On-line Coupling of HPLC or SFC

On-line coupling of SFC with gas chromatography has sporadically been used for the investigation of volatiles from aromatic herbs and spices. The requirements for instrumentation regarding the pumps, the restrictors, and the detectors are similar to those of SFE-GC. Additional parts of the device are the separation column and the injector, to introduce the sample into the mobile phase and successively into the column. The most common injector type in SFC is the high-pressure valve injector, similar to those used in HPLC. With this valve, the sample is loaded at ambient pressure into a sample loop of defined size and can be swept into the column after switching the valve to the injection position. The separation columns used in SFC may be either packed or open tubular... 

The coupling of SFE with gas chromatography has also been described, as has chromatography with supercritical fluids (SFC). Examples include the. separation and determination of PCBs, PAHs. and pesticides in such environmental samples as soil and sediments [291]-[294]. A frequently employed technique for determining traces of pesticide in aqueous samples is the on-line coupling of solid-phase extraction or dialysis with HPLC [242]. [243], [295]. 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

In on-line extraction the process is coupled directly ( hyphenated ) to the analytical technique used for further analysis of the extract (either spectroscopy or, more frequently, chromatography, because of the limited selectivity of extraction). Common examples include SFE-GC, SFE-SFC, SFE-HPLC, SFE-FTIR,... 

In on-line collection, the analytical instrument is usually coupled to the extraction experiment. The analyte is trapped at the head of a GC, SFC, or HPLC column, and then analyzed. Thus, an advantage of the on-line approach is good sensitivity because the analyte is concentrated a disadvantage is that all of the analyte is directed into the analytical instrument and none can be analyzed by other techniques without performing another extraction. On-line extraction can also deposit many contaminant(s) onto the column head. There is a risk of overloading the column. The on-line approach minimizes operator intervention. 

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