3.3 Supercritical fluid chromatography  (Page 4/5)

Oven

The oven, as referenced before, exists to heat the mobile phase to its desired temperature. In the case of SFC, the desired temperature is always the critical temperature of the supercritical fluid. These ovens are precisely controlled and standard across SFC, HPLC, and GC.

Restrictor

As suggested by its name, the restrictor aims to restrict the flow through the columns. In altering the flow, the speed at which the sample elutes, the resolution at which it elutes, and the properties of the supercritical fluid can be altered, thus allowing for each SFC column to be tailored to the sample in question.

Detector

So far, there has been one largely overlooked component of the SFC machine: the detector. Technically not a part of the chromatographic separation process, the detector still plays an important role: identifying the components of the solution. While the SFC aims to separate components with good resolution (high purity, no other components mixed in), the detector aims to define what each of these components is made of.

The two detectors most often found on SFC instruments are either flame ionization detectors (FID) or mass spectrometers (MS):

• FIDs operate through ionizing the sample in a hydrogen-powered flame. By doing so, they produce charged particles, which hit electrodes, and the particles are subsequently quantified and identified.
• MS operates through creating an ionized spray of the sample, and then separating the ions based on a mass/charge ratio. The mass/charge ratio is plotted against ion abundance and creates a “fingerprint” for the chemical identified. This chemical fingerprint is then matched against a database to isolate which compound it was. This can be done for each unique elution, rendering the SFC even more useful than if it were standing alone.

Sample

Generally speaking, samples need little preparation. The only major requirement is that it dissolves in a solvent less polar than methanol: it must have a dielectric constant lower than 33, since CO ₂ has a low polarity and cannot easily elute polar samples. To combat this, modifiers are added to the mobile phase.

Stationary phase

The stationary phase is a neutral compound that acts as a source of “friction” for certain molecules in the sample as they slide through the column. Silica attracts polar molecules and thus the molecules attach strongly, holding until enough of the mobile phase has passed through to attract them away. The combination of the properties in the stationary phase and the mobile phase help determine the resolution and speed of the experiment.

Mobile phase

The mobile phase (the supercritical fluid) pushes the sample through the column and elutes separate, pure, samples. This is where the supercritical fluid’s properties of high density, high diffusivity, and low viscosity come into play. With these three properties, the mobile phase is able to adequately interact with the sample, quickly push through it, and strongly plow through the sample to separate it out. The mobile phase also partly determines how it separates out: it will first carry out similar molecules, ones with similar polarities, and follow gradually with molecules with larger polarities.