Report
Chromatography from the Mobile-Phase Perspective upercritical fluid chromatography (SFC) is often described as having characteristics of GC and LC, but the ramifications and opportunities of this melding are not clear to the casual observer. To make matters more confusing, many other chromatography techniques initially appear to be unique, including several newer techniques such as subcritical (or near-critical) fluid chromatography, enhanced-fluidity and elevated-temperature enhanced-fluidity LC (1,2), and hightemperature LC (3-5) and GC (6). It's no wonder that many people, seeing the apparent complexity of the new chromatographies, are slow to embrace change. This Report demonstrates that the barriers between the various chromatographic techniques are imaginary and artificial and that GC and LC are fundamentally related and are limiting examples of a more general case. Phase diagrams are used to illustrate unified chromatography and to map old and new separation techniques according to mobile-phase properties.
Thomas L. Chester Procter & Gamble 0003-2700/97/0369-165A/$14.00/0 © 1997 American Chemical Society
offlexibilityand control to the When viewed from mensions chromatographic retention process. the outlet pressure above the perspective of the 1 atmElevating allows us to take advantage of properties of mobile-phase fluids that are not acmobile phase, the cessible at atmospheric pressure. For example, using LC as our starting point, we perceived complexities can use liquids at temperatures well above their normal boiling points (4,5). Viscosity of old and new modes drops significantly when liquids are heated, causing a concomitant reduction in the of chromatography pressure drop on a column and improving solute diffusion rates. Thus the result is are not so complex both faster optimum mobile-phase velocities and the possibility of accessing these after all velocities with a small column pressure The situation is much simpler than is implied by the apparent multiplicity of techniques. The most important feature of the newer techniques is the use of elevated column outlet pressure. We can begin to understand this importance by realizing that LC and GC developers simply used what amounts to a default pressure 1 atm, by venting their column outlets to the air. However, just as temperature selection is important in these techniques controlling the outlet pressure can also add new di-
drop Furthermore when heated sufficientlv liquids gain enough compressibility that pressure can be used as a solvent strength control parameter Finallv elevating the outlet pressure allows the selprrion of mobile-phase fluids t h a t a r e rvff limits in nfitrriQl T C* cnr»Vi QO K n t a n o
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Thp«p fluirk can hp uspH neat or
mixed with ordinary liquids to reduce viscosity, improve diffusion rates, or make other desirable changes in the behavior of AX. I_-I I_ ^ i 11 the mobile phase that would not, . 1be possible at atmospheric outlet pressure.
Analytical Chemistry News & Features, March 1, 1997 165 A
Report the term "supercritical fluid chromatography". The critical temperature and pressure of a fluid are the coordinates of the critical point, that is, the highest temperature and pressure at which separate liquid and vapor phases can coexist. The supercritical fluid region for a pure substance (defined by decades of usage and by IUPAC [7] and the American Society for Testing and Materials [8]) exists when both the critical temperature and critical pressure of the substance are exceeded (Figure la) This unfortunate and arbitrary definition has had the effect of walling off the shaded section of the figure and imparting the "suoercritical fluid region" with unique pronerties that occur only when it is entered There is x\c\ 0*1 riot* at**
Figure 1 . Two-dimensional phase diagram. (a) Arbitrary definition of the supercritical fluid region for a neat material, (b) In actuality, there is no separate supercritical fluid region. The critical point is at the highest temperature and highest pressure where separate liquid and vapor phases coexist. Dashed lines or shading as in (a) cause confusion, (c) There are no boundaries or discontinuities in the one-phase region shown here as long as the phase transitions are avoided. This entire region is available for chromatographic mobile phases and is limited only by the physical capabilities of the equipment and by the chemical stability of the system's components.
When the outlet pressure is elevated and pressure and temperature are controlled, the resulting techniques are similar and the behaviors of conventional LC and GC are completely and seamlessly bridged. Names for these techniques depend only on where they fall in a phase diagram that describes the mobile phase. Supercritical fluids and phase diagrams
A great deal of confusion stems from the word "supercritical" and its significance in
unique supercritical fluid region in the nh'ase diagram a fact that is more accurately illustrated in Figure lb. If we observe the properties of coexisting liquid and vapor under conditions somewhere on the boiling line near the middle of its range and then move on that line toward the critical point by appropriately raising the pressure and temperature, we would observe the separate liquid and vapor phases becoming more and more alike. All properties of the fluid merge at the critical point, and only one fluid phase exists beyond that. This single phase is compressible, like a gas, and will expand to uniformly fill its container. Yet when sufficiently compressed it exerts significant intermolecular forces and can dissolve solutes like a familiar liquid The diffusion rates of a fluid in its supercritical fluid retrion are faster than for the liquid and slower than for t h e gas state and also depend fVte fluirl den sity and temperature
We never want to do chromatography near the mobile-phase critical point or near any mobile-phase phase boundary. We are free, however, to move anywhere in the shaded area in Figure lc in choosing or programming mobile-phase parameters because a liquid-vapor phase separation is not possible anywhere in this area. Although the ordinary liquid and vapor regions are included, we can travel anywhere in the shaded area without ever undergoing a phase change or observing any kind of discontinuity in physical or chemical properties
166 A Analytical Chemistry News & Features, March 1, 1997
In the "liquid" region well below the critical temperature, the mobile phase is nearly incompressible. Varying the pressure alone of a liquid well below its critical temperature has little effect on mobilephase strength (as long as the pressure is not reduced so much that the liquid boils). However, as the temperature is increased (with a pressure increase to prevent boiling), the mobile phase becomes less viscous diffusion rates inand the liquid becomes increasingly compressible At temperatures just above the critical point the fluid is highly compressible At sufficiently low pressures the fluid behavior approaches that of a perfect gas with no significant intermolecular forces and no solvent strati crfh Yet liquid-like solvent results when the fluid is compressed to linuid-like densities Th s we have the abiilty to con t>
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