Open tubular columns optimum velocity

The efficiency obtained from an open tubular column can be increased by reducing the column radius, which, in turn will allow the column length to be decreased and, thus, a shorter analysis time can be realized. However, the smaller diameter column will require more pressure to achieve the optimum velocity and thus the reduction of column diameter can only be continued until the maximum available inlet pressure is needed to achieve the optimum mobile phase velocity. 

Equation (13) is the first important equation for open tubular column design. It is seen that the optimum radius, with which the column will operate at the optimum velocity for the given inlet pressure, increases rapidly as an inverse function of the separation ratio (cc-1) and inversely as the square root of the inlet pressure. Again it must be remembered that, when calculating (ropt)5 the dimensions of the applied pressure (P) must be appropriate for the dimensions in which the viscosity (r)) is measured. 

In practice, the mobile-phase linear velocity is set slightly higher than optimum SO s to quickcn the chromatographic run time (i.e., the time between injection and separation, and then detection). Figure 4.5 is a plot of H versus u for a packed GC column and for an open tubular GC column. H is much lower for the open tubular column because multiple flow paths are eliminated. Note also that the curvature in the plot in Fig. 4.5 for the open tubular column is much less than that of the packed column. This... 

The measurement and control of carrier gas flow is essential for both column efficiency and for qualitative analysis. Column efficiency depends on the proper linear gas velocity which can be easily determined by changing the flow rate untU the maximum plate number is achieved. Typical optimum values are 75 to 90 mL/min for 1/4" outside diameter (o.d.) packed columns 25 mL/min for 1/8" o.d. packed columns and 0.75 mL/min for a 0.25 fim i.d. open tubular column. These values are merely guidelines the optimum value for a given column should be determined experimentally. 

The optimum mobile phase velocity for an open tubular GC column is given in chapter 13, equation (14). Reiterating this equation,... 

GC capillary coliunns have a uniform geometry and in the absence of a stationary phase H in is directly proportional to the column internal diameter dc and retention characteristics, k, of the stationary phase film including polar character and film thickness [9-11]. Column design may be evaluated for efficiency and optimum mobile phase velocity using the following equation for proposed by Golay when he developed the theory for open tubular (WCOT) columns [9] ... 

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