Solvent vapor exit

E. C. Goosens, D. de Jong, G. J. de Jong and U. A. Th Brinkman, Reversed-phase liquid cliromatography coupled on-line with capillary gas chromatography. II. Use of a solvent vapor exit to ina ease introduction volumes and introduction rates into the gas cliromato-graplT, J. Microcolumn Sep 6 207-215 (1994). 

Figure 4.13. Schematic diagrams of large-volume injection systems. Top on-column configuration with solvent vapor exit. (Drawing courtesy of Agilent Technologies.) Bottom programmed temperature vaporization configuration. (Drawing courtesy of ATAS, International.)...

Figure 3.9. Schematic diagram of an automated on-line solid-phase extraction-gas chromatography system. Large volume on-column injection with an early solvent vapor exit and retaining precolumn is used for sample transfer. (From ref. [117] Elsevier).

Once these analytes have moved ahead of the film boundary they are no longer retarded and are lost through the solvent vapor exit. The loss of partially trapped analytes is more critical in on-line SPE-GC than for conventional large volume injection because of the nonuniform distribution of analytes in the solvent film. The introduction of a small volume of organic solvent, presolvent, immediately in front of the sample plug to ensure that a solvent film is already present in the retention gap when sample transfer starts, can be used to minimize the loss of volatile analytes. Acceptable recovery of analytes as volatile as xylene and chlorobenzene was obtained using an on-column interface when 30-50 jl1 of methyl acetate was introduced immediately in front of 50 xl of the same desorption solvent. 

Figure 4 Scheme of a pressurized liquid extraction-LC-GC setup. 1, nitrogen 2a-c, pump 3, eluent 4, oven 5, preheating coil 6, extraction vessel 7, cooling coil 8, eluent coil 9, trapping and LC column 10, restrictor 11, precolumns 12, analytical column 13, solvent vapor exit 14, flame ionization detector VI, extraction valve V2-V4, multiport valves. (Reproduced with permission from Hyotylainen T, Andersson T, Hartonen K, Kuosmanen K, and Riekkola ML (2000) Pressurized hot water extraction coupled online with LC-GC Determination of polyaromatic hydrocarbons in sediment. Analytical Chemistry72-. 3070-3076 American Chemical Society.)... 

Figure 3 LC-GC instrument autosampler, two syringe pumps (main pump and slave pump), LC-switching box with injection valve, valve with first LC column and backflush loop (filled from pressurized bottle, not shown), second LC column on heart cutting valve, LC detector (mostly UV), transfer valve directing to waste or into GC, on-column injector or other inlet device, precolumn (coated or uncoated) ending on T-piece directing to solvent vapor exit or separation column.

To achieve solvent evaporation rates suitable for LC at flow rates of 200-400 pi min vapors must be discharged at a rate of 50-300 ml min This presupposes a solvent vapor exit (SVE) between the precolumn (sometimes consisting of two pieces, mostly of 0.53 mm i.d.) and the separation column. The SVE is fully open during solvent evaporation and switched to a strong restriction to achieve a small purge flow during analysis. 

FIGURE 2.8 Basic arrangement of an HPLC-GC device with a sample loop interface. RG, retention gap Cl, retaining column C2, analytical column LVl, large volume injector and SVE, solvent vapor exit. 

Several studies have been published on the use of special stationary phases (using packed GC columns) or special types of deactivated retention gaps [11,12]. Among these, a series of papers by Goosens et al. [13-15] on the use of a carbo-wax-deactivated retention gap to transfer acetonitrile-water eluents from the LC to the GC part of the system, merit attention. When using an on-column interface and a solvent vapor exit (SVE), up to 200 Xl of aqueous-organic eluent could be introduced, provided that the water content of the eluent did not exceed that of the azeotropic mixture (16 vol%). Otherwise, water will be left in the retention gap after evaporation of the azeotropic mixture and will mar the analysis. In order... 

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