Packing material change

Occasionally, samples are run that adsorb onto the packing material. Generally, if one of the performance characteristics of the column changes by 10% or more, it is prudent to clean the column. These performance characteristics are (1) asymmetry factor, retention time, resolution, and theoretical plates. 

Note that when packing is changed from one material of construction to another, it is important to recognize the effect on minimum wetting rate for the new condition. 

In the pharmaceutical industry, chromatographic QC methods are often used with no changes over long time frames. The basic reason for this is the amount of effort necessary for a complete revalidation of a method on a new column brand. Because of these constraints, chromatographic columns should be chosen from reliable and reputable manufacturers that can sustain a reproducible production of the columns over such extended time periods. Until recently, the capability of column manufacturers to reproduce the packing materials was not known, and users needed to rely on their intuition and trust the column manufacturers. 

Gradient elution requires packing materials that can withstand the accompanied changes in solvent composition. 

The selectivity of a column, a, is also a function of the packing material but can be varied by changing the temperature or the nature of the mobile phase. The value for a can range from 1 if the retention times of two peaks are identical to infinity if the first peak is eluted in the void volume. 

Capacity parameters are not often used as primary optimization parameters in chromatography. Therefore, they are only included in table 3.10 in those cases in which they are used with some frequency. It should be noted, however, that changing one of the capacity factors usually involves the use of a completely different column and is therefore unattractive. Although changing the capacity parameters affects retention in an essentially predictable way, changing the column (packing material, film thickness, etc.) may give rise to unexpected second order phenomena. This is a second reason for which capacity parameters should not be recommended as primary optimization parameters. 

In packed columns, there are two parameters which may be varied independently in order to optimize the column characteristics, i.e. the diameter of the column and the diameter of the particles. In open columns, only the column diameter may be varied. Additionally, the phase ratio may be varied by changing one of the capacity parameters (see section 3.5). For packed columns these parameters include the surface area of the packing material, the column porosity and the stationary phase film thickness. For open columns only the latter parameter is relevant. 

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