Supercritical fluid chromatography hexane separation

The technology alternative to chiral LC is supercritical fluid chromatography (SFC) (Liu et al., 2002 Mangelings and Vander Heyden, 2008a, 2008b). SFC is essentially an NP format where CO2 is substituted for hexane. Gradients work well with SFC and usually span 5—50% B (alcohol) and facilitates fast method development. The combination of gradient and low viscosity often facilitates fast separations (usually at least three times faster than NP-LC). SFC works when combined with MS detection, but historically has not been easy to perform and has not worked as well as RP-LC/MS (White and Burnett, 2005). 

Supercritical fluid chromatography (SFC) is a hybrid of GC and HPLC in which the mobile phase is a compound that is held above its critical temperature Tc) and critical pressure Pc)- By far the most common compound that is used as an SFC mobile phase is CO2, for which Tc 31°C and Pc 73 atm. The diffusivities of many compounds in supercritical CO2 are high enough that either packed-column or open-tubular columns may be used. The solvent strength of supercritical CO2 is roughly intermediate between those of n-hexane and benzene thus, many compounds that are too nonvolatile to be separable by GC can be separated by SFC. Detection of solutes separated by SFC is also easy, since GC detectors such as the FID and HPLC detectors such as UV absorption detectors can both be used. 

The mobile phase is the phase which moves in a definite direction. It may be a liquid (LC and CEC), a gas (GC), or a supercritical fluid (supercritical-fluid chromatography, SFC). The mobile phase consists of the sample being separated/analyzed and the solvent that moves the sample through the column. In the case of HPLC the mobile phase consists of a non-polar solvent(s) such as hexane in normal phase or polar solvents in reverse phase chromotagraphy and the sample being separated. The mobile phase moves through the chromatography column (the stationary phase) where the sample interacts with the stationary phase and is separated. 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

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