Retaining precolumn

In this way, the liquid can be transferred at a speed corresponding to the evaporation speed. The fraction to be analysed is contained in a loop (see Eigure 2.5), connected to a switching valve. By opening the valve, the sample in the loop is driven by the carrier gas into the GC unit (8), instead of the LC pump. An early vapour exit is usually placed after a few metres of the deactivated precolumn (9) and a short piece (3-4 m) of the main column (retaining precolumn). This valve is opened during solvent evaporation in order to reduce the amount of solvent that would reach the detector, and at the same time, to increase the solvent evaporation rate (6). 

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.

However, the most frequently used system is trace enrichment in a short LC column or SPE precolumn. The column is usually filled with C18 or PLRP-S and dried with nitrogen prior to elution. The analytes are eluted with an organic solvent, usually ethyl acetate, which is injected into the gas chromatograph through an on-column interface by a retention gap. In the chromatograph there is also a retaining precolumn, to minimize losses of the most volatile compounds, an analytical column and, between the precolumn and the analytical column, a solvent vapour exit (SVE) to eliminate the vapour (Figure 13.18). 

The sample must be introduced at a rate slightly exceeding the evaporation rate. Slower injection causes the loss of solvent trapping because all solvent evaporate concurrently, whereas a too high introduction rate results in flooding of the solvent into the retaining precolumn and, eventually, the analytical column and the vapor exit. 

Triazine herbicides represent another class of herbicides that are found widely in groundwater and surface water. This method uses the automated analysis of 10-mL water samples by SPE followed by analysis directly by GC/MS (Brinkman, 1995). The method uses innovative technology to interface the SDU of the PROSPEKT with a precolumn in the GC. The GC is modified such that the sample may be injected onto a precolumn, which is essentially a retention gap column of uncoated deactivated fused-silica capillary that is several meters in length, with the analytical column off-line (see Fig. 10.20). Then the GC may be turned on and analyze the sample automatically. Because the retention-gap column is uncoated, there is refocusing of the analyte on the retaining precolumn, even with large injection volumes of up to 100 pL. 

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).

Alternatively, the fraction is pushed by the LC pump into a 5-10 cm fused silica capillary containing a piece of wire and heated to 250-350°C ( wire interface ). The vapors escape through a retaining precolumn of some 3 m X 0.32 mm i.d. The oven temperature must again be above the solvent boiling point. 

Since the vapors are diluted with carrier gas, this permits lower oven temperatures. Performance for volatile solutes is improved, but the suitable oven temperature is more difficult to predict. The vapors are released through a 50 cm x 0.53 mm i.d. retaining precolumn. 

Figure 3 Scheme of on-line SPE-GC-MS system. Abbreviations AC, analytical column RG, retention gap RP, retaining precolumn R, restriction, Vl-4, valves. 

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