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a complex surface. In our XPS experiments, the measured 0 1s binding energy a t 532 eV is consistent with oxidized graphite (15) and supports the LITD results which showed large amounts of COP Both the AES and XF’S spectra showed evidence for Si, yet it is clear that the LITD results are far more useful for identifying the molecular nature of the Si species. These experiments do not permit us to estimate the detection sensitivity of LITD, but prior work in our laboratory has shown submonolayer sensitivity for a variety of organic species on platinum (9-11). One of the advantages of LITD over conventional rapid heating techniques is that the vaporized species are removed in microseconds rather than the several tens of seconds that are required to resistively heat the sample. Thus,even if there is a very small amount of material on the surface, it can be detected by LITD because the short laser pulse produces a large flux of desorbed material. Conceptually, this can be thought of as a type of high-resolution chromatography because the sample is compressed into a narrow pulse of high flux density. An essential requirement, of course, is a detector that can capture the complete mass spectrum, and this makes FT mass spectrometry well suited for these experiments.
ACKNOWLEDGMENT We thank F. J. Feher for helpful discussion of the LITD results. Registry No. 02,7782-44-7; Si, 7440-21-3;Clz, 7782-50-5; Nz, 7727-37-9; Na, 7440-23-5; Fz, 7782-41-4; K, 7440-09-7; COz, 12438-9; HzO,7732-18-5; Co, 7440-48-4; Cr, 7440-47-3. LITERATURE CITED (1) Hall, R. 6. J . Phys. Chem. 1987, 9 1 , 1007-1015. (2) Novak, F. P.; Balasanmugam, K.; Viswanadham, K.; Parker, C. D.; Wilk, 2. A.; Mattern, D.; Hercules, D. M. Int. J . Mass Spectrom. Ion P h y ~ 1983, . 5 3 , 135-149. (3) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1988, 5 8 , 1102-1108.
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Cotter, R. J.; Van Breemen, R.; Yergey, J.; Heller, D. Int. J . Mass Spectrom. Ion. Fhys. 1983, 46, 395-398. Van der Peg, G.J. Q.; Van der Zande, W. J.; Kistemaker, P. G. Int. J . Mass Spectfom. Ion Processes 1984, 62, 51-71. Hall, R. 6.; DeSantolo, A. M.; Bares, S. J. Surf. Sci. 1985, 161, L533-L542. Brand, J. L.; George, S. M. Surf. Sci. 1988, 167, 341-362. Stair, P. C.; Viswanathan, R.; Weitz, E. Burgess, D. R., Jr.; Hussla, I.; Rev. Sci. Instrum. 1984, 5 5 , 1771-1776. Sherman, M. G.;Kingsley, J. R.; Dahigren, D. A,; Hemminger, J. C.; McIver, R. T., Jr. Surf. Scl. 1985, 148, L25-L32. Kingsley, J. C.; Hemminger, J. C.; McIver, R. T., Jr. Sherman, M. 0.; Anal. Chlm. Acta 1985, 178, 79-89. Sherman, M. G.; Land, D. P.; Hemminger, J. C; McIver, R. T., Jr. Chem. Phys. Len. 1987, 137, 298-300. Sato, I . I€€€ Trans/. J . Magn. Jpn. 1987, 2 , 4. Translated from Sato, I.J . Magn. SOC.Jpn. 1988, 10, 6. Handbook of Auger Nectron Spectroscopy, 2nd ed.; Physical Electronics Division, Perkin-Elmer Corp.; Eden Prairie, MN. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics Division, Perkin-Elmar Corp.; Eden Prairie, MN. Barber, M.; Evans, E. L.; Thomas, J. M. Chem. Phys. Len. 1973, 18, 423.
Donald P. Land Tsong-Lin Tai John M. Lindquist John C. Hemminger* Robert T. McIver. Jr.* Department of Chemistry University of California Irvine, California 92717 RECEIVED for review April 27, 1987. Accepted September 1, 1987. Support for this research was provided by the National Science Foundation under Grant CHE8511999 and the donors of the Petroleum Research Fund, administered by the American Chemical Society. J.C.H. wishes to acknowledge support from the Alfred P. Sloan Foundation in the form of an Alfred P. Sloan Research Fellowship. J.M.L. and D.P.L. wish to acknowledge support in the form of IBM Research Fellowships.
AIDS FOR ANALYTICAL CHEMISTS Detection of Aerosol Formation in the Effluent of a Supercritical Fluid Chromatograph Steven R. Goates,* Norman A. Zabriskie, John K. Simons, and Bahram Khoobehi’
Department of Chemistry, Brigham Young University, Provo, Utah 84602 Supercritical fluid techniques continue to grow in importance in analytical chemistry, especially supercritical fluid chromatography (1). Accompanying this growth is the development of postcolumn detection methods, including flame ionization detedion (2,3),inductively coupled plasma emission ( 4 ) ,mass spectrometry (5,6),and supersonic jet spectroscopy (7-9). In each of these detection methods, the behavior of the fluid at the point of decompression, especially in regard to nucleation processes, can severely affect the performance of the detector. Giddings (10) pointed out the “fogging” problem in supercritical fluid expansions almost 2 decades ago, and the problem has been examined recently by others, including Smith et al. (11). The problem is not widely appreciated, however, and statements such as “when a supercritical fluid is introduced to the vacuum it immediately converts to a molecular beam” are not uncommon. Current address: Department of Ophthalmology, Eye and Ear Infirmary, University of Illinois College of Medicine at Chicago.
For convenience in discussion, we have categorized aggregate-forming processes into three primary types: (1) the formation of liquid droplets of the carrier solvent at the point of expansion, which we have termed aerosol formation, (2) microscopic clustering of carrier molecules in weakly bound van der Waals complexes with solute molecules, and (3) precipitation of solute molecules into relatively stable particulates. However, whether the first two categories really represent distinct processes is open to question. We have discussed the problem of solute precipitation elsewhere (7). It occurs when the sample approaches saturation in the carrier (7,10, 12) as can occur in a long restrictor where the change in pressure is significant and places important constraints on nozzle design for superson’ic jet detection. There has been some confusion in the analytical chemistry literature about the different types of clusters represented by categories 2 and 3 and the effect of the Mach disk (13) in a supersonic expansion; weakly bound van der Waals clusters may form in the jet but will be broken up at the Mach disk, whereas sample
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aggregation may occur in the turbulent region of the Mach disk. Solute aggregation is the major contributor to the spiking which has been observed with flame ionization detection, but, as we discuss later in this paper, erratic "spitting" of large aerosol droplets can Occur with extended restrictors and will also produce noise spikes. However, even fine aerosol mists can severely interfere with spectrmopic detection, not only increasing the noise level but also inhomogeneously broadening spectral lines; thus the characterization of aerosol formation has been particularly important to our investigation of supersonic jet spectroscopy with supercritical fluid carriers (7). We report here the conditions for aerosol formation in the effluent jet, as monitored by laser-light scattering, for five fluids, which cover a wide range of critical constants and chemical properties. EXPERIMENTAL ,SECTION Equipment. A modified ISCO high-preasure syringe pump delivered fluids at pmures up to 400atm through '/le in. stainless steel tubing to the nozzle interface shown in Figure 1, which emptied into a small vacuum chamber. Fluid reservoir pressures were monitored with a calibrated Celesco pressure transducer: the pressure drop along the column was estimated by the Hagen-Poiseuille law to be less than 2 atm for the greatest flow rates. The chamber was pumped with an Alcatel ZM2033 vacuum pump which maintained pressures in the chamber between 30 and 100 mTorr, depending on the sample flow rate. Some experiments were also done in a larger chamber, described elsewhere (7)in to lod Torr. which pressures were maintained at Two types of restrietors were used in the nozzle: laser-drilled pinholes in stainless steel disks (Optimation, Inc.) and fused silica capillaries. Scientific Glass Engineering unions and butt connectors were employed in the nozzle assembly (see Figure 1); capillary restridors were connected in the usual manner with a graphite ferrule. and pinhole restrietors were sealed to the nozzle tip with graphitized vespel gaskek W E ) and a stainless steel cap with a 30' exit cone. Separate heaters were employed for the column and the nozzle for better temperature control. A 3/,-in.-diameter copper sheath wrapped with heating wire and covered by fiberglass tape served as the nozzle heater; it was machined to fit snugly on the end cap of the nozzle. The transfer column was heated with standard heating wire and insulated with a fiberglass wrap. Ironfconstantan thermocouples were placed on the stainless steel tubing of the column and into a well near the tip of the copper sheath Procedure. The presence of aerosol droplets in the jet was easily detected hy crossing the jet with a laser beam and observing light scattering off the jet (see Figure 2). The W n m light from a Control Laser Corp. Model 551A argon ion laser was directed perpendicularly to the sample flow, usually about 5 mm downstream from the nozzle. and allowed detection of droplets larger than or about the size of this wavelength. Smaller clusters could be detected in the laser-indueed fluorescencespectra of fluomcent sample noolecules seeded into the carrier fluid (7).Scattered light was monitored at right angles to the jet and laser beam as fluid temperature was increased a t constant hacking pressure. The limiting temperature for aerosol formation at a particular pressure
F ~ U 2. W (a) Photographs of progressive disappearance of aeroool in effluentjet with increasing fluid temperature. In mi$ cas8 ltm capillary restrictor extends about 1 mm beyond ltm heated nozzle assembly. (b) Photomultiplier tube output for disappearance of CO, aerosol for an expansion at 78 atm through a 5-pm pinhole
was taken as the point at which output from the photomultiplier tube dropped to background level (see Figure 2b). Disappearance of aerosol in the jet could also be observed visually and because limiting temperatures determined visually agreed within experimental error with measurements made with the photomultiplier tube, much of the data was colleded bv using the more convenient visual technique. In order to achieve good thermal equilibration,the column and the nozzle were first heated to about 15 "C below the estimated limiting temperature, and maintained at this lower temperature for 15-20 min. The column and nozzle temperatures were then increased at about lo C/min until the aerosol disappeared; the column temperature was maintained within 2 or 3 "C of the nuzzle temperature throughout each run. No difference in limiting temperatures was observed for slower heating rates. Reagents. Five fluids, chosen to cover a range of critical constants, were tested with this method. Spectrograde benzene was obtained from Fisher Scientific and OmniSolv n-pentane from EM Sciences. SFC-grade carbon dioxide, nitrous oxide, and sulfur hexafluoride were purchased from Scott Specialty Gases. RESULTS A N D DISCUSSION Comparison of results in the high-vacuum and mediumvacuum chambers, as well as a few rough experiments performed in the open atmosphere, revealed that formation of aerosol in the jet was not affected by background pressure in the expansion chamber. Quantitative measurements of light scattering intensity at points near the nozzle could not be made due to scattering of light off the nozzle itself, but no distinguishable breaks in the a e m l scattering were obmrved, thus it appears that the aerosol forms either within the nozzle or within a distance iust a few nozzle diameters away from the nozzle tip. Aerosol Formation Beyond t h e Critical Point. Aerosol limitina temperatures for n-pentane as a function of backina pressure for-various pinhole restrictors ranging in size from 2.5 to 25 pm are depicted in Figure 3: these results are representative of the general behavior of all the fluids. Also plotted is the equilibrium vapor-liquid curve for n-pentane and an extrapolation of that curve beyond the critical point. It can be seen that for subcritical conditions. the aerosol limiting data lie near to the vapor-liquid phase line, as might be expected. Of course, here we mean aerosol droplets as operationally defined to be those which are large enough (greater than about the wavelensh of light employed) to be detectable. However, with exceptions to he noted later in this paper, the drop in apparent aermol is precipitous enough near the observed temperature of disappearance to suggest that little molecular aggregation persists at greater temperatures. I t is interesting to note that the aerosol limiting data continue to follow the extrapolated liquid.vapor phase line up to temperatures well above the critical temperature rather than turning sharply upward at the critical point. Therefore, whether or not the supercritical fluid stream converts completely to a molecular heam in the expansion can be simply
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Flgure 3. Limits for aerosol formation by n-pentane expanding through (*) 2.5-pm, (0)5-pm, (X) 12.5-pm, and (0)25-pm pinhole restrictors. The solid line is the vapor-liquid equillbrlum curve, and the dashed line is an extrapolation of that curve.
Figure 5. Aerosol limits for CO, through (*) 5 - ~ m ,(0)7-pm, (X) lO-Mrn, and (0)25-pm capillary restrictors.
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Figure 4. Aerosol limits for n-pentane through (0)a 25-pm pinhole and (*) a 25-wrn capillary restrictor.
but conveniently predicted on the basis of its “gaslike” or “liquidlike” state prior to expansion up to about 10% above the critical temperature for n-pentane. Joule-Thompson Contributions. With the smaller diameter pinholes (2.5 pm to 12.5 pm), no dependence of the aerosol limiting temperature on nozzle diameter is evident. However, at 25 pm there is a marked displacement to higher limiting temperatures. It is possible that with larger diameter restrictors, supersonic cooling effects begin to make a significant contribution to macroscopic clustering. JouleThompson cooling of the fluid in the throttled flow through capillary restrictors also leads to the need to heat the nozzle to higher temperatures in order to offset the resulting lack of equilibrium between nozzle and fluid. This JouleThompson contribution is clearly evident in the comparison of aerosol limiting temperatures for a 25-pm capillary (4 cm in length) and a 25-pm pinhole shown in Figure 4. As shown for COz in Figure 5, a pronounced dependence of aerosollimiting temperatures on bore size was observed, decreasing capillary diameters leading to lower aerosol limiting temperatures. This behavior can be understood in terms of increased residence time and decreased mass of the fluid in the smaller diameter capillaries, allowing better thermal equilibration of the fluid with the nozzle. An extension of the capillary beyond the heated nozzle assembly of only about 1 mm (as was done for Figure 2a) required temperatures higher by tens of degrees to eliminate aerosol. Proper modeling of the JouleThompson contribution requires more information about heat transfer than is practical for most nozzles, but for nozzles heated to the tip, the effect is small. Erratic Flow in Capillaries. Increasing the length of capillary restrictors similarly leads to lower aerosol limiting
1 .o
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Flgure 6. Comparison of aerosol limits in reduced variables for all five fluids: (0)SF,, (0),n-pentane, (A)benzene, (e)C02, and (X) N,O. The solid line Is the vapor-liquid equilibrium curve for COP, and the dashed line is an extrapolation of that curve. The dot-dash line is the prediction for CO, from ref 11.
nozzle temperatures, but the effect is less pronounced than might be expected. For example, with n-pentane a difference of about 8 O C was observed between the limiting temperatures with a 0.5-cm and a 5-cm capillary of 25-pm diameter. More important, however, was that as the length of the capillary increased (and also as the diameter decreased for a fixed length) aerosol droplets coalesced into larger and larger droplets, and the flow became more and more erratic. This spitting behavior made it difficult to obtain measurements with long, narrow capillary restrictors; along with solute precipitation, it can contribute to large noise spikes in extracolumn detectors. Variations among Fluids. Figure 6 shows a comparison of all five fluids for expansion through a 10-pm pinhole nozzle; also plotted are the vapor-equilibrium curve for COz and the predictions of Smith et al. (11)for COP. Wide variations are evident among the several fluids, which cannot be simply related to either absolute or reduced pressures and temperatures. Although a simple extrapolation of vapor-liquid behavior above the critical point is useful in estimating upper limits for aerosol formation, it provides no information about how far beyond the critical point such behavior persists. In fact, we have examined heat capacity data for supercritical COP (14) and found that aerosol formation of COz persists beyond the gaslike versus liquidlike behavior evident in these data. While simpler models do not allow for prediction of the extent of aerosol formation, the more rigorous treatment for short restrictors given in ref 11 and based on isenthalpic expansion of the fluid is seen to work very well for COP,except for some deviation at the highest pressures. This model would
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be expected to account for the behavior of the more complex carrier fluids as well, because increasing the number of internal degrees of freedom decreases the maximum temperature a t which a two-phase separation occurs in an isenthalpic expansion. Differences in the behavior of C 0 2 and N 2 0 with respect to clustering were further evident in spectroscopic studies of supercritical fluid/supersonic jet expansions (7), where these fluids with a low number of molecular degrees of freedom were found to form microscopic, van der Waals aggregates about solute molecules more readily than were SFs or n-pentane. We hypothesize that additional clustering due to supersonic cooling contributes to formation of aerosol in jets of C02and N20,especially in higher pressure expansions. In such cases the distinction between microscopic and macroscopic clusters is blurred. In fact, at the higher pressures with C02and N20 the light scattering did not abruptly "wink outn, as with the other fluids, but rather faded gradually away. Reference 11 considers flow which is sonic at the tip of the nozzle but does not cover supersonic flow beyond the nozzle. Simple carrier molecules which supersonically cool well, with low critical temperatures will be the ones which are most prone to this additional clustering effect. For many postcolumn detectors the fine microscopic mists will cause no serious interference, but they will particularly impair spectroscopic measurements. Other Restrictor Types. Finally, we have briefly examined two other types of restrictors: short tapered capillaries (15) and fritted capillary plugs from Lee Scientific. The tapered capillaries show no marked difference in aerosol formation characteristics from those of straight capillaries, and, while they are somewhat less prone to cause sample precipitation, microscopic sample aggregation is still a problem. We did not find sample precipitation to be a problem for the fritted plugs, but microscopic solvent cluster formation is very extensive with this type of restrictor; we are investigating its characteristics further.
CONCLUSIONS Wide variations in aerosol-forming behavior preclude using any particular fluid as a guide for the behavior of other fluids, although outer limits can be set by a simple extrapolation of vapor-liquid equilibria. Furthermore, lack of knowledge about heat conduction and other kinetic effects with most nozzle systems prevents precise modeling for extended restrictors.
The careful treatment of fluid flow and aerosol formation by Smith et al. (11) gives much insight into characteristics of restrictors, but the thermodynamic data required for such predictions are not readily available for many fluids. However, laser light scattering provides a simple diagnostic means for establishing the conditions for aerosol formation of any particular nozzle system.
ACKNOWLEDGMENT We wish to acknowledge the work of Alan J. Barker in the early stages of this study and to thank Steven M. Fields and Milton L. Lee for help with supercritical fluid technology. We are also indebted to Bob W. Wright for pointing out to us the effect of degrees of freedom on the isenthalpic expansion model. Registry No. COz,124-38-9;NzO,10024-97-2;SF,, 2551-62-4; n-pentane, 109-66-0; benzene, 71-43-2.
LITERATURE CITED Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1984, 56, 619A-627A. Chester, T. L. J . Chromatogr. 1984, 299, 424-431. Fjeldsted, J. C.; Kong, R. C.; Lee, M. L. J. Chromatogr. 1983, 279, 449-455. Oleslk, J. W. Anal. Chem. 1987, 5 9 , 796-799. Randall, L. G.; Wahrhaftig, A. L. Anal. Chem. 1978, 50, 1705-1707. Smith, R. D.; Fjeldsted, J. C.; Lee, M. L. J . Chromatogr. 1982, 247, 231-243. Goates, S. R.; Barker, A. J.; Zakharia, H. S.: Khoobehi, 6.; Sheen, C. W. Appl. Specfrosc., in press. Fukuoka, H.; Imasaka, T.: Ishlbashi, N. Anal. Chem. 1988, 58, 375-379. Sin, C. H.: Pang, H. M.; Lubman, D. M. Anal. Chem. 1988, 5 8 . 487-490. Giddings, J. C.: Myers, M. N.; McLaren, L.; Keller, R. A. Science 1968, 162, 67-73. Smith, R. D.; Fulton, J. L.; Petersen, R. C.; Kopriva, A. J.; Wright, 6 . W. Anal. Chem. 1986, 58,2057-2064. Rawdon, M. G. Anal. Chem. 1984, 5 6 , 831-832. Ashkenas, H.; Sherman, F. S. I n RarefieM Gas Dynamics, Suppl. I I I , Vol. I I : 4th InternafionalSymposium;de Leeuw, J. H., Ed.; Academic: New York, 1966: pp 84-105. International Tables of the Fluid State, Vo/. 3: Carbon Dioxide ; Angus, S., Armstrong, E., de Reuck, K. M., Eds.; Pergamon: New York, 1976; pp 84-177, 373. Guthrie, E. J.; Schwartz, H. E. J. Chromatogr. Sci. 1986, 2 4 , 236.
RECEIVED for review May 18,1987. Accepted August 7,1987. This work was carried out with the support of the U. S. Department of Energy, under Grant Nos. De-FG22-83PC60807 and DE-FG22-86PC90534. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of DOE.
Gas-Dtffuslon Unit wlth Tubular Microporous Poly(tetrafluoroethy1ene) Membrane for Flow-Injection Determination of Carbon Dioxide Shoji Motomizu,* Kyoji Tiiei, Tohru Kuwaki, and Mitsuko Oshima
Department of Chemistry, Faculty of Science, Okayama University,Tsushima-naka, Okayama-shi 700, Japan In flow-injection analysis (FIA), many separation systems have been installed in a flow line to increase the selectivity of determination, including separations by precipitation, solvent extraction, dialysis, and gas evolution (diffusion). Of these separations, the gas-diffusion technique is considered to be highly selective, because fewer species are generated as gases a t room temperature. Baadenhuijsen e t al. applied a gas-diffusion technique to determine carbon dioxide in plasma ( I ) . A dialysis unit was used with grooves of 0.5 mm depth and 15 cm length and equipped with a nonwettable, gas-permeable membrane of dimethylsilicon rubber. This silicon rubber can be replaced by microporous poly(tetrafluoroethy1ene)(PTFE) membrane, which is more versatile and effective in FIA.
Aoki et al. used a new type of gas-diffusion unit that was constructed by using a tubular microporous PTFE membrane and applied in the determination of ammonia ( 2 ) and free chlorine (3). Nagashima et al. also used a tubular PTFE membrane to determine nitrate and nitrite (4). A gas-diffusion unit with a tubular PTFE membrane is more versatile than a dialysis-type unit with PTFE membrane sheet, because a gas-diffusion unit with arbitrarily long tubings can be constructed. All the gas-diffusion units with tubular PTFE membrane, however, were constructed with glass or PTFE tubing and glue. In this paper, the authors present a gas-diffusion unit with a tubular microporous PTFE membrane that is constructed without glue. As a result, gas-diffusion units of various lengths
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