Column resolving power

To compensate for, what appeared to be very misleading efficiencies values, the effective plate number was introduced. The effective plate number uses the corrected retention distance, as opposed to the total retention distance to calculate the efficiency. Otherwise the calculation is the same as that used in the normal calculation of theoretical plates. In this way the effective plate number becomes significantly smaller than the true number of theoretical plates for solutes eluted at low k values At high k values, the the two measures of efficiency tends to converge. In this way the effective plate number appears to more nearly correspond to the column resolving power. In fact, it is an indirect way of trying to define resolution in terms of the number of effective plates in the column. 

For the use of the plate theory to determine peak widths and column efficiency, see Columns Resolving Power, p. 491. 

Most conventional applications in GC use temperature-programming rates that are smaller than need be, resulting in substantially longer analysis times with little increase in column resolving power relative to that obtained with faster temperature programming and shorter analysis times. This is a result of the unavailability of necessary instrumentation optimized in component design with acceptable simplicity and performance levels. Sacks has prepared a definitive treatment of fast GC, including all instrumental aspects (19). 

Equation (19) shows that the resolving power of the column (using the definition proposed by Giddings) is directly proportional to the square root of (Ne), the number... 

As a secondary consideration, the chromatographer may also need to know the minimum value of the separation ratio (a) for a solute pair that can be resolved by a particular column. The minimum value of (a) has also been suggested [8] as an alternative parameter that can be used to compare the performance of different columns. There is, however, a disadvantage to this type of criteria, due to the fact that the value of (a) becomes less as the resolving power of the column becomes greater. Nevertheless, a knowledge of the minimum value of (cxa/b) can be important in practice, and it is of interest to determine how the minimum value of (aA/B) is related to the effective plate number. 

An expression for the maximum charge that can be placed on a column without impairing resolution has already been derived, but the approach, when dealing with an overloaded column for preparative purpose, will be quite different. For preparative purposes the phase system is chosen to provide the maximum separation of the solute of interest from its nearest neighbor. It should be pointed out that the separation may, but probably will not, involve the closest eluting pair in the mixture. Consequently, the maximum resolving power of the column will not be required for the purpose of separation and the excess resolution of the solute of interest from its nearest neighbor can be used to increase the column load. 

In our early evaluations, three parameters were utilized for the resolving power of the columns (3,4,7). These were the valley-to-peak height ratio, v, the peak separation parameter, P, and the parameter mentioned earlier, Djcr. The valley-to-peak height ratio is defined as... 

In 1993, Jorgenson s group improved upon then earlier reverse phase HPLC-CZE system. Instead of the six-port valve, they used an eight-port electrically actuated valve that utilized two 10-p.L loops. While the effluent from the HPLC column filled one loop, the contents of the other loop were injected onto the CZE capillary. The entii e effluent from the HPLC column was collected and sampled by CZE, making this too a comprehensive technique, this time with enhanced resolving power. Having the two-loop valve made it possible to overlap the CZE runs. The total CZE run time was 15 s, with peaks occurring between 7.5 and 14.8 s. In order to save separation space, an injection was made into the CZE capillary every 7.5s,... 

In 1995, Moore and Jorgenson used the optically gated CZE system to obtain extremely rapid separations with HPLC coupled to CZE. The rapid CZE analysis made possible more frequent sampling of the HPLC column, thus increasing the comprehensive resolving power. Complete two-dimensional analyses were performed in less than 10 min, with the CZE analyses requiring only 2.5s. A peak... 

For example, a 30-m column (regardless of diameter) should have a tR for argon or butane of approximately 100 sec. It appears better to set the linear velocity higher than the optimum rather than lower than the optimum to obtain good column efficiency. Determine the column temperature where the most difficult-to-separate compounds elute and set the linear velocity at that temperature. Now the column will exhibit its maximum resolving power at the point where it is needed most. 

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