Cryogenic focusing

Figure 8.22 Schematic diagram of the Suprex MPS/225 integrated aupercritical fluid extractor, cryogenically focused interface and supercritical fluid chromatogra d>. The bold lines represent the direction of fluid flow in the load and inject positions.

Peak width is usually reasonably sharp on the chromatogram. Peak width can sometimes be tightened by using cryogenic focusing, although this is usually unnecessary. 

Fig 1 The effect of injection volume and cryogenic focusing, head space on column injection test mixture of (Table 1). Operating conditions temrera-ture 30 °C 3 min., 8°/min to 100 hydrogen carrier at 50 cm/sec (lOO °C), chart speed 1 cm/min. A - 50 4.injection without liquid nitrogen B -100/4. 

Another way to increase sensitivity is to increase the volume injected into the column. Lowering the temperature of the entire column in cryogenic oven trapping (COT) methods or for the injection port at the inlet of the column in cryogenic focusing (CF) methods allows the injection of as much as 10 times the typical sample volume. Liquid nitrogen or liquid carbon dioxide is used to lower the temperatures to well below 0 °C (—180 °C for nitrogen and 90 °C for carbon dioxide). These techniques result in better peak shapes in addition to increased sensitivity. 

The sorbant trapping process in purge and trap analysis is essentially a concentration step, but combined with cryogenic focusing this affords better injection on to the chromatography column and loss of very volatile substances, e.g. vinyl chloride which may not be trapped on the absorbant. 

Figure 33-11. A splitless interface with cryogenic focusing.

Many factors may influence the method precision the place of the fiber in the injector (for each desorption it must be in the warmest region), the temperature of the cryogenic focusing (stabile and quite low) the time between extraction and desorption (the volatile analytes may get lost in the fiber). Table 1 presents the SPME fibers and their recommended use. 

The systems designed for workplace atmosphereswhich are usually monitoring for ppm levels, are relatively simple compared to the ambient air monitoring systems which are used for low levels of compounds in a complex mixture. These systems often utilize multiple desorption transfer, cryogenic focus-... 

The sample introduction system for subsequent GC/MS analyses of urine headspace volatiles contained within these pressurized stainless steel canisters involved the initial release of the volatiles from the canister and their adsorption onto an in-line Tenax trap. In turn, desorption from the Tenax, employing a six-port valve in line with a U-tube cryogenic trap (0.125 in OD x 9 in) containing 60/80 mesh glass beads, was followed by cryogenic focusing on this loop. Compounds were then released from the loop by heat, and separated by gas chromatography. 

Cold traps are used for two reasons enrichment purposes and solute band concentration. There are two typ>es of cryogenic trapping by cryogenic condensation and cryogenic focusing. 

In cryogenic focusing, volatile compxmds are trapped in the liquid phase of a chromatographic column at a low temperature which, however, preserve its chromatographic properties. In other words cryogenic focusing is based in the same... 

Wylie, P. L. (1986) Headspace analysis with cryogenic focusing a procedure for increasing the sensitivity of automated capillary headspace analysis. Chromatographia, 21, 251-8. 

Jacobsson and Falk [56] suggested determination of low (ng/ml) concentrations of hydrogen sulfide in aqueous solutions with ALOT column (PoraPLOT Q). The detection limit is about 1 ng/ml (see Fig. 2-11 [56]). If greater sensitivity is needed stripping analysis in combination with a cryogenic focusing device should be used. A prerequisite of such a method, however, is that water has to be removed prior to the trap (for example, by calcium chloride [57]) in order to prevent plugging of the cold trap. Thus, ALOT columns can be recommended for trace analysis of polar compounds. 

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