Chromatography is an analytical technique used for the separation of complex chemical mixtures into individual components. In Supercritical Fluid Chromatography (SFC), the sample is carried through a separating column by a supercritical fluid (typically carbon dioxide) where the mixture is divided into unique bands based on the amount of interaction between the individual analytes and the stationary phase in the coumn. As these bands leave the column their identities and quantities are determined by a detector.
2.High Pressure Pump
Supercritical fluid chromatography (SFC) is a relatively recent chromatographic technique, having been commercially available since only about 1982. As a result there is a large amount of research currently underway both in SFC method development and in hardware development. What differentiates SFC from other chromatographic techniques (gas chromatography (GC) and high performance liquid chromatography (HPLC)) is the use of a supercritical fluid as the mobile phase.
Supercritical fluid chromatography has several main advantages over other conventional chromatographic techniques (GC and HPLC). Compared with HPLC, SFC provides rapid separations without the use of organic solvents. With the desire for environmentally conscious technology, the use of organic chemicals as used in HPLC could be reduced with the use of SFC. Because SFC generally uses carbon dioxide collected as a byproduct of other chemical reactions or is collected directly from the atmosphere, it contributes no new chemicals to the environment. In addition, SFC separations can be done faster than HPLC separations because the diffusion of solutes in supercritical fluids is about ten times greater than that in liquids (and about three times less than in gases). This results in a decrease in resistance to mass transfer in the column and allows for fast high resolution separations. Compared with GC, capillary SFC can provide high resolution chromatography at much lower temperatures. This allows fast analysis of thermolabile compounds.
For every substance there is a temperature above which it can no longer exist as a liquid, no matter how much pressure is applied. Likewise, there is a pressure above which the substance can no longer exist as a gas no matter how high the temperature is raised. These points are called the supercritical temperature and supercritical pressure respectively and are the defining boundaries on a phase diagram for a pure substance. Beyond which the substance has properties intermediate between a liquid and a gas and is called a supercritical fluid. In this region the fluid has good solvating power and high diffusivity, which make it a good choice as a mobile phase in chromatography.
Part of the theory of separation in SFC is based on the density of the supercritical fluid which corresponds to solvating power. As the pressure in the system is increased, the supercritical fluid density increases and correspondingly its solvating power increases. Therefor, as the density of the supercritical fluid mobile phase is increased, components retained in the column can be made to elute. This is similar to temperature programming in GC or using a solvent gradient in HPLC.
Supercritical fluid chromatography can most easily be described as an adaptation of either high performance liquid chromatography (HPLC) or gas chromatography (GC) where the major modification is the replacement of either the liquid or gas mobile phase with a supercritical fluid mobile phase. In general there are two hardware setups used: 1) An HPLC like setup with two reciprocating pumps designed to provide a mixed mobile phase with a packed analytical column placed in an oven followed by an optical detector in which the pressure and flow rates can be independently controlled, 2) A GC like setup with a syringe pump followed by a capillary column in a GC oven with a restrictor followed by a flame ionization detector, where the pressure is controlled by the flow rate of the pump.
In SFC the mobile phase is initially pumped as a liquid and is brought into the supercritical region by heating it above its supercritical temperature before it enters the analytical column. It passes through an injection valve where the sample is introduced into the supercritcal stream and then into the analytical column. It is maintained supercritical as it passes through the column and into the detector by a pressure restrictor placed either after the detector or at the end of the column. The restrictor is a vital component as it keeps the mobile phase supercritical throughout the separation and often must be heated to prevent colgging; both variable and fixed restrictors are available.
There are a number of possible fluids which may be used in SFC as the mobile phase. However, based on its low cost, low interference with chromatographic detectors, and good physical properties (nontoxic, nonflammable, low critical values) carbon dioxide is the standard. The main disadvantage of carbon dioxide is its inability to elute very polar or ionic compounds. This can be overcome by adding a small portion of a second fluid called a modifier fluid. This is generally an organic fluid which is completely miscible with carbon dioxide (alcohols, cyclic ethers) but can be almost any liquid including water. The addition of the modifier fluid improves the solvating ability of the supercritcal fluid and sometimes enhances selectivity of the separation. It can also help improve separation efficiency by blocking some of the highly active sites on the stationary phase. Modifier fluids are commonly used, especially in packed column SFC.
In general, the type of high pressure pump used in SFC is determined by the column type. For packed columns reciprocating pumps are generally use while for capillary SFC syringe pumps are used. Reciprocating pumps allow easier mixing of the mobile phase or introduction of modifier fluids. Syringe pumps provide consistent pressure for a neat mobile phase.
For packed SFC a typical LC injection valve is commonly used. In capillary SFC small sample volumes must be quickly injected into the column and therefor pneumatically driven valves are used. The ovens used in SFC are generally conventional GC or LC ovens.
Once the sample is injected into the supercritical stream it is carried into the analytical column. The column contains a highly viscous liquid (called a stationary phase) into which the analytes can be temporarily adsorbed and then released based on their chemical nature. This temporary retention causes some analytes to remain longer in the column and is what allows the separation of the mixture. Different types of stationary phases are available with varying compositions and polarities.
There are two types of analytical columns used in SFC, packed and capillary. Packed columns contain small deactivated particals to which the stationary phases adhears. The columns are conventionally stainless steel. Capillary columns are open tubular columns of narrow internal diameter made of fused silica, with the stationary phase bonded to the wall of the column.
SFC is compatible with both HPLC and GC detectors. As a result, optical detectors, flame detectors, and spectroscopic detectors can be used. However, the mobile phase composition, column type, and flow rate must be taken into account when the detector is selected as they will determine which detector is able to be used. Some care must also be taken such that the detector components are capable of withstanding the high pressures of SFC.
The results of a run in SFC are a chromatogram; the printout of the detector signal versus time. Below are two chromatograms obtained using SFC for the separation of polymers and of pesticides
Sampling & Monitoring Primer Table of Contents
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Copyright © 1997 Daniel Gallagher
Last Modified: 09-10-1997