Anal. Chem. 1900, 60,390-394
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necessity of examining the spectral profile of a peak. Furthermore, the detection of a coeluting component that might otherwise be missed may be achieved automatically.
CONCLUSIONS The utility of this method is that recurring components can be identified, or at least recognized. As more samples of similar components are analyzed, the selectivity of this approach improves. This technique does not replace spectral searching, but it does provide confirming evidence to the identity of a component. If the signal-to-noise ratio of a component is consistently low, a good spectrum of that compound may never be recorded from a single GC/FT-IR analysis. Nonetheless, if successive samples are shown to contain the same unidentified component, the spectra from all the analyses can be coadded to produce a high signal-tonoise ratio spectrum. The identity of the unknown compound can then be determined, either by searching or by interpretation. Clearly, if a component is repeatedly found in a series of analyses, its identity is important. It is interesting to note that all the searches presented in this paper were done by using spectral libraries. The searches could have been accomplished by using interferometric data. Based on other work in this laboratory, the identity of a few
more components may have been found by using such a search, but the value of the correlation method is in no way diminished by a better search. The correlation method serves to confirm the identity of components from analysis to analysis, regardless of the original method by which the components were identified.
LITERATURE CITED (1) Azarraga, L. V.; Potter, C. A. HRC CC, J. Hhh Resolut. Chromatogr. Chromatogr. Commun. 1981, 4 , 60-69. (2) Wehrli, A.; Kovats, E. Helv. Chlm. Acta 1959, 42, 2709-2736. (3) de Haseth, J. A.; Isenhour, T. L. Anal. Chem. 1977, 49, 1977-1981. (4) de Haseth, J. A; Azarraga, L. V. Anal. Chem. 1981, 53, 2292-2296. (5) Lowry, S. R.; Huppler, D. A. Anal. Chem. 1981, 53, 889-893. (6) Forman, M. L.; Steel, W. H.; Vanasse, G. A. J. Opt. SOC.Am. 1986, 56, 59-63. (7) Azarraga, L. V.; Hanna, D. A. "GIFTS Gas Chromatography Fourier Transform Infrared Software Package and User's Guide"; U.S. EPA/ ERL, Athens, GA, 1979.
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RECEIVED for review January 27, 1987. Resubmitted October 12,1987. Accepted October 27,1987. The support of the U.S. Environmental Protection Agency under Cooperative Agreement No. CR-807302 is gratefully acknowledged. This work was presented in part at the 34th Annual Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1983.
Direct Monitoring of Supercritical Fluids and Supercritical Chromatographic Separations by Proton Nuclear Magnetic Resonance L. A. Allen, T. E. Glass, and H. C. Dorn* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
The direct coupling of 'H nuclear magnetic resonance ('H NMR) as a detector to a flowing supercritical fluid (SF)apparatus is described. The SF/'H NMR system is capable of elevated temperatures (- 100 "C) and pressures (-3500 psi). One application of this apparatus is the measurement of 'H NMR relaxation t h e (T,'s) over a wide range of temperatures and pressures. A second application is the monitorhg d supercrttlcaifluids contalnhg quadrupdar nudel (e.g., "N) where reduced line widths are obtained. Another application described in this paper Is supercritical fiuld chromatography dlrectly coupled to 'H NMR (SFC/'H NMR). The direct Identlkatbn of mnponats present in a model system Is demonstrated and comparison Is made to resuits obtained by using normal phase HPLCINMR conditions.
Supercritical fluids (SF) are becoming increasingly important in a number of different applications because of their unique properties. For example, supercritical fluids have relatively high densities in comparison with normal gases, while solute diffusion coefficients place them intermediate to liquids and gases, as illustrated by the data in Table I (I). The separation of high molecular weight and/or thermally labile species and the coupling of extraction-separation analysis are important applications of supercritical fluid chromatography 0003-2700/88/0380-0390$0 1S O / O
Table I. Typical Values for Fluid Physical Properties property
gas
SCF
diffusivity, cm2/s viscosity, g / ( c m s)
lo-'
2X 2x 0.4-0.9
density, g / m L
liquid
5
X
lo6
1.0
(2). The solvating ability of supercritical fluids is related to the density of the fluid, which is readily varied by changes in temperature and pressure. This increased flexibility can be an advantage of SFC over normal liquid and gas chromatography. To operate in the supercritical region, temperatures and pressures above the critical point (Tc,p,) must be used. A common SFC solvent, COP,has the advantage of relatively low critical point parameters, T, = 31 O C and P, = 1072 psi. 'H nuclear magnetic resonance has been shown to be an effective detector for liquid chromatography (LC/'H NMR) [3-10). Solvent systems such as halocarbons (e.g., Freon 113) and deuteriated solvents have been used in normal-phase chromatography to help alleviate 'H NMR background signals. However, for the case of reverse-phase chromatography, a large residual 'H Nh4R background signal is usually present for the solvents typically used (e.g., D,O, CD,OD, etc.) (8). Multipulse techniques can be used to suppress this residual 'H signal; however, certain spectral regions of interest are normally not @ 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 5, MARCH 1, 1988
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Figure 1. Block diagram of the SFC/'H NMR system.
observable (9, 10). An alternative chromatographic approach is the use of supercritical fluids (SF)(e.g., COJ which have no 'H NMR background signal. The use of supercritical fluids, however, requires a flow system capable of operation at elevated temperatures and pressures. Evilia (11)and Jonas (12) have also shown that significant NMR line narrowing can be obtained for quadrupolar nuclei (e.g., 14N, "0,etc.) when dissolved in supercritical fluids. This is presumably due to the reduced molecular correlation times (7,) for molecules dissolved in supercritical fluids in comparison with normal liquids. In this paper, we describe an apparatus enabling the direct monitoring of supercritical fluids (SF)by a high field superconducting NMR instrument. Of particular interest is a home-built NMR flow probe possessing variable temperature and pressure capabilities. The custom-built NMR flow probe described is capable of operating at temperatures as high as 100 OC and pressures of -3500 psi. One important application of this apparatus is the direct coupling of supercritical fluid chromatography to 'H NMR (SFC/'H NMR). Multinuclear NMR observation (e.g., 14N)is also demonstrated.
EXPERIMENTAL SECTION A diagram of the complete SFC/'H NMR apparatus is presented in Figure 1. An important component in this apparatus is the pump for delivering the supercritical fluid. Reciprocating piston pumps can be modified and used effectively as solvent delivery systems for supercritical fluids (13). Pulsing, while present, can be minimiied by use of appropriate techniques. While syringe pumps are typically used in SFC studies, the associated high expense and the need for higher flow rates made them less amenable. For our work, a single-piston reciprocating pump (Scientific Systems Incorporated (SSI) Model 200 liquid chromatography pump) has been effectively modified to deliver the higher flow rates needed in analytical and semipreparative scale packed column SFC. Together with a gas chromatography oven, an effective SFC system has been assembled (Figure 1). In cases where separations were not required, the supercriticalfluids were monitored with the chromatographic columns excluded from the apparatus. Cooling of the exposed pump head to -10 "C with a dry ice/ethanol/water bath facilitatesefficient delivery of liquid COP Linearity of solvent delivery was achieved over a wide range of flow rates. Carbon dioxide was supplied to the pump directly from a cylinder equipped with an eductor tube or from an inverted gas cylinder previously charged with COP Custom composition mixtures can similarly be delivered by first introducing a known weight of solute (e.g., polar modifier) followed by charging the cylinder with COz and monitoring the appropriate increase in cylinder weight. Modifiers have also been introduced by a second pump downstream prior to injection. Pulsing, inherent to reciprocating piston pumps, was reduced through a diaphragm type SSI Model 210 pulse dampener which also required an important modification. The fluorocarbon polymer diaphragm failed to isolate the pumped COZ from the compressible liquid. Increasing
Figure 2. Custom flow probe for use in directly coupled SFC/NMR: (a) Insulated glass transfer line; (b) glass Inserts; (c) Cu/constantan thermocouple; (d) stainless steel equillbratlon coil; (e) brass shield; (f) Helmholtz coil; (g) ceramic flow cell; (h) brass Swageiok fitting.
back pressure due to the permeating COz caused premature diaphragm failure. However, replacement with a convoluted stainless steel diaphragm with the necessary increase in internal volume eliminated seal failure with no loss in pulse dampening ability. Flow modulation remained at less than 2%. Flow rates up to 8.0 mL/min and pressures up to 4500 psi (measured at the pump) have been used with no seal or pump failures. Supercritical conditions for the chromatographic separations were established by use of a Varian series 1200 gas chromatography oven prior to detection by NMR. The solvent was preheated to the preset operating temperature by use of a coil of in. X 0.05 in. stainless steel tubing placed inside the oven. Separation within the oven followed injection using a standard Valco valve. Oven dimensions allow for packed columns (analytical or semipreparative) up to 25 cm long; two 25 cm long columns placed in tandem are also possible. An Econosphere CIS250 X 4.6 mm analytical scale column with 5-rm packing was used for the separations presented. Chromatographic separation thus occurs under the controlled temperature of the oven with the pressure controlled by the flow rate and restrictor osifice. After separation, detection utilizes a high field nuclear magnetic resonance spectrometer with the custom variable temperature probe maintained at the same temperature as the GC oven &2 "C (vide infra). Due to the higher flow rates used, effective back-pressure restriction was possible with a stainless steel microvernier GC valve at ita lowest settings. Carbon dioxide deposition due to adiabatic expansion at the orifice and subsequent clogging was eliminated by immersion of the restrictor in a hot water bath. Pressure programming can be accomplished by using a variable flow rate with a fixed orifice setting. The spectrometer employed was a JEOL FX200 with a 'H resonance frequency of 199.5 MHz. In order to directly couple SFC to a 'H NMR spectrometer, a custom flow probe was developed capable of withstanding high pressures and maintaining stable elevated temperatures. A diagram of the probe is shown in Figure 2. Nonmagnetic materials were used exclusively in the probe construction. Equilibration of the Boltzmann magnetization at the magnetic field employed was required prior to NMR
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measurement; therefore, a coil of in. X 0.05 in. stainless steel tubing precedes the observation cell (- 1mL internal volume). A 0.25 in. 0.d. alumina ceramic tube (Omegatite 450) was used for the observation cell. Pretreatment of the ceramic tube with concentrated nitric acid ensured no residual iron present m a result of the extrusion method of manufacture which could degrade spectral resolution. The stainless steel tubing of the equilibration coil was connected to the ceramic tube with brass Swagelok reducing fittings; the brass ferrules were replaced with a graphitelvespel blended ferrule. The latter maintains a highpressure seal while allowing easy interchange of ceramic cells. Different internal diameter tubes allow a range of observation volumes (20-120 pL). Orthogonal Helmholtz coils, tuned to the respective frequency of interest (e.g., 'H, 199.5 MHz; 14N,14.4 MHz) were employed. Static 'H resolution was typically 1.4 Hz (0.01 ppm) due to nonspinnii conditions. Air was passed through a blast tube connected to the base of the probe and this heated air then traveled through an insulated glass line to the preequilibrium region prior to passage into the observation region. Forced air flow rates on the order of 20 L/min facilitated rapid and stable temperature equilibration. A concentric tube design, in which a sheath of heated air was passed along the outside of the observation region, minimized radial temperature gradients. The operating temperature was monitored with a copperconstantan thermocouple adjacent to the observation volume and controlled with a JEOL JNM-VT unit for a temperature stability of f l O C . The eluent sample then passed out of the spectrometer to the back-pressure restrictor.
RESULTS AND DISCUSSION For most of the supercritical fluid studies to date, a ceramic tube with an internal diameter of 1.6 mm in the NMR probe (Figure 2) was used for most experiments. A static 'H NMR line width (Au,,,) of 1.4 Hz was obtained for this configuration.
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This line width is slightly higher than that usually obtained (
