Transfer of LC Fractions

On-line transfer must be capable of introducing many hundreds of microlitres of LC eluent into the GC. While this has been routine for almost a decade for normal-phase LC eluents (typically based on pentane or hexane), transfer of water-containing eluents is still at an experimental stage, both as a result of technical difficulties, and because of limited applicability.

Routine transfer in on-line LC-GC is mostly achieved by on-column techniques. Usually an early vapour exit is used, releasing the solvent vapours through an outlet installed after a pre-column system. This protects the GC detector and accelerates the discharge of the large volume of vapours (increases the evaporation rate). An uncoated and/or a retaining pre-column is used. The uncoated pre-column serves to evaporate the eluent and reconcentrate the initial bands of higher boiling solutes by the retention gap effect. The coated pre-column retains the solutes during release of the solvent.

Distinction must be made between fully and partially concurrent solvent evaporation. Fully concurrent evaporation means that all the eluent is evaporated during introduction into the GC, i.e. no liquid accumulates in the pre-column system. Volumes of up to several millilitres can be transferred, but volatile components are lost. Partially concurrent evaporation leaves behind unevaporated solvent that must be retained by a relatively long uncoated pre-column and vaporized after the end of the transfer. It is used when solvent trapping is needed for the retention of the volatile components during release of the solvent vapours; components as volatile as heptane can be analysed quantitatively in pentane.

The most obvious alternative to on-column transfer, programmed temperature vaporizing (PTV) solvent splitting, has been proposed, but not described as a routine technique so far.

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Solar Panel Basics

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