Chromatography Equations

General

Column Efficiency: 

In general, N = number of Theoretical Plates, a is a constant depending on the method used, t_R = retention time of peak, and W = the peak width at a given peak height.  N is unitless.  The peak width at ½ height is the most commonly used method for calculating HPLC Column efficiency

Capacity Factor (also known as Retention Factor or Relative Retention)   

The Capacity Factor, k, of a sample component is a measure of the degree to which that component is retained by the column relative to an unretained component.  Where t_R is the elution time of the retained component, and t₀ is the elution time of the unretained sample.  Uracil is often used to measure t₀

Separation Factor (also know as Selectivity)

The selectivity parameter, is the spacing between two peaks.

Resolution

Rs is defined as the amount of separation between two adjacent peaks, where k is the average value for the two peaks.

GC

Phase Ratio

Where d = column internal diameter and uf = film thickness.  Phase Ratio encompasses both the film thickness and column internal diameter to give a value that can characterise all column internal diameters and film thickness combinations.

TLC

Retention Factor

Equation for describing the solvent migration through the sorbent layer 

Z_f is the distance from the immersion line to the solvent front, k is the flow constant (or velocity coefficient) and t is the time of development.

Solvent front velocity, V_f is defined by 

Relationship between pH and pKa

According to the Henderson-Hasselbach equation, the relationship between pH, pKa, and the relative concentrations of an acid and its salt is as follows:

where [A^-] is the molar concentration of the salt (dissociated species) and [HA] is the concentration of the undissociated acid.  When the concentrations of the salt and acid are equal, the pH of the system equals the pKa of the acid.

VWR Chromatography 2004