Zero-length column chromatographic

More recently, a detailed study of diffusion of the xylene isomers in large crystals of NaX and natural faujasite was undertaken by both sorption rate and tracer exchange.(11-14) The data obtained by both these techniques using several different crystal sizes were entirely consistent but the diffusivities were much smaller than the values derived for the same systems by NMR PFG measurements. In an attempt to resolve this discrepancy we have developed a new chromatographic technique (zero length column or ZLC) which is less sensitive than conventional sorption methods to the intrusion of external heat and mass transfer resistances and which is therefore useful for following relatively rapid diffusion processes. The method has now been applied to study the diffusion of a range of different hydrocarbons in both A and X zeolite crystals and the results of these studies are summarized here. 

Macroscopic, such as the analysis of uptake curves, Wicke-Callanbach methods based on steady-state or transient diffusion cell, time lag method, chromatographic methods, zero length column (ZLC) method, and FR method... 

Figure 5.2. Schematic diagram of a standard liquid chromatograph modified for use with a packed capillary column. Typical pump settings are 300-400 p,l / min with flow splitting of 1 2000 to give a column flow of 150-200 nl / min. The microinjection valve has a 40 nl internal loop and an additional T.IO split creates an injection volume of 2-3 nl (larger volumes can be injected by on-column focusing with gradient elution separations). The detector uses a U- or Z-shaped flow cell with a 3 nl volume and 8 mm path length. Fused-silica capillary tubing with an internal diameter < 20 pm and zero-dead-volume connectors are used for column connections. (From ref [8j. American Chemical Society).

One instrument configuration utilized in this laboratory is shown in Figure 5. In this instrument the column was mounted in a constant temperature gas chromatograph oven, which also served to heat the air circulated through the DFI probe. A zero dead volume union is typically used to connect the column to a short length of 4-8 ym i.d. or contoured (tapered) fused silica restrictor. The restrictor and probe tip are heated to compensate for cooling due to decompression of the fluid during the DFI process. 

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