Mechanical Stability and Hydrodynamic Properties

Mechanical and chemical stability of novel stationary phases are basic requirements concerning their application. A lack in stability generally causes a loss in resolution and thus reduces column efficiency. In addition, the reproducibility of retention times, being important for qualitative analysis, may be affected. Evaluation of the mechanical stability of polymeric stationary phases is usually accomplished by the determination of the pressure drop across the column, when employing solvents of different polarity within a wide range of flow rates. A stationary phase can be considered as mechanically stable if a linear relationship between applied flow rate and resulting back pressure is obtained. 

An important measure concerning column characterization in LC is the column permeability, which represents the capacity of the support to transport the mobile phase as consequence of a pressure drop occurring over the column. In other words, the permeability of a column determines the required pressure to achieve the desired flow rate. The linear flow velocity (u) across an empty cylindrical column is given by 

Assuming a column fllled with stationary phase, only a certain volume, namely the interparticulate or interstitial volume (e ), is accessible to the mobile phase by convection. Thus, the volumetric flow rate through a packed column can be formulated as 

Since chromatographic flow is laminar in nature, the specific permeability (Bq) of a packed column can be calculated using the Darcy equation [131,132] 

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