Retention gap

The retention gap method (1, 2) represents the best approach in the case of qualitative and quantitative analysis of samples containing highly volatile compounds. The key feature of this technique is the introduction of the sample into the GC unit at a temperature below the boiling point of the LC eluent (corrected for the current inlet pressure), (see Eigure 2.2). This causes the sample vapour pressure to be below the carrier gas inlet pressure, and has two consequences, as follows ... 

These effects refer to the reconcentration obtained with an uncoated inlet. In fact, the term retention gap means a column inlet of a retention power lower than that of the analytical column. This retention gap is placed in front of the analytical column, thus allowing different reconcentration mechanisms to occur. 

Together with this solvent effect, another effect, called phase soaking, occurs in the retention gap technique if a large volume of solvent vapour has saturated the carrier gas, the properties of the stationary phase can be altered by swelling (thicker apparent film), a change in the viscosity or changed polarity. The consequence is that the column shows an increased retention power, which can be used to better retain the most volatile components. 

Phase ratio focusing is based on the higher migration speed of components through the retention gap compared to that through the analytical column. Reconcentration depends on the ratio between the retention power in the pre- and in... 

Transfer of an LC fraction of 300 jl1 volume occurred by the conventional retention gap technique. In fact, Figure 2.3fb) shows the GC chromatogram obtained after the transfer of the LC fraction. 

The retention gap techniques, essential for the analysis of very volatile components, are often replaced by concurrent eluent evaporation techniques, due to their simplicity and the possibility of transfering very large amount of solvent. In this case, the solvents are introduced into an uncoated inlet at temperatures at or above the solvent boiling point. 

The main advantages of this technique are that short retention gaps are sufficient (3-20 m), the evaporation rate is faster than that obtained with transfers at a temperature below the boiling point of the solvent, and the requirement that solvent... 

Figure 2.10 Schematic representation of a vaporizing chamber/precolumn solvent split/gas discharge interface, where the vaporizer is packed and heated at a suitable temperature for solvent evaporation. The vapour exit can be positioned at the end of the retention gap.

Figure 2.12 Schematic representation of an on-line SPE-GC system consisting of three switching valves (VI-V3), two pumps (a solvent-delivery unit (SDU) pump and a syringe pump) and a GC system equipped with a solvent-vapour exit (SVE), an MS instrument detector, a retention gap, a retaining precolumn and an analytical column. Reprinted from Journal of Chromatography, AIIS, A. J. H. Eouter et al, Analysis of microcontaminants in aqueous samples hy fully automated on-line solid-phase extraction-gas chromatography-mass selective detection , pp. 67-83, copyright 1996, with permission from Elsevier Science.

K. Grob and Z. Li, Intr oduction of water and water-containing solvent mixtures in capillary gas clir omatogr aphy. I. Eailure to produce water-wettable precolumns (retention gaps) , J. Chromatogr. 473 381-390 (1998). 

Figure 4.5 (a) By placing a short retention gap column before the cryortap, solute will be... 

Many applications involving the study of the composition of essential oils are based on the use of the on-column interface and retention gap techniques because of the high volatility of the components to be analysed. 

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

The on-eolumn interfaee is the one whieh is most often used in LC-GC of aqueous samples beeause it ean be applied to a wider range of eompounds.The loop-type interfaee is limited for determining volatile eompounds that are volatilized together with the solvent and not retained in the retention gap. Several attempts at solving this problem have been made. One option is to add a eo-solvent whieh enters the retention gap before the analytes and thus forms a eo-solvent film in front of the eluate. 

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