An Introduction to Supercritical Fluid Chromatography Part 1: Principles and Instrumentation Margo D. Palmleri National Bureau of Standards, Gaithersburg, MD 20899
In recent years, there has been tremendous growth in research involving Supercritical Fluid Chromatography (SFC). Klesper, Corwin, and Turner reported the first SFC separation in 1962 (I), and until 1982, only two to five papers per year were writtenon SFC. In 1986 over 60 papers were published on SFC, and 115 papers were published in 1987 (2). Technological advances have improved SFC instrumentation and have allowed researchers t o take advantagr of some of the unirluc chnrarteristirs of supercritical fluids. SFC has heen the subicct of several detailed r w i w s 11 . -161. . This review will " eive t h e reader some basic knowledge ahout supereritieal fluids and their use in chromatography. Supercritical Fluld Properties The phase change of a substance from a gas to a liquid depends upon the temperature and pressure applied to a system. Above a certain temperature, called the critical temperature, a substance cannot he liquified, regardless of the applied pressure. The minimum pressure required to liquify a
Margo Paimlerl received a E istry from Northern Illinois Un8versny m 1982 and a PhD in analytical chemistry from Iowa State Universily in 1987. She is Me recipient of a NatioMl Research Camcii Postdonoral Fellowship and is currently working at the National Bureau of Standards. Her current research interests include high-performance liquid chromatoe raphy, supercriticel fluid chromatography. solvent extraction. and metal complexation chemistry.
Certain commerclai equipment, instruments, or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards nor does it imply that the materials or equipmem Identifiedare necessarily me best available for the purpose.
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Journal of Chemical Education
substance a t its critical temperature is the critical pressure. The critical temperature and pressure combine to define a unique point on the phase diagram known as the critical point (Fig. 1). When a substance is subjected to pressures and temperatures above its critical point, a highly compressed gas known as a supereritieal fluid is formed. Figure 1shows the phase diagram of carbon dioxide, which is a commonly used supercritical fluid. Figure 2 shows the pressure-density isotherms of carbon dioxide. Above the critical temperature (31.3 'C), large changes are seen in the density of carhon dioxide as the pressure is varied. A supercritical fluid has properties that are intermediate to liquids and ordinary gases. Table 1 compares some of the physical properties of gases, liquids, and supercritical fluids (8).The density affects the solvation power of the solvent: the higher the density, the greater the solvent strength. At high densities, supercritical fluids have solvent strengths similar to those of liquids. Both liquids and supercritical fluids have high solvent strengths and can dissolve many different types of solutes. Due t o their lower density, gases are much poorer solvents than supercritieal fluids. By changing the density of the fluid through temperature and pressure variation, the solvation strength of a supercritieal fluid can be altered. Raising the pressure increases the density of the supercritical fluid and causes i t to become more liquidlike. When the temperature is increased, the density of the supercritical fluid decreases, and the phase becomes mare gaslike. Depending upon thedenatty, therisror~tieaofw p w c ~ i tical fluids can he similar r u gaser or inwrmrdiatr between rates and liquid?. Finnllv, solute diffusion coefficients id supercritical fluids are intermediate between those in gases and in liquids. Table 2 lists some chemicals that have potential use as supercritical fluids for chromatography. Supercritical fluid extraction has been used as an alternative to distillation and liquid extraction (11-17). Supercritical fluid extraction occurs in a manner analogous to liquid extraction except that one of the solvents is a su~ercriticalfluid. Several advantngm rrier in using a supercritical cluid a\ a srdvrnt in extraction ~ 1 1 ) Vir.1, . ioprrcriti-
Figure 1. Phase diagram of carbon dioxide: tp = triple paint, cp = critical point.
( s l fluid wlvents arc mure easilv rrmowd from the extracted material The rxtructrd matmal i\ readily concrntrarcd by simpv decreasing the pressure; the solvent is removed as a gas eliminating a time-consuming evaporation procedure. Second, the solvent strength of the supercritical fluid can he varied by changing the density. When carbon dioxide is the solvent, supereritieal fluid extraction has the added advantage of being nontoxic. No residual toxic solvent remains in the extracted material. Third, the higher diffusion coefficients found in supercritical fluids allow shorter equilibration times in extraction. Finally, supercritical fluid extraction has the advantaee over distillation a t atmos~hericoressurein that the compose. Supercritical fluid extraction does have same drawbacks in comparison to liquid extraction and distillation, Supercritical fluid extraction must he carried out a t high pressures, which makes the process mare complicated and potentially more dangerous. (Continued on page A256)
The choice of sunereritical fluid tends t o he limited t~. o those with low critical -.~~~~... - ~suhatances ~-points. Finally, a problem for both supercritical fluid extraction and SFC is the paucity of fundamental data on supercritical fluid systems. More information is needed to explain and predict solute behavior in supercritical fluids. Supercrltlcal Fluld Chromatography
Cornoarison of SFC to Gas (GOand Liquid ~hrornato~raphy Chromatowa~hv - . . (LC) . . In analytical chemistry, supercritical fluids find their ereatest aooliestian as the mobile phase in rhrumntography. Bolh packed and capillary columns are used in SFC. The solvotion power of zuperrritical fluids has a major effect on the way analytes are separated in SFC. As in LC, SFC separation of analytes occurs primarily through a difference in affinity of the aualyte for the mobile and stationary phases. In GC, however, separation depends upon the affinity of the analyte for the stationaryphase relative to the volatility of the analyte. The difference in the separation mechanism between GC and SFC or LC is due t o the solvation power of liquids and supercritical fluids. As a result of the differences in physical properties among gases, liquids, and supercritical fluids, different mobile phase characteristics are seen in GC. ~~.LC.~,and SFC. In order to understand the relationships between column efficiency. solute effects in supercrirical fluids,nnd flow rate in uhnmatography, one needs t o understand some basic chromatographic theory. An in-depth description of chromatographic theory can be found in several textbooks (e.g., 18-21). Column efficiency is evaluated using a unit called a theoretical plate. The greater the number of theoretical plates (N), thegreater the efficiency. Column efficiency is related t o flow rate by the Van Deemter equation, which, in its simplified form, is ~
~
..
~
~
Figure 2. Pressure-densily behavior of carbon dioxide at various temperatures. (From ref 2. Diagram provided courtesy of Lee Scientific. Inc.)
~~~
where H is the column efficiency term, u is the linear flow rate, and A, B, and C are coefficients related t o peak broadening. The t e r n H, called the height equivalent of a theoretical plate (also abbreviated HETP), is related to N by the equation
H = LIN
(2)
where 1. is the lengthof the column. For the greatest efficienr).. H should be as small as ~osaihle.The coefficient A takes into a r count the different paths analyte molecules take through a column. In packed columns, molecules take different paths through the packing. In capillary columns, different solute flow rates occur due t o the characteristics of laminar flow in an open tube. Those solutes near the capillary wall travel a t lower flow rates than those a t the center of the capillary. B is the term used t o describe solute diffusion along the direction of flow. The C coefficient represents the resistance
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Table 1. Cornparloon of Typlcal Physlcal Properties of Llqulds, Gases, and Supercritlcal Physical Ropeny of me mase Typical Solute Oil'sinn~.
Density Phase -3
(NY
SupercriticalFluid Liquid
.
(g cm-9 TOP
0.3-0.9 1
to mass transfer of the solute between the stationary and mobile phases. The faster the equilibration of the solute between the mobile and stationary phases, the smaller the value of C. The ideal flow rate far minimizing peak broadening occurs a t the minimum of the H versus u plot where the system has the highest number of theoretical plates. Diffusion coefficients in supercritical fluids are intermediate between those found in liauids and eases. These intermediate values rive rise Lo differences between SFC and or LC. The s & ~ t r d i f f u s i ~ i t y strongly affects the Ccuefficient in t h Van ~ Deemter equation (6). The greater the diffusivity, the faster the exchange of the solute between the mobile and stationary phases and the smaller the C value. In LC the solute diffusion coefficient is low. and the value of C
ti? -~
~~
~~
Coefficients (cmZsC') lo-' tO-3-10P
