Precolumn hyphenation

Recovery procedures have traditionally involved some form of solvent, gas or heat extraction from the bulk sample matrix. Some of these lend themselves to precolumn hyphenation (e.g. SFE, TD, Py, HS), as opposed to others (e.g. Soxhlet, ultrasonics). Extraction of additives should not be considered as an isolated step, because it may strongly influence the subsequent chromatographic separation. The success of an analysis may very often depend more on the extraction procedure than on the chromatographic separation. In hyphenation there should be compatibility between the sample preparation and subsequent chromatographic analysis. 

Ion exchange Solid-phase extraction Size-exclusion Multidimensional chromatography 

The solvents used in extraction may affect subsequent chromatography. Precolumn hyphenation is considered to be much more difficult and critical than the more developed postcolumn hyphenation. Sample preparation and injection are considerable bottlenecks in terms of ruggedness. 

The most common precolumn chromatographic techniques discussed here are SFE (Section 3.4.2), SPE (Section 3.5.1) and SPME (Section 3.5.2). However, sampling methods such as thermal desorption, pyrolysis and headspace (Section 4.2.2) may also be classified in this category. 

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 

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Compatibility with instrumental analysis (precolumn hyphenation)... 

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