Column, capillary multipath

It is seen that the Golay equation produces a curve identical to the Van Deemter equation but with no contribution from a multipath term. It is also seen that, the value of (H) is solely dependent on the diffusivity of the solute in the mobile phase and the linear mobile phase velocity, It is clear that the capillary column can, therefore, provide a simple means of determining the diffusivity of a solute in any given liquid. 

The open-tubular column or capillary column is the one most commonly used in gas chromatography (GC) today. The equation that describes dispersion in open tubes was developed by Golay [1], who employed a modified form of the rate theory, and is similar in form to that for packed columns. However, as there is no packing, there can be no multipath term and, thus, the equation only describes two types of dispersion. One function describes the longitudinal diffusion effect and two others describe the combined resistance to mass-transfer terms for the mobile and stationary phases. 

A second difference, between gas and liquid chromatography, lies in the mode of solute dispersion. In the first instance, virtually all LC columns are packed (not open tubes) which introduces a dispersion process into the column that is not present in the GC capillary column. In a packed column the solute molecules will describe a tortuous path through the interstices between the particles and obviously some will travel shorter paths than the average, and some longer paths. Consequently, some molecules will move ahead of the average and some will lag behind, thus causing band dispersion. This type of dispersion is called multipath dispersion and is an additional contribution to longitudinal diffusion, and the two resistance to mass transfer contributions, to the overall peak variance. 

The A term is a constant, independent of flow rate, which accounts for the effects of multipath flows in a column, and also may include the effects of injection and detection (i.e., extra column effects), which serve to broaden a peak. Note that this term is taken as zero for a capillary column. A = 2Xdp, where dp is particle diameter and 2 is a packing factor. 

Golay Equation a modification of the van Deemter equation for a nonpacked (usually capillary) column, i.e., no multipath dispersion effect. When applied to modern capillary gas chromatography the original Golay equation should be corrected for the gas compressibility (use u jit to replace u) and for the film thickness of the stationary phase (Equations [3.53-3.54]). 

Figure 2.118 Path differences for laminar flow in capillary columns (multipath effect, caused by turbulence at high flow rates).

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