Resolving Power of a Column

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In addition, even if peak area measurements are used, a separation of 4a will usually 

Although not apparent at this time, it will become clear that two adjacent peaks from solutes of different chemical type, or significantly different molecular weight, are not likely to have precisely the same peak widths (i.e., exhibit the same efficiency). Nevertheless, in most cases, the difference will be relatively small and, in fact, likely to be negligible. As a consequence, the widths of closely adjacent peaks will, at this time, be assumed to be the same. 

If the widths of the two peaks are the same, then the peak width in volume flow of mobile phase will be 

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In Section 30E-7, we show how we use the retention factor to compute the resolving power of a column. 

In order to define the resolving power of a column in tenns of molecular weight, there must be an equation that relates A and This can be calculated by an equation defining the linear portion of the SEC calibration curve ... 

Gives the resolving power of a column with regard to the separation of components i and j (isothermally). t- = retention time of substance i ... 

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. 

The separating power of a column can Ise expressed as its peak capacity defined as the number of peaks that can be resolved, at any specified resolution level, in a given separation time. For the general case it can be calculated using equation (1.49)... 

Equation (2) shows that the resolution is a function of three different factors (1) the resolving power of the column as measured by the plate number that expresses the relative width of bands (2) the relative retention of the two compounds that measures how far apart the bands are from each other and (3) the magnitude of retention, as separation is a result of retention. The relative influence of these factors has been discussed by Snyder (72,13) in a form very easy to use in practice. 

It is seen from equation (9) that the resolving power of the column, as defined by Giddings, will be directly proportional to the square root of the number of effective plates. As a consequence (R) can be used by the chromatographer to directly compare the resolving power of columns of any size, or type. However, the value of (R) will vary with the value of (k ) for the solute, and so comparisons between columns must be made using solutes that have the same (k ) value. 

It is also of interest to the chromatographer to know the minimum (a) value of a pair of solutes that can be separated on a particular column. In fact, this has been suggested, (11), as a basis for comparing the resolving power of different columns. The disadvantage of this type of criteria is that the value of (a) becomes smaller the higher the resolving capacity of the column. Nevertheless, the minimum value of (a) is important in practice and it is of interest to see if it can be related to the effective plate number of the column. 

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