Concurrent eluent evaporation

LOOP-TYPE INTERFACES (CONCURRENT ELUENT EVAPORATION)... 

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. 

While partially concurrent eluent evaporation is easier to use, and is preferred for the transfer of normal phase solvents, concurrent eluent evaporation with co-solvent trapping is the technique of choice for transfer of water-containing solvents, because wettability is not required. 

Figure 2.7 (11) shows a gas chromatogram obtained by co-solvent trapping and concurrent eluent evaporation after injecting 500 p.1 of diluted gasoline. The main solvent was -pentane with 5% of -heptane as co-solvent. It is noteworthy that without the co-solvent, higher-boiling compounds could be lost. 

Coupled liquid chromatography-gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC-GC transfer methods that are now available (1, 43-52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent. 

K. Grob, Concurrent eluent evapor ation with co-solvent Capping for on-line reversed-phase liquid cliromatography-gas clir omatogr aphy. Optimization of conditions , J. Chromatogr. 477 73-86 (1989). 

The analysis of sterols, sterols esters, erythrodiol and uvaol, and other minor components of oils and fats, is usually carried out by normal-phase HPLC-HRGC by using a loop-type interface and the concurrent eluent evaporation technique, as reported in the papers cited by Mondello et al. (48) (up to 1995) and in more recent papers (49, 50). More recently, reversed-phase LC-GC methods have been... 

Figure 2.5 Schematic representation of a loop-interface scheme for concurrent eluent evaporation. The sample is first loaded in a loop and then, after switching the valve, directed by the carrier into the GC column. The solvent evaporates from the front end of the liquid, thus causing band broadening. Since the column is not flooded, very large amount of liquid can be introduced.

Figure 2.7 Gas chromatogram obtained for 500 p.1 of diluted gasoline in n-pentane introduced by concurrent eluent evaporation, using n-heptane as the co-solvent. Reprinted from Journal of High Resolution Chromatography, 11, K. Grob and E. Muller, Co-solvent effects for preventing broadening or loss of early eluted peaks when using concurrent eluent evaporation in capillary GC. Part 2 n-heptane in n-pentane as an example , pp. 560-565, 1988, with permission from Wiley-VCH.

M. Biedermann, K. Grob and W. Meier, Partially concurrent eluent evaporation with an early vapor exit detection of food irradiation through coupled LC-GC analysis of the fat , /. High Resolut. Chromatogr. 12 591-598 (1989). 

Measurement of performance 102-103 see also gas chromatography Comprehensive (coupled) liquid chromatography 253-254 Concurrent eluent evaporation 22-25, 26, 28-29... 

Retention gap techniques with on-column interfaces are used to analyze those analytes that are poorly retained during full concurrent eluent evaporation [98-102,105]. The eluent fraction is pushed by the liquid chromatography pump into the thermostatted retention gap where it is mixed with carrier gas and evaporated at a temperature below the pressure corrected eluent boiling point. A layer of condensed solvent (flooded zone) is formed in front of the evaporation site that acts as a temporary retaining liquid phase. Partial concurrent eluent evaporation during transfer allows the use of shorter retention gaps and an increase in the eluent volume that can be transferred. A 10 m x 0.53 mm... 

Figure 3.11. Schematic diagram of a loop type interface with concurrent eluent evaporation for LC-GC. (From ref. [100] Elsevier).

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