Milos Novotny Stephen R. Springston Chemistry Department Indiana University Bloomington, Ind. 47405
Paul A. Peaden John C. Fjeldsted Milton L. Lee Chemistry Department Brigham Young University Provo, Utah 84602
Capillary Supercritical Fluid Chromatography Rapid separations of respectable efficiency for nonvolatile solutes can be achieved by use of capillary columns in conj u n c t ion w i t h super cr i ti ca 1 f 1u i d s
Capillary columns were first described for gas chromatography (GC) in 1957 by M.J.E. Golay, who utilized long, open tubular columns uniformly coated with a thin layer of stationary phase ( I ) . Although many technological improvements were necessary in the years following Golay’s invention, the method has recently revolutionized many areas of scientific research. This is primarily due to its high resolving power, allowing the separation, detection, and quantitation of hundreds of components from a sample in a single chromatogram. The practical separating capability of capillary gas chromatography is presently unparalleled. However, the method is somewhat restricted by the limited volatility and thermal stability of many organic compounds. Mixtures of less volatile compounds can be analyzed by high performance liquid chromatography (HPLC),but large numbers of theoretical plates can only be obtained a t the expense of a long analysis time. Achievement of the maximum number of theoretical plates in capillary HPLC is presently complicated by the extremely small column diameters required for low plate heights. This requirement is a consequence of solute diffusivities and mobile phase viscosities. These are the two most important properties governing chromatographic efficiency (2, 3 ) and are primarily a function of the mobile phase and its physical properties. While encouraging results were recently obtained ( 4 ) with open microtubular columns as small as 30 pm id., theoretical analysis by Knox and 0003-270018 11035 1-407A$01.OO/O @ 1981 American Chemical Society
Gilbert ( 5 ) indicates the need to work with columns of about 10 pm and detectors with volumes of a few nL to maximize efficiency, a formidable challenge at present. Clearly, other alternatives are needed for highly efficient separations of large, less volatile molecules. A system is needed that provides somewhat faster solute diffusion than in liquids (permitting larger, more “reasonable” column diameters) and mobile phase viscosities sufficiently low so as to permit long columns to be exploited. Simultaneously, such a system should be suitable for the separation of large and thermally labile molecules. Qualitatively, these conditions are met by use of open capillary columns in conjunction with a supercritical fluid as the mobile phase. It is reasoned in this article and supported by preliminary data that, under certain experimental conditions, this combination can provide a nearly ideal chromatographic system for the above-mentioned molecular species. The pioneering work on supercritical fluid chromatography in packed columns by Klesper et al. (6) clearly demonstrated the potential of this approach in separating thermally labile substances. Later work of Giddings et al. (7-9) and Sie and Rijnders (10-13) with both dense gases and supercritical fluids extended this interesting direction in separation science both theoretically and experimentally. However, these developments of the late 1960s were largely overshadowed by the advent of HPLC. Furthermore, technical difficulties with supercritical
fluids discouraged additional research. The most recent advances have been reviewed by Klesper (14). A very serious limitation of supercritical fluid chromatography with packed columns is the pressure gradient generated by the column packing, as discussed by Novotny et al. ( 1 5 ) . Ironically, while a decrease in particle size leads to much greater efficiency in liquid chromatography (where the mobile phase is practically noncompressible), the reverse is observed with supercritical fluids ( 1 5 ) .This point was reemphasized by Gouw and Jentoft (I6),who compared the effects of an increasing pressure gradient along the column in supercritical fluid chromatography to a decreasing temperature program in gas chromatography. Both have disastrous consequences as far as the column efficiency is concerned. The advantages of capillary column “openness” are well known in GC and will become even more evident with supercritical fluid chromatography, if the whole system (from the point of sampling to the point of detection) can be maintained under proper conditions of high pressure and low pressure drop. Supercritical fluid chromatography should provide superior migration of labile and less volatile substances through a capillary column when compared to GC. Fluids compressed to their critical points and above exhibit an “extraction effect” on chromatographed samples similar to solvation in HPLC. The effect of “extraction” may be predicted based on the second virial coefficient of the system (10) or
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
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the Hildehrand solubility theory (9), although such predictions are first approximations a t best. The point of practical interest is that while ordinary gases can he compressed to produce these effects (9), similar conditions can he met with some other fluids at pressures and temperatures well within the limits of ordinary HPLC technology. Some examples are shown in Table I. Attention will he focused on fluids with low critical temperatures, since they will he useful for separating thermally labile compounds. Optimum conditions for capillary supercritical fluid chromatography will now he briefly considered. If we introduce the initial restriction of a maximum allowable pressure drop, P,.,, between the inlet and outlet of a capillary column of radius r , the maximum column length, L,. is given by the integrated Poiseuille equation:
where 7 is the mobile phase viscosity and F is the average linear velocity of the fluid. The number of theoretical plates, N, for such a column is:
Since both L and H (plate height) are functiuns o f i , the maximum efficiency, Nma,,can easily he found (3). However, the maximum number of plates applies here to conditions of infinite length and zero flow.If, instead, the column is operated under conditions of optimum efficiency per unit length, more realistic efficiencies may he calculated. The minimum plate height, H,,,, can he derived from the Colay equation, assuming that resistance to mass transfer in the stationary phase is negligible:
Hm,"= 0.577 r
(3)
This cundition occurs at:
where D , is the diffusion coefficient of a solute in the mobile phase. Sub408 A
stituting equations (l),(3), and (4) into equation (2) yields the maximum number of plates that can he generated from a column of fixed length:
This equation is similar to that derived earlier hy Giddings (3) for the limiting number of plates used to compare the relative merits of gas and liquid chromatography. The difference is that equation (5) assumes ii = popt,whereas Giddings's expression holds only for L = m and ii= 0. For a given pressure drop, operation at ii = poptrather than L = m and = 0 decreases the maximum efficiency by a factor of two. While the above considerations are only valid for a nonretained chromatographic peak (capacity ratio, k = O), some interesting observations can he made. Estimated efficiencies per unit time are quite impressive since D , and q are more favorable to a rapid chromatographic separation in a supercritical fluid than in a liquid mobile phase. Whereas literature data on both these physical quantities for supercritical fluids are rare, only approximate calculations can he made a t this time. The diffusion coefficient of n-pentane in carbon dioxide a t 48 atm and 40 "C was estimated to he 1.5 X cmz s-l (17), while a viscosity value of 3.59 X g cm-1 s--1 at 90 atm and 40 "C is available from another literature source (18).Actual diffusion coefficients may he as much as one or two orders of magnitude smaller for large solutes a t supercritical pressures, hut no one seems to have measured such quantities as yet. If we arbitrarily choose a pressure gradient restriction of 1atm, a maximum of 8.1 X lo6 theoretical plates could be generated in 18 h for a nonretaiced component with a 0.24 mm i.d. capillary column (length, 570 m). Through reduction of the column diameter to 100 pm, 1.4 X 106 theoretical plates could be obtained in just 0.54 h (length, 41 m). Practical realization of optimized capillary supercritical fluid chromatography is dependent on at least three crucial criteria:
ANALYTICAL CHEMISTRY, VOL. 53. NO. 3. MARCH 1981
The system must he designed so the entire column and the detector both operate at the same high pressure: instrumental contributions to hand broadening must he minimal, and * stationary phase films of optimum thickness must he permanently bonded to the capillary wall so the resistance to mass transfer in the stationary phase is small, and so the phase cannot he stripped off by the passing fluid. Development of capillary supercritical fluid chromatography has recently become feasible. Designs of microcolnmn HPLC equipment (19-22) have made certain instrumental components more readily adaptable to capillary supercritical fluid chromatography. Among them, a low-volume loop injector and a miniaturized spectroscopic cell are particularly useful. A number of fluids are available, such as n-pentane, isopropanol, or carbon dioxide, that allow the migration of relatively heavy molecules under reasonable pressures and temperatures. These mobile phases, combined with the small-volume requirements of capillary techniques, make safety hazards almost negligible. Glass capillary columns with wall thicknesses similar to those used in conventional capillary gas chromatography can be easily used. Advantage can also he taken of recently developed bonded phases (22).
The capillary supercritical fluid chromatograph operating in our lahoratory possesses the basic features illustrated in Figure l. A high-pressure syringe pump operating a t rmm temperature delivers, a t constant pressure, a liquid (n-pentane) or compressed gas (carbon dioxide) that is converted into a supercritical fluid in a preheating coil. The fluid suhsequently sweeps through a loop valve where the sample is introduced directly into the glass capillary column. The column is heated to temperatures consistent with the desired fluid conditions in a thermostat. The thermostat also contains a flow-through cell (either a UV or spectrofluorimetric detector) used to monitor chromatographic effluents under high pressure. A length of 50-pm glass capillary after the detection cell is used to maintain system pressure. Solutes can also he detected directly in the last section of the column. As this comprises the optical compartment, no hand spreading occurs due to interconnection deadvolume. Increasing the pressure is the most convenient means to influence solute retention (15-27):therefore, pressure programming will ultimately he used for resolution of complex mixtures as shown in Figure 1.
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Flgure 1. Block diagram showing the components of a capillary supercritical fluid
chromatograph We wish to report some preliminary results on capillary supercritical fluid chromatography, using n-pentane as the mobile phase a t 210 "C and 32 atm. The glass capillary columns used were fabricated with phenylmethyl polysiloxane film bonded to the capillary wall similar to the procedure of Blomberg and W h n m a n (22).As a compromise with currently available sampling conditions, 0.2 and 0.3 mm i.d. glass capillary columns were used. The plate height vs. linear velocity curves were measured for the fluorimetrically detected pyrene peak (A, = 335 nm; ,A, = 425 nm, 10-nm bandwidth). Figure 2 shows the curves obtained for the two column diameters. While pyrene is only slightly retained in the phase system used, the plate height values are larger than theory predicts. Reasons for this discrepancy are not currently obvious;
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ANALYTICAL CHEMISTRY, VOL. 53. NO. 3. MARCH 1981
while it would he useful to have more accurate values for the diffusion coefficient, D,, under the conditions used, we believe that the current design of the injection device is the largest contributor to the plate height increase. Efforts are being made a t present to eliminate such problems. The direct insertion of the capillary tube into the spectral measurement cell reduces similar difficulties at the outlet. A model mixture of polycyclic aromatic hydrocarbons is shown in Figure 3. A 58 m X 0.2 mm i.d. glass capillary column was used to separate this mixture under isobaric conditions with n-pentane as the carrier fluid. While this result is only a preliminary one [capillary gas chromatography can achieve better separation of such compounds (23)],capillary supercritical fluid chromatography clearly merits further exploration.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
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lure 2. Plot of HETP vs. linear velocity for pyrene on a 0.30 mm 1.d. (blue) and 1.20 mm i.d. (green) column. The mbile phase was Rpentane at 210 "C and 32
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n The high efficiency demonstrated ove with capillary supercritical fluid romatography is a good reason for vsuing this research. Other facts of iportance also appear. For separaIns where both selectivity and high lumn efficiency are needed (e.g., resition of certain isomeric submces), it may be of advantage that e interaction of solute and mobile w e molecules ean he sensitively adited by pressure. In addition, while adient elution techniques are needed "program" solute retention in liq-
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Dum 3. isobaric run of several PAH andards in a 0.20 mm 1.d. X 58 m lpillary column. Conditions were as In jure 2. The standards were: 1) anracene. 2) pyrene, 3) benzo[k]fluonthene, 4) benzo[e]pyrene, 5)dlbenztbclanthracene* 6, benzo[ghilpe~ le, and 7 ) caonene
ANALYTiCAL CHEMISTRY, VOL. 53, NO. 3. MARCH 1981
uid chromatography, pressure programming in supercritical fluid chromatography can he used to meet similar goals. As the former programming technique is incompatible with certain detection systems, fewer problems are anticipated with supercritical fluids under increasing pressure. For example, one can envision a complex mixture of nonvolatile substances in supercritical carbon disulfide being separated hy pressure programming, while the resolved components are introduced into a high pressure cell of an infrared detector. Some attention has already been given to the coupling of supercritical fluid chromatography with maas spectrometry through a jet molecular separator (24). While the technological problems that exist here are formidable, they appear no more serious than with the now extensively studied
LCIMS. As research in capillary supercritical fluid chromatography proceeds, different fluids and pressure regions can he explored to achieve chromatographic migration of various nonvolatile and unstable molecules. As pointed out hy Giddings and co-workers (9),the Hildebrand solubility parameters can be employed to predict approximate suitability of mobile phasea for various molecular separations. As our capability of performing precise measurements in supercritical fluid chromatography gradually improves, meaningful measurements of solut* solvent interactions in the vicinity of the critical point and beyond will a b become available. References (1) Golay, M.J. E.A w l . Chem. 1957,29, 92-32,
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Flhersciimc
(2) Giddings, J. C. “Dynamics of Chroma-
tography”; Marcel Dekker: New York, 1QfiS
(3) Giddings, J. C. Anal. Chem. 1964.36. 1890-92. (4) Tsuda, T.; Nakagawa, G. J. Chromatogr. 1980,199,249-58. (5) Knox, J. H.; Gilbert. M. T. J. Chromatog?. 1979.186. 405-18. (6) Klesper, E.;Corwin, A. H.;Turner, D. A. J . Ore. Chem. 1962,27,7WI. (7) Myers. M. N.; Giddings, J. C. Separation Sci. 1966. I, 761-76, (8) McLaren. L.; Myers. M. N.;Giddings, J . C . Srimee 1968,159,197-9. (9)Giddings. J. C.;Myers, M. N.; MeLaren, I..: Keller, R. A. Science 1968. 162, 67-73.
(10) Sie, S. T.: van Beersum, W.; Rijnders,
G. W.A. Separation Sei. 1966,I . 459wl (IyYSie, S. T.; Rijnders, G. W.A. Separa-
tion Sei. 1967.2, 729-53. (12) Sie, S.T.;Rijnders. G. W. A. Separation Sei. 1967,2,755-77. (13) Sie. S.T.;Bleumen. J. P.A,; Rijnders.
Milos Novotny is currently professor of chemistry at Indiana University. A native of Czechoslovakia, he received his undergraduate education in chemistry and a doctoral degree in biochemistry at the University of Brno. Nouotny’s research interests include capillary gas chromatography, HPLC, and GC-MS, as well as biomedical and environmental applications of these methods.
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Paul A. Peaden received his BS in chemistry at Brigham Young University, and is presently working on his PhD i n analytical chemistry. His research topic is the development of liquid Chromatographic and supercritical fluid chromatographic techniques for the analysis of high molecular weight polycyclic aromatic compounds.
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G. W. A. In “Gas Chroma raphy 1968”; Harbourn, C. L.A.%d.; Institute of Petroleum: London, 1969: p 235. (14) Klesper. E. Angew. Ckem. I n t . Ed. Engl. 1978.17.738.16. (15) Novotny, M.; Rertseh, W.; Zlatkis, A. J . Ckromatogr. 1971,61,17-28. (16) Gouw. T. H.; Jentoft, R. E.J . Chromatogr. 1972.68, 303-23. (17) Sie. S.T.:Riinders. G. W. A. A n d . Chim.’Aeto ‘1967.3R,31-44. (18) Kestin. J.; Whitelaw,J. H.;Zien,T. F. Phy.vieo 1964.30, 161-81. 119) . . Ishii. D.:Asai. K.: Hibi. K.:Jonokuchi, T.; Nagaya. M.J. Ckromatogr. 1977, 144, 157-68. (20) Hirata Y.;Novotny, M. J . Chromotogr. 1979,186.521-8. (21) Scott. R. P.W.: Kucera. P.J . Chromotogr. 1979.169.51-72. (22) Blomberg. L.;Wannman, T.J. Chromatogr. 1979.168.81-8. (23) Lee, M. L.; Novotny. M.; Bartle, K. D. Anal. Ckem. 1976.44 156672. (24) Randall, L. G.; Wshrhsftig, A. L. Anal. Chem. 1978.50, 1703-5.
ANALYTICAL CHEMISTRY, VOL. 53. NO. 3. MARCH 1981
Milton L. Lee received his BA at the University of Utah. and earned a PhD in analytical chemistry at Indiana University in 1975. He spent one year at MIT performing post-doctoral research before accepting his present position on the faculty at Brigham Young University. His research interests include the surface chemistry of glass and silica as it applies to GC, GC-MS. LC, and supercritical fluid chromatography,and the application of these techniques to the analysis of complex mixtures in environmental, coal, and coal-derived samples. Stephen R.Springstan receiued his BS degree from Virginia Polytechnic Institute and State University, Blacksburg, Va., in the field of chemistry. He is currently a doctoral degree student at Indiana University. His thesis research is concerned with the fundamental principles, instrumentation, and applications of supercritical fluid chromatography.
