Supercritical fluid chromatography

The supercritical or supercritical liquid chromatography ( SFC abbr, Eng. Supercritical fluid chromatography) is an analytical detection method of chromatography.

SFC at mobile phases are used, which are located outside the material-specific critical temperature and the critical pressure. Many physical properties of these fluids are between those of gases and liquids. Particularly noteworthy are the density, viscosity and the diffusion coefficient. , Can be treated with supercritical fluid chromatography of the analytical problems are no longer applicable for gas or liquid chromatography.

Physical Basics

The most crucial feature of the SFC is the physical state of the mobile phase. The fabric used for this purpose is in a supercritical state. This is, simply put, a state between gas and liquid. This can be explained so that the supercritical fluid is at such a high temperature that can be transformed into the pure liquid state it through no matter how high pressure again. This is called the lowest for accurate temperature critical temperature TC. However, in order to delineate the supercritical region exactly, one must still define the critical pressure pC. If this is exceeded, can be combined with any of the most high temperature of the pure gaseous state reach. Both sizes are material-specific constants. The following table lists some examples.

It is assumed, for example, a chemical compound such as carbon dioxide and changes in a closed vessel pressure and temperature, one can obtain the corresponding states of aggregation. One moves ( with the pressure and temperature values ​​) at the phase boundaries along, there are two aggregate states, the so-called triple point, there are even three. Exceeds one the critical point cp, one comes to the supercritical range.

Starting from this state, you can now do the following observation: one isothermal Reduces the pressure, the supercritical fluid is continuously transferred to the gaseous state. On cooling from isobaric ( at the same pressure ), you get into the liquid region. There is here no abrupt crossing a phase boundary between gas and liquid. The material properties are varied continuously.

Thus, supercritical fluids have properties of gases and liquids. They can be just as compress gases, so that you can adjust the pressure on the density. The density can also be increased by lowering the temperature at constant pressure.

As supercritical fluids have variable solubility, which may be quite small, but also much higher than that of pure liquids. For example, ammonia and water in this state even able to break the glass.

Most important for the chromatographic separation are diffusion coefficient, viscosity and density of the mobile phase.

The diffusion coefficient has an effect on the exchange kinetics of the sample components between the stationary and mobile phases. The bigger, the faster sets the partition equilibrium. The dissolving capacity of the mobile phase depends on the density. A high density allows for a dissolution of the analysis component. Thus, there is no need to evaporate the sample, such as in gas chromatography (GC). If the viscosity is relatively large, it comes in long capillary columns and packed columns for high pressure drop. With increasing density, the viscosity of the supercritical fluid is more similar to that of the gases ( is therefore relatively small).

Mobile and stationary phase

As described above, the mobile phase is a supercritical fluid at the SFC. There are only compounds, which are relatively easy to transfer to the supercritical state for the simplest possible handling apparatus. The critical pressure and critical temperature may therefore not be too high. A selection of such substances is listed in the following table.

In addition to aliphatic hydrocarbons, such as pentane, carbon dioxide is currently probably the most commonly used because it is inert and non-toxic, has good solvent power and is very inexpensive to obtain. A disadvantage is the low polarity. Therefore often other components of the mobile phase must be added. Such modifiers are for example, methanol, acetone, hexane, and methylene chloride. By polar solvent and the solvent power can be improved. Xenon and other noble gases are higher for reasons of cost used rarely. Apart from ammonia used is also not highly polar compounds, such as hydrohalic acids, alkyl bromides, and higher nitrogen oxides due to their physiological properties, high aggressiveness and instability.

Special stationary phases have not yet been developed for the SFC. In general, using the known materials for gas chromatography and HPLC. For example, unmodified and chemically modified silica gels, such as those used in the RP-HPLC. Or you can take polymers such as polysiloxanes from the GC for the coating of carrier particles and capillary columns. The stationary phase can be a solid or liquid to be in the form of a thin film on a substrate.

Construction of apparatus

The equipment have been taken mostly from the fields of gas chromatography and HPLC. The pressure control and promotion of the mobile phase is usually about Langhubkolbenpumpen. The injection system for the sample is a multi-port valve with small internal sample loop as in HPLC. It basically has two ways of capillary column ( capillary SFC, CSFC ) or the packed column ( PSFC ) and in the SFC. Due to the increased pressure drop, the packed columns are relatively short - from 5 to 25 cm. In capillary lengths of 5 to 20 m is common.

Such as in gas chromatography, the temperature can be controlled by a column oven. A very important additional component in the SFC apparatus is the restrictor. This is a precision valve or a capillary tube at the end of the column. The restrictor is designed to ensure the required minimum pressure for the supercritical state until the end of the column. Particularly important is the restriction in capillary columns because of the very small flow rates.

The detection is dependent in part on the material used for the mobile phase. For example ( FID) is very good for pure carbon dioxide the flame ionization detector. However, when added to a modifier, the FID is less useful.

Most detectors are used in conjunction with capillary GC, wherein the packed columns mainly from the HPLC, such as UV / VIS detector.

• Chromatography