Supercritical fluid chromatography

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The supercritical or supercritical liquid chromatography ( abbr. SFC , engl. Supercritical Fluid chromatography ) is an analytical detection methods of chromatography .

The SFC uses mobile phases that are beyond the substance-specific critical temperature and pressure . Many physical properties of such fluids lie in between those of gases and liquids . Particular mention should be made of the density , viscosity and the diffusion coefficient . For example, supercritical fluid chromatography can be used to deal with analytical problems for which gas or liquid chromatography can no longer be used.

Physical basics

The most decisive characteristic of the SFC is the physical state of the mobile phase. The substance used for this is in a supercritical state . Simply put, it is a state between gas and liquid. This can be explained by the fact that the supercritical fluid is at such a high temperature that it cannot be converted back into a purely liquid state by applying any pressure, however high. The lowest for accurate temperature critical temperature T is called C . However, in order to precisely delimit the supercritical area, the critical pressure p C must still be defined. If this is exceeded, the purely gaseous state cannot be reached at any temperature, however high. Both quantities are substance-specific constants. The following table shows some examples.

substance critical temperature critical pressure
° F ° C lb / sq.in atm.
air −220 −140 573 39
Ethanol (C 2 H 6 O) 421 216 956 65
Ammonia (NH 3 ) 266 130 1691 115
Benzene (C 6 H 6 ) 554 292 735 50
Carbon dioxide (CO 2 ) 88.2 31 1132 77
Carbon monoxide (CO) −222 −141 528 35.9
Diethyl ether (C 4 H 10 O) 381.2 194 544 37
Hydrogen (H 2 ) −402 −242 294 20th
Nitrogen (N 2 ) −236 −149 514 35
Oxygen (O 2 ) −180 −118 735 50
Water (H 2 O) 706-716 375-380 3200 217.8

For example, if you take a chemical compound such as carbon dioxide and change the pressure and temperature in a closed vessel, you can achieve the corresponding states of aggregation. If you move (with the pressure and temperature values) along the phase boundaries, there are two states of aggregation, at the so-called triple point there are even three. If you exceed the critical point c p , you get into the supercritical area.

Starting from this state, the following consideration can now be made: If the pressure is reduced isothermally, the supercritical fluid is continuously converted into the gaseous state of aggregation. If you cool down isobarically (at the same pressure), you get into the liquid area. There is no abrupt crossing of a phase boundary between gaseous and liquid. The material properties are continuously changed.

Supercritical fluids therefore have the properties of gases and liquids. They can be compressed just like gases, so that the pressure can be used to adjust the density. The density can also be increased by lowering the temperature at constant pressure.

Supercritical fluids also have a variable solubility , which can be smaller, but also very much larger than that of pure liquids. For example, ammonia and water in this state are even able to dissolve glass .

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

The diffusion coefficient influences the exchange kinetics of the sample components between the stationary and mobile phase. The larger, the faster the distribution equilibrium is established. The solubility of the mobile phase depends on the density. A high density allows the analysis components to dissolve. There is no need to evaporate the sample as in gas chromatography (GC). If the viscosity is relatively high, there is a high pressure drop in long capillary columns and packed columns. With increasing density, the viscosity of the supercritical fluids is more similar to that of the gases (i.e. it is relatively small).

Mobile and stationary phase

As already described, the mobile phase in the SFC is a supercritical fluid. For the simplest possible handling of apparatus, only those compounds come into consideration that are relatively easy to convert to the supercritical state. The critical pressure and temperature must therefore not be too high. A selection of such substances is listed in the following table.

mobile phase T C (° C) p C (MPa) Density (g / cm³)
Carbon dioxide (CO 2 ) 30.05 7.37 0.47
Propane (C 3 H 8 ) 96.8 4.26 0.23
n -pentane (C 5 H 12 ) 196.5 4.22 0.24
Trifluoromethane (CHF 3 ) 23 4.81 0.52
Xenon (Xe) 16.6 5.83 1.11
Ammonia (NH 3 ) 132.4 11.27 0.24

In addition to aliphatic hydrocarbons such as pentane, carbon dioxide is probably the most frequently used at the moment because it is inert and non-toxic, has good dissolving properties and can be obtained very inexpensively. One disadvantage is the low polarity . This is why other components of the mobile phase often have to be added. Such modifiers are, for example, methanol , acetone , hexane and methylene chloride . Polar solvents can also improve the dissolving power. Xenon and other higher noble gases are rarely used for reasons of cost. Apart from ammonia, no highly polar compounds such as hydrohalic acids , alkyl bromides and higher nitrogen oxides are used because of their physiological properties, high aggressiveness and instability.

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

Apparatus construction

The equipment technology was largely taken over from the areas of gas chromatography and HPLC. The pressure control and delivery of the mobile phase is usually carried out using long-stroke piston pumps . As with HPLC, the injection system for the samples is a multi-way valve with a small internal sample loop.
With the SFC you basically have the two options of the capillary column (capillary SFC, cSFC) or the packed column (pSFC). Because of the greater pressure drop, the packed columns are only relatively short - 5 to 25 cm. For capillary columns, lengths of 5 to 20 m are common.

As with gas chromatography, the temperature can be regulated using a column oven.
Another very important component in the SFC apparatus is the restrictor. This is a precision valve or capillary at the end of the column. The restrictor is intended to ensure the required minimum pressure for the supercritical state up to the end of the column. The restriction on capillary columns is particularly important because of the very low flow rates.

The detection is partly dependent on the substance used for the mobile phase. For example, the flame ionization detector (FID) is quite suitable for pure carbon dioxide . However, if a modifier has been added, the FID is less useful.

Detectors from GC are mostly used in connection with capillary columns, in the case of packed columns mainly those from HPLC, for example the UV / VIS detector.

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