell is used, HC1 concentrations can be decreased from the 9 M needed at room temperature to perhaps 6 M, a step that alone may improve the detection limit by a factor of 2 due to less dilution by the concentrated HCl that has to be added to the sample. Work is in progress that will test the ideas outlined above. ACKNOWLEDGMENT We thank Jiri Dedina for building the atomizer cell and the batch hydride generation apparatus used in these experiments, Anden Cedergren for valuable discussions, and Michael Sharp
for linguistic revision of the manuscript. LITERATURE C I T E D (1) (2) (3) (4) (5) (6) (7) (8)
(9)
(IO) (11) (12) (13) (14) (15) (16) (17) (18)
Holak, W. Anal. Chem. 1989, 4 7 , 1712. Nakahara, T. Prog. Anal. At. Spectrosc. 1983, 6 , 163. Aggett, J.; Aspell. A. C. Analyst (London) 1978, 707, 341. Smith, A. E. Analyst (London) 1975, 700, 300. Kirkbright, G. F.; Taddia, M. Anal. Chim. Acta 1978, 700, 145. Pierce, F. D.; Brown, H. R. Anal. Chem. 1978, 48, 693. Agterdenbos, J.; Bax, D. Fresenius’ Z . Anal. C I ” . 1986, 323,783. Welz, E.; SchubertJacobs, M. J . Anal. At. Spectrosc. 1988, 1 , 23. Schmidt, F. J.; Royer, J. L. Anal. Leff. 1973, 6 , 17. Welz, E.; Melcher, M. Analyst(London) 1984, 709, 569. Aggett, J.; Hayashi, Y. Analyst (London) 1987, 772,277. Dedina, J. Anal. Chem. 1982, 5 4 , 2097. Myron, A. G. J . Am. Chem. SOC. 1957, 79, 1865. Nelson, F.; Kraus, K. A. J . Am. Chem. SOC. 1955, 7 7 , 4508. Welz, E.: Melcher, M. Analyst (London) 1984, 709, 573. Sinemus, H. W.; Melcher, M.; Welz, E. At. Spectrosc. 1981, 2,81. Pierce, F. D.; Brown, H. R. Anal. Chem. 1977, 4 9 , 1417. Dedina, J. Frezenius’ Z . Anal. Chem. 1988, 323,771.
RECEIVED for review March 23,1988. Accepted May 23,1988. This work was supported by The Swedish Natural Science Research Council through Grant E-EG 8724-103.
Nuclear Magnetic Resonance Studies of Nitrogen-14-Containing Species in Supercritical Fluids Jan
M.Robert’ a n d Ronald F. Evilia*
Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148
14N NMR spectra of solutes dlssolved in supercritlcal fluid (SCF) solvents are examlned by using a sapphire high-pressure sample cell In an unmodlfled spectrometer. Examples uslng carbon dloxlde, ethane, and ethylene as solvents are reported. The 14Nline widths are reduced In the SCF solvents compared to those In typlcai llquld solvents. Solubilities large enough to yleld signal-to-noise ratios >5 In 6-24 h are obtained for a varlety of ilquld solutes at reduced denslties of 1.0-2.0, but solld compounds had only limlted solubilitles. Sample pressures and temperatures were varied to observe thelr effects on the spectral line wldth. The spectral line-width Improvement appears to have a functional group dependence. Line-width enhancement factors of 3-25 were obtained experlmentaily and varied between approxlmately 10 % and 70% of the enhancements predicted from the dlfferences in solution vlscoslty. The results are Interpreted to lndlcate a substantial vlscoslty-Independent contributlon to the rotational correlatlon time.
Fourier transform nuclear magnetic resonance (FT-NMR) solution studies are among the most prevalent of all instrumental measurements reported in today’s chemical literature. Despite this enormous popularity, high-resolution NMR is generally limited to the study of only a handful of spin 1 / 2 nuclei and higher spin nuclei in extremely symmetrical environments. With the advent of high-field NMR instrumentation came increased sensitivity and better spectral resolution than previously possible a t lower magnetic field Current address: Department of Chemistry, S. G. Mudd Bldg. #6, Lehigh University, Bethlehem, PA 18015.
0003-2700/88/0360-2035$01 SO/O
strengths. To date, however, most quadrupolar nuclei ( I 1 1) remain largely unstudied by high-resolution NMR techniques. For the most part, this is because the highly efficient quadrupolar relaxation mechanism leads to extreme line broadening, which obscures spectral features. In addition, some nuclei such as I7O and 33Ssuffer from low NMR receptivity values, and thus, routine observation is time prohibitive without isotopic enrichment. In nonviscous liquid solution, the quadrupolar line width is directly proportional to the molecular rotational correlation , as such, temperature elevation and solution time, T ~ and viscosity minimization can and have been used to sharpen the resonance signal (1-3). Furthermore, it has been demonstrated in our laboratory that very low viscosity supercritical fluids can be used as solvents to substantially narrow quadrupolar line widths beyond those obtainable with the lowest viscosity organic solvents such as acetone (4-6). In this paper we report the results of a study of 14N line widths of various nitrogencontaining functional groups in several solvent gases using a high-pressure sapphire sample cell in an unmodified spectrometer. Other researchers have examined 14N-,33S-,and 55Mn-containing species in supercritical fluids by using a specially constructed high-pressure probe with a wide-bore, 4.2-T superconducting magnet (7). Recent intense interest in 15N ( I = 112; NA = 0.36% (NA is natural abundance)) spectroscopy, including various sophisticated pulse sequences, 2-D experiments, and reversepolarization techniques (8,9) suggests that the ability to obtain high-resolution 14N ( I = 1;NA = 99.64%) spectra would have an enormous impact on the future focus of FT-NMR spectroscopy. The implications of high-resolution 14Nobservation for biochemical investigations of amino acids, peptides, proteins, etc., are dramatic. Therefore, our efforts have been directed in the area of methods development for high-pressure, 0 1988 American Chemical Society
2036
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
high-resolution NMR studies of 14N-containingcompounds dissolved in supercritical phases. This report summarizes some of our data and conclusions regarding solubility measurements, solvent variations, the effects of temperature, line-width enhancement factors, and functional group effects observed for a variety of nitrogenous compounds dissolved in nonpolar supercritical fluids. The results of the various studies are interpreted in such a manner as to establish some guidelines for the application of supercritical fluid solvents to high-resolution FT-NMR studies of 14N-containingspecies. EXPERIMENTAL SECTION Apparatus. High-pressure, supercritical fluid samples were contained in one of several specially constructed, high-pressure sample cells similar to one reported by Roe (10). Each sample cell consisted of a thick-walled sapphire (single-crystal aluminum oxide) tube epoxied onto a titanium alloy pressure valve assembly. Minor variations were made in the original valve design to make the cap lighter and to make it easier to remove the needle valve pin after use. Although this type of tube was successfully spun by Roe, our replications and modifications would not spin in our instrument. Because the additional line width introduced by the lack of spinning was negligible compared to our sample line widths, the samples were run without spinning. Annealed sapphire tubes (14.5 cm in length, 0.5-cm o.d., 0.11-cm wall thickness sealed at one end with a 0.5-cm end plug) were purchased from Tyco Saphikon, Inc. (Milford, NH). Tube volumes (ca. 0.7 cm3)were determined gravimetrically. Tubes were epoxied onto the titanium alloy pressure valves with Aremco Bond 515 thermosetting epoxy from Aremco Products (Ossining, NY). Valves were machined at the University of New Orleans of 6Al, 4V titanium alloy donated by Timet, Inc. (Grand Prairie, TX). A removable glass-to-metal connection was used to attach the sample cells to a vacuum manifold equipped with a Wallace-Tiernan pressure gauge. Samples were prepared by condensing the solvent gas into the tube from the manifold while the system pressure was monitored with the Wallace-Tiernan gauge. When the desired quantity of gas was delivered, the tube valve was closed and the tube removed from the manifold. The cold tube was taken to an explosion-safe area where it was heated by an air convection heater for pressure testing. All spectra were obtained on a JOEL FX-9OQ FT-NMR spectrometer operating at a 14Nfrequency of 6.42 MHz, using a IO-" probe insert. An external deuterium lock accessory supplied by the manufacturer was used t o stabilize the field drift to less than 0.2 Hz/h at the 14Nfrequency. Temperature regulation was accomplished with a NM-VTS variable temperature controller, calibrated to &2 "C of the set temperature. Reagents. All solvent gases were obtained in lecture bottles from Matheson, Inc., and were used as received. Absolute ethanol was procured from the Florida Distillers Co. Liquids used as solutes were obtained from various suppliers and were of the highest commercial purities available, usually reagent grade or better. Several organic liquids, including nitrobenzene, nitromethane, formamide, and diethylformamide, were redistilled according to standard procedures prior to their use (11). Free base amino acids were purchased from Sigma; caffeine and Proton Sponge [ 1,8-bis(dimethylamino)naphthalene] were supplied by Aldrich. Other high-grade solids were obtained from Kodak. Sample Preparation. Aliquots of liquid solutes were added to the empty tube by microsyringe. Solid substances were added to the tubes as powders that were tapped into the sapphire tubes. In some cases, absolute ethanol was added to the samples t o facilitate dissolution. The sample cells containing the solutes were then attached to the vacuum manifold and subjected to several freeze-pump-thaw cycles before vacuum condensation of the solvent gases. Gases were condensed into the tubes under liquid nitrogen until the predetermined quantity was delivered. The quantity of solvent gas was calculated from the tube volume and the desired solvent density. The sample tubes were sealed, removed from the manifold, and allowed to slowly come to thermal equilibrium at ambient temperature. Pressure testing for 12-18 h at 50-110" C followed. Solubility was noted by visual observation before acquiring NMR spectra of liquid solutes. For some
solid solutes, spectra were acquired with excess solid in the tube. Data Acquisition. Most spectra were acquired at ambient probe temperature (ca. 28 "C) or slightly higher; variable temperature studies were performed over the range of 0-looo C. All data were acquired without sample tube spinning. Instrumental parameters employed included a 90" pulse width, a short pulse delay (
